Wastewater Re-use and Groundwater Quality (Proceedings of symposium HS04 held duriim IUGG2003 al Sapporo. July 2003). IAI IS Publ. 285. 2004." 5

Agricultural use of treated wastewater: the need for a paradigm shift in sanitation and treatment

JULES B. VAN LIER Subdepartment of Environmental Technology, Wageningen University, PO Box 8129, 6700 EV Wageningen, The Netherlands [email protected]

FRANS P. HUIBERS Irrigation and Water Engineering Group, Wageningen University, Nienwe Kanaal If 6709 PA Wageningen, The Netherlands

Abstract Shortages in irrigation water in and around urban areas call for alter­ native sources, particularly in the (sub)tropical zones. Domestic sewage represents such an alternative source and its nutritional value has been known by farmers for a long time. In many situations even raw (diluted) domestic sewage is used for agricultural purposes, especially in those areas that cannot afford extensive sewerage and treatment systems. However, discharge or re­ use of non-treated effluents gives rise to serious environmental problems, including threats to human health. Whenever agricultural use of urban effluents is considered, an integrated approach should be pursued, taking into account the agricultural requirements as well as the possible technological solutions for cost-effective sanitation and treataient. Such an integrated set-up questions the existing paradigms in sanitation and treatment and will call for a more decentralized approach, minimizing the requirements for large-scale infra-structural investments, such as sewerage systems. Also, with respect to the available treatment techniques, economic sustainability is often disregar­ ded in making the final choices. Amongst the available compact technologies for wastewater treatment, the anaerobic (pre-)treatment is seen as an appropriate technology, but so far, its potential has hardly been utilized. Most interestingly, anaerobic treatment is ideal for implementation in a decentral­ ized mode. The products of anaerobic Ueatment consist of nutrient-rich effluents, stabilized digested sludge and energy rich biogas. In particular, the former two can be used beneficially by local fanners, whereas the biogas produced can be used on the site if it is produced in sufficient quantities. The present paper discusses the prospects of anaerobic (pre-)treatment, embedded in centralized and decentralized treatment and re-use concepts. Its cost- effectiveness may lead to a more rapid implementation of productive sewage reclamation, while the environmental problems are concomitantly addressed.

Key words agricultural re-use; anaerobic treatment; cosl-elïecliveness; decentralization; sanitation paradigm; source separation; wastewater reclamation

INTRODUCTION

Appropriately treated domestic sewage can be regarded as ideal for irrigation and fertilization purposes, particularly in the (semi)arid climate region. In addition to an increased availability of an additional source of irrigation water, treated sewage contains valuable plant nutrients, such as nitrogen, phosphorous, and potassium (NPK) at levels of interest (e.g. Feigin et al, 1991; Darwish et al, 1999). Feigin et al (1991) 6 Jules B. Van Lier & Frans P. Huibers

Table 1 Benefits and constraints of effluent use in irrigated agriculture.

Benefits Constraints Increased availability of irrigation water resources Health hazards through water contact (farm Secure and year around supply labourers) Reduced need for fertilization Environmental risks if not properly managed Increased crop yield Soil degradation in case of high salinity Effluent can be marketed (cost optimization in Crop quality hazards treatment and re-use chain) Irrigation operational hazards (clogging, supply- Alleviation of high-quality water scarcity (increase demand difference) of regional self-sufficiency) Negative economic impact (consumer's Alternative tertiary wastewater treatment perception, governmental crop restriction regulations) Social and cultural norms and values

