FIELD ASSESSMENT OF WASTEWATER TREATMENT FACILITIES
IN THE
OAXACA VALLEY, MEXICO
HUMBOLDT STATE UNIVERSITY
By
Peter H. Haase
A Master’s Project
In Partial Fulfillment
Of the Requirements for the Degree
Master of Science Environmental Systems International Development and Technology
May 2010
FIELD ASSESSMENT OF WASTEWATER TREATMENT FACILITIES
IN THE
OAXACA VALLEY, MEXICO
HUMBOLDT STATE UNIVERSITY
By
Peter H. Haase
Approved by the Master’s Project Committee
Robert Gearheart, Major Professor Date
Brad Finney, Committee Member Date
Arne Jacobson, Committee Member Date
Chris Dugaw, Graduate Coordinator Date
Jená Burges, Vice Provost Date
ABSTRACT
FIELD ASSESSMENT OF WASTEWATER TREATMENT FACILITIES
IN THE
OAXACA VALLEY, MEXICO
By Peter H. Haase
Latin America and the Caribbean is the most urbanized region in the developing
world. However, less than 10 percent of domestic and industrial wastewater is properly
treated, causing severe environmental, health, social and economic problems.
Over the past several years, Mexico has undertaken significant sanitation projects
including wastewater treatment projects. State and federal water agencies have assisted both semi-urban and rural communities construct over 80 municipal wastewater treatment plants in the State of Oaxaca in southern Mexico.
From July 2006 through February 2008, an investigation was conducted to evaluate ten (10) wastewater treatment systems in Oaxaca, Mexico. Nine (9) systems utilized vegetated gravel bed treatment systems and the tenth system used wastewater stabilization ponds. The objectives of the investigation were to understand the factors that motivated the communities to install wastewater treatment systems, select the type of systems, costs of the systems, and to review the design and construction of the projects.
The evaluation also evaluated the social, political, economic and technical factors
affecting the performance, operation and maintenance of the plants.
The study found that the majority of the systems were in poor operating condition.
The poor operating systems resulted from various factors including engineering designs
using dated and inaccurate design criteria, inadequate operation and maintenance, and in
some instances substandard construction practices.
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TABLE OF CONTENTS
ABSTRACT………………………………………………………………………………iii
LIST OF TABLES………………………………………………………………………. vii
LIST OF FIGURES…………………………………………………………………….. viii
INTRODUCTION…………………………………………………………………………1 Literature Review………………………………………………………………….4 Setting…………………………………………………………………………….. 4 Methods……………………………………………………………………………4 Study Findings……………………………………………………………………. 5 Conclusions and Recommendations……………………………………………… 5
LITERATURE REVIEW………………………………………………………………… 6 Economic Impacts of Wastewater Discharge……………………………………...6 Investment to Water Supply and Sanitation Sector………………………………. 9 Level of Wastewater Treatment…………………………………………………. 13 Constraints to Improved Sanitation and Wastewater Treatment………………... 16 Inadequate and Poorly Maintained Infrastructure……………………….. 16 Inappropriate Technology……………………………………………….. 18 Financial Constraints……………………………………………………. 21 Insufficient Operation and Maintenance………………………………… 22 Conclusions……………………………………………………………………… 23
SETTING………………………………………………………………………………... 25 State of Oaxaca………………………………………………………………….. 25 Oaxaca Valley…………………………………………………………………… 29 Setting…………………………………………………………………… 30 Physiography……………………………………………………………..31 Mountain Zones…………………………………………………. 31 Piedmont………………………………………………………… 31 High Alluvium……………………………………………………32 Low Alluvium…………………………………………………… 32 Climate…………………………………………………………………... 33 Temperature……………………………………………………... 34 Rainfall…………………………………………………………... 34 Evaporation……………………………………………………… 37 Annual Water Deficit……………………………………………. 37 Surface Water Resources………………………………………………... 38 River (Rio) Atoyac………………………………………………. 38 River (Rio) Salado/Tlacolula……………………………………. 39 Groundwater Resources…………………………………………………. 40 Etla Valley Groundwater Basin…………………………………. 40 Tlacolula Valley Groundwater…………………………………... 41 Zimatlan Valley Groundwater…………………………………... 42
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Water Use………………………………………………………………... 42 Economics……………………………………………………………….. 42 Governance……………………………………………………………… 45 Education…………………………………………………………………47 Health Care……………………………………………………………… 48 Water and Sanitation Infrastructure……………………………………... 49 Water Supply……………………………………………………………. 49 Wastewater Treatment…………………………………………………... 50 Project Costs……………………………………………………...54
METHODS……………………………………………………………………………… 56 Community Selection Process……………………………………………………56 Procedures for Community Visit, Facility Inspection and Data Collection……...57 Technical Evaluation of Wastewater Treatment Plants…………………………. 60 Agency Interviews………………………………………………………………. 61 Communities Visited……………………………………………………………. 61
RESULTS……………………………………………………………………………….. 63 Overview………………………………………………………………………… 63 Community Description…………………………………………………………. 65 Peri-Urban Communities………………………………………………... 65 San Andres Huayapam………………………………………….. 65 San Sabastian de Tutla…………………………………………... 73 Santa Lucia del Carmen…………………………………………. 77 Regional Towns…………………………………………………………. 81 Tlacolula de Matamoros………………………………………… 81 San Francisco Telixlahuaca………………………………………85 Rural Communities……………………………………………………… 89 Santo Domingo de Tomaltepec………………………………….. 89 San Tomas Mazaltepec………………………………………….. 92 Teotilan del Valle………………………………………………... 95 San Dionisio de Ocotepec………………………………………. 99 San Pablo de Mitla……………………………………………... 103 Summary of Findings…………………………………………………………... 107 General Findings……………………………………………………….. 107 Community Field Assessment Summary………………………………. 109 Community Factors…………………………………………….. 109 Governance……………………………………………...109 Education………………………………………………..110 Economic Status………………………………………...110 Employment and Income………………………………. 110 Technical Factors………………………………………………. 113 Engineering Design and Plant Sizing………………….. 113 Construction…………………………………………… 113 Operation and Maintenance……………………………. 114 Operation Status………………………………………... 114 Disposal and Reuse 114
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Ambient Water Quality………………………………… 116 Managerial and Financial Factors……………………………… 118
CONCLUSIONS AND RECOMMENDATIONS…………………………….. 122 Conclusions…………………………………………………………….. 122 Recommendations……………………………………………………… 125
REFERENCES………………………………………………………………… 127
APPENDIX A – TECHNICAL EVALUATION OF WASTEWATER TREATMENT SYSTEMS……………………………………………………...130
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LIST OF TABLES
Table Page
1 Investment by IDB and WB in Water Supply and Sanitation Project…...... 11
2 Sanitary Sewers and Wastewater Treatment Coverage in Latin America and Cuba………………………………………………………….. 15
3 Type of Wastewater Treatment in Latin America and Cuba………………. 17
4 Summary of Various Constraints or Factors Leading to Water and Sanitation Project Failures…………………………………………………. 24
5 The Background Statistics on the Demographics for the Country of Mexico and the State of Oaxaca……………..…………………………….. 29
6 Summary of Historic Climatic Data for Oaxaca Valley Region………….... 36
7 Water and Sanitation Coverage in Oaxaca and Other States of Mexico…... 49
8 Type and Number of Community Wastewater Treatment Plants……...... 51
9 Capital Costs for Five Three-Stage Anaerobic Treatment Systems……….. 55
10 Location and Population Information for Each Community………………. 62
11 Water Quality Data for Wastewater Treatment for Tlacolula……………... 84
12 Community Factors Assessment…………………………………………… 112
13 Summary of Evaluation of Plant Sizing and Design Criteria……………… 115
14 Technical Factors Assessment……………………………………………... 117
15 Managerial and Financial Factors………………………………………….. 120
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LIST OF FIGURES
Figure Page
1 Relief Map of Oaxaca and Region…………………………………………. 26
2 Municipal Boundaries in the State of Oaxaca……………………………… 27
3 Map of Oaxaca Valley and Major Rivers………………………………….. 30
4 Map of the Valley of Oaxaca to Show Main Physiographic Zones………... 33
5 Map of Oaxaca Valley and Predominant Soil Types………………………. 34
6 Mean Monthly Temperature in Oaxaca City………………………………. 35
7 Mean Monthly Precipitation in the City of Oaxaca………………………... 36
8 Mean Monthly Evaporation in the City of Oaxaca………………………… 37
9 Pattern of Annual Water Deficit for the City of Oaxaca…………………... 37
10 Photo of Rio Atoyac in the City of Oaxaca………………………………... 39
11 Photo of Rio Salado, Oaxaca, Mexico……………………………………...40
12 Typical Layout of First Generation Wastewater Treatment System with Anaerobic Digester and Vegetated Gravel Bed………………………. 53
13 Typical Layout of a Three-Stage Anaerobic Treatment Plant at Santo Domingo Tomaltepec……………………………………………….. 54
14 Map of Community Site Visits…………………………………………….. 62
15 Satellite Image of San Andreas Huayapam………………………………... 66
16 Aerial Photo of Wastewater Treatment Plant ………….………………….. 68
17 Treatment Plant Headworks (bar screen and grit chamber) and Anaerobic Digester…………………………………………………………68
18 Upflow Anaerobic Rock Filter……………………………………………..69
19 Vegetated Gravel Beds……………………………………………………..69
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20 Effluent from Upflow Anaerobic Rock Filter and Vegetated Gravel Bed…69
21 Outlet Control Box with Adjustable Pipe outlet…………………………… 70
22 Covered PVC Piping Detail at Treatment Plant…………………………… 71
23 Satellite Image of San Sebastian de Tutla………………………………….. 74
24 Aerial Photo of Wastewater Treatment Plant for San Sebastian De Tutla……………………………………………………………………. 75
25 Photos of Vegetated Gravel Bed and Surfacing Effluent………………….. 75
26 Final Effluent Quality San Sebastian de Tutla……………………………... 76
27 Satellite Image of Santa Lucia del Camino………………………………… 78
28 Inlet and Outlet Weir Configuration for Up Flow Anaerobic Rock Filter…………………………………………………………………. 79
29 Effluent from Up Flow Anaerobic Rock Filter…………………………….. 80
30 Vegetated Gravel Bed under Construction for Santa Lucia del Camino…... 80
31 Satellite Image of Tlacolula………………………………………………... 82
32 Wastewater Treatment System at Tlacolula de Matamoros……………….. 83
33 Satellite Image of San Francisco Telixlahuaca…………………………….. 86
34 Aerial Photo of Wastewater Treatment System……………………………. 87
35 Headworks and Anaerobic Digester………………………………………..87
36 Upflow Anaerobic Digester Manifold and Overflow Weir………………...88
37 Vegetated Gravel Beds……………………………………………………..88
38 Satellite Image of Santa Domingo de Tomaltepec………………………… 90
39 Aerial Photo of Wastewater Treatment Plant ……………………………... 90
40 Bar Screen and Grit Chamber and Anaerobic Digester…………………….91
41 Upflow Anaerobic Rock Filter……………………………………………...91
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42 Vegetated Gravel Bed and Chlorine Contact Tank………………………...91
43 Satellite Image of San Tomas Mazaltepec…………………………………. 93
44 Wastewater Treatment System for San Tomas Mazaltepec……………….. 94
45 Photos of Anaerobic Digester and Upflow Anaerobic Rock Filter At San Tomas Mazaltepec…………………………………………………. 95
46 Satellite Image of Teotilan del Valle………………………………………. 96
47 Wastewater Treatement System at Teotilan de Valle……………………… 97
48 Headworks and Outlet of Upflow Anaerobic Rock Filter with Excessive Solids Teotilan de Valle………………………………………... 98
49 Gravel Beds and Final Effluent at Teotilan deValle……………………….. 98
50 Location Map of San Dionisio Ocotepec…………………………………... 99
51 Satellite Image of San Dionisio de Ocotepec……………………………… 100
52 Aerial Photo of Wastewater Treatment System……………………………. 100
53 Effluent Quality from BRAIN System entering the Vegetated Gravel Beds………………………………………………………………… 101
54 Vegetated Gravel Beds and Effluent Quality San Dionisio Ocotepec……... 102
55 Location Map Mitla………………………………………………………... 103
56 Satellite Image of San Pablo Mitla………………………………………… 105
57 Aerial Photo of Wastewater Treatment Plant……………………………… 105
58 Headworks (Screens and Sedimentation Channel) and Anaerobic Digesters………………………………………………………... 106
59 Upflow Rock Filter Inlet Distribution Box and Outlet Overflow Weir……. 106
60 Vegetated Gravel Bed System……………………………………………... 107
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INTRODUCTION
Water quality impacts to critical and often limited water resources are prolific throughout Latin America and other developing regions in the world. The discharge of untreated or poorly treated wastewater to rivers and bays is impacting surface and groundwater systems, marine resources, and resulting in severe environmental health impacts to the local populations.
Three countries in Latin America, including Mexico, Cuba, and Columbia have invested significant funds and undertaken hundreds of wastewater treatment projects to
begin to address these issues. A range of different wastewater treatment processes have
been implemented, however, many systems are not operating satisfactorily or have been
abandoned. Several factors have lead to the failure or limited success of wastewater
treatment projects, including: inadequate and poorly maintained infrastructure, selection
or application of inappropriate technology, financial constraints, insufficient operation
and maintenance, lack of community support or ownership of projects, and lack of training and educational support.
“Low-tech” approaches for wastewater treatment are important for the long-term success of water pollution control in both developed and developing countries.
Generally, wastewater treatment systems that fall under the category of “low-tech” treatment systems are simple non-mechanical treatment processes that rely on natural treatment processes to purify the wastewater water. These systems are characterized by their simple construction, minimal use of energy and chemicals, and ease of operation and maintenance. Examples of these technologies include septic tanks, Imhoff tanks, or
1 2 anaerobic digesters, upflow anaerobic sludge blanket digesters (UASBs), pond type treatment systems, and engineered natural treatment systems that can include surface water wetlands and vegetated gravel bed systems.
Over the past 15 years, the Mexican Government and the State of Oaxaca have invested in over 43 small community wastewater treatment projects. Many of these projects are employing “low-tech” treatment systems that utilize anaerobic digesters, upflow rock filters, vegetated gravel beds treatment systems, and multi-stage pond treatment systems.
A field based assessment study was begun in July 2006 and continued through
August 2008, as a Master’s Project in Environmental Systems with an emphasis in
International Development and Technologies at Humboldt State University. A series of field visits were conducted to review the design, cost, condition, operation and maintenance of ten community wastewater treatment plants constructed in the Oaxaca
Valley, in southern Mexico. During this period of time, several meetings were held with
Federal and State agencies and community members to understand why the communities installed wastewater systems, who funded, designed and constructed the systems, and to assess community support or sense of ownership of the project.
Initially, the principal focus of the field study was to evaluate the design, condition, and performance of the wastewater treatment projects. However, after the initial visits the author discovered that many of the projects were in disrepair and/or poorly maintained. The goal of the study was expanded to understand the underlying
3 causes of the poor success rate of the projects to try and understand the cultural, social, political, and factors that impact wastewater treatment projects in the region. Additional analysis of the technical design criteria was also conducted for eight of the ten projects visited.
A review of an inventory of wastewater treatment plants completed in Mexico indicated that numerous “wetland” based treatment systems had been constructed in the
State of Oaxaca. Based on this finding a field based study was designed to understand why these treatment schemes were selected and to understand how they were performing.
The specific objectives of the field assessment and evaluation were to:
1. Understand why a particular treatment system was selected as a preferred
technology for many communities;
2. Establish the design criteria for these systems;
3. Determine how the projects were funded;
4. Estimate the project costs of the different treatment systems;
5. Review the construction and condition of the projects visited;
6. Evaluate the performance, operation, and maintenance of these systems;
7. Determine if the projects were supported by the communities; and
8. Determine if the projects were supported by the Mexican State and
Federal government agencies.
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Literature Review
The literature review provides an overview of wastewater treatment initiatives in
Latin America, economic impacts related to wastewater pollution problems, and constraints impacting the success of wastewater treatment in developing countries.
Setting
Oaxaca is both a culturally rich and financially poor region of Mexico with diverse and numerous indigenous cultures and beliefs. These and other factors, such as educational and political conditions, impact the implementation and support of community wastewater projects in the region. Recognizing the importance of these various factors, a review of the geography, demographics, economics and migration, system of local governance, education, health care, and water and sanitation infrastructure in the state and central valley of Oaxaca has been prepared. This information is helpful to better understand some of the constraints or impediments to advancing community development in the Oaxaca.
Methods
A field based study was conducted over a two year period in the Central Valley of
Oaxaca, Mexico. The field study involved visiting ten (10) community wastewater treatment systems and interviewing local, state, and national agencies, private consultants, university staff and non-governmental organizations involved in wastewater management in the Oaxaca Valley The study was initiated in July in 2006 and continued through August of 2008. A technical evaluation of eight projects was conducted by
5 evaluating the size of the unit processes as compared to criteria used by the State of
Oaxaca Water Commission and US Environmental Protection Agency.
Study Findings
The results of the field visits, interviews, and technical evaluation are summarized for each community. Key findings have also been summarized and tabulated to allow a comparative analysis of each community surveyed in the study. A summary of the various interviews and key topics discussed are also presented.
Conclusions and Recommendations
The last section of the project report presents the study conclusions and recommendations.
LITERATURE REVIEW
Latin America and the Caribbean is the most urbanized region in the developing world. However, less than 10% of domestic and industrial wastewater is properly treated
in Latin America, causing severe environmental, health, social and economic problems in the region. Although the majority of the urban population in this region is connected to public water supplies and sewerage systems, less than 5 percent of municipal wastewater is treated (Idelovitch and Ringskog 1997).
This unhealthy and unsustainable situation has largely resulted from the low priority given to wastewater treatment. More urgent needs of the population, such as the provision of potable water and the sanitary collection of sewage have prevailed over the
past two decades, and wastewater treatment has been deferred. Collecting the sewage
from urban areas may be beneficial by moving the potential health threat away from
populated areas, but in many instances this practice causes severe water quality problems
impacting riverine and coastal ecosystems and water supply projects located downstream
of the sewage discharges.
Major constraints impeding wastewater management have included ineffective political strategies to improve water quality, the selection of inappropriate technologies, funding limitations for wastewater treatment, the lack of public support to invest in wastewater management including directly funding capital projects and the operation and maintenance of government sponsored or subsidized projects.
Economic Impacts of Wastewater Discharges
Municipal wastewater discharges are one of the most significant threats to sustainable development worldwide. The effects of wastewater discharge are usually
6
7 localized, but they are a major source of coastal and marine contamination in all regions and therefore a global issue. For example, pathogenic organisms in domestic wastewater contaminate marine and estuarine waters causing massive transmissions of infectious diseases to bathers and consumers of raw and undercooked shellfish with a global economic impact recently estimated at $10 billion per year (UNEP 2001). In developing countries, approximately six children per minute die from diseases caused by unsafe water and inadequate sanitation. An average of 250 million cases of gastroenteritis occur every year worldwide due to bathing in contaminated water and between 50,000 –
100,000 deaths occur every year from infectious hepatitis (UNEP, 2004). The global burden of human disease caused by sewage pollution of coastal waters is estimated at 4 million lost ‘man-years’ every year, which equals an economic loss of approximately 16 billion US$ a year (UNEP 2004).