also commented on the positive effects of the various micronutrients on crop produc­ tion. From the environmental engineering point of view it is of interest that by using treated effluents in irrigated agriculture, the agricultural field can be considered as a tertiary treatment step, while non-controlled environmental pollution is prevented if well managed (Asano, 1998). The fact that domestic sewage is validated as a resource with an economic value means that treated effluents can potentially be marketed. The latter will increase the financial feasibility of a treatment and re-use scenario, since the overall costs can then be divided over the polluters (community) and beneficiaries (fanners). Once treated effluents are directed to agricultural fields, the pressure on high-quality water resources decreases, contributing to solutions related to water availability problems. Another important advantage is the alleviation of scarcity high- quality water, in those areas where clean water is difficult to obtain. As such, the use of domestic effluents has interesting economic benefits as listed in Table 1. The general constraints in irrigation with treated wastewater (Table 1) focus on human and environmental health hazards, which include: (a) the fate of pathogens; (b) the fate of excess nutrients in the environment (e.g. outside the season); and (c) the fate of micro-pollutants. Also crop productivity and quality hazards should be consid­ ered; these are related to bio-accumulation of pollutants and/or vegetative growth induction owing to excess dosage of nitrogen. Problems of soil salinization and sodification may occur in the long term causing loss of productivity (Abo Gobar, 1993). When working with treated wastewater, irrigation systems should be amended and potential risks must be minimized, which could lead to higher costs for the farmer, as explained below. Another constraint hampering a direct link between wastewater production and agricultural use is the gap between treated wastewater supply and agricultural demand with respect to day and night flow, but more importantly to seasonal variations. Waste­ water will be produced continuously while the demand for irrigation water depends on the season and the crop growth stage. Social constraints include a negative perception on the side of the consumers, possibly leading to lower market values of the crops produced, while socio-cultural norms might simply restrict the official use of treated effluents. Last but not least, governmental crop restriction regulations might affect the economic sustainability of the farmers that use the treated sewage for crop irrigation. Agricultural use of treated wastewater: the need for a paradigm shift in sanitation and treatment 1

Although some constraints are well documented, it must be noticed that in various countries and/or regions, the common practices already overrule the constraints, and farmers even use raw sewage to meet their water and nutrient demands. Therefore, it is recognized that there is a strong need to develop cost-effective treatment technologies for the reclamation of domestic wastewater. The treatment systems to be developed must be complemented with appropriate agricultural engineering and proper manage­ ment in order to address some of the above-mentioned constraints, since the complete solution cannot be found in the treatment technology alone. For example, crop choice could be amended when there is only a limited removal of pathogens in the treatment system. Various countries (e.g. in the Middle East and northern Africa) apply restricted irrigation criteria making a division between: (a) crops eaten raw; (b) crops that need cooking; (c) animal fodder; (d) industrial crops (e.g. cotton); and (e) tree crops (FAO, 1992). Such an approach suppoits the acceptability of wastewater irrigation by consumers. In addition to crop choice, fanners could also anticipate the irrigation water quality by applying the proper irrigation technology and water management. For instance, with pathogen-rich wastewater the use of sprinkler irrigation is discouraged, while (sub­ surface) drip irrigation, although expensive and only applicable to certain crops, clearly has advantages, since it prevents contacts with the pathogen-rich wastewater (Oron et al, 1991). On the other hand, suspended solids-rich treated wastewater might cause clogging problems at the water emitters of these micro-irrigation systems (Ravina et al, 1997). Although risk minimization can be attained by using proper irrigation techniques, care still should be taken when dealing with pathogen rich wastewater, since risks are not eliminated but mitigated. The same is true for potential salts accumulation in the soil that incontrovertibly requires periodic leaching to prevent an irreversible loss of the agricultural land. It is also clear that the change in irrigation technology will increase the financial burden to the farmers and require higher skills and will not likely take place on a voluntary basis. The above options for intervention illustrate that the topic "treated sewage for agri­ cultural use" needs an interdisciplinary approach, which has not yet been developed, or has only been developed to a limited extent. Most likely, by recognizing this, the status quo of "non-controlled sewage disposal" vs "lack of irrigation water and nutrients" will be more quickly addressed.