Deterioration of the aquatic environment is visible around the globe. The discharge of untreated domestic wastewater has been identified as a major source of environmental pollution. Over 70% of coral reefs are affected by discharges of untreated sewage (Lee and Floris 2003). Other damages attributed to inadequate management of wastewater include:
• Higher costs for production of drinking and industrial water, resulting in
higher fees or tariffs;
• Loss of income from fisheries and aquaculture;
• Poor water quality deters tourists, immediately lowering income from
tourism;
• Loss of valuable biodiversity; and
8
• Loss in real estate value, when the quality of the surrounding area
deteriorates, especially important for slum dwellers, where housing is the
primary asset.
The cholera epidemic in Latin America in the early 1990’s serves as a grim
reminder of the importance of wastewater treatment in the control and prevention of
certain water related diseases. The direct and indirect economic impacts of the cholera epidemic in Peru were substantial. The country had to spend sharply more than usual in
both curative and preventive health care. The epidemic led to nearly 3,000 mortalities,
which implied a loss of economic production in addition to the suffering and hardship of
the sick and their families. The outbreak led to substantial economic losses due to the
ban of exportation of food crops and a drop in tourism. Studies estimated the costs in
Peru during 1991, the first year of the epidemic, ranged from $180 million to $500
million. The average of the estimates yields a figure of about $340 million for the first
year alone, or about 1.5 percent of Peru’s gross domestic product (GDP) (Idelovitch and
Ringskog 1997). (Please note that a citation at the end of a paragraph encompasses all of
the data and information cited in the paragraph.)
The level of economic losses of 1.5 percent of GDP merits comparison with the
level of investment in the Peruvian water supply and sewerage sector. Over the period
1971-78, Peru invested annually only $1.3 per capita in water supply and sewerage,
equivalent to 0.18 percent of GDP. During 1985-89, at the height of the debt crisis of the
1980s, investment dropped further to only 0.15 percent of the country’s GDP. Such low
levels imply that the country was effectively disinvesting from the water and sanitation
sector over this period (UNEP 2001).
9
Investment in Water Supply and Sanitation Sector
Between 1980 and 1994, about 2 billion more people obtained access to an improved water supply and 400 million more urban people gained access to sanitation facilities worldwide. However, aside from these gains, actual sanitation coverage declined over this period from 67 percent to 63 percent in urban areas, and 33 percent to 18 percent in rural areas, respectively (Briscoe 1999).
In the developing world, the historical tradition has been, and the current situation
remains, characterized by insufficient and fluctuating investment in water supply and
sanitation (Lee and Floris 2003). In 1999, water supply and sanitation projects accounted
for $25 billion dollars worldwide. About 90 percent of investment was obtained from domestic sources and 10 percent from external sources. World Bank investment in water and sanitation pre-1991 was approximately 4.5 percent of total funds in the sector, and in
1997 it was 5.7 percent. Although external funding has increased; the cost of doing
business has also increased – many regions are challenged by the fact that the cost of raw
water is rising due to two main factors:
• Growing populations and increasing economic activity against a finite water resource base; and • As cities grow, so do the ‘pollution halos’ around the city. Within the context of this study a ‘pollution halo’ refers to the uncontrolled discharge
of sewage, trash or solid waste, and other potentially toxic materials that occur as urban centers experience unplanned growth with limited or no infrastructure. On the urban fringe, informal communities, such as slums, have become more prevalent in the largest urban cities in most developing countries. In many instances industrial development is also located in fringe areas with minimum environmental control measures.
10
The ‘pollution halo’ often requires cities to relocate water supply intakes at substantial costs. The net effect of these factors is substantial, with the cost of raw water increasing by a factor of two to three each time a new water source is tapped.
Ringler et al. (2000), provide a comprehensive review of data from the Inter-
American Development Bank (IDB) and the World Bank (WB) together with more limited data on national government investments. This review indicates that investments in the water sector as a whole, including investment in irrigation, hydroelectric power and water and sanitation projects has declined. During the years 1961-96, the IDB funded US
$28.7 billion of investment in the water sector in Latin America and the WB funded
US$20.1 billion (in 1990 constant US$). Combined total water sector investments in the region peaked three times during the last 30-years, at US$2.3 billion in 1974, US$2.5 billion in 1983 and US$2.4 billion in 1993, but the general trend has been downward since 1983 in investment in irrigation and hydroelectric projects. The dollars values presented have been adjusted for inflation and represent the value of the US$ in 1990.
Over the full period, 1962-95, investments in the water sector as a whole by the two funding organizations increased at 1.86 percent per year. However, the growth in real investment occurred in the 1960s and 1970s, with annual growth of 6.35 percent between
1962 and 1982. Thereafter, investments declined at 4.68 percent per year during 1982-
95. The largest drop in investments occurred during the debt crisis of the 1980s. During this period, WB funding was still growing at a low rate of 1.10 percent per year, whereas,
IDB investments declined at a rate of 11.71. These trends reversed during the early
11
1990s with IDB funding picking up again, at 2.70 percent per year, but the WB funding declining rapidly, at 8.45 percent annually (Ringler et al., 2000).
Funding by the IDB and WB for urban water supply, sanitation and related sectors has continued to grow during the last 30 years (with the exception of the debt-crisis years in the 1980s), at 3.97 percent per year during 1962-95. Investments in rural and urban sanitation and sewerage projects increased at 5.55 percent annually during 1962-82 and dropped to 1.59 percent per year during the peak of the debt crisis, between 1982 and
1995. Rates of growth in investment were especially high at the beginning of the 1990’s, at 10.30 percent per year in the case of IDB funding, and 5.77 percent per year for WB funding (Ringler et al., 2000). Table 1 summarizes the investments by the IDB and WB in water supply and sanitation projects in Latin America from 1961-95.
Although investment in the water supply and sanitation sector has increased in the
1990s, the level of investment has not kept pace with the level of growth and demand in the region due to water shortages and pollution problems (Ringler et al., 2000). For example, in Lima, Peru, the average cost to meet short- and medium-term water supply needs has cost residents approximately US$0.25 per cubic meter; However, because of over pumping (overdraft) of the local aquifer, to meet long-term needs, a transfer of water from a different watershed is planned that will cost residents approximately US$0.53 per cubic meter. Similarly, in Mexico City, water pumped from the Mexico Valley aquifer cost approximately US$0.60 per cubic meter; however, due to groundwater pollution problems in the valley, a pipeline was constructed that conveys water 180 kilometers from the Cutzamala River at a cost of US$0.82 per cubic meter (Ringler et al., 2000).
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Table 1. Investment by IDB and WB in Water Supply and Sanitation Project (after Ringler et al., 2000)
In the past, loans from multi-national lending institutions have been important in funding water and wastewater projects in developing countries. However, many developing countries currently estimate that, due to the size of the investments needed, they can finance at most 10 percent of the needed infrastructure costs (Ringler et al.,
2000). The Inter-American Development Bank committed 20 percent of its total lending budget (US$3 billion) to water and wastewater projects in Latin America and the
Caribbean between 1997 and 2000. It does not seem likely that central and local governments in developing countries will be able to make up the difference from their own resources any more in the future than they have in the past (Ringler et al., 2000).
In 2000, the United Nations (UN) adopted the UN Millennium Declaration, which committed the UN member nations to a “new global partnership” to reduce extreme poverty and setting out a series of eight target areas or goals with a deadline of 2015.
The eight areas include ending poverty and hunger, expanding primary education, promoting gender equality, reduce child mortality, improve maternal health, combating
HIV/AIDs, malaria and other diseases, ensure environmental sustainability, and develop a global partnership for development (UNDP 2010). The World Bank estimates that investment in developing countries must double, from US$15 billion annually to US$30
13 billion, if the Millennium development goals for water and sanitation are to be met: halving the number of people with unsafe sanitation by 2015 (Lord and Isreal, 1996).
At present, approximately 75 percent of capital funding is provided by governments, but it is doubtful that most developing countries can more than double their share, or that more can be expected from external support agencies or financial institutions. Lee and Floris (2003) argue that the needs to meet the Millennium
Development goals can only be met by recourse to the private sector.
Level of Wastewater Treatment
Data were compiled from three principal data sources to estimate the number and type of wastewater treatment systems in place in several Latin American countries. A majority of the wastewater data were obtained from a regional evaluation conducted in
2000 by the Panamerican Health Organization’s (PAHO) Center for Environmental
Protection and Investigation in Health (CEPIS) based in Lima, Peru. Additional data sources included country reports and other sources available from the PAHO and the
Mexican National Water Commission (ConAgua).
Table 2 shows the coverage of sanitary sewers and wastewater treatment for 16 countries in Latin America and the Caribbean (Cuba). Ten countries participated in the
PAHO Regional Inventory and reported the number of sanitary sewers and treatment plants installed. The inventory is based on a survey conducted by CEPIS in the end of
1999. Survey forms were sent to national and regional health organizations based in all
Latin American and Caribbean Countries. While there was only moderate participation in the survey, the data provide an indication of the level of service for countries of varying population size and economic status.
14
These data indicate that the percent coverage of cities (cities are urban areas having a resident population greater than 50,000 inhabitants) with sanitary sewer may range from almost 90 percent in Mexico, Colombia, and Cuba, to approximately 40 to 50 percent coverage in Guatemala and Chile, and less than 15 percent in the remaining countries. It is important to note that the data presented are only the number of sewers installed, but does not report the total number of people (or population) served by the systems. In many instances, particularly in large urban areas, sanitary sewer systems may only serve portions of a city, such as affluent neighborhoods and not marginal or fringe slum areas. Therefore, it is difficult to ascertain the potential health benefits derived by these data.
The data also show that the number of wastewater treatment systems installed is very low. Comparing the number of treatment plants to the number of cities in several countries indicate that the highest level of service is approximately 85 percent in Cuba and approximately 20 to 25 percent in Mexico, Colombia, and Chile. Brazil has installed a number of wastewater treatment systems over the past two decades; however, an inventory of the number of plants constructed was not available. Wastewater treatment occurs in less than 10 percent of the cities in the remaining countries.
15
) ua 2003 ua g eatment Coverage in Latin America and Cuba CEPIS 1999 and ConA CEPIS 1999 , PAHO 2000 ( Table 2. Sanitary Sewers and Wastewater Tr
16
Table 3 presents information on the type and number of different treatment systems installed in several Latin American Countries and Cuba. With the exception of
Mexico, the information indicates that a majority of the wastewater treatment systems installed are pond type systems. Pond type treatment is typically the least cost system to install and operate and requires low skilled level operators to maintain.
Constraints to Improved Sanitation and Wastewater Treatment
Briscoe (1999) reported that the water supply and sanitation industry is deeply
affected by the changing economic paradigm. The sector has long been undergirded by
publicly and in many cases poorly financed, government-run utilities, which have
performed poorly in terms of efficiency, quality of services, coverage and environmental
impacts.
Inadequate and Poorly Maintained Infrastructure
In several of the Latin American countries, Lord and Isreal (1996) found minimal
commitment on behalf of governments or responsible agencies for maintenance of
existing infrastructure. Because of limited financial resources, the absence of
competition for the agencies providing the services, poor accountability, low priority, and
a lack of skilled personnel, much of the infrastructure, from pipelines to treatment plants
were in poor condition. New projects were found to be more desirable to fund than
rehabilitation and maintenance of existing systems. In general, wastewater treatment
facilities were often found to be in states of ill-repair or not functioning at all. Most major
urban centers discharge untreated wastewater directly into the receiving waters.
Table 3. Type of Wastewater TreatmentTable 3. Type in Latin America and Cuba
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Inappropriate Technology
Worldwide experience in wastewater management has numerous examples of
complete or partial failure. The most common management actions are, unfortunately, no
action at all. By one estimate, as much as two-thirds of the wastewater generated in the
world receives no treatment at all (Marino and Boland 1999). Even more conspicuous is
that many newly built wastewater treatment plants are not properly operated or
maintained, or, in some cases, are not being operated at all. Often, inappropriate
treatment levels or treatment technologies have been selected, leading to excess costs,
disappointing results, or both. Often such investments are poorly targeted, providing
abatement for low-priority effluents, while more hazardous discharges go untreated.
Many of these investments, responding to a piecemeal approach to river basin pollution
problems, may also result in costly and ineffective solutions (Marino and Boland 1999).
The consequences of these failures are that projected environmental and social benefits have not been realized; and water resource availability, already scarce, is further depleted by pollution, even after large sums of funds are committed and spent. This has had an added strong negative impact on the already weak financial situation of water agencies, and has squandered public and political goodwill without producing expected results. As a result, these failures have jeopardized and delayed the timely adoption of appropriate and needed remedies (Marino and Boland 1999).
Failures in water pollution-control investments are often attributed to a lack of
financial capacity in the operating agencies, or to a general lack of institutional competence. Marino and Borland (1999) argue that on first approximation these diagnoses are only partially valid and in many instances overlook common problems: the
19 responsibility of the designer or politician who did not take into account the specific socio-economic and environmental conditions in which the facilities or solutions must operate; the particular characteristics and impacts of the pollution problems that “had to be solved”; and/or the tendency of local authorities to request the “best” and “most modern” available technologies even if they are not the most appropriate solution for the problem. These highly mechanized systems tend to be more complicated and costly to operate and maintain. In these circumstances, there is a distinct preference for equipment-intensive solutions that generally require have a smaller land area, but usually utilize imported equipment and have a relatively high energy demand. An opposing approach is low (soft) technology alternatives that use local resources, lower energy, but may take more land area; however, in the long run this later option may have been more appropriate. This bias to higher levels of technologies is clearly exacerbated by foreign financing. For example, some foreign financing requires that equipment be purchased from the lending country resulting in high costs due to duties, taxes and overall high costs for goods (Marino and Borland 1999).
Insufficient scientific knowledge of receiving water conditions and uses also leads to the selection of inappropriate treatment levels and wastewater management practices.
Where unnecessarily high levels of treatment are attempted, operating costs often exceed available funds and the technical capacity of the operators who manage the treatment facility. Consequently, the facilities are not operated and maintained correctly, and tend to fall out of use shortly after completion. The result is idle infrastructure that can itself become a public health hazard, creating conditions worse than those that prompted the investment in the first place. This problem is wide spread in many countries, and
20 particularly affects medium and small cities because (a) they tend to have weaker financial and technical capacity; and (b) in these medium and small cities the pollution problems created by inadequate wastewater management solutions constitute a smaller embarrassment to the central government and go unattended whereas if this condition occurred in a capital or major city then action may be undertaken (Marino and Borland
1999).
Ludwig et al. (2003) attribute many of the failures of wastewater projects to the fundamental lack of understanding by the staff of the assistance agencies. Their findings indicate that the fundamental design criteria for the facilities must be developed to suit the socio-economic status of the developing country. Ludwig et al. (2003) further report that since the establishment of the large international assistance agencies (IAA)
(including the multilateral development banks, UN affiliates, and bi-laterals) after the second world war, they have invested large sums to help finance planning and construction of community sewage and water supply projects in the developing countries.
In practically all cases the control of the planning and design of facilities has been in the hands of the IAAs using planners/engineers who are experts in planning and design of facilities for developed countries. Unfortunately, only a few of these officials are competent in understanding that most developing countries cannot afford sophisticated treatment systems and many of the IAAs have chosen not to invest in post construction monitoring of actual project performance (because it is much more easier to assume satisfactory performance and to collect facts to the contrary would be politically disturbing) (Ludwig et al. 2003).
21
Financial Constraints
The issue of political risk and poor governance seems to be the most important
constraint that impedes the flow of finance into the water supply and sanitation sector
investments (UNEP 2004). The risks reflect the fact that in many developing countries
political interference or favoritism and weak or unstable regulatory bodies do not have
the support to enforce environmental protection measures or demand the need to secure
financing for water pollution efforts.
In low and medium income countries environmentally-related expenditures as a share
of national income may be comparable with higher-spending countries, though absolute
levels are very low (Ludwig et al., 2003). This suggests that it is often not the
willingness, but the ability to pay linked to low income, that is the main constraint
towards higher level of domestic environmentally related expenditures, particularly with
the water and sanitation sector (Ludwig et al., 2003).
Additional financial related constraints identified by the UNEP (2004) are:
• Environmental protection has been a low priority for many developing
countries due to competing interests with other sectors, such as health and
education, as a result of acute scarcity and accumulated external debt burden.
• Low fees or tariffs for environmental protection measures have resulted in
weak revenue generation, which are too small to create substantial funds to
support significant investment in this sector.
• Low levels of funding from the IAAs for wastewater projects have resulted
from weak demand by countries for environmental assistance.
22
• Centralization of funds by national governments for environmental projects
has limited financial autonomy at the local government/municipality level,
which has hampered incentives for local governments to undertake long-term,
responsible environmental management projects.
• There has been a lack of accounting for the costs of externalities from
environmental degradation such as health costs, loss of ecosystems, loss of
tourism, etc.
Insufficient Operation and Maintenance
A comprehensive evaluation of operation and maintenance of urban services
(water, sewer and utilities) was conducted in the cities of Colombo in Sri Lanka; Cuttack in India, Karachi and Faisalabab both in Pakistan. The evaluation was undertaken by the
Water, Engineering and Development Department of Loughborough University of the
United Kingdom (Sohail et al. 2001). The purpose of the study was to identify the constraints and potential strategies to improve the sustainability of urban services in poor
communities. The constraints on effective operation and maintenance, at both municipal
and community levels that were highlighted in the country assessments found that O&M
was generally given a low priority by policy makers. Many municipalities did not have
sufficient resources to cope with O&M requirements. Political interferences or changes
made sustainability difficult to achieve. Government priorities were directed to
construction rather than O&M. Inappropriate engineering standards and technologic
choices created unnecessary O&M difficulties and increased costs. There was a lack of
training and understanding of O&M requirements by municipal workers.
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Community level constraints were attributed to the lack of community involvement in project design, lack of training and understanding of O&M requirements, insufficient funds for O&M needs, and a lack of responsibility and ownership for systems. Table 4 presents a summary of the key constraints leading to project failures reported by Sohail et al., 2001).
Conclusions
The discharge of untreated wastewater is resulting in significant impacts to water quality, the environmental and socio-economic conditions in many developing countries.
Investment and implementation of wastewater management programs have had limited success due to many constraints, including inadequate funding, inadequate or poorly maintained infrastructure, inappropriate technology and insufficient operation and maintenance. The following sections describe site specific evaluation of wastewater management in the state of Oaxaca in Southern Mexico. Many of the findings of the field based assessment in Oaxaca reflect the literature reviewed in this section.
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Table 4. Summary of Various Constraints or Factors Leading to Water and Sanitation Project Failures (after Sohail et al., 2001)
Constraints Causes Engineering and Technology • Selection of inappropriate technology • High technology with high O&M requirements – high rate of failure/breakdown • Poor construction or materials • Lack of monitoring and assessment Project Planning and Management • Lack of guidelines • Passive community involvement in project planning and management • Poorly defined roles for stakeholders • Lack of clear strategy • Poor coordination with overlapping responsibilities/duplication of effort or lack of follow through/accountability • No proper systems for developing a public, private and community partnership in O&M of services Training and Education • Lack of skills, capacity, and trained staff related to planning, administration and O&M needs • Inadequate health education • Poor information or data for O&M • Inadequate regulatory mechanism for promotion of community-based O&M Financial • Communities not contributing sufficient funds for O&M needs • Negative behavior of users/users reluctant to pay for services/illegal service connections Lack of Ownership • Community has little trust in utility or central government • Mismanagement leading to collapse of services • Community taking responsible for small repairs, but leaving large repairs to Central Government • Community used to policy of central government intervening in all O&M • Lack of ownership at all levels • Shortage of staff • Lack of official involvement of communities
SETTING
State of Oaxaca
A field based study focused on wastewater management was undertaken in the
State of Oaxaca in southern Mexico. The purpose of the assessment was to evaluate
social, economic, cultural and technical factors affecting the design, construction,
operation and maintenance of ten community wastewater projects in the region. This
section provides an overview of the physical, economic, political (governance),
education, health care, water and sanitation infrastructure of the State and Central Valley
of Oaxaca where the study was based. This background information is important to
support and explain several of the findings reported in the project report.
Oaxaca (wɑˈhɑkə) named for its largest city, is one of the 31 states of Mexico, located in the southern part of the country, west of the Isthmus of Tehuantepec. Oaxaca borders the states of Guerrero to the west, Puebla to the northwest, Veracruz to the north,
Chiapas to the east, and the Pacific Ocean in the south. The state of Oaxaca is located between 15o38’ and 18o48’ latitude north and between 93o52’ and 98o30’ longitude west.
With an area of 36,820 km² (95,364 mi.²), Oaxaca is the fifth largest state in the
Republic. According to the 2005 census it had a population of 3,506,821 people.
The state of Oaxaca has been a functional region for at least three millennia
(Perraton 1998). The name Oaxaca was derived from the Aztec word “Huaxyacac”,
which means “the location of the Guale”. The tree Guaje (Leucaena esculenta) is
abundant in the central valleys and its fruit is edible.
Due to its large size, rough terrain and the tendency of the indigenous
communities to identify strongly with their village as opposed to their region, Oaxaca is
25 26 divided in 571 municipalities, the most of any one state, accounting for almost 1/4 of all the municipalities in the country. Figure 1 is a relief map of Oaxaca and surrounding states and Figure 2 shows the municipal boundaries of the State of Oaxaca. Within municipalities are many towns and villages that are self ruled with a system called Usos y
Costumbres (uses and customs), a system that advocates for retention of culture and practical ways of conducting daily business.
Figure 1. Relief Map of Oaxaca and Region (Google Map, 2009)
Oaxaca is located at the convergence of the Sierra Madre Oriental and the Sierra
Madre del Sur mountain ranges, resulting in a rugged and mountainous terrain with a large temperate central valley. The average altitude is 1,500 meters (5,085 feet) above sea level. The area is a distinct physiographic section of the larger Sierra Madre del Sur province, which in turn is part of the larger Sierra Madre System physiographic division.
27
Oaxaca, the historic home of the Zapotec and Mixtec peoples, contains more speakers of indigenous languages than any other Mexican state (Murphy et al., 1992). Its
3.5 million people are made up of descendents from 15 different groups including the
Zapotec, Mixtec, Chatino, Trique and Mixe peoples. They speak over 14 languages and
90 dialects. The terrain of the state of Oaxaca directly influences this cultural diversity as the mountain ranges and valleys separate the villages, and these isolated areas have developed their own languages, foods and customs.
Figure 2. Municipal Boundaries in the State of Oaxaca (INEGI 2000)
The Zapotecs are the largest ethnic group, numbering approximately 400,000.
They have lived in the Central Valleys (Valles Centrales) of Oaxaca for at least 2,500 years and they built the hilltop city of Monte Alban. This city was the center of Zapotec culture and reached its peak between 300 and 700 AD. After this time, the city declined
28 until it came under Mixtec influence around 1200 AD. The majority of the rural population in the State is farmers and local artisans that make handicrafts, mescal, and other products (Schmal 2007).
The Mixtecs have the second largest ethnic population with approximately
300,000 people. They are farmers, potters, and metalworkers and their culture thrived in harmony with the Zapotecs until they were both conquered by the Aztecs in the 15th century (Schmal 2007).
The Mixe (pronounced MEE-hey) number around 90,000 and live in the northeast portion of the state. The Mixe are well known in Oaxaca for their large brass bands and it is their music that is considered to bind them together as a people (Lipp 1991).
Each of the indigenous groups believes fiercely in being allowed to follow the traditions of its ancestors. Many of them still wear traditional clothing and prepare food following methods devised long ago. While most people follow Catholicism, they also believe in including their original traditions. These intertwined beliefs combine to form very colorful and interesting villages and people (Schmal 2007).
Table 5 presents some basic statistics regarding the population and demographics of Mexico as a whole, and the State of Oaxaca. In 2007, approximately 52 percent of the population of the State of Oaxaca was living in rural communities and 48 percent were concentrated in urban cities. The data shows that the annual growth rate in the State of
Oaxaca is lower than the rate in the overall country, which is likely attributed to the high migration rate, as discussed below. The median age and life expectancy in Oaxaca is
29 lower than the national average as well as the level of education attained in Oaxaca is lower than the national average. Several of these statistics are in more detail discussed in the following sections.
Table 5. The background statistics on the Demographics for the Country of Mexico and the State of Oaxaca
Parameter Mexico Oaxaca Population (2000) 97, 483,400 3,438,765 Annual Population Growth 1.85% 1.3% Population in urban areas 74.6% 44.5% Median age 22.0 20.0 Educational Attainment (years) 7.3 5.6 Life expectancy at birth 74.9 73.5 Average number of children 2.9 3.3 Average hourly wage (pesos) : Male 21.5 12.3 Female 19.8 13.7
Oaxaca Valley
The Valley of Oaxaca is a geographic region located within the modern day State
of Oaxaca in southern Mexico. The valley, which is located within the Sierra Madre
Mountains, is shaped like a distorted and almost upside-down “Y”, with each of its arms
bearing specific names: the northwestern Etla arm, the central southern Valle Grande or
Zimatlan Valley, and the Tlacolula arm to the east, as shown in Figure 3 a map of the
Oaxaca Valley. The Oaxaca Valley is actually the conglomeration of three smaller
valleys that converge at the City of Oaxaca: the Etla Valley to the northwest; the
Tlacolula Valley to the east and the Zimatlan Valley to the south. A number of
important and well-known archaeological sites are found in the Valley of Oaxaca,
including Monte Alban, Mitla, and San José Mogote. The state capital, Oaxaca City, is
located in the central portion of the valley.
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Figure 3. Map of the Oaxaca Valley and Major Rivers (Riley, Murphy and Mendez-Rosado 1998)
Setting
Central valley communities share many features. Brick and cement block homes
of one or two stories with red tiles radiate in standard grids from central plazas. The
plazas are constructed around churches, government buildings and small market areas.
Circling the communities are farmlands that households depend on to produce maize for
self-consumption. The majority of the land is not irrigated and is rain-fed, supporting
seasonal or temporal cropping and production. Households typically hold an average of
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1.7 hectares of land that produces maize during an approximately 4 to 5 months growing season.
Physiography
According to Kirby (1973), the Oaxaca Valley region can be divided into four
physiographic zones: mountain, piedmont, high alluvium, and low alluvium, as shown in
Figure 4. Figure 5 also shows the predominant soils of the valley floor and piedmonts
areas mapped by Kirkby (1973).
Mountain Zones. The mountain zones have the lowest temperatures and highest
rainfall with steep sloped land. The mountain regions are used primarily as an area for
water supply, wood collection and making prepared charcoal. In general they are
covered with high pine and oak forests which show a zonation of dominant species with
increasing altitude from a broad-leafed oak woodland at 2,000 to 2,500 meters, to a mixed pine-oak forest at 2,500 to 2,700 meters, to a pine forest above 2,700 meters
(Kirby 1973).
Piedmont. In different parts of the Oaxaca Central Valley the piedmont is wide
and forms a continuous surface with deep gullies cut up to 20 meters into it rather than a
series of separate spurs. The average slope of the piedmont from mountain to alluvium is
1 to 2 degrees with gulling producing side slopes up to 20 to 30 percent. The soils
developed on the piedmont are generally poor for traditional agricultural production
mainly because they are very stony, and where the surface is cut on bedrock the soils are commonly thin with out-crops of rock scattered over the slopes. Although the piedmont
soils are marginal for agricultural cultivation, they are primarily used for cultivation and
grazing so that the natural vegetation in these regions is partly or wholly destroyed.
Between the 1,800 to 2,000 meter elevations the open mesquite grassland gives way to a
32 thorn forest which is almost impenetrable when well developed, with different several species of cacti and agave. Above 2,000 meters, piedmont vegetation is more open with the thorny species giving way to narrow- and broad-leafed oaks (Kirby 1973).
High Alluvium. The high alluvium forms the main part of the valley floor and is the most important zone for agriculture both in quantity and quality. It varies in total width from 1 kilometer just north of Oaxaca City to 17 kilometers near Ocotlan to the south. In contrast with the mountain and piedmont zones, soils on the valley floor are always greater than 1 meter in depth, and soil thickness is not a limiting factor for agriculture. Throughout the valley floor, soil profiles are poorly developed and retain their alluvial structure almost unaltered below the A horizon soil. Some salt accumulation is found in the B horizon soil, especially in the widest parts of the Valley where low gradients allow shallow depressions to contain standing water for several days or weeks after storms in the summer months. Slope gradients in the high alluvium are generally less than 1 degree trending north to south (Kirby 1973).
Low Alluvium. The main rivers of the valley are generally incised into the high alluvium to a depth of 1 to 3 meters, although below dams downcutting has reached 6 to
7 meters and a new flood plain is presently forming. The flood plain, or low alluvium, is very restricted in area, occurring as a distinct geomorphic unit along less than 50 percent of the courses of the Rio Atoyac and Rio Salado and attaining a maximum width of only
2.5 kilometers (in the Zaachila Valley) (Kirby 1973).
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Figure 4. Map of the Valley of Oaxaca to show main physiographic zones (Kirkby 1973)
Climate
The Central Valley of Oaxaca is surrounded by mountain ranges of over 3,000 meters (10,000 feet) or more, which creates a rain shadow in the valley. The valley
watershed is a closed system, cut off from externally replenished sources of surface water
and groundwater.
Physical isolation and lack of water have been major factors in the development
of the valley since its settlement more than 10,000 years ago. The Oaxaca Valley is a dry
semi-humid region and suffers a pattern of severe droughts, on average one every four
years (Riley et al., 1998).
34
Figure 5. Map of Oaxaca Valley and predominant soil types (Kirkby 1973)
Temperature. The mean annual temperature in City of Oaxaca is 20.6oC and the
mean monthly low temperature is 17oC and high is 24oooC. Figure 6 shows the monthly
high, mean and low monthly temperature for the City of Oaxaca (Kirkby 1973).
Rainfall. In the summer, masses of tropical air coming from the Gulf of Mexico and from the Caribbean Sea penetrate all of the territory up to the mountain range of the
Sierra Madre Occidental. These humid winds called the “trade winds (Alizes) are the cause for the heavy rains that take place regularly from May to the end of September throughout the country east of the Sierra Madre Occidental.
35
These rains are also influenced by the displacement of the “thermal equator” to the north in the summer, accompanied by a zone of low pressure and convective winds producing abundant rains during their cooling ascent. The “thermal equator” is defined as an imaginary line round the earth running through the point on each meridian with the highest average temperature. It lies mainly to the north because of the larger landmasses and therefore greater summer heating. In most of the valleys in Mexico, when such a weak pressure system takes place, a diurnal variation of wind direction occurs. The wind blows upslope in the daytime and down slope at night. Intense rainfall events from periodic tropical storms are commonly the only occasions when the region experiences water surpluses.
30
25
C o 20
15 Temperature. 10
5
0 JFMAMJJASOND Month
Figure 6. High, Mean, and Low Monthly Temperature in Oaxaca City (Kirkby 1973)
Similarly, the very dry months in winter are the result of cold air masses coming from the North. These dry cold air currents travel at very high altitude and become warmer in their descent while remaining relatively dry. The general precipitation pattern
36 in the central valleys of Oaxaca is directly related to the movement of global air masses
(Perraton 1998).
Annual rainfall varies across the Central Valley’s of Oaxaca. Average annual rainfall in the central part of the valley is 630 mm, rainfall in the eastern valley of
Tlacolula is 550 mm, and in the mountainous region of Cuajimoloyas it is 1000 mm.
Table 6 presents a summary of historic climatic data for the Valley of Oaxaca prepared by Kirkby (1973) and Perraton (1998). Figure 7 presents the monthly mean rainfall for the City of Oaxaca (Kirkby 1973).
Table 6. Summary of historic climatic data for Oaxaca Valley Region (Kirkby 1973 and Perraton 1998)
37
Figure 7. Mean Monthly Precipitation in the City of Oaxaca (Kirkby 1973)
Evaporation. Mean annual evaporation for the Oaxaca Valley is reported to be
2,544 mm (Kirkby 1973). Figure 8 presents the mean monthly evaporation in the City of
Oaxaca (Kirkby 1973).
Figure 8. Mean Monthly Evaporation in the City of Oaxaca (Kirkby 1973)
Annual Water Deficit. As previously discussed, above the Central Valley of
Oaxaca generally is in a water deficit, with mean annual evaporation exceeding the mean annual precipitation. This relationship is illustrated in the annual water deficit budget shown in Figure 9 for the City of Oaxaca (Kirkby 1973).
Potential ET
Water Deficit = Pot. ET ‐ R
Months of the year
38
Figure 9. Pattern of annual water deficit for City of Oaxaca (Kirkby 1973)
Surface Water Resources
The Central Valley of Oaxaca is drained by two principal river systems. The
largest river is the Rio Atoyac, which flows north-south through the Etla Valley. The
smaller river, the Rio Salado flows east to west through the Tlacolula Valley. The City
of Oaxaca was constructed at the confluence of both rivers. The rivers combine to form
the Rio Verde and the largest watershed in the State of Oaxaca, encompassing an area of
18,812 square kilometers. The total length of the river is 342 kilometers with a mean
runoff rate of approximately 5,937 million cubic meters per year.
River (Rio) Atoyac. The Rio Atoyac drains the Etla valley. The river’s main
tributaries include the Atoyac, Mazaltepec, San Agustín, San Gabriel and San Pablo
rivers, which drain from the western slopes of the Sierra Madre range. The rivers
generally have year around or perennial flow. Flows in the main Rio Atoyac ranges less
than 0.83 m3/s in the dry season to over 1.67 m3/s in the rainy season. In the valley the
river is severely contaminated by both point and non-point pollution sources. Point
source pollution is from the discharge of partially or untreated municipal sewage, and
small industries on the north side of the City of Oaxaca. Non-point pollution is primarily
from agricultural runoff from farm lands in the Etla Valley north of Oaxaca City (Flores-
Marquez et al., 2008). During the dry season the river is anaerobic, black, and very
odorous from hydrogen sulfide generation. Figure 10 is a photo of the Rio Atoyac in the
City of Oaxaca .
39
Figure 10. Photo of Rio Atoyac in City of Oaxaca (Ponce 2002)
River (Rio) Salado/Tlacolula. The Rio Salado drains from east to west joining the Rio Atoyac in the City of Oaxaca. The Rio Salado starts from the eastern tributaries of the Rio Grande de Mitla and the Diaz Ordaz. The stream flows westerly to
Lambityeco where the stream channel becomes very broad and highly braided and becomes more of a seasonal marsh during the rainy season. For a distance of six kilometers downstream, the river forms a broad wetland area that over time has accumulated salts. Due to excessive flooding this area has been abandoned for agricultural use. Below this area the river becomes channelized and flows to the City of
Oaxaca. Flows in the river are highly variable and dependent on rainfall, discharge of agricultural return flows, and partially or untreated municipal wastewater. The river is highly contaminated from the discharge of municipal sewage, industrial discharges and non point sources of pollution. During the dry season the river is anaerobic, black and
40
smelling of hydrogen sulfide. Figure 11 is a photo of the Rio Salado east of the City of
Oaxaca.
Figure 11. Rio Salado, Oaxaca, Mexico (Word 2007)
Groundwater Resources
There are three principal groundwater basins in the Central Valley region of
Oaxaca, which are located in the three valleys that intersect at the City of Oaxaca. These basins are named the Etla, Tlacolula and Zimatlan groundwater basins.
Etla Valley Groundwater Basin. The Etla groundwater basin has formed in the
Etla Valley as a result of the Rio Atoyac. According to Flores–Márquez et al. (2008), the hydrological system is composed of two aquifers: a shallow one 20 to 60 m thick, and another one at depths below 60 m. The upper aquifer is in Neogene–Quaternary sand and gravel sediments containing water of good quality, whereas, the second aquifer is probably made up of Cenozoic lacustrine sediments, epiclastic tuffs and fluvial deposits with a thickness of some hundreds of meters. Mesozoic–Precambrian rocks underlay the aquifer system. A clay body between both aquifers, with an average thickness of 36 m
41 constitutes an aquitard. This aquifer system provides about 80 percent of total water for domestic and agricultural uses in the Etla Valley region.
By 1984, approximately 75 wells were used to extract fresh water for agricultural and human uses. Later more than 125 wells were drilled next to the city of Oaxaca and other small towns in the valley, mainly for domestic use.
Tlacolula Valley Groundwater. The geology of the Tlacolula Valley is complex and highly variable, which controls the occurrence and quality of groundwater in the region. Generally, the basin consists of alluvium deposits located in the more central regions of the valley that are 30 to 40 meters thick, composed of sand, gravels and silt of good relative permeability. Extrusive rocks of the middle tertiary age composed of volcanic rocks of rhyolitic, andesitic constitution underlay the alluvium and are surficial in the piedmont areas as tuff or tuff-breccias outcroppings. Below the extrusive rocks of the Tertiary clay, sediments which contain large quantities of salt (the results of evaporates) have been encountered that can exceed thicknesses of 150 meter to 300 meters deep (Perraton 1998).
There is a long history of groundwater use in the Tlacolula Valley. Shallow hand dug wells have been in wide use throughout the region for several hundreds of years.
Deep well drilling has been more widely practiced over the past 40 years (Perraton 1998).
The majority of shallow wells are developed in the alluvium deposits and groundwater is routinely very shallow, under 1 to 4 meters below ground surface. Drilled wells are common in the volcanic formations and in many locations have encountered artesian conditions resulting in free flowing water at the surface.
42
Zimatlan Valley Groundwater. The Zimatlan Valley groundwater system consists of alluvium deposits from the Rio Atoyac. The alluvium material consists predominantly of sands, gravels and silts that can be as deep as 250 meters below ground surface. Underlying the alluvium deposits is shallow metamorphic bedrock primarily granites (Chavez-Guillen 1977).
The aquifer is relatively limited and does not produce significant quantities of groundwater in the Central Valley. Shallow groundwater has been used historically employing hand dug wells to depths of 10 to 15 meters deep. Large production wells are not common due to the low permeable material encountered in the deeper zones.
Water Use
According to the National Water Commission (CNA 2008), agricultural water use
represents approximately 73 percent of the total annual water use in the region.
Municipal and domestic water supplies are the second largest user, representing
approximately 25 percent. Industrial use is the lowest reported, accounting for
approximately 2 percent of the total.
Economics Overall, Oaxaca is a very poor state with its many natural resources underutilized.
The lack of economic development opportunities in rural areas has forced many indigenous people to migrate to the cities. This migration has caused the city of Oaxaca to double in size in the past 20 years (Van et al. 2005). Migration out of the rural areas and into the cities has become a major source of rural income. Many indigenous people move "temporarily" from their traditional homes to the cities to work and they then send their income back to their communities. This method of increasing income has also brought many migrants from the Oaxaca area to the United States. Migrants come and
43 may work for as long as 10 years to send enough money (remittance) home to buy a house (Blendford 2003).