EXISTING PARADIGM

Long term application of wastewater treatment, reclamation and re-use has been experienced at various locations in the arid and semiarid climate zone. The state of the art and current perspectives were published by Asano (1998) describing all possible aspects of wastewater reclamation and re-use, particularly in western societies (USA, Israel). The author states that in addition to adequate treatment strategies, the institutional capacity is crucial for safely applying treated effluents for further usage. From the various overviews, the existing paradigm can be deduced i.e.: safe agricultural use of treated domestic sewage requires centralized sewage collection, followed by extensive treatment to eliminate any possible environ­ mental and/or human health threats. 8 Jules B. Van Lier & Frans P. Huibers

Indeed, if abundant financial means are available, the public simply demands "zero" risks. On the other hand, tolerable risks should be accepted when financial constraints are setting the agenda. The way wastewater treatment and agricultural effluent use is implemented is very much dependent on the local and/or national economic situation. Therefore, by discussing the issue on a global level, care must be taken to adopt the above-cited paradigm for policy making. In many places, agricultural use of (partially) treated, or even non-treated, domestic wastewater is a basic necessity for farmers to sustain their own livelihood, owing to the existence of a strong water demand for agricultural purposes and an insufficient or too costly water supply, particularly for the poor farmers, including city dwellers. Moreover, in many parts of the world the available financial means are vastly insufficient to construct centralized sewage collection, high-tech treatment systems and distribution networks that comply with the regulations that are valid under "Western" conditions and that on many occasions are adopted by local authorities. As a result, in many places, illegal, unguided, and/or unplanned direct and indirect use of raw, partially treated, or diluted wastewater occurs (Huibers etal, 2004). Therefore, if agricultural effluent use can only be discussed whenever the effluent characteristics are below the restrictive guidelines to eliminate all potential risks, the illegal and uncontrolled practices will continue and open dialogue is completely paralysed. An opening in this debate can be forced by accepting risk minimization to an acceptable level as a criterion, which is in agreement with the prevailing local socio-economic conditions. It goes without saying that this is an extremely sensitive issue. Responsibilities for sanitary and civil constructions are generally separated from responsibilities related to agricultural production, environment, health, socio-economic development, etc. An institutional debate covering the various angles of wastewater treatment and water re-use in detail is not very common. However, for the proposed risk minimization in effluent use, an open dialogue between the various responsible authorities is essential. In fact, even without the required financial resources that are needed to develop a so-called "zero- risk" concept, cost-effective optimization and safe usage of the limited water and nutrient resources could very well become feasible by following a multidisciplinary approach, integrating the various fields of interest such as, sanitary and environmental engineering, irrigation and water management, drainage, agronomy, environmental management, sociology, economy and political sciences. In the example below we illustrate how a joint approach could contribute to minimize risks related to human health, while allowing treated effluent use for crop cultivation.

PATHOGENS: VARIOUS LEVELS OF INTERCEPTION

Infection of products by pathogenic organisms is considered to be one of the major risks in effluent use. The generally accepted approach is to treat the wastewater up to levels where the infection potential is practically eliminated. Consequently, advanced wastewater treatment systems with related costs are required. However, following the water from household to the cultivated crop, various levels of interception can be distinguished showing that pathogen removal does not necessarily have to be linked to advanced treatment: Agricultural use of treated wastewater: the need for a paradigm shift in sanitation and treatment 9