Central valley communities have experienced rapid population growth patterns over the past 50 years. Since the 1950’s many communities have doubled in population.
This rapid growth has created a demand of wage labor, education, services (water, electricity, wastewater, roads, etc.) and medical care. Unfortunately, the economy of many of these rural communities and regions has not expanded and generally is depressed. Given this situation, there are few opportunities for wage labor, professionals
(doctors, engineers and architects, teachers), and in turn, limited local access to market goods. All of these factors are important motivation for migration. According to the
National Statistics Agency (INEGI 2000) on average 80 percent of the households in the state of Oaxaca made less than twice the minimum living wage ($5.00 per day as of
2000).
Oaxaca's modern period of engagement with the world economy began with the paving of the Pan-American Highway into the city in the 1950’s. The improvement of the road made it possible for merchants and manufacturers from central Mexico to transport their goods into Oaxaca. Within a few years, competition from Mexico City had eliminated much of the local industry. Today, the city of Oaxaca is the capital of one of the two poorest states in Mexico. Although it contains nearly 4 percent of the nation's population, the state produced only 1.5 percent of the nation's gross national product
(INEGI 1990). Perhaps a more salient measure of the state's poverty is that the mortality rate for the state is 9.5/1000 compared to a national figure of 5.6 (INEGI 1990; Murphy et al., 1992).
44
As in many other secondary cities of Mexico, this increased engagement with the metropolitan center has also increased linkages between regional centers and rural areas.
Transforming rural/urban relationships has lead to increased centralization and migration as the roads were built to move people and goods between urban centers and the hinterland (Murphy et al., 1992).
Over the past 30 years the migration stream into Oaxaca City has increased significantly. Prior to the 1970’s Oaxacans looking for work to help maintain their households left for Mexico City, or one of newly developing oil cities such as
Villahermosa or Salina Cruz. Oaxaca City, with only a service and government economy was not an attractive destination for migrants. But even so, high immigration rates continued to maintain high percentages of household heads born outside the city. For example, in 1977 fifty seven percent of the household heads were from some place other than Oaxaca City (Hendricks and Murphy 1981). By 1987 that percentage had increased to 73 percent.
It has been estimated that Oaxacans return more than 50 million dollars a year to the Central Valley region. Remittances to rural households in Mexico may constitute anywhere from 75 to 90 percent of local incomes (Blenford 2003).
The economy of the state of Oaxaca is also largely dependent upon tourism.
Visitors to the city of Oaxaca and the coastal communities of Huatulco, Puerto Escondido and Puerto Angel are the single most important source of income for these towns besides remittance money. In addition to working in the service sectors, many indigenous people make handicrafts to be sold to the tourists. Oaxaca’s handicrafts are beautiful and highly sought after.
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Coffee is the second largest economic producer and accounts for 30% of the state’s exports. Government subsidies have helped farmers improve coffee yields and
Mexico has become the world’s largest producer of organic coffee (grown with no pesticides or chemical fertilizers). Mangos and pineapples are grown for domestic markets outside of the State of Oaxaca. Many farmers grow corn, beans, squash, bananas, oranges, and avocados either to sell locally or for their own use. Productivity however, is low because the quality of the soil has been degraded due to the decades-long incorrect use of pesticides and because water availability is limited due to a lack of infrastructure for irrigation.
Forestry has also become a source of income for some communities and Ejidos.
Ejidos are small villages with communally owned land. Oaxaca is currently one of
Mexico’s main timber producers (Mader 2007).
Governance
A large proportion (63.9 percent) of the state’s population lives in communities of less than 5000 inhabitants, and 415 of the state’s 570 municipalities follow traditional forms of community organization known as Usos y Costumbres (uses and customs)
(VanWey et al., 2005). Under Usos y Costumbres, the governance of the community is usually divided into two separate but equally important committees – the County Council
(Asemblea de Comuneros) and the Assembly of Citizens, along with various subcommittees. The Asemblea de Comuneros, under the elected leadership of the municipal president, manages communal lands, forests and other natural resources owned by the community. The Comuneros are traditionally composed of adult males and
46 widows and also oversee the nomination of members to civil-religious committees responsible for community festivals and church maintenance.
The Assembly of Citizens and its subcommittees, under the leadership of the municipal president, manage political functions and programs related to civic activities under the state and federal governments. The participation of women and non-native residents in the assembly is supported by law (Vanwey et al., 2005)
In communities that abide by Usos y Costumbres, citizens are expected to provide service to the community under the cargo system. Generally, the cargo system practices in these communities dictates that community members must serve on a governance committee or one of the subcommittees on a regular basis. The Assembly of Comuneros and its committees determine work that needs to be done in the community through
“voluntary” communal labor known as the tequio (often obligatory in practice). For example the Comuneros will determine when maintenance needs to done on local infrastructure, such as roads, water systems, wastewater treatment plants, drainage channels, and other communal projects. The work will then be carried out by the tequios, which are traditionally all adult male members of the community (Vanwey et al., 2005).
Traditionally, each household in a community is required to participate in the tequio system and provide a certain amount of communal service each year. Depending on the community arrangements, members may pay a fee in advance or in lieu of labor or find a substitute worker when they cannot attend the tequio (Vanwey et al., 2005).
Migration in the State of Oaxaca has had a profound impact on the system of
Usos y Costumbres in many rural communities. The migration of working-age males from rural communities in the state to the urban centers of Oaxaca, Mexico City, and the
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United States has dramatically changed the demographic makeup of many communities and substantially reduced the traditional labor pool. Cohen and Rodriguez (2004) reported that by the year 2000, Oaxacans were well represented in the migrant stream heading for the U.S. and on average 46 percent of the central valley community’s households included at least one migrant. The majority of U.S. bound migrants from the central valley are men (76 percent) (Cohen and Rodriguez 2004). This demographic shift has reduced the working age male populations in many rural communities in the
Central Valley of Oaxaca, which has also impacted the traditional tequio system.
Larger communities and peri-urban communities around Oaxaca City have adopted representative local governments that include officials and town or county councils that are elected to represent the communities. Typically, local mayors and councils are elected to serve single 18 month or 36 month (3 year) terms. The short term limits in turn can mean severe turn over in city staff and severely impact the operation of a wastewater facility if there is no continuity or transfer of experience from one administration to another.
Once a new mayor is elected, it is their responsibility to appoint staff to key departments, such as public works, parks and markets, education, and the environment.
Under this scenario, most wastewater treatment facilities will be managed by either the public works department or the environmental department or both.
Education
Historically, the level of education in Oaxaca is low. Six year primary education
is compulsory by the government. Completion of this level of education is low. Cohen
and Rodriguez (2004) found that on average 51 percent of the adults over the age of 15 in
48 the central valley had not completed primary education, although men have a slightly higher rate of education (about ½ year) than women on average.
In most rural areas local educational opportunities are limited to primary education. To seek training beyond the primary level, students must travel to larger cities or the state capital.
The problems with rural education are many. Teacher salaries are lower in
Oaxaca than anywhere else in Mexico, and qualifications are well below the level compared to national educational levels. Other limitations include a lack of schools for indigenous communities. In Oaxaca, there are no indigenous higher schools, colleges or universities, which have apparently created a gap between indigenous and non- indigenous teachers. The indigenous students do not have the opportunity to learn their culture or language in the public schools. In general most teachers do not speak the local languages and the schools are required to follow the State standard curriculum, which are very “mainstream-like” teaching methods that do incorporate traditional or local cultural or language in to classrooms (Lars Hojer Sorenson 2007).
Health Care
Healthcare is deficient throughout the state, and only 23 percent of Oaxaca’s
population has direct access to medical services (INEGI 2002). The national health care system implemented by the Secretary of Health has established health care centers in larger towns and cities which provide public health care coverage. However, coverage by these centers is constrained by limited funding, staffing and supplies.
Infant mortality in Mexico related to poor water and sanitation is highest in
Oaxaca and Chiapas. In 2005, the rate of mortality for children five and under was 32
49 children per 100,000, as compared to 12 children per 100,000 in the Federal District of
Mexico (National Water Commission 2008).
Water and Sanitation Infrastructure
Infrastructure coverage for water and sanitation in the State of Oaxaca is
relatively low compared to other states in Mexico. Table 7 presents the percent coverage
of water and sewer improvements in the state of Oaxaca and other states in Mexico.
In general, municipal projects are typically self-funded or funded through a
combination of local, state and national monies. To cover the costs of small development
projects, village leaders assess fees from households in their communities.
Table 7. Water and Sanitation Coverage in Oaxaca and other states of Mexico
Potable Water Coverage (%) Sanitary Sewer Coverage (%) State Regional Urban Rural Regional Urban Rural Oaxaca 73.3 84.7 63.4 60.0 84.0 39.2 Mexico 93.2 95.6 77.4 91.2 96.0 59.9 Nuevo Leon 95.6 97.7 60.5 95.3 97.5 57.8 District Federal 96.5 97.0 85.4 97.2 98.1 78.0 Yucatan 96.1 96.7 93.7 68.2 74.9 36.5 Aguascalientes 97.8 99.2 92.0 96.9 98.8 88.4 Baja California 93.8 95.9 67.5 88.9 91.8 51.7 Chiapas 73.5 86.2 61.9 74.7 94.1 57.0 Durango 90.9 98.9 74.8 82.6 95.4 56.9 Jalisco 93.3 95.8 77.9 95.8 98.2 81.0 Puebla 85.4 90.3 74.0 79.0 89.9 53.6 Tabasco 76.4 88.7 61.5 93.4 97.8 88.1
Water Supply
Water supply is provided by both surface water from streams originating in the
Sierra Madre and groundwater in the valley. Groundwater has historically been supplied
by shallow hand dug wells, which are numerous throughout the Oaxaca Valley. Over the past fifty years deep water wells have become more common to serve larger communities and suburban areas around the city of Oaxaca.
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Wastewater Treatment
Over the past 20 years, Mexico has implemented hundreds of community
wastewater projects. A majority of projects have been completed to comply with water
pollution control legislation (NOM-001-Semarnat-1996), adopted by the National
Government in 1997. NOM-001 required that cities with populations over 50,000 people
were required to have a wastewater treatment plants installed by 2000. Communities
with populations from 20,000 to 50,000 people were required to install treatment facilities by 2007, and communities with populations less than 20,000 people are required to install treatment plants by 2010. In general, most states are behind in complying with these requirements.
Within the State of Oaxaca, approximately 60 community wastewater projects have been constructed during this period. The majority of projects have been completed at the request of the municipal governments and in many instances to resolve disputes between two communities. Few projects have been completed to comply with NOM-
001; however, the State and National Water agencies are now pushing to meet the compliance dates, but it is unlikely they will.
Approximately 40 percent of the wastewater projects constructed in the State of
Oaxaca employ “subsurface flow wetlands” (vegetated gravel bed treatment systems
(VGBTS)), and a majority of these types of systems have been constructed in the Oaxaca
Valley. Table 8 presents a summary of the type and number of the different treatment
plants constructed in the State of Oaxaca as of 2003. This information was obtained by
the Mexican National Water Commission (Comision Nacional de Agua (CNA 2003)).
The data summarized in Table 8 indicates the type, number and volume treated by the
51 various type of wastewater treatment. The data set provided by the CNA only indicates the total volume treated by the various treatment scheme, not the total volume per treatment plant.
Table 8. Type and Number of Community Wastewater Treatment Plants (CNA 2003)
Total Wastewater Type of Treatment Plant Number Flow (l/s) Trickling Filters 1 75 Facultative Laguna 7 43 Activated Sludge 11 377 Imhoff Tank 1 5 Subsurface Flow Vegetated Gravel Bed 20 94 Others 3 9 Total 43 603
Beginning in the late 1980’s, the State Water Commission of Oaxaca (SWCO) began to implement wastewater treatment projects. During the initial 15 years, the state water agency was responsible for the design and supervision of the construction of community wastewater projects. Only recently, in the past 5 years, has the private sector undertaken a more active role in the design and, in some instances, the operation of community wastewater projects in the State.
In July 2006, the chief engineer with SWCO, Jose Luis Zaragoza, was interviewed as part of this study by Peter Haase. Mr. Zaragoza has been the chief engineer overseeing all community wastewater projects in State over the past 20 years.
The SWCO is responsible for engineering and/or technical oversight and permitting of all wastewater projects in the state. Mr. Zaragoza described what treatment technologies were being utilized in the region, and why these systems were being used.
In the late 1980’s, the U.S. Agency for International Development (USAID) funded a series of workshops on the design of “low-tech” community wastewater
52 treatment projects. The majority of the workshops were conducted by Michael Ogden, a consulting design engineer from Santa Fe, New Mexico, United States. At that time, the technology promoted by Mr. Ogden and USAID were VGBTS. The design criteria presented in the workshops was based on the USEPA Design Manual-Constructed
Wetlands and Aquatic Plant Systems for Municipal Wastewater Treatment (USEPA
1989).
These types of systems were promoted because they were considered to be:
1. Relatively inexpensive to construct;
2. Simple to operate and maintain;
3. Reliable;
4. Produce well treated effluent; and
5. Would not breed mosquitos.
The original design scheme developed by the SWCO, with support from Mr.
Ogden was a simple three stage system that included head works (screens and coarse sedimentation), an anaerobic digestor (septic tank), followed by vegetated submerged gravel beds. Figure 12 presents a typical layout of this treatment plant that was installed at San Sabastian de Tutla, located southeast of the City of Oaxaca.
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Influent
Effluent
Legend ‐ Screen and Grit Chamber ‐ Anaerobic Digester
‐ Vegetated Gravel Bed ‐ Sludge Drying Beds ‐ Flow Direction
Figure 12. Typical Layout of first Generation Wastewater Treatment System With Anaerobic Digestor and Vegetated Gravel Bed
After several years of operation, the SWCO determined that the systems were clogging up rapidly, and more frequently than originally anticipated. Further investigation by SWCO determined that the organic loading to the plants was higher than originally planned. SWCO attributes the higher loading rates to overall lower water usage by community members. To address the higher loading rates, SWCO developed an intermediate treatment process designed to reduce the organic loading rate to the VSBTS.
The intermediate treatment process developed by the SWCO was an up flow anaerobic rock filter. Figure 13 shows the layout of the second generation of treatment plants using a three-stage (anaerobic digester, upflow anaerobic rock filter and vegetated gravel bed) - anaerobic treatment system.
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Influent
Effluent
Legend ‐ Screen and Grit Chamber ‐ Anaerobic Digester ‐ Upflow Anaerobic Rock Filter ‐ Vegetated Gravel Bed ‐ Sludge Drying Beds ‐ Flow Direction ‐
Figure 13. Typical Layout of a Three-Stage Anaerobic Treatment Plant At Santo Domingo Tomaltepec
Project Costs. Table 8 presents cost data that were obtained from Mr. Jose Luis
Zaragoza (2005) with the SWCO for five community wastewater treatment projects that utilize the three stage anaerobic treatment plant shown in Figure 13. The data indicates that the systems have a relatively low total capital cost, with a per capita cost ranging from $69 per person for the larger communities and $92 to $110 dollars per person for the smaller communities. As a point of reference a simple pour-flush latrine can cost between $100 to $200 dollars per household member. The data indicates that the costs of these projects have an economy of scale - the larger the population served the lower the per capita costs. The capital costs per for each gallon treated per day range from $1.70 to approximately $3.
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Table 9. Capital Costs for Five Three-Stage Anaerobic Treatment Systems (Source: Jose Luis Zaragoza 2005)
METHODS
A field based study was conducted by the author in the Central Valley of Oaxaca,
Mexico. The field study involved visiting ten (10) community wastewater treatment systems and interviewing local, state, and national agencies, private consultants, university staff and non-governmental organizations involved in wastewater management in the Oaxaca Valley. The study was initiated in July of 2006 and continued through
August of 2008.
Community Selection Process
The community selection process was based on several factors:
1. The type of wastewater system utilized by the community;
2. The location and size of the community to the main urban center, the City
of Oaxaca; and
3. The willingness of the Community leaders to meet with the author.
An inventory of wastewater systems installed in the Central Valley of Oaxaca was prepared by the National Water Commission of Mexico in 2000. The inventory included
the type of wastewater treatment installed and identified that 25 “wetland” type
wastewater systems had been constructed in the Central Valley Region of Oaxaca.
Using the inventory data, the author identified the location of 20 wastewater
treatment plants in the Central Valley of Oaxaca. With the assistance of a local resident
and an engineer with the State Water Commission of Oaxaca (SWCO) all of the 20
communities were contacted by telephone from City of Oaxaca in July 2006. During this
initial visit in July, eight (8) communities were willing to meet with the author.
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Of the 8 communities, three projects were located very close to the City of
Oaxaca, and are located in peri-urban communities. One community was a relatively large regional town and three towns were small rural communities that are located over one hour away from the City of Oaxaca. In March 2007, two additional towns were visited, one town was another larger regional town and the second a smaller rural community. The towns were selected to allow the author to compare the results of the study for three different communities, peri-urban towns which are located just outside of large urban center of the City of Oaxaca; regional towns which are large regional centers that include primary and secondary schools, large health centers or small hospitals, and regional markets; and rural towns which are commonly small towns with limited educational and public facilities. A more detailed description of the different types of towns is presented in the Results section.
All of the towns were contacted prior to the site visit in order to make sure the author had access to the treatment facility, and was able to arrange interviews with the town leaders and the system operators in each community.
Procedures for Community Visit, Facility Inspection and Data Collection
Prior to conducting the community visits, a set of survey questions were prepared to guide the interviews. During the community visits there was an attempt to interview the mayor, the public works director and the wastewater operator/worker. Survey questions were prepared to guide the discussion and to obtain data relevant to the study.
The questions asked to the town mayors and public works director included the following:
1. When did the town install a wastewater treatment plant?
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2. Who designed and constructed the facility?
3. Why did they install a wastewater treatment plant?
4. Have they visited the wastewater treatment plant?
5. Do they know how much the wastewater system cost to construct?
6. Is the wastewater plant working?
7. Is the wastewater treatment system in good working order or well maintained?
8. Is the wastewater plant improving water quality in the town?
9. How much did the wastewater treatment system cost?
10. Does the community pay for water or wastewater service?
11. Does the town have a wastewater operator? If so, are they paid, and if they are
paid how much?
12. How much does it cost to operate the wastewater system on a monthly basis?
13. Have they done any water quality testing to see how well the system is
performing?
14. Is any of the treated water reused for a beneficial purpose, such as, for irrigation
of crops?
15. Are there any problems with the wastewater systems?
16. Do you have any suggested improvements to the current wastewater system?
Another important part of the site visits was to obtain community information related to the structure and operation of the local government (governance), how water and sewer services are provided and maintained, information about the local economy, and the level and/or access to education in the community. This community information
59 was obtained from interviews with the mayor and/or public works director, when they were willing to share it.
During the field visits the wastewater operator/workers were interviewed and the following questions were typically asked:
1. How long have you been the operator of the plant?
2. What type of maintenance is conducted on a routine basis (daily, monthly
quarterly or annually)?
3. Have you had any training prior to starting your job?
4. How much are you paid for your work?
5. What problems to you have with the system?
6. Do you think the plant is working well? If not what are the problems?
7. What improvements would you make to the plant?
8. Would you be interested in obtaining any additional training and education in
wastewater operation and maintenance?
9. Do you know any other wastewater operators in other communities? And if so do
you discuss problems or issues related to the plants?
During each field visit, several factors were observed and noted to evaluate the condition of the facility and discharge. The factors considered included:
1. The design of the treatment system;
2. The construction of the treatment plant and related improvements;
3. The operation and maintenance of the new system;
4. The visual quality of the influent and effluent; and
5. The visual quality of the receiving water and/or disposal area.
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The interviews were conducted in Spanish and the author is bilingual, so an interpreter was not necessary. The results of the site visits and interviews were maintained in a field notebook. A formal survey form was not used.
Technical Evaluation of the Wastewater Treatment Plants
An additional technical evaluation of eight of the ten wastewater treatment systems was conducted. The technical evaluation was conducted based on the estimated dimension and design flows calculated by the author based on the population served and unit flows prescribed by the State Water Commission of Oaxaca (SWCO).
In general, the evaluation was performed to determine the size of each treatment system and to estimated if it was operating within the acceptable range of State of
Mexico’s design criteria, as well as the USEPA’s recommended criteria for these types of systems (USEPA 1988 and USEPA 2000). More specifically, the technical evaluation calculated theoretical hydraulic residence time (HRT) in each unit process; the organic loading rate; the hydraulic loading rate; and lastly, comparing the minimum vegetated gravel bed area recommended based on design criteria adopted by the SWCO and the
USEPA (2000).
The physical dimensions of the treatment systems was estimated based on scaled aerial photos of the treatment facilities, including the anaerobic digesters, the up flow anaerobic rock filters, and the vegetated gravel beds. The volume of the treatment systems were estimated based on design information provided by the SWCO. Daily wastewater flow was estimated for each community using population census data obtained from the National Statistics and Geographic Institute of Mexico.
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Agency Interviews
During the study period several meetings were held with Federal, State and Local agencies to obtain regulatory, programmatic, design and cost data. The author interviewed staff from the National Water Commission (NWC) and the SWSO.
Regulatory information related to water quality standards, permit requirements and timelines was obtained by the national and state agencies. Programmatic information related to project selection and approval, funding, implementation, operation, maintenance and training was discussed. The SWCO provided copies of engineering design plans and reports for two community wastewater systems completed in the Oaxaca
Valley and provided cost data for several wastewater projects completed over the past five years. Staff from the NWC provided copies of the national standards that outlined the adopted timeline for communities to install wastewater treatment plants.
Communities Visited
During this period, ten community wastewater systems were inspected and evaluated. The inspections focused on the design, cost, construction, operation, maintenance and overall condition of the treatment systems.
All ten wastewater plants were located in the Oaxaca Valley in the State of
Oaxaca in Southern Mexico. Table 10 presents information pertaining to the location and size of each community visited, and Figure 14 is a map showing the community locations.
Nine of the ten plants utilize anaerobic digesters and vegetated gravel bed treatment systems. One system evaluated utilizes a two-stage pond system, including an anaerobic pond followed by a facultative pond.
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Table 10. Location and population information for each community
Location Altitude Total Male Female Community Name Longitude Latitude mts Population Population Population Peri-Urban Communities San Andrés Huayápam 963954 170606 1710 4104 1922 2182 Santa Lucía del Camino 964136 170345 1540 42570 20758 21812 San Sebastián Tutla 964020 170335 1530 4157 1970 2187 Regional Towns Tlacolula de Matamoros 962845 165715 1600 14074 6588 7486 San Francisco Telixtlahuaca 965415 171740 1700 9322 4500 4822 Rural Communities San Dionisio Ocotepec 962345 164746 1620 4942 2296 2646 Santo Domingo del Tomaltepec 963722 170338 1590 2061 996 1065 San Pablo Villa de Mitla 962142 165515 1680 7829 3613 4216 Santo Tomás Mazaltepec 965215 171020 1660 1878 870 1008 Teotitlán del Valle 963112 170145 1670 4427 2088 2339 Source: XII General Population and Demographic Census of Mexico, 2000, Instituto Nacional de Estadistica Geografia e Informatica (INEGI)
Figure 14. Map of Community Site Visits (Source: base map from Google Map, 2009)
RESULTS
Overview
Ten wastewater plants located in the Oaxaca Valley in the State of Oaxaca in
Southern Mexico were studied. As previously discussed the plants visited were constructed in communities representing three distinct types: peri-urban communities located within close proximity (within 30 minutes) to Oaxaca City; regional towns; and rural communities.
Peri-Urban Communities. The peri-urban communities, which are closer to the
City support a demographic population made up of more semi-professional and professional residents that choose to live just outside the City to gain access to lower land costs or to live in a more “rural and tranquil” setting. The majority of the residents in these communities work in Oaxaca City. This population base also has access to better educational opportunities in the urban area, as well as easy access to state and federal water agencies. The three peri-urban communities visited include: San Andres
Hauyapam, San Sebastian Tutla, and Santa Lucia del Camino. The economic base of these communities is primarily derived from incomes from employment in the City. The peri-urban towns tend to have abandoned the traditional model of Usos y Costumbres
(Uses and Customs) governance and adopted a more central governing model with a mayor, town employees and local town council to approve budgets, projects and expenditures.
Regional Towns. Regional towns represent larger communities (>5,000 people) that are the county seats for the municipalities. These larger communities tend have health centers, and primary and secondary schools through high school. Some of these
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64 towns include vocational programs and junior college level courses. These communities tend to be the economic centers of the municipalities and support commerce, such as banks, commercial centers, light industry, professional services (medical, dental and other special medical practices) and several churches. The two regional towns visited are
Tlacolula de Matamoros and San Francisco Telixtlahuaca. The economic base of these communities is primarily derived from small commercial and services industries, public markets, agriculture, tourism and remittance from migrant family members. The system of governance in the regional towns appears to be shifting from the traditional Usos y
Costumbres (Uses and Customs) system to a central model with a municipal mayor, public departments, and a county council.
Rural Communities. Rural communities are generally represented by small towns (<2,500 people) and are located in fairly remote locations over one hour from a large urban center, such as Oaxaca City. These towns typically have one or possibly two primary schools, a central town center with a central park and small market place, possibly one bank, a few small restaurants and corner stores, and a central church. The economic base of these communities is primarily derived from agriculture, tourism, and remittance from migrant family members. The rural towns tend to be more traditional and governed under the Usos y Costumbres (Uses and Customs) system.
The following sections describe each community and the results of the site assessments. The community descriptions includes when available satellite imagery of the towns and wastewater treatment facilities and photographs of different treatment processes and conditions. The satellite images we obtained from Google Earth and the digital photographs were taken by the author.
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Community Descriptions
Peri-Urban Communities
San Andres Huayapam. San Andres Huayapam is an indegious community
located in the foothills north east of the City of Oaxaca. The town is less than a one half
hours from the city center. Figure 15 shows a satellite image of the town and the location
of the wastewater treatment plant.
Based on discussions with town residents, over the past 30 years the town
demographics have shifted from a predominantly indigenous population to a mixed
population. Given the town’s proximity to the City of Oaxaca, small population (~4,200
people) and picturesque setting, it has become a very desirable area to live for both
professional Mexicans and U.S. ex-patriots working in the city.
Given the town’s close proximity to a major urban center, many residents have
access to higher education and better employment opportunities. Due to the influx of a
population with a higher economic and educational status, the community demands that
the local government provide a relatively high level of service. Based on these factors,
the municipal government encountered was found to be more professional than typically
found in a more rural setting.
The SWCO designed and installed a three-stage wastewater treatment system for
the town of San Andres Huayapam. The system includes a headworks and screens, an
anaerobic digester followed by an up-flow anaerobic rock filter. Effluent from the rock
filter flows to five vegetated gravel beds that are plumbed in parallel.
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Town Center Scale: 1” = 900 mts
Wastewater Treatment Plant
Figure 15. Satellite Image of San Andreas Huayapam
The wastewater treatment system was constructed by a private contractor under
the supervision of the town and the SWDO. The head works, screens, anaerobic digester,
and up-flow anaerobic rock filter are constructed of reinforced concrete. The vegetated
gravel beds are constructed using synthetic liners supported by reinforced concrete walls.
Figure 16 shows the layout of the wastewater treatment system and Figure 17 shows the
headworks and the anaerobic digester. Figure 18 and 19 show the upflow anaerobic rock
filter and the vegetated gravel beds respectively.
The anaerobic digester is divided into two primary trains and each train is divided into three. Each digester compartment receives influent through a distribution box that uses overflow weirs and piped inlets to equally distribute the wastewater to each
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67 compartment. Influent enters each compartment by two inlet pipes that are installed approximately about one third from the bottom of each digester compartment. One outlet pipe is installed in three compartments to collect and convey effluent to the up flow anaerobic rock filters via gravity. There are two outlet pipes exiting the digester.
Effluent flow from the digester is divided by a distribution box to two weir distribution boxes constructed on the top of the rock filter. Multiple pipe outlets from the weir distribution boxes distribute influent to bottom of the rock filter so that influent will flow upward and out small outlet canals constructed in the center of the filter.
Effluent from the rock filter flows via gravity to a distribution box, which distributes the wastewater to five vegetated gravel bed systems. Each VGB is filled with approximately 60 cm of 3/4 to 1 inch diameter washed river gravel and rock. A PVC pipe distribution manifold is installed at the inlet and outlet side of each VGB. Each manifold is installed at the bottom of the bed. Tail water control boxes are installed on the outlet side of each box to control the water level in each basin. Figure 20 shows the effluent from the upflow anaerobic rock filter and the final effluent from the vegetated gravel bed. A movable standpipe allows the operator to adjust the water level by rotating and adjusting the height of the pipe and corresponding water level in the VGB, as shown in Figure 21. Effluent from the VGB cells is collected and discharges to a small chlorine contact chamber before discharging to a small nearby stream or being used to irrigate alfalfa or other pasture grasses in adjacent fields.
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Scale: 1” = 50 mts
Figure 16. Aerial Photo of Wastewater Treatment Plant
Figure 17. Treatment Plant Headworks (bar screen and grit chamber) and Anaerobic Digester
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Figure 18. Upflow Anaerobic Rock Filter
Figure 19. Vegetated Gravel Beds
Figure 20. Effluent from Upflow Anaerobic Rock Filter (left photo) and from Vegetated Gravel Filter (right photo)
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Figure 21. Outlet Control Box with Adjustable Pipe Outlet
During various site visits the plant was found to be well constructed. All exposed
PVC piping was covered by cement mortar, as shown in Figure 22. This is an important construction detail in the Oaxaca Valley, which is at a relatively high elevation (over
1,500 meters). Any exposed PVC is vulnerable to rapid UV degradation, becoming very brittle and subject to breakage within a few years of installation. The outlet and distribution weirs were also constructed well and appeared to be working effectively to provide good distribution and collection of effluent from the basins.
Figure 22. Covered PVC Piping Detail at Treatment Plant
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The plant appeared to be in good working order and well maintained. Visually comparing the effluent from the up flow rock filter and the vegetated gravel bed system did not show significant difference in quality. The effluent from the treatment plant was relatively clear with no observable septic odors, indicating that the system was providing a reasonable level of treatment for carbonaceous biochemical oxygen demand and total suspended solids. Given that the three stage treatment systems are all anaerobic processes, there was no measurable difference in nitrogen levels through the treatment system measured using portable ammonia test strips from the HACH Company.
During the visit, a few wetland plants were pulled out of the VGB to check the condition of the gravel beds. It was found that the gravels beds were anaerobic and that the plant roots were very shallow and did not penetrate into the bed due to the anoxic conditions.
The community employs a full-time maintenance person to operate and maintain the plant. The operator had been in this position for many years and had not been replaced even after a change in local government. The community also maintains an ongoing contract with a local engineering firm to routinely inspect and monitor the treatment plant.
The Community has imposed a monthly service fee for the operation and maintenance of the sewage treatment system. In 2005, the monthly fee was approximately 5 pesos per household, which generated approximately 5,000 pesos per month. The treatment plant operator was paid 2,000 pesos per month.
The evaluation of the wastewater treatment system indicates that the anaerobic digester has a hydraulic retention time (HRT) of 20 hours, which is lower than the design
71
72 criteria (24 hours) established by the State Water Commission of Oaxaca (SWCO). The upflow anaerobic rock filter (UARF) has a HRT of 16 hours, which meets the SWCO design criteria. The estimated organic and hydraulic loading rates are also within the
SWCO criteria.
The SWCO utilized the design criteria presented in the USEPA’s Design Manual for Constructed Wetlands and Aquatic Plant Systems for Municipal Wastewater
Treatment (USEPA 1988). This design approach utilizes a first order reaction rate equation developed by Reed et al. (1987), and allows the designer to calculate the area of the vegetated bed based on the influent and effluent BOD concentration; however, after several years of data analysis, the updated USEPA Design Manual (USEPA 2000) reported that “none of the relationships were found to reasonably fit all of the data.” The updated manual indicated that the design approach recommended is to use the maximum pollutant areal loading rates to size a vegetated gravel bed system for the reduction of biochemical oxygen demand (BOD) and total suspended solids (TSS).
The subsurface flow vegetated gravel bed system installed has a surface area of
1,406 square meters (m2). Using the SWCO design approach the surface area should
2,130 m2 and using the recommended areal loading rates the gravel bed should have a surface area of 8,750 m2. These calculations indicate that the gravel bed is undersized for
the quantity of flow it is likely receiving, which is one factor that likely results in the anaerobic conditions observed.
In summary, the technical evaluation presented above indicates that the anaerobic digester and the vegetated gravel bed were undersized and partially clogged. These
problems negatively affect the performance, operation and maintenance of the treatment
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73 system. These problems may have occurred as a result of several factors, including inadequate funding to construct a larger plant, the use of dated or inappropriate design criteria, higher strength wastewater than anticipated, incorrect engineering and design, and insufficient operation and maintenance practices. A summary of these results are presented in Table 13 and a spreadsheet with these calculations is presented in Appendix
A.
San Sebastian de Tutla
San Sebastian de Tutla is a peri-urban community located approximately 30 minutes south of the center of Oaxaca City. The community is also a small barrio of
Oaxaca with a population of approximately 4,200 people. Figure 23 is a satellite image of the town. The economic status of the community is mixed with low and lower middle class residents. Most residents work in government jobs, teaching, construction, auto repair shops, restaurants and other service sector type employment. The community has a primary school and access to secondary schools and universities in Oaxaca City.
The wastewater system was designed and constructed by the SWDO, and includes a two-stage system that includes a multi-compartment septic tank followed by a series of vegetated gravel beds in series. The system was one of the first systems of this kind designed and constructed by the State, and is considered to be the first generation system.
A layout of the system is shown in Figure 24. (The second generation design includes the up flow rock filter such as used by the community of San Andres Huayapam.)
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Town Center
Wastewater Treatment Plant
Scale: 1”=600 mts
Figure 23. Satellite Image of San Sebastian de Tutla
Scale: 1” = 100 mts
Figure 24. Aerial Photo of Wastewater Treatment Plan for San Sebastian de Tutla
The system was visited on several occasions throughout the study period.
Although the system was generally well maintained, close observation found that VGBs
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75 were organically overloaded and not working well, as shown below in Figure 25. The wetland plant coverage was relatively thick; however, when the plants were pulled out of the bed the root systems were very shallow (less than 6-inches deep) and did not penetrate into the anoxic zone of the bed.
Figure 25. Photos of Vegetated Gravel Bed and Surfacing Effluent
Visually, the effluent from the treatment plant was relatively clear indicating a reduction of carbonaceous BOD and total suspended solids, as shown in Figure 26. As observed at San Andreas Huayapam, and as would be expected for this type of treatment system, testing the water using ammonia test strips indicated that the plant was not removing any ammonia or nitrogen in the wastewater.
Figure 26. Final Effluent Quality San Sebastian de Tutla
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The Community employees a full-time maintenance person who maintains the plant and pump system. The operator is paid 3,000 pesos a month and the Community charges the residents approximately 5 pesos a month for this service.
A technical evaluation of the wastewater treatment plant was conducted. Using design criteria established by the SWCO, the size of each unit process (anaerobic digester and vegetated gravel beds) was assessed to determine if it was properly sized for the population served. The population of San Sebastian de Tutla is approximately 4,157 people. The per capita design flow for wastewater used by the SWCO is 100 liters per day (lpd). Based on this unit flow, the total daily flow to the treatment plant is approximately 416 cubic meters per day (m3/day).
As previously described, the wastewater treatment system utilizes a two-stage
treatment system including an anaerobic digester and a multi-bed vegetated gravel bed
system. The anaerobic digester is estimated to be 10 meters wide, 10 meters long and 3
meters deep. The volume of the digester is approximately 300 m3. Based on a daily flow
of 416 m3/day, the theoretical hydraulic residence time for the anaerobic tank is
approximately 17 hours, which is approximately 60 percent less than the SWCO design
criteria, set at 24 hours.
The wastewater system has 12 vegetated gravels beds. Each bed is approximately
25 meters long and 12.5 meters wide. The total surface area of the vegetated bed system
is 3,750 m2. Based on the SWCO the size of the vegetated bed should be 2,110 m2;
however, using the updated USEPA criteria based on areal loading rates the gravel bed
should be 8,667 m2.
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In summary, the analysis indicates that the anaerobic digester and the vegetated gravel beds were undersized to accommodate the estimated flows to the plant. This condition has contributed to the failure of this system. A summary of this analysis is summarized in a spreadsheet in Appendix A.
Santa Lucia del Camino
Santa Lucia del Camino is a barrio or suburban district approximately 20 minutes south of the City of Oaxaca. The community is relatively large with a resident population of approximately 43,000 people. Figure 27 presents a satellite image of the community.
The economic status of this community is mixed with low, lower and upper middle class residents. During various site visits it appeared that the community was experiencing growth and a lot of new residential housing projects were under construction.
In 2000, the Community and SWCO completed the construction the first phase of a wastewater treatment system, which included an anaerobic digester and up flow anaerobic rock filter. In 2005, the Community was nearing completion of the construction of the second phase of the treatment plant including the installation of several vegetated gravel beds. Once the system is completed, it will be identical to the plant used in San Andreas Huayapam.
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Scale: 1” = 600 mts
Figure 27. Satellite Image of Santa Lucia del Camino
The wastewater system was designed by the SWCO and constructed by private contractors. As noted, the system is being constructed in phases and possibly by different contractors. During the site visit it was apparent that the quality of construction and attention to detail improved in subsequent phases of construction. This could have been attributed to the State Engineer demanding a higher level of construction or the experience of the particular contractor.
The anaerobic digester was completed prior to 2000 and the majority of the PVC piping used in the plant is exposed, vulnerable to UV degradation and will likely need to be replaced in the near future. Whereas, in the up flow anaerobic rock filter all PVC piping was covered with mortar. The inlet and outlet weirs in the rock filter appeared to
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28.
Figure 28. Inlet and Outlet Weir configuration for Up Flow Anaerobic Rock Filter
Visual inspection of the influent and effluent from the rock filter do not show significant improvement in quality and the effluent was not very clear (Figure 29), as compared to the effluent quality observed at the plant in San Andres Hauyapam, as shown in Figure 20. The solids loading from the digesters did not appear to be excessive and the digesters appeared to be operating well and solids were being properly managed and removed every three months. The marginal performance of the rock filter may be attributed to the system being hydraulically overloaded.
Figure 29. Effluent from Up Flow Anaerobic Rock Filter
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The VGBs was under construction during the site visit, as shown in Figure 20.
The system shown is typical for most of the VGB systems visited in the Valley. The
VGB is constructed with synthetic liner anchored by a low curb or wall constructed with reinforced concrete. The inlet and outlet pipes are installed the full length of the bed and the flow of water is across the bed. As seen in the photo in Figure 30, the gradation of the gravel was highly variable from very fine gravels or sands to small cobbles. The gravel appeared to be clean; however, it contained the significant amount of fines, which will likely cause the VGB to clog faster than normal once operation is started.