Sanitation The general concept is to apply flush-through toilets, which are connected to the sewerage network. In this way, the concentrated hazardous waste (human faeces) is diluted by a factor of 100-200 or more, creating an immense pool of diluted infectious wastewater. Instead of combined sewage systems, the introduction of source separated sewerage where the black (toilet) water is separated from the rest, will result in a "grey" wastewater which is relatively easy to treat and which contains considerably fewer pathogens. Particularly in new housing estates, hotels, hospitals, and recreational areas, source separating sewer systems are relatively easy to construct. Treatment technology Disinfection of wastewater could be part of the applied treatment technology. Considering the negative impact of chlorine (formation of persistent carcinogenic chlorinated organics) other methods (e.g. UV, Ti02, membranes, etc.) should be searched for in order to reduce the pathogen load. The latter is particularly of importance in an agricultural re-use scheme. It must be noted that advanced treatment, including pathogen removal, is generally a very costly part of the treatment scheme. Water distribution After the treatment systems, the water is distributed to an irrigation network. In order to regulate peak demands as well as fluctuations in effluent flow, the irrigation network should be preceded by storage in a reservoir. Such a reservoir could be combined with pathogen removal, reducing the costs for the overall layout. Crop selection Depending on the expected quality of the effluent, the crops to be cultivated can be classified by the degree of consumer contact, in which the highest degree of purification is required with crops eaten raw. Irrigation technology Pathogen-rich wastewater should be handled with care to avoid direct contact by farm labourers and people around. Use of sprinkler irrigation is discouraged when only limited or no pathogen removal is obtained, as pathogen loaded droplets (aerosols) may drift over large distances. On the other hand, high-tech irrigation such as the application of (sub-)surface drip irrigation limits the contact of the wastewater with the irrigators and crops. Since pathogens do not enter the plant tissue—unless the plant is damaged—crop infection can be effectively prevented. The costs for safe re-use are then transferred from the community (wastewater treatment system) to the farmer. Whether or not this is acceptable depends on the local situation. Crop handling Well-purified effluents do not give a guarantee that a crop is completely free of pathogens when sold, as crop handling is also a major source of infection. Safe handling requires knowledge transfer and education to farmers, field labourers, transporters and merchandizers. On the other hand, one could also use the field for disinfection, e.g. by fixing a required minimum period between the last irrigation and the crop harvesting. In this way sunlight (UV) and field-drying are used as a means to reduce the pathogen levels on the crops. The previous analysis illustrates that by following an integrated approach, constraints related to the use of sewage effluents in irrigated agriculture can be dealt with at various locations in the water chain. Subsequently, related costs can be spread over the various stakeholders leading to a more rapid access to the alternative water and nutrient resources. A similar exercise can be done for environmental impacts of e.g. salts, nutrients, micro-pollutants, etc. 10 Jules B. Van Lier & Frans P. Huibers

BASIC WASTEWATER TREATMENT

In most cases, agricultural use of effluents is performed with treated, partially treated, or non-treated domestic sewage that is centralized or decentralized, collected by means of an open or closed sewerage system. The collected sewage has an offensive smell, is infectious and has a greyish-brownish colour. Considering the health threats that occur from human contact with non-treated sewage, the UN Johannesburg Summit (2002) explicitly addressed the need for basic treatment of municipal sewage before discharge or use of the effluents in irrigated agriculture. However, what can be regarded as basic treatment? The general set-up of a "Western" treatment and reclamation system consist of the following steps (Metcalf & Eddy, 1995): (a) Primary treatment: Screening, grit removal, solids sedimentation, leading to a 25-50% biological oxygen demand (BOD) removal. (b) Secondary treatment: Biological treatment (e.g. activated sludge, trickling filters), solids-liquid separation, resulting in effluent BOD of 20-30 mg l"1. (c) Tertiary treatment: Removal of colloidal matter and suspended solids (coagula- tion/flocculation, membranes) and nutrients (N, P). (d) Advanced treatment: Disinfection (e.g. chlorination, UV, ozonation, reverse osmosis etc.) In huge areas of the developing world, basic wastewater treatment has not yet been implemented and in large parts sewerage networks have not yet been installed. Before introducing the costly "western solutions" a critical evaluation of the applied technologies is needed. From the above-described treatment steps it follows that the largest fraction of the organic matter (expressed as BOD, or chemical oxygen demand (COD)) is removed during secondary treatment using biological treatment systems. The most commonly applied technology worldwide is the so-called activated sludge system, which converts organic matter with oxygen (in air) to water and CO?. The basic layout of an activated sludge process is depicted in Fig. 1.

sludge sludge digestion dewatering

sludge

grit screens chamber

raw sewawage' ^ treated effluent

primary activated secondary clarifier clarifier sludge tank

Fig. 1 Basic principle of the aerobic activated sludge process for sewage treatment. Agricultural use of treated wastewater: the need for a paradigm shift in sanitation and treatment 11