Figure 30. Vegetated Gravel Bed under construction for Santa Lucia del Carmen
The system is operated by a full-time maintenance person, who is employed by the community. In 2005, the maintenance person was paid 2,500 pesos per month to conduct basic cleaning and maintenance of the system by the town. The each household in the community is charged for both water and sewage service. The number of households paying for both water and sewer services was roughly 40 to 50 percent; however, the Mayor of the community indicated that he believed that the number of
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81 homes paying for wastewater service would increase as the economic conditions in the community improve. However, he did not indicate how and when this may occur.
Regional Towns
Tlacolula de Matamoros
Tlaculola is a regional town and the county seat for the municipality of Tlaculola.
It is located approximately 30 kilometers east of Oaxaca City. The town is situated on
the valley floor and is surrounded by both agricultural and underdeveloped or natural
lands. Figure 31 is a satellite image showing the town and location of the wastewater
treatment system.
The town has a residential population of approximately 14,000 people. The town
is a very important financial center for the Oaxaca Valley and operates one of the largest
public markets in the region. The town has undergone significant growth over the past
ten years and this is apparent by the new construction projects observed throughout the
community. Based on discussions with the city staff, a significant amount of new
construction is attributed to an influx of new residents from Mexico City who desire to live in a smaller community that is within relatively close proximity to the City of
Oaxaca.
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Wastewater Treatment Plant
Town Center
Scale: 1” = 2,000 mts
Figure 31. Satellite Image of Tlacolula
In 2000, the State of Oaxaca and the community installed a two-stage waste stabilization pond system. The system includes an anaerobic and facultative pond, as shown in Figure 32. The pond is located west of the community and has been designed and constructed as a gravity flow system.
The system includes a head works, with two-stage coarse screening and sedimentation chamber. After the head works the wastewater enters a distribution box that distributes the wastewater to three inlet weir boxes constructed in the anaerobic pond and at equal lengths along the width of the pond basin. Similarly, there are three outlet weir boxes on the opposite side of the basin that convey wastewater to three inlet weir boxes in the facultative pond. Three outlet weir boxes are installed at the far end of the facultative pond. The weir boxes use steel plate flash boards to control the water level in
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83 each pond. Effluent from the facultative lagoon enters a small serpentine chlorine contact tank, which has not been in use for many years.
Figure 32. Wastewater Treatment System at Tlacolula de Matamoros
The system was initially visited in March 2005 and found to be operating in marginal condition. The system appeared to be organically over loaded. Water quality testing completed by the National Autonomous University of Benito Juarez in Oaxaca verified that the system is receiving high strength wastewater and is organically overloaded. The results of water quality testing is presented in Table 11, which presents results of raw influent, effluent from the anaerobic pond, and final effluent from the facultative pond.
During a subsequent site visit in February 2008, the color of the facultative pond had changed from a brownish green color to a reddish color indicating that the system is organically overloaded with predominantly sulfur bacteria. The anaerobic pond may be
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84 overloaded with solids, which may be overflowing and impacting the biology and performance of the facultative pond.
The town employs a full time person to conduct basic maintenance of the plant.
The town pays this person (worker) 2,000 pesos per month. Routine maintenance activities include the removal of trash and sediment from the sedimentation chamber, and weeding of the banks of the ponds. The facility appears to be well maintained; however, the solids in the anaerobic pond need to be removed. The Community charges 2.5 pesos per month for sewer service.
Based on the test results and observations it is apparent that the ponds are undersized given the strength of waste entering the system. The system does not have any redundancy (i.e. multiple ponds) to allow for routine maintenance, and there are no facilities (sludge drying beds) to remove and dry solids from either pond.
Table 11. Water Quality Data for Wastewater Treatment for Tlacolula (All values reported concentrations in mg/L)
BOD5 Total Suspended Solids Anaerobic Pond Facultative Anaerobic Facultative Date Influent Effluent Pond Effluent Influent Pond Effluent Pond Effluent 12/19/2007 822 328 218 458 188 468 12/20/2007 796 464 262 280 269 467 12/22/2007 810 400 210 659 186 467 12/23/2007 890 460 270 171 180 529 1/21/2008 872 396 376 407 241 438 1/24/2008 792 424 348 380 220 471 1/26/2008 864 428 296 626 217 451 3/3/2008 758 444 182 413 227 380 3/5/2008 956 442 262 461 251 409 3/12/2008 800 400 300 561 225 375 Source: Toledo, A.M., K.C. Carrasco de la Cruz, and j.J. Villalobos-Lopez. 2008. Wastewater Evaluation Studyof Oxidation Ponds for Tlacolula de Matamoros, Oaxaca . Thesis for Biochemistry Degree. UABJO
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San Francisco Telixlahuaca
San Francisco Telixlahuaca is a regional town approximately one hour north of
Oaxaca City. The population in the town is approximately 9,400 people. Figure 33 is a satellite image of the town and wastewater treatment system.
The Community and State of Oaxaca installed a wastewater treatment plant in
1997. The system includes a three stage process including two anaerobic digesters in parallel, an up flow anaerobic rock filter and ten vegetated gravel beds. The system also includes two sludge drying beds. Figure 34 shows the layout of the wastewater treatment system. Figure 35 shows the headworks and the anaerobic digester.
The community does not charge for sewer service. The community employs an operator that they pay 4,000 pesos per month. Unfortunately, the operator that was employed at the time of the site visit did not received any training on the operation and maintenance of the plant and was primarily maintaining the grounds around the plant clean, but was not trained or confident enough to repair the plant.
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Town Center
Scale: 1” = 1,500 mts
Wastewater Treatment Plant
Figure 33. Satellite Image of San Francisco Telixlahuaca
The digester and rock filters were found to be very poor condition. The digesters were full of solids and most of the plumbing from the rock filter was degraded, broken, and dysfunctional. When the plant was constructed the piping was left exposed and appeared to be degraded from UV exposure and was very brittle and broken as shown in
Figure 36. Although, the digester and rock filter were not working properly the vegetated beds were well vegetated and many of the beds were being used to raise flowers commercially as shown in Figure 37.
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Scale: 1” = 100 mts
Figure 34. Aerial Photo and Layout of Wastewater Treatment Plant
Figure 35. Headworks and Anaerobic Digester
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Figure 36. Upflow Anaerobic Rock Filter Distribution Manifold (left photo) and Overflow Weir (right photo)
Figure 37. Vegetated Gravel Beds
An analysis of the treatment system compared to design criteria adopted by the
SWCO and the USEPA (2000) has been conducted. Based on the population of the community (9,322 people) the daily flow is estimated to be 932 m3/day. The results
indicate that the anaerobic digester has a HRT of 22 hours, which is just under the SWCO
criteria of 24 hrs. Similarly, the HRT of the up flow anaerobic rock filter has a HRT of
12 hours, which is also below the SWCO criteria of 16 hours. The surface area of the
vegetated ravel bed system is 2,530 m2. Based on the estimated design flow of the
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89 system the SWCO design criteria recommend 4,728 m2 and using the updated USEPA
criteria would require 19,421 m2.
In summary, the analysis indicates that the anaerobic digester, the upflow
anaerobic rock filter, and the vegetated gravel beds were undersized to accommodate the
flows to the plant. This condition has likely `contributed to the failure of this system. A
summary of this analysis is summarized in a spreadsheet in Appendix A.
Rural Communities
Santa Domingo de Tomaltepec
Santa Domingo de Tomaltepec is a rural town located approximately 45 minutes northeast of the City of Oaxaca. Figure 38 is a satellite image of the community. The town population is approximately 2,061 people. The economy of the town is based on local agriculture and remittance.
The Community and State Water Institute constructed and started up a community wastewater treatment system in 2001. The system includes a head works, lift station, an anaerobic digester, an up flow anaerobic rock filter, five vegetated gravel beds, and a chlorine contact chamber. The layout of the wastewater treatment system is shown in
Figure 39.
The town employs a full time wastewater operator/maintenance worker, who is employed primarily to remove trash and sediment from the sediment chamber, to cut back vegetation on the VGBs and to operator the chlorination system. The operator is paid 4,000 pesos per month. The town also charges 20 pesos per household for sewer charges that cover the cost for electricity and the operator.
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The system was found to be reasonably good working condition, as compared to the other systems visited. The digester appeared to be routinely cleaned; however, at the time of the inspection it was in need of cleaning and appeared to be wasting sludge to the up flow rock filter.
Scale: 1” = 1,500 mts
Town Center Wastewater Treatment Plant
Figure 38. Satellite Image of Santa Domingo de Tomaltepec
Scale: 1” = 100 mts
Figure 39. Aerial Photo and Layout of Wastewater Treatment Plant
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Figure 40. Bar Screen and Grit Chamber (left) and Anaerobic Digester (right)
Figure 41. Upflow Anaerobic Rock Filter
Figure 42. Vegetated Gravel Bed and Chlorine Contact Tank
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The VGBs were actively maintained and routinely cleaned. Vegetation in two of the beds had recently been cut. Three of the beds had thick vegetation as shown in Figure
42.
The original chorination system included a liquid feed chlorinator and serpentine type chlorine contact tank as shown in Figure 42. The liquid feed chlorinator was not operating, and the operator was manually adding chlorine to the system every third day.
The operator had a very basic understanding of the physical operation of the plant; however, he did not have a good understanding of basic treatment processes, including proper disinfection procedures.
Given the population of the community (2,061 people) the design flow is estimated to be 206 m3/day. An evaluation of the design flow and size of the treatment plant indicates that the anaerobic digester and the upflow anaerobic rock filter have HRTs of 35 hours and 30 hours, respectively. The size of the vegetated gravel bed is 1,680 m2, whereas using the SWCO criteria a 1,045 m2 bed would be recommended. Using the updated USEPA criteria based on areal loading rates the bed size should be 4,294 m2.
In summary, the treatment system was properly sized according to the SWCO
criteria and as previously noted the system seemed to working better than most systems
visited. The system would likely work better if the size of the vegetated gravel bed was increased. A summary of this analysis is presented in a spreadsheet in Appendix A.
San Tomas Mazaltepec
San Tomas Mazalltepec is a small rural colonial community located approximately 1-1/2 hours northwest of the City of Oaxaca in the Etla Valley. Figure 43
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93 is a satellite image of the town and wastewater treatment facility. The town has a population of approximately of 2,000 people.
Wastewater Treatment Plant
Town Center
Scale: 1” = 1,000 mts
Figure 43. Satellite Image of San Tomas Mazaltepec
The Community and State installed the wastewater treatment plant in 1997. The plant includes an anaerobic digester, up flow anaerobic rock filter, seven vegetated gravel filter and two sludge drying beds. The system was designed by the SWCO. The layout of the system is shown in Figure 44.
The system is maintained by the community based on the tequio system of community service. The community also charges 60 pesos per household per year for sewage service. The community staff indicated that they collect about 50 percent of the annual user fees.
During the site visit the treatment plant was found to be in poor condition. The anaerobic digester was full, the anaerobic rock filter was clogged (see Figure 45) and the 93
94 vegetated gravel bed filters were being bypassed and did not have any established wetland plants. The plant appeared to be abandoned and not maintained. It was clear that no one in the community knew how to operate or maintain the system.
Figure 44. Wastewater Treatment System for San Tomas Mazaltepec
The town charges 60 pesos per household per year for sewage service and the town staff indicated that they are able collect about 50 percent of the annual user fees.
The population of the community is approximately 1,878 inhabitants and the calculated design flow is 188 m3/d. Based on the design flow, the HRTs of the anaerobic
digester and the upflow anaerobic rock filters was calculated to be 38 hours for both
stages of treatment, which is substantially higher than the SWCO criteria. The area of the
vegetated gravel bed is 2,400 m2, which is higher than the area required by the SWCO
criteria, and calculated to be 952 m2. Based on the USEPA areal loading rates the
recommended surface area is 3,913 m2.
In summary the treatment plant size exceeds the SWCO criteria for all three unit
processes installed. The vegetated gravel bed system is approximately 50 percent of the
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size recommended by the USEPA (2000). A summary of this analysis is also presented
in a spreadsheet in Appendix A.
Figure 45. Photos of Anaerobic Digester and Upflow Anaerobic Rock Filter at San Tomas Mazaltepec
Teotilan del Valle
Teotilan del Valle is a rural community approximately 45 minutes east of the City of Oaxaca. The town has a population of approximately 4,500 people. A satellite image of the town is shown in Figure 46.
The town is renowned for traditional weaving and art. The weaving, tourism, and remittance from relatives in the Mexico City and the U.S. are the primary economic base for the community. Most families also maintain small agricultural plots for production of corn and beans for non-commercial domestic use only.
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Town Center
Wastewater Treatment Plant
Scale: 1” = 1,000 mts
Figure 46. Satellite image of Teotilan del Valle
The community and the SWCO constructed the wastewater treatment system in
1999. The system design is similar to many of the systems previously described and includes a head works, two anaerobic digesters, an up flow anaerobic rock filter and nine gravel beds. The system is entirely a gravity flow system. An aerial photo of the treatment plant is shown in Figure 47.
The treatment system was in marginal condition. The up flow anaerobic rock filter contained excessive solids and was overloaded with solids, indicating that the anaerobic digesters were not being properly maintained (Figure 48). The system operator had been working on the system for several years, but did understand nor had he received any training on the operation and maintenance requirements of the system.
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Scale: 1” = 100 mts
Figure 47. Wastewater Treatment System at Teotilan de Valle
The gravel beds were not planted with vegetation and were being operated as single pass or intermittent gravel filters. Many of the gravel beds were clogging due to the excessive solid loadings from the rock filters. However, the effluent quality from the beds appeared to be very clear, as shown below in Figure 49, indicating even though system is not operating optimally it is still providing reasonable treatment of carbonaceous BOD and TSS. The effluent from the non-vegetated gravel beds is substantially clearer in color than the effluent from the VGB, since there is no internal organic load.
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Similar to other communities in the Valley, the town employs a laborer to conduct menial labor around the plant. At the time of the visit the laborer was paid 3,000 pesos a month to work on the plant. The community charges an annual fee of 60 pesos per year for sewer service.
Figure 48. Headworks and Outlet of Upflow Anaerobic Rock Filter with Excessive Solids Teotilan de Valle
Figure 49. Gravel Beds and Final Effluent at Teotilan de Valle
The population of the community is approximately 4,427 inhabitants and the calculated design flow is 443 m3/d. Base on the design flow, the HRTs of the anaerobic
digester and the upflow anaerobic rock filters was calculated to be 33 hours and 24 hours,
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The area of the vegetated gravel bed is 3,168 m2, which is higher than the 2,245 m2 area recommended using SWCO criteria. Based on the USEPA areal loading rates the recommended surface area is 9,223 m2.
In summary the treatment plant size exceeds the SWCO criteria for all three unit
processes installed. The vegetated gravel bed system is approximately 30 percent of the
size recommended by the USEPA (2000). A summary of this analysis is also presented
in a spreadsheet in Appendix A.
San Dionisio de Ocotepec
San Dionisio de Octepec is a rural community located approximately 60
kilometers south east of Oaxaca City, as shown in Figure 50. The population of the town
is approximately 5,000 people. Agriculture and remittance is the economic base for the
community. A satellite image of the community is shown in Figure 51 and an aerial
photo of the wastewater treatment system is shown in Figure 52.
Figure 50. Location Map of San Dionisio Ocotepec
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Town Center
Wastewater Treatment Plant
Scale: 1” = 600 mts
Figure 51. Satellite image of San Dionisio de Ocotepec
Scale: 1” = 60 mts
Figure 52. Aerial Photo of Wastewater Treatment System
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In 2002 the town completed the construction of a two stage wastewater treatment system. The plant was designed by the Center of Investigation for Regional Integral
Development (CIDIR). The system uses a modified up flow anaerobic sludge (UASB) bioreactor design, referred to as “hybrid anaerobic integrated bioreactor” (BRAIN) followed by vegetated gravel beds. The system uses two BRAINs followed by twelve vegetated gravel beds (VGBs). The VGBs are 2 meters wide by 60 meters long.
At the time of the site visit the plant was found to be poorly maintained and in marginal operating condition. The quality of effluent leaving the BRAIN appeared to be clear and providing reasonably effective treatment, as shown in Figure 53.
Figure 53. Effluent Quality from BRAIN System entering the Vegetated Gravel Beds
The VGBs were clogged with ponding and solids on the surface. Approximately
60 percent of the beds were covered with established vegetation (Figure 54). The
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Figure 54. Vegetated Gravel Beds and Effluent Quality San Dionisio Ocotopec
Members of the community stated that they do not charge for sewer service and that they do not pay anyone to maintain the system. The system is apparently maintained by the community volunteers (tequio). The community members interviewed were not aware of receiving any guidance or training on the operation and maintenance of the system, and they assumed that it was working properly.
The population of the community is approximately 4,942 inhabitants and the calculated design flow is 494 m3/d. Base on the design flow, the HRT of the anaerobic
digester was calculated to be 35 hours, which is higher than the SWCO criteria. The area
of the vegetated gravel bed is 1,440 m2, which is lower than the 2,506 m2 area required
using SWCO criteria. Based on the USEPA areal loading rates the recommended surface
area is 10,296 m2.
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In summary, the treatment plant size exceeds the SWCO criteria for the vegetated bed surface and is approximately 13 percent of the size recommended by the USEPA
(2000). A summary of this analysis is also presented in a spreadsheet in Appendix A.
San Pablo de Mitla
San Pablo Mitla (Mitla) is a rural community located approximately 45 kilometers east of the City of Oaxaca, as shown in Figure 55. A satellite image of the town is shown in Figure 56. The population of the town is approximately 8,000 people. The town is primarily an agriculturally based economy supported by remittances from the family members located in Mexico City and the U.S. The town is also a popular and important tourist destination because of the well preserved archeological ruins on the outskirts of town.
Figure 55. Location Map of Mitla
The Community and SWCO recently completed a new wastewater treatment plant for the town. The plant design is similar to many systems already described and includes a head works (coarse screens and grit chamber), two anaerobic digesters, an up flow
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A site visit was conducted approximately one year after the new plant had been started and the system was found to be well constructed, but in poor condition due to a lack of operation and maintenance. The solids in the digesters were not being cleaned out frequently enough and solids were overflowing into the rock filter and VGBs.
Apparently, the SWDO had trained the public works department staff employed at the time the plant was started up. However, within six months of plant startup a new city administration was voted into power and the previous staff was replaced with new staff who were not trained in the operation and maintenance of the system. The new employees did not have any understanding of the operation and maintenance requirements of the system.
The population of the community is approximately 7,829 inhabitants and the calculated design flow is 783 m3/d. Base on the design flow the HRTs of the anaerobic
digester and the upflow anaerobic rock filter were calculated to be 22 hours and 12 hours,
respectively. In both cases the HRTs are below the SWCO criteria. The area of the
vegetated gravel bed is 2,028 m2, which is lower than the 3,970 m2 area required using
SWCO criteria. Based on the USEPA recommended areal loading rates, the bed surface
area is 16,310 m2.
In summary the treatment plant is undersized based on the SWCO criteria. The
vegetated bed surface and is approximately 15 percent of the size recommended by the
USEPA (2000). A summary of this analysis is also presented in a spreadsheet in
Appendix A.
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Town Center Scale: 1” = 1,000 mts
Wastewater Treatment Plant
Figure 56. Satellite Image of San Pablo Mitla
Scale: 1” = 60 mts
Figure 57. Aerial Photo of Wastewater Treatment Plant
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Figure 58. Mitla Headworks (Screens and Sedimentation Channel) And Anaerobic Digesters
Figure 59. Mitla Upflow Rock filter Inlet Distribution Box and Outlet Overflow Weir
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Figure 60. Mitla Vegetated Gravel Bed System
Summary of Findings
The primary focus of the wastewater treatment assessment system was to visit and evaluate community wastewater projects. In addition, several staff from the national and state waster agencies were intervieweed to obtain information on the overall planning and implementation of projects. The following section provides a summary of the general findings and the results of the community field assessment work.
General Findings
Based on interviews and meetings with national, state and local agencies, and consulting engineers, several general finding are noted:
1. The selection and implementation of wastewater projects tend to reflect the
political importance of the town and desire of the community and are not targeted
or strategic to improve water quality employing a watershed based approach.
2. Public and Private wastewater engineers working in the Oaxaca Valley continue
to employ treatment schemes, primarily the three stage anaerobic treatment
system employing a anaerobic digester, an upflow anaerobic rock filter and a
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vegetated gravel bed system. They seem to use this system because they are
familiar with the technology and construction, regardless of the systems limited
effectiveness.
3. The design criteria used by local engineers to size the vegetated gravel beds is
dated and not technically based, and as a result, the surface area of all of the
vegetated gravel beds is too small and leading to clogging within a relatively short
period of time (typically less than 5 years after plant startup).
4. Many projects were constructed entirely with national and state funding without
any cost sharing requirements of the local municipal governments or towns.
Since the local communities do not have substantial financial obligations at stake,
there is no pressure imposed to require long-term maintenance and operation of
the systems.
5. It is apparent that local professional engineers designing community wastewater
systems in the State of Oaxaca are working in isolation and selecting a narrow
range of technologies for wastewater treatment. Based on discussions with
several local design engineers, they were found to be reluctant to consider
different technologies.
6. Due to a constant change in the local municipal government and in some cases a
lack of participation by community members, several projects were found to be
essentially abandoned. These cases were good indictors that projects were
undertaken without good planning and outreach by the local and state agencies.
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Community Field Assessment Summary
The results of the community assessment study have been compiled and
summarized into three tables. The results have been summarized into three different
categories: community factors, technical factors, and managerial factors.
Community Factors
Table 12 presents the community factors that have been identified, including: the
type of governance system in place (representative verses traditional system (Usos y
Costumbres – tequio system), the level of educational accessible to the community, the
economic status of the community, and the primary source of income for community
members. (As previously discussed in the Methods Section, this information was
generally obtained during interviews with the town mayors and public works directors.)
Table 12, 14, and 15 are set up into different categories and uses a simple “X” to
denote whether the particular factor is present (or absent) in the town. For example,
under the category of economic status there are five subcategories ranging from upper to
lower economic status or income brackets. A “high” category implies that there are
residents in the community that have a high income and low economic status implies a
low income bracket for residents. The information is provided to inform the user if a
community has a mixed income base or a middle to low income status.
Governance. The community data indicates that eight of the ten communities
have adopted a more modern and conventional central government approach to run the
municipality and municipal infrastructure and have established public works departments
and have environmental coordinators. The local governments are elected for short
periods of time, typically 18 to 36 months. Once they change, one significant problem is
109
110 that the new department heads may or may not understand the wastewater system and may not know that it requires ongoing operation and maintenance. This was clearly the case with the system at San Pablo Villa de Mitla. After only three months of operation of the new plant, a new government resumed power and the plant was unattended for over nine months before we visited it and observed the problems documented above. The two remaining rural communities are using the traditional form of community government,
Usos y Costumbres.
Education. All of the peri-urban communities have access to primary, secondary and advanced education. One regional community, Tlacolula has access to primary, secondary and the first two-years of university courses that are offered as extension courses through the national university. Three rural communities only have access to primary education.
Economic Status. All of the communities surveyed appeared to be economically stable and none of the communities were impoverished. All seemed to be economically mixed with residents earning low to middle incomes. The economic status of residents is broader in the larger communities. For example Teotilan de Valle includes low, middle, upper middle, and high income residents, which is attributed to both professional
Mexican nationals seeking to live outside of the city in a more rural setting and expatriate residents from the United States (U.S.) and other foreign countries. Employment in the towns is quite varied and is based on the size, location and level of services provided.
Employment and Income. Agriculture has historically and continues to be a main source of employment of many indigenous residents in the Valley communities.
All of the peri-urban and regional towns also support professionals, including doctors,
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111 lawyers, dentists, accountants, and others. The regional towns and a few towns support service jobs at small cafes, restaurants, bakeries, hotels and a few towns have artisans who work in arts and crafts primarily sold to tourists in Oaxaca City and/or exported to the U.S. and Europe. Remittance from family members located in the Mexico City, the
U.S. and other parts of Mexico is also an important source of income to many residents in all of the communities surveyed.
111
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Remittance
Agriculture
Artisan
Service Jobs Service Professional Jobs Professional
Primary Sources of income Sources Primary
Low
Low Middle Low
Middle Upper Middle Upper
Economic Status Economic
High
Advanced
Secondary Primary Access to Education Access Government Representative Representative Department Public Public Department Works orEquivalent Governance Table 12. Community Factors Assessment Table 12. Community Factors Usos y Usos y Tequio Tequio System Costumbres Name y Communit San Andrés Huayápam Andrés San del Camino Lucía Santa Tutla Sebastián San Matamoros de Tlacolula Telixtlahuaca Francisco San Ocotepec Dionisio San Santo Domingo del Tomaltepec de Mitla Pablo San Villa Mazaltepec Tomás Santo Teotitlán del Valle X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X Peri-Urban Communities Peri-Urban Towns Regional Communities Rural
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Technical Factors
Table 13 presents a summary of the evaluation of the treatment plants based on a review of the design criteria used by the SWCO, as well as criteria developed by the
USEPA (2000). Table 14 summarizes the assessment of technical factors identified in each community. The technical factors considered included the adequacy of the engineering design and construction, the condition of the treatment system in terms of the operation and maintenance of the system, the performance or status of the treatment system, the method of disposal or reuse, and the impact of ambient water quality as a result of the discharge.
Engineering Design and Plant Sizing. The field assessment found that six plants appeared to be properly designed and sized for the communities served by the system in accordance with the design criteria established by the SWCO. Four treatment plants were poorly designed or undersized to accommodate the flows and wastewater strength entering the plants. All of the vegetated gravel bed systems are substantially smaller than the USEPA 2000 design criteria based on aerial loading rates.
Construction. Six treatment plants were found to be well constructed. All of the exposed piping was covered for protection from ultra violet degradation. All of the structures and control valves were well constructed and easily accessible for operation and maintenance.
Several other plants were found to be undersized and both hydraulically and organically overloaded. At two plants PVC piping was not covered, had suffered UV degradation and was broken in several locations rendering the plants inoperable. Several
114 of the vegetated gravel bed systems were not properly constructed and contained a high fraction of fines that most likely causes the beds to clog more rapidly.
Operation and Maintenance. Two plants were found to be well maintained. At one location, San Andres Huayapam, the community has retained a private engineering company to oversee the operation and maintenance of the plant. Although, the treatment plant at San Sabastian de Tutla was overloaded, the system operator maintained the facility in good condition and was attempting to operate the system adequately.
Four plants were found to be in moderate condition with the overall facilities being maintained; however, at most of these plants the operator needed to remove solids from the primary digesters or the anaerobic pond to prevent carryover of solids to the subsequent treatment process, an upflow anaerobic rock filter or facultative pond.
Four plants were found to be poor condition and three of these systems appeared to be abandoned by the community. At all of these sites the entire treatment systems were full of solids and had not been maintained for a prolonged period of time.
Operation Status. The results of the field assessment found that two of ten wastewater treatment systems were operating as designed. One system was operating marginally and producing a relatively clear effluent. The remaining seven treatment plants were operating poorly as result of a lack of maintenance or construction problems.
Disposal and Reuse. All of the treatment plants discharge to surface water courses that eventually discharge to the Rio Salado or Rio Atoyac. Three communities also periodically reuse treated wastewater for irrigation of pasture lands in the proximity to the treatment plants.
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115
2 Criteria USEPA 2000 2000 USEPA SWCO SWCO Criteria VGB Installed Rock Filter Rock Plant Sizing and Design Criteria. Upflow Anaerobic Anaerobic Upflow Hydraulic Residence Time, HRTTime, Residence (hrs)Hydraulic Gravel Bed Vegetated m Area, Surface Anaerobic Digester Anaerobic /d) 3 (m Design Flow Flow Design Table 13. Summary of Evaluation of Table 13. Summary of Evaluation Community 1. SWCO = HRT24 Digesters hrs Anaerobic for = 162. Rock Filters SWCO Anaerobic hrs HRT Upflow for San Andres Huayapam Andres San Tutla de Sabastian San Telixtlahuaca Francisco San Tomaltepec de Domingo Santa Mazaltepec Tomas San Valle de Teotilan 420 932 206 Ocotepec de Dionisis San 416 Mitla de Villa Pablo San Notes: 188 21 494 24 35 17 783 443 38 35 16 22 16 30 33 NA 38 NA 1,406 2,530 1,680 3,750 12 24 2,130 4,728 1,045 2,110 2,400 1,440 2,028 19,421 8,750 3,168 4,294 952 2,509 8,667 3,970 2,245 10,296 3,913 16,310 9,223
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Ambient Water Quality. Based on the quality of effluent observed at each site, the effluent from six plants was clear, which indicates that the treatment plants were reducing the carbonaceous or organic loading to the stream. The four poorly operated plants were not reducing the organic loads to the stream. Nine of the ten plants utilize anaerobic treatment processes in series and do not reduce the ammonia-nitrogen levels in the wastewater. The vegetative gravel beds may also increase the ammonia concentration in the final effluent resulting from the decaying plant material or biomass in the system that includes nitrogenous compounds.
Based on the marginal performance of the treatment plants, and the elevated concentration of ammonia-nitrogen in the final effluent of the plants, the discharge of the plants is degrading the water quality of the downstream surface water resources.
All of the treatment plants discharge to small streams or canals that flow to the main rivers, Rio Salado and Rio Atoyac. Based on the arid condition of the Oaxaca
Valley, flows in these streams are typically very low and do not provide dilution of the discharges. The discharge of partially or poorly treated effluent is impacting the rivers and both are anoxic as they flow through the valley floor.
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Load Ammonia-Nitrogen in Reduction No
No Reduction in Organic loading and Ammoni and loading Organic in Reduction No a Reduction in Organic Loading (BOD & TSS) & (BOD Loading Organic in Reduction
Ambient Water Quality Ambient
Source of Irrigation Water Irrigation of Source
Surface Water Discharge Water Surface
Poorly Operated Poorly Operating Moderately Operating
Status Disposal/Reuse
g Condition g Operatin Good
actors Assessment
Poorly Maintained Poorly
Moderately Maintained Moderately
Maintenance Maintained Well
Operation and and Operation
Poor Construction Poor
Table 14. Technical F Construction Moderate Construction Construction Good System System Marginal Marginal Design & Design Overloaded Engineering Design Engineering within Typical Typical Criteria Designed Designed Name y Communit San Andrés Huayápam Andrés San Camino del Lucía Santa SebastiánSan TutlaTlacolula de Matamoros Telixtlahuaca Francisco San X X Ocotepec Dionisio San Tomaltepec del Domingo Santo PabloSan Villa de Mitla Mazaltepec Tomás Santo XTeotitlán del Valle X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X Peri-Urban Communities Peri-Urban Towns Regional Rural Communities
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Managerial and Financial Factors
Table 15 summarizes the managerial and financial factors assessed during the community field assessment. These factors indicate the how the system is administered, whether the community employs a wastewater operator or maintenance worker, the operator/worker’s skill level, the monthly sewer fees charged by each community, the monthly wages paid to the operator, operator training status, and identifies if the community conducts routine water quality testing of the wastewater treatment system.
The majority of the wastewater systems are administered by the local town government that includes a department of public works with a director who oversees the day to day operation of the wastewater system. Two rural systems visited were administered under the traditional community service system of Usos y Costumbres and the system is supposed to be maintained by a community volunteer group (tequio).
Seven communities charge a monthly sewer fee, which is used to pay the operator, materials, parts and electricity bills if the system has any pumps. The monthly fees ranged from 2.5 to 20 pesos ($0.20 to $1.50) per month. A majority of the communities charge 5 pesos per month.
Seven communities employ full-time operators/maintenance workers to conduct basic maintenance of the facilities. Two communities rely on the tequios to maintain the systems; however, the field visits found that both of these systems were not being maintained. One system does not have an operator or tequio and is in disrepair.
Operators were found to be paid between 2,000 to 4,000 pesos per month, which is equivalent to about $200 to $400 dollars. It was interesting to note that the wealthiest
119 community (San Andres Huayapam) paid the operator the lowest wage; however, this community also paid for additional oversight services by a local engineering consultant.
The operators maintaining the peri-urban systems were found to have a basic understanding of the function of the treatment system. The operator for the plant serving
San Andres Huayapam was the most informed and best trained operator and was provided ongoing support from a private engineering firm that was hired by the community to oversee the operation and maintenance of the facility. The operators in the other two communities obtained periodic support from the SWCO. The operators maintaining the four other systems were not trained and were employed to complete basic tasks, such as cleaning the screens and sedimentation chamber, cleaning weir boxes, adjusting outlet pipe heights, weeding and cutting the vegetation in the gravel beds.
Only one community (San Andres Huayapam) indicated that they conduct routine quarterly water quality testing to check the performance of the plant. The testing is apparently conducted by the private engineering firm that supervises the operation and maintenance of the plant; however, the community was unwilling to provide copies of the test results. No routine testing is conducted at the other communities.
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120
g
Monitorin Water Quality Water Quality Training Operator Provided ) Pesos/Month ( Operator Wages ) Fee Pesos/household ( Monthly Sewer Monthly d Financial Factors Worker Level Skill Operator/Maintenance Operator/Maintenance System System Worker Operator/Maintenance Operator/Maintenance Table 15. Managerial an System AdministratorSystem Departmental Tequio Name y Communit San Andrés Huayápam Andrés San Camino del Lucía Santa Tutla Sebastián San Tlacolula deMatamoros San Francisco Telixtlahuaca X X Ocotepec Dionisio San del Tomaltepec Domingo Santo San Pablo Villa de Mitla X X XMazaltepec Tomás Santo TeotitlánValle del X X Yes Yes X Yes X Yes Yes X Yes Moderate Moderate No No Moderate No Low Low Low 5 Yes 5 NA 5 NA NA 2.5 0 2,000 2,500 20 Low 3,000 0 Yes Limited Unknown Unknown 4,000 quarterly Yes - 5 Limited 4,000 No No NA 5 No No Limited NA No NA No No No No 3,000 No No No No No No
Peri-Urban Communities Peri-Urban Towns Regional Communities Rural Not Applicable NA-
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Comparing the different factors in the summary tables indicates the following findings:
1. The highest income community, San Andrés Hauyápam, has the best maintained
and operated wastewater system. This community is the only community that has
trained the wastewater operator and monitors the performance of the treatment
plant via water quality testing.
2. The peri-urban systems were found to be the best operated and maintain systems
in the study group.
3. All of the peri-urban and regional town systems have paid wastewater operators.
4. All of the peri-urban and middle to upper income level rural communities have
established the highest monthly sewer fees (5 pesos per connection).
5. Three out of four of the high income bracketed communities reuse the treated
wastewater for irrigation of crops and pasture.
6. The wastewater systems constructed in the three poorest rural towns, San Dionisio
Ocotepec, Santo Domingo del Tomaltepec and Santo Tomás Mazaltepec were
found to be in the poorest condition and not well maintained. These three towns
included the study that only had primary schools within the communities. The
other seven communities also had secondary schools.
CONCLUSIONS AND RECOMMENDATIONS
Conclusions
1. Several communities and/or local governments visited were not aware that they
had treatment plants that needed to be maintained. This lack of local knowledge
indicated that there was a lack of ownership or community support for wastewater
treatment in these locations.
2. The National and State water agencies have invested a substantial amount of
money and engineering resources to develop and promote a wastewater treatment
scheme that is not achieving substantial improvements in water quality in the
Oaxaca Valley. The national and state agencies responsible for wastewater
management in the Oaxaca Valley have prioritized their efforts on the
construction of new projects without supporting the long term operation,
maintenance and administration of the projects. The lack of institutional support
for ongoing O&M and community project administration has had a significant
negative impact on the success of wastewater management and pollution control
improvements in the Oaxaca Valley.
3. The emphasis of the National and State agencies has been in the construction of
new projects without the critical or thorough evaluation of the performance of the
systems being installed.
4. The two agencies have not put a concerted effort in the education and training of
communities and operators. Very little funding is provided on the operation and
maintenance of wastewater projects.
122 123
5. Based on interviews with National and State agency staff it is evident that the
selection and implementation of projects are based on the political and economic
status of the community and projects are not targeted or strategic to improve
water quality based on a watershed based approach.
6. Public and Private wastewater engineers working in the Oaxaca Valley continue
to employ treatment schemes, primarily the three stage anaerobic treatment
system employing a anaerobic digester, an upflow anaerobic rock filter and a
vegetated gravel bed system. They seem to use this system because they are
familiar with the technology and construction, regardless of the systems limited
effectiveness. The design criteria used to size the vegetated gravel beds is dated,
not technically based, and as a result the surface area of the gravel beds is
commonly too small for the populations served.
7. The short duration of the local governments in their elected posts can have a
significant impact on the ongoing operation and maintenance of the facility. The
frequent turnover of staff and responsibilities disrupts the institutional history of a
community, and has caused some projects to be abandoned when a new city
government takes over and wastewater treatment is not a priority of the new
mayor and their appointees.
8. Using a volunteer community service group (tequio) to provide periodic operation
and maintenance of the treatment systems does not appear to be a viable method
because routine maintenance and observation of the treatment works does not
occur.
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9. Although, the national, state and local governments have invested a substantial
amount of money and material resources to install and treat municipal wastewater
in the Oaxaca Valley, the efforts have not substantially improved water quality in
the region. The quality of the two major rivers, Rios Ayotac and Salado is very
poor, in part as a result of the discharge of partially or untreated municipal
sewage.
10. All three of the peri-urban communities had developed and implemented effective
operation, maintenance and administrative wastewater services. These projects
serve as good examples for other communities in the Oaxaca Valley region.
11. Based on the results of the agency interviews it was determined that many
projects were constructed entirely with national and state funding without any
cost sharing requirements by communities. Many communities do not charge for
sewer service or operations of the treatment plants. This lack of financial
obligation at the community level is a key impediment to the long term ownership
and maintenance of the system.
12. There did not appear to be any regulatory oversight of the projects. Neither the
national or state agencies appeared to be very knowledgeable about the problems
observed in the field or engaged in any potential remedy. The only cases that
were potentially being pursued were related to potential legal actions being taken
by communities located downstream of discharges.
13. There appeared to be an overall lack of water quality monitoring both at the
treatment plants and in the streams and rivers. In the absence of data there is
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limited opportunity to conduct a critical assessment and evaluation of the overall
wastewater management programs underway in the Oaxaca Valley.