Although the process is efficient in COD/BOD removal, the technology has several severe drawbacks for repetitive implementation in developing countries like: (a) Extremely high energy demand (aeration): ~1 kWh kg"1 COD (or 2 kWh kg 1 BOD); (b) Complex process design that involves many process units; (c) Expensive equipment is involved (compressors, pumps, blowers etc.); (d) Generally there are no local spare parts available; (e) Skilled personnel are required. A somewhat more energy efficient variety of the activated sludge process is the oxidation ditch (Metcalf & Eddy, 1995). As an alternative to the energy intensive aerated systems, lagoon or pond systems were developed which in some aspects seem to be ideal for implementation in developing countries, since investment costs are low, no high-grade energy is required, and pathogens are removed satisfactorily (Mara, 1998). On the other hand, the area of land required is very high which will be problematic in the vicinity of populated areas, and thus, large and expensive conveyance systems are necessary to make the system economically feasible. Another negative aspect is the surface based design criterion, meaning that the system is rapidly overloaded if the influent flow or concentration increases. As a result, pond systems are often accompanied with odour nuisance, while the greenhouse gas methane is emitted to the atmosphere. When use of the treated effluent in irrigated agriculture is considered, the pond system has the striking disadvan­ tage that in hot climates high amounts of fresh water are evaporated, caused by the large surface area. For instance, the large-scale pond system Khirbet As Samra near Amman, Jordan (Duqqah, 2002) occupies a surface of 181 ha and consequently evaporates 13- 18 000 mJ day ' of water in summer when the need for water is highest. At the influent design flow of about 68 000 mJ day"1, corresponding to a hydraulic retention time (HRT) of 40 days, this volume accounts for 20-25% of the water flow. As a consequence, the concentration of salts in the remaining effluent increases by at least 25% in summer. As mentioned before, the increase in salinity has a negative effect on the suitability of the water for agriculture (Abo Gobar, 1993). Moreover, additional water losses are experienced caused by leaching to the soil and water losses during conveyance after treatment. Advantages and disadvantages of the pond system are tabulated in Table 2. An emerging technology that has the advantages of both pond systems and activated sludge processes, i.e. high treatment performance at no energy costs, is the anaerobic high-rate process for sewage treatment (Van Lier & Lettinga, 1999; Chernicharo & Machado, 1998). The most important advantages of anaerobic treatment are:

Table 2 Advantages and disadvantages of pond systems.

Advantages Disadvantages Low investment costs (if land price is low) High demand for flat land No energy consumption Often accompanied by odour nuisance Relatively easy to maintain (apart from desludging) Requires conveyance network (generally applied as Efficient for pathogen removal off-site treatment) Often loss of valuable greenhouse gas methane Evaporation of large volumes of freshwater Concomitant increase of salts Non-flexible to population growth 12 Jules B. Van Lier & Frans P. Huibers

(a) No use of fossil fuels for treatment; (b) Production of energy (-2.5 kWh kg"1 COD converted); (c) Reduction of greenhouse gas emissions (if dissolved CH4 is removed from the effluent); (d) Reduction of excess sludge production (up to 90% with industrial wastewater); (e) Production of stabilized sludge; (f) Up to 90%) reduction in space requirement (compact system); (g) No, or very little use of chemicals; (h) No destruction of nutrients; (i) Plain technology. Anaerobic treatment, however, cannot be regarded as a sole treatment system since it needs an additional treatment step to reach discharge or re-use standards (Chernicharo et al, 2001). As a mineralization process, anaerobic systems merely convert and remove organic matter. In the post-treatment step, residual organic matter, any excess nutrients and pathogens should be dealt with. An important disadvantage is the lack of information and know-how on anaerobic wastewater treatment systems at the responsible authorities, leaving its potentials largely unexploited. Related and frequently cited disadvantages are the possible odour problems, high sensitivity for toxic compounds in wastewater streams, and operational problems during start-up. However, once knowledge transfer has reached an adequate level, the latter disadvantages will automatically vanish. Although the high-rate anaerobic wastewater treatment system is more sophisticated with respect to design and operation than the above-mentioned pond systems, its compactness and potential for construction in the vicinity of populated areas contributes to the preference of anaerobic high-rate reactors over pond systems on various locations. Compared to activated sludge systems, the anaerobic high-rate technology can be regarded as ideal for the removal of organic matter, particularly in areas with warm climates (van Haandel & Lettinga, 1994). The carbon/energy flow of activated sludge and anaerobic high-rate treatment illustrates the prime advantages of the latter technology over the first-mentioned (Fig. 2).

Biogas Heat loss 35 m3 (285 kWh)

10-20 kg 2-10 kg COD COD 100 kg COD ¥ Sludge, 30-60 kg Sludge, 5 kg

Fig. 2 Fate of carbon and energy in aerobic (left) and anaerobic (right) wastewater treatment. Agricultural use of treated wastewater: the need for a paradigm shift in sanitation and treatment 13

anaerobic Effluent polishing grit high-rate screens chamber reactor

treated effluent

sludge sludge drying

Fig. 3 Basic principle of the anaerobic wastewater treatment (sewage).

The carbon/energy-flow principles largely affect the set-up of the corresponding wastewater treatment system. Compared to the basic plant layout of activated sludge processes (Fig. 1), the general process layout of anaerobic wastewater treatment is much more simple (Fig. 3). If appropriately applied, the anaerobic high-rate reactor of Fig. 3 comprises four process units of the conventional activated sludge process as depicted in Fig. 1, namely: the primary clarifier, the biological reactor (= activated sludge tank), the secondary clarifier and the sludge digester. This is particularly true for the treatment of domestic sewage. In the case of industrial wastewater, the local conditions determine whether the anaerobic reactor requires an additional pre- and/or post-treatment step. At present, the so-called upflow anaerobic sludge blanket (UASB) reactor is the most frequently applied system for anaerobic sewage treatment (Chernicharo & Machado, 1998; Seghezzo et al, 1998). Full-scale experience exists particularly under (sub­ tropical conditions (Draaijer et al, 1994; Schellinkhout & Osorio, 1994). Generally, a reduction of the biological oxygen demand (BOD) of between 75 and 85% is realized, with effluent BOD concentrations of less than 40-50 mg F1. Total removal rates with regard to chemical oxygen demand (COD) and total suspended solids (TSS) are up to 70- 80% and sometimes even higher (Van Haandel & Lettinga, 1994; Chernicharo & Machado, 1998). In order to comply with local regulations for discharge, the UASB system is generally accompanied by a proper post-treatment system, such as: facultative ponds, sand filtration, constructed wetlands, trickling filters, physico-chemical treatment, and activated sludge treatment (Schellinkhout & Osorio, 1994; Chernicharo et al, 2001). The UASB and the post-treatment step can be implemented consecutively or in a more integrated set-up. The treatment of dilute types of domestic wastewater under tropical conditions is being accepted more and more as a proven technology, with a more or less standardized design, construction, operation and maintenance (Van Haandel & Lettinga, 1994). However, it should be noted that local climate conditions might require alterations in designs that have been applied so far. Moreover, dependent on the local sewage characteristics, completely different reactor types might be more appropriate. Recent research in the Middle East shows the limitations of the "tropical designed" single stage UASB reactors in areas that are characterized by a very concentrated sewage (COD up to 2-2.5 g l"1) and large fluctuations in summer and winter temperature (13-28°C) (El 14 Jules B. Van Lier & Frans P. Huibers

Mitwalli et al, 2000; Hallalsheh, 2002; Mahmoud étal, 2004). However, amended process designs, such as the UASB-Digester concept of Mahmoud et al. (2004), show very promising features and results. In contrast to off-site anaerobic treatment of domestic wastewater, only limited information is available regarding the application of anaerobic on-site treatment at community and/or household scale (Zeeman & Lettinga, 1999). Particularly, at this very low scale, where the required sewerage is reduced to a minimum, separation of the black and grey wastewater streams is technically simple. While the most hazardous fraction (the black toilet wastewater) is separately treated with compact anaerobic technologies, the largest quantity of household wastewater can be recovered by using simple means for further agricultural use on a small scale (Burnat et al, 2004). From the presently available technologies to reduce the organic pollution load of domestic/municipal wastewaters, anaerobic high-rate treatment is more and more recognized as the most ideal primary-secondary treatment system. For linking the treatment system to agricultural effluent use, obviously an additional post-treatment step is required treating the water up to re-use levels (Chernicharo et al, 2001; Tawfik et al, 2002). The level of pathogens is of particular concern since removal of pathogen-indicator organisms like E. coli is less than one log unit in UASBs, although, the sludge bed in the UASB reactor acts as an effective filter for parasitic eggs like Helminth's eggs. As discussed in the previous sections, the level to which the effluent should be post-treated depends on the type of application and the overall management.

WASTEWATER TREATMENT FOR EFFLUENT USE IN IRRIGATED AGRICULTURE

In selecting the most proper treatment method with the aim to use the treated effluent as irrigation water, a number of criteria are of importance the weight of each is dependent on the local socio-economic situation: Cost effectiveness From a country's or region's perspective, the availability of cost-effective treatment systems will increase the accessibility of sewage as a water and nutrient resource and will reduce the need for farmers to use raw sewage in agriculture. Obviously the costs of the required investment is a community burden and can never be mitigated to the poor farmers that now use the raw sewage. However, by implementing an integrated approach, a shared cost concept could lead to a feasible solution. It is also clear that implementation and control of sewage treatment requires a well-developed institutional framework, irrespective the type of technology that is applied.

Compactness An important advantage of compact systems is the possibility to construct the treatment in the vicinity of the populated area, preventing the need for expensive conveyance networks and improving the control on the origin of wastewater flows. Extensive systems such as pond systems, constructed wetlands, and slow sand filtration require large areas of land while potential odour problems require a treatment on distance. On the other hand, the extensive systems mentioned do have their advantages as post-treatments, since then the pollution load to be treated is much less Agricultural use of treated wastewater: the need for a paradigm shift in sanitation and treatment 15 and thus the required land is distinctly reduced. From the agricultural point of view it is obvious that evaporation of large quantities of clean water and a concomitant salt concentration increase must be prevented, giving preference to compact technologies over extensive treatment systems. Robustness A treatment system must be reliable for all circumstances. Irregularities in flow and load should not disturb the processes of the selected treatment system. In addition, the dependence on infrastructure investments, such as electricity supply, should be as low as possible. Also, the costs of electricity could be prohibitive, while in the developing countries the availability of electricity supply is not guaranteed 24 h a day. This criterion is also of considerable importance in politically unstable regions like the Middle East. Electricity plants are obvious military targets, having the indirect consequence of non-functioning treatment systems. Flexibility It is not very likely that at present developing countries will be in the position to copy "Western standards" and treatment capacity when implementing their local policy. Probably a similar approach will and needs to be followed as in the industrialized world. This means, a gradual increase of discharge standards allowing the implementation of basic treatment first, followed by a gradual extension of the treatment plant to more stringent legislation. It is well known that the removal of 80% of the pollution load can be obtained with limited costs, while the costs for the removal of the last 20% do not merit its purpose if in the same region raw sewage is discharged owing to limited fund availability. But the chosen system must allow future extension to more stringent legislation.

Acceptability The technology to be applied, with clear reference to affordability, must be accepted by the various stakeholders involved, including the executing institu­ tions, responsible engineers, local community, farmers, etc., as this will guarantee the sustainability of the investment. Often a discrepancy is observed between the wishes of responsible institutions and the capabilities of the local community, particularly in donor projects where technology-push is a consequence of a supply driven set-up of a project. On such occasions, the high operation and maintenance costs may lead to a failure of the entire investment.

Decentralization As the generally applied "Western" concept is based on centralized services, huge infrastructure investments are required. For developing countries, the construction of extensive sewerage networks is absolutely out of the question, forcing a change in conceptual thinking. In a decentralized approach, a city, region, or country can be served with sanitation and treatment step by step, without the need to firstly accomplish a regional infrastructure investment. In this sense, decentralization will immediately increase the accessibility of sewage as an alternative water and nutrients resource.

Management plan For the sake of sustainable development, a master-manage­ ment plan must underlie the individual projects. In considering a decentralized set-up, such a management plan might look more complex, but could also be developed stepwise. An interesting reference is the city of Recife, Brazil, that recently adopted the decentralized approach, including decentralized wastewater treatment as the most feasible alternative in their specific situation (Florencio & Kato, 2001). 16 Mes B. Van Lier & Frans P. Huibers

EFFLUENT TREATMENT FOR AGRICULTURAL RE-USE

By linking the foreseen treatment system to agricultural production, the set-up and technology to be applied needs some further consideration. For instance, some typical wastewater engineering parameters like COD, are of lesser importance, while agricultural parameters, like electric conductivity (ex.), need to be included. Some more examples are described below:

Location In order to avoid the need for large conveyance networks for the treated water, the treatment systems should most ideally be constructed close to agricultural fields where the treated water will be re-used. From the civil engineering point of view, treatment systems are generally located at the lowest point, which, in coastal areas, is near to the sea where there are no agricultural activities. This is particularly the case in a centralized set-up of the master-management plan, e.g. in the case of Tunisia (Chenini et al, 2003).

COD removal By directing the treated effluent to the agricultural field the COD level can be more relaxed than the generally applied stringent values, particularly considering the positive effect of organic matter on soil structure.

Nutrient removal Nutrients are resources that can beneficially be re-used by farmers and should not be removed. However, off-season or whenever nutrients are in excess, periodic nutrient removal could be a strategy in the treatment system to be applied. Depending on the commodities that are grown, the available nutrients are sufficient for crop cultivation, saving farmer's expenses for artificial fertilizers. Recent research in Jordan shows that farmers could save €650-2000 per season (Boom, 2001). Pathogen removal If needed, a pathogen removal step must be included in the treatment scheme. However, the problem with pathogens can also be addressed at other levels in the sanitation, treatment, and re-use scheme as discussed earlier in this paper.

Solids removal Considering the impact of solids, particularly on micro-irrigation, addition of a solids removal step must be considered. Solids removal might be included in the wastewater treatment systems, or alternatively, mitigated to additional filtration steps in the water distribution and irrigation network.

Treatment technology selection In addition to the advantages of compact systems (no evaporation, no salt increase) already discussed, the potential removal of specific micro-pollutants must also be considered. For instance, a high degree of heavy metal removal is expected when an anaerobic step is included, causing precipitation of (bivalent) metal-sulphides. Other micro-pollutants (e.g. containing oxidized azo-linkages) need consecutive reducing and oxidative conditions for complete mineralization.

Applied scale Aside from the advantages of the decentralized approach already discussed, it is obvious that decentralized and/or (community) on-site systems will minimize or even prevent the mixing of industrial wastewater with the domestic flows. This will reduce the risks of spreading micro-pollutants to the agricultural field. Agricultural use of treated wastewater: the need for a paradigm shift ill sanitation and treatment 17

CONCLUSIONS

The existing paradigm in sanitation and domestic sewage treatment hampers the imple­ mentation of wastewater treatment and agricultural effluent use, particularly in poor, developing economy countries. Our present work calls for a solution that includes institutional reform and technological innovation as described in the following two conclusions: - Appropriately treated domestic sewage represents an interesting resource of both water and nutrients for irrigated agriculture. However, constraints related to human and environmental health hazards, and related aspects, paralyse the debate between the need for these resources and the official approval for usage from the governmental institutions. An integrated approach in which "agricultural effluent use" directs the water management chain from the household to the agricultural field demands pollution prevention at the source and opens the perspectives for cost-optimization in treatment and conveyance costs. - Cost reduction in wastewater treatment can be accomplished by implementing cost-effective technologies. At present high-rate anaerobic wastewater treatment for the primary and secondary treatment of domestic/municipal sewage