14. During the study the author co-coordinated and co-taught two, week-long courses
on wastewater management in the City of Oaxaca in March 2007 and in February
2008. Eight of the ten towns sent staff to attend the two different courses. The
majority of the information and findings presented in this project report was
presented during each course and served as a point of reference to present
strategies to address the problems encountered during the study.
Recommendations
1. The national and state water agencies should conduct an objective technical
review of their projects and engineering design criteria used to size and design
wastewater treatment plants in the State of Oaxaca.
2. The national and state agencies should review alternative technologies in addition
to the current design being promoted in the region. Several technologies, such as
upflow anaerobic sludge blanket digesters, and multi-pond systems would be
appropriate and likely more reliable technologies for the area. Both of these
options could be combined with the use of free surface water wetlands to achieve
a high level of treatment likely at a comparable or lower cost than the current
treatment schemes employed.
3. Ongoing education and training courses should be conducted or supported by the
national, state and local governmental agencies. Several key topics include
watershed protection and water quality, municipal infrastructure administration,
operation and management.
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4. The national and state agencies should initiate a wastewater operator program to
train and support wastewater operators working in the state.
5. The state water agency should develop a tracking program that schedules and
tracks routine field inspections of wastewater projects.
6. The national and state water agencies should develop and implement a baseline
water quality program to provide ongoing water quality monitoring of the local
receiving waters and wastewater projects.
7. The state water agency should require the local communities to provide some
level of cost sharing in the construction of a new wastewater project. The State
should also assist the local communities to develop wastewater operations
departments that would establish tariffs and administer an operation and
maintenance program.
8. The state and local designers should adopt more current design criteria, such as
the criteria established in the USEPA’s 2000 design manual.
REFERENCES
Blenford, L. 2003. Migrant Remittances and (Under) Development in Mexico. Critique of Anthropology 23(3)
Briscoe, J. 1999. The financing of hydropower, irrigation and water supply infrastructure in developing countries. Water Resources Development, Vol. 15, No. 4
Cohen, J. H. and L. Rodriguez. 2004. Remittance Outcomes in Rural Oaxaca, Mexico: Challenges, options, opportunities for Migrant Households. Working Paper 102. The Center for Comparative Immigration Studies, University of California, San Diego
Chavez-Guillen, Ruben. 1977. Hydrogeology of the Zimatlan Valley, Oaxaca, Mexico. Bulletin of the Geologic Society of Mexico, Vol. XXXVIII, No. 2, pp 65-82, December
Comision Nacional de Agua (CNA). 1996. Official National Standards to Control Wastewater Discharges to Water Bodies, (NOM-001-Semarnat-1996).
CNA. 2008. Water Statistic of Mexico
Hendricks, Janet and Arthur D. Murphy. 1981. From Poverty to Poverty: The Adaptation of Young Migrant Households in Oaxaca, Mexico, Urban Anthropology 10(1): 53-70.
Idelovitch, E. and K. Ringskog. 1997. Wastewater Treatment in Latin America: Old and New Options. The World Bank, Directions in Development Series
Instituto Nacional de Estadistica Geografica e Informatica (INEGI). 2000. XII General Population and Demographic Census of Mexico
Kirkby, Anne V. 1973. The Use of Land and Water Resources in the Past and Present, Valley of Oaxaca, Mexico, Volume 1. Memoirs of the Museum of Anthropology University of Michigan, Number 23. Prehistory and Human Ecology of the Valley of Oaxaca, Mexico
Lars Hojer Sorensen. 2007. Report on Education in Oaxaca: The social conflict between the Mexican government and the Teachers Union. Universidad De La Tierra
Lee, T. and V. Floris. 2003. Universal access to water and sanitation: Why the private sector must participate. Natural Resources Forum. Vol. 27
Lipp, F.J. 1991. The Mixe of Oaxaca: religion, ritual and healing, University of Texas
Lord, W.B. and M. Isreal. 1996. A Proposed Strategy to Encourage and Facilitate Improved Water Resource Management in Latin America and the Caribbean, Prepared for the Environmental Division, Social Programs and Sustainability Department, Inter-American Development Bank
127 128
Ludwig, H. F., K. Mohit and D. Gunaratham. 2003. The Truth about Public Health Protection and Community Sewerage and Water Supply Sanitation in Developing Countries. The Environmentalist, Vol 251
Mader, R. 2007. Forests of Mexico. Planeta.com, Global Journal of Practical Ecotourism
Marino, M and J. Boland. 1999. An Integrated Approach to Wastewater Treatment: Deciding where, when and How Much to Invest. Directions in Development, The World Bank
Murphy. A.D., M. Winter, E.W. Morris and M. W. Rees. 1992. The Socio-demographic Effects of the Crisis in Mexico. Latin American Network Information Center. University of Texas, Austin
Perraton, Etienne. 1998. Hydrogeologic and Agroclimatic Considerations for the Development of a Water Management Model for the Tlacolula Subbasin, Oaxaca, Mexico. Doctoral Thesis. McGill University, Department of Agricultural and Biosystems Engineering
Personnel Communication, Jose Luis Zaragoza, Chief Engineer with the State Water Commission for Oaxaca, July 2005 and February 2007
Ponce, V.M. 2002. Photographs of the Rio Atoyac, Oaxaca, Oaxaca, Mexico
Riley, B. M., A.D. Murphy, and M.A.Mendez-Rosado. 1998. A Reversal of Tides: Drinking Water Quality in Oaxaca de Jaurez, Mexico. In Water, Culture & Power: Local Struggles in a Global Context. 1998. Island Press. Washington, D.C.
Ringler, C., M.W. Rosegrant and M.S. Paisner. 2000. Irrigation and Water Resources in Latin America and the Caribbean: Challenges and Strategies. Environmental and Production Technology Division, International Food Policy Research Institute
Schmal, J.P. 2007. Oaxaca: Land of Diversity. LatinoLA, January 2007
Sohail, M., S. Cavill and A.P. Cotton. 2001. Operation, Maintenance and Sustainability of Services for the Urban Poor, Water, Engineering and Development Centre, Loughborough University
United Nations Development Programme (UNDP). 2010. About the Millennium Development Goals: Basics What are the Millennium Development Goals
United Nations Environment Programme (UNEP). 2004. Financing wastewater collection and treatment in relation to the Millennium Development Goals and World Summit on Sustainable Development targets on water and sanitation. Note by Executive Director, Governing Council of the United Nations Environment Programme
UNEP. 2001. Global Programme of Action for the Protection of the Marine Environment from Land-Based Activities. United Nations Environmental Program, Global Programme of Action Report Series No. 5 129
UNEP. 2004. Financing wastewater collection and treatment in relation to the Millennium Development Goals and World Summit on Sustainable Development targets on water and sanitation. Note by Executive Director, UNEP/GCSS.VIII/INF/4
U.S. Environmental Protection Agency (USEPA). 1988. Design Manual-Constructed Wetlands and Aquatic Plant Systems for Municipal Wastewater Treatment. EPA 625/11l-88/022. U.S. EPA CERI, Cincinnati, OH.
USEPA. 2000. Manual Constructed Wetlands Treatment of Municipal Wastewaters. EPA/625/R- 99/010, USEPA Office of Research and Development, Cincinnati, Ohio
VanWey, L.K., C.M. Tucker and E.D.McConnell. 2005. Community Organization, Migration, and Remittances in Oaxaca. Latin American Research Review, Vol.40. No. 1, February
Word, T. 2008. Natural Wastewater Treatment and Outreach, Tlacolula, Oaxaca, Mexico. Winzler and Kelly Consulting Engineers
APPENDICES
Appendix A
Technical Evaluation of Wastewater Treatment Systems Design Criteria from the State of Oaxaca Water Commission & USEPA
1. General Criteria Per capita flow = 100 lpd Typical BOD Concentration = 264 mg/L 0.264 kg/L Typical COD Concentration = 500 mg/L 0.5 kg/L
2. Anaerobic Digesters HRT = 24 hrs
3. Upflow Anaerobic Rock Filter HRT = 16 hrs Bed depth = 2.5 mts Maximum organic loading rate = 2.0 kg DQO/m3‐day Maximum hydraulic loading rate = 5.0 m3/m3‐day Rock size 6 to 8 cm
4. Vegetated Gravel Bed 1988 USEPA Design Manual for Constructed Wetlands and Aquatic Plant Systems for Municipal Wastewater Treatment (EPA/625/1‐88/022) Ci = 125 mg/L
Ce1 = 30mg/L; Ce2 = 75 mg/L Q = daily fow, m3 o KT @ 22 C = 1.33 Ks for sandy gravel = 500 m3/m2‐d S = 0.01 d = 0.6 mts n= 0.35 Bed Cross‐sectional area = Ac = Q/(ksxS)
Bed Surface Area = As = [QX(lnCi‐lnCe)]/[KTxdxn) 2000 USEPA Design Manual for Constructed Wetland Treatment of Muncipal Wastewaters (EPA/625/R‐99/010 Surface Area based on areal loading rate (ALR) = 6 g/m2‐d
A1 San Andres Huayapam Population = 4,200 Per Capita Flow = 100 lpd Design Flow, Q = 420 m3/d
Dimensions
Organic Loading Hydraulic Rate (Kg Loading Rate Unit Process Length (mts) Width (mts) Depth (mts) Volume (m3) HRT (hrs) DQO/m3‐d) (m3/m3‐d) Guidelines Anaerobic Digester 12 10 3 360 20.6 CEA HRT USEPA (HRT) Upflow Anaerobic Rock Filter 14 10 2 280 16.0 0.75 1.5 24 36 Length (mts) Width (mts) # beds Total Area (m2) 16 Subsurface Flow Vegetated Gravel Bed 22.5 12.5 5 1406 Bed Sizing per 1988 USEPA Design Guidelines
Case 1: Ce1 = 30 mg/L Case 2: Ce2 = 75 mg/L Gravel Bed Surface Area 2130 765 Total Width (mts) 139 Length (mts) 15 Number of Cells 5 Width of Each Cell 28 Bed sizing per 2000 USEPA Design Guidelines Surface Area (m2) based on ALR (m2) 8750
Results 1. Anaerobic Digester is undersized with a low HRT, 70% of the minimum recommended HRT of 24 hrs. 2. The upflow anaerobic rock filter appears to properly sized, within the recommended OLR and HLR. 3. The surface area of the vegetated gravel bed is small compared to 1988 and 2000 USEPA Design Guidelines. 4. Per USEPA 1988 design guidance the gravel bed is approximately 66% of the recommended area. 5. Per aerial loading rates per the USEPA 2000 design guidance (6 g/m2‐d) the bed is only 16 % of the recommended area.
A2 San Sabastian de Tutla Population = 4157 Design Flow, Q = 416
Dimensions Guidelines
Organic Loading Hydraulic Rate (Kg Loading Rate Unit Process Length (mts) Width (mts) Depth (mts) Volume (m3) HRT (hrs) DQO/m3‐d) (m3/m3‐d) CEA USEPA Anaerobic Digester 10 10 3 300 17.1 24 36
Length (mts) Width (mts) # beds Total Area (m2) Subsurface Flow Vegetated Gravel Bed 25 12.5 12 3750 Bed Sizing per 1988 USEPA Design Guidelines
Case 1: Ce1 = 30 mg/L Case 2: Ce2 = 75 mg/L Gravel Bed Surface Area 2110 758 Total Width (mts) 139 Length (mts) 25 Number of Cells 12 Width of Each Cell 13 Bed sizing per 2000 USEPA Design Guidelines Surface Area (m2) based on ALR (m2) 8667
Results 1. Anearobic digester is within the recommended design guidelines based on HRT. 2. The UFAR filter is also with the recommended HRT, OLR and HLR criteria. 3. The area of the vegetated bed is below the 1988 criteria ~67%. 4. Based on surface areal loading rates recommended in 2000 Design Guidance bed area is only 16% of the recommended area.
A3 San Francisco Telixtlahuaca Population = 9322 Design Flow (m3) , Q = 932
Dimensions Guidelines
Organic Hydraulic Loading Rate Loading Rate Unit Process Length (mts) Width (mts) Depth (mts) Volume (m3) HRT (hrs) CEA USEPA (Kg DQO/m3‐d) (m3/m3‐d) Anaerobic Digester 12 24 3 864 22.2 24 36 Upflow Anaerobic Rock Filter 20 10 2.5 500 12.9 16 0.93 1.86 Length (mts) Width (mts) # beds Total Area (m2) Subsurface Flow Vegetated Gravel Bed 23 10 11 2530 Bed Sizing per 1988 USEPA Design Guidelines
Case 1: Ce1 = 30 mg/L Case 2: Ce2 = 75 mg/L Gravel Bed Surface Area (m2) 4728 1698 Total Width (mts) 139 Length (mts) 34 Number of Cells 5 Width of Each Cell 28 Bed sizing per 2000 USEPA Design Guidelines Surface Area (m2) based on ALR (m2) 19421
Results 1. Anaerobic digester HRT is approximately 91 % of the recommended HRT 2. UARF HRT is approximately 81% of the recommended HRT 3. Per 1988 criteria the Ssrface area of vegetated bed is approximately 0.54% of the recommended area. 4. Per 2000 criteria the surface area of vegetated bed is approximately 13% of the recommended area.
A4 Santa Domingo de Tomaltepec Population = 2061 Design Flow (m3) , Q = 206
Dimensions Guidelines
Organic Hydraulic Loading Rate Loading Rate Unit Process Length (mts) Width (mts) Depth (mts) Volume (m3) HRT (hrs) CEA USEPA (Kg DQO/m3‐d) (m3/m3‐d) Anaerobic Digester 10 10 3 300 35 24 36 Upflow Anaerobic Rock Filter 13 8 2.5 260 30 16 0.40 0.79 Length (mts) Width (mts) # beds Total Area (m2) Subsurface Flow Vegetated Gravel Bed 28 12 5 1680 Bed Sizing per 1988 USEPA Design Guidelines
Case 1: Ce1 = 30 mg/L Case 2: Ce2 = 75 mg/L Gravel Bed Surface Area (m2) 1045 375 Total Width (mts) 69 Length (mts) 15 Number of Cells 5 Width of Each Cell 14 Bed sizing per 2000 USEPA Design Guidelines Surface Area (m2) based on ALR (m2) 4294
Results 1. Anaerobic digester HRT is approximately 145% of the recommended HRT 2. UARF HRT is approximately 187% of the recommended HRT 3. Per 1988 criteria the surface area of vegetated bed is approximately 160% of the recommended area. 4. Per 2000 criteria the surface area of vegetated bed is approximately 40% of the recommended area.
A5 San Tomas Mazaltepec Population = 1878 Design Flow (m3) , Q = 188
Dimensions Guidelines
Organic Hydraulic Loading Rate Loading Rate Unit Process Length (mts) Width (mts) Depth (mts) Volume (m3) HRT (hrs) CEA USEPA (Kg DQO/m3‐d) (m3/m3‐d) Anaerobic Digester 10 10 3 300 38 24 36 Upflow Anaerobic Rock Filter 15 8 2.5 300 38 16 0.31 0.63 Length (mts) Width (mts) # beds Total Area (m2) Subsurface Flow Vegetated Gravel Bed 25 12 8 2400 Bed Sizing per 1988 USEPA Design Guidelines
Case 1: Ce1 = 30 mg/L Case 2: Ce2 = 75 mg/L Gravel Bed Surface Area (m2) 952 342 Total Width (mts) 63 Length (mts) 15 Number of Cells 5 Width of Each Cell 13 Bed sizing per 2000 USEPA Design Guidelines Surface Area (m2) based on ALR (m2) 3913
Results 1. Anaerobic digester HRT is approximately 158% of the recommended HRT 2. UARF HRT is approximately 238% of the recommended HRT 3. Per 1988 criteria the surface area of vegetated bed is approximately 252% of the recommended area. 4. Per 2000 criteria the surface area of vegetated bed is approximately 61% of the recommended area.
A6 Teotilan de Valle Population = 4427 Design Flow (m3) , Q = 443
Dimensions Guidelines
Organic Hydraulic Loading Rate Loading Rate Unit Process Length (mts) Width (mts) Depth (mts) Volume (m3) HRT (hrs) CEA USEPA (Kg DQO/m3‐d) (m3/m3‐d) Anaerobic Digester 20 10 3 600 33 24 36 Upflow Anaerobic Rock Filter 20 9 2.5 450 24 16 0.49 0.98 Length (mts) Width (mts) # beds Total Area (m2) Subsurface Flow Vegetated Gravel Bed 22 16 9 3168 Bed Sizing per 1988 USEPA Design Guidelines
Case 1: Ce1 = 30 mg/L Case 2: Ce2 = 75 mg/L Gravel Bed Surface Area (m2) 2245 806 Total Width (mts) 148 Length (mts) 15 Number of Cells 5 Width of Each Cell 30 Bed sizing per 2000 USEPA Design Guidelines Surface Area (m2) based on ALR (m2) 9223
Results 1. Anaerobic digester HRT is approximately 138% of the recommended HRT 2. UARF HRT is approximately 150% of the recommended HRT 3. Per 1988 criteria the surface area of vegetated bed is approximately 141% of the recommended area. 4. Per 2000 criteria the surface area of vegetated bed is approximately 34% of the recommended area.
A7 San Pablo Villa de Mitla Population = 7829 Design Flow (m3) , Q = 783
Dimensions Guidelines
Organic Hydraulic Loading Rate Loading Rate Unit Process Length (mts) Width (mts) Depth (mts) Volume (m3) HRT (hrs) CEA USEPA (Kg DQO/m3‐d) (m3/m3‐d) Anaerobic Digester 24 10 3 720 22 24 Upflow Anaerobic Rock Filter 19 8 2.5 380 12 16 1.03 2.06 Length (mts) Width (mts) # beds Total Area (m2) Subsurface Flow Vegetated Gravel Bed 26 13 6 2028 Bed Sizing per 1988 USEPA Design Guidelines
Case 1: Ce1 = 30 mg/L Case 2: Ce2 = 75 mg/L Gravel Bed Surface Area (m2) 3970 1426 Total Width (mts) 261 Length (mts) 15 Number of Cells 5 Width of Each Cell 52 Bed sizing per 2000 USEPA Design Guidelines Surface Area (m2) based on ALR (m2) 16310
Results 1. Anaerobic digester HRT is approximately 92% of the recommended HRT 2. UARF HRT is approximately 75% of the recommended HRT 3. Per 1988 criteria the surface area of vegetated bed is approximately 51% of the recommended area. 4. Per 2000 criteria the surface area of vegetated bed is approximately 12% of the recommended area.
A8 San Dionisis de Ocotepec Population = 4942 Design Flow (m3) , Q = 494
Dimensions Guidelines Unit Process Length (mts) Width (mts) Depth (mts) Volume (m3) HRT (hrs) CEA USEPA Anaerobic Digester 22 11 3 726 35 24 36 Length (mts) Width (mts) # beds Total Area (m2) Subsurface Flow Vegetated Gravel Bed 60 2 12 1440 Bed Sizing per 1988 USEPA Design Guidelines
Case 1: Ce1 = 30 mg/L Case 2: Ce2 = 75 mg/L Gravel Bed Surface Area (m2) 2506 900 Total Width (mts) 165 Length (mts) 15 Number of Cells 5 Width of Each Cell 33 Bed sizing per 2000 USEPA Design Guidelines Surface Area (m2) based on ALR (m2) 10296
Results 1. Anaerobic digester HRT is approximately 145% of the recommended HRT 3. Per 1988 criteria the surface area of vegetated bed is approximately 57% of the recommended area. 4. Per 2000 criteria the surface area of vegetated bed is approximately 14% of the recommended area.
A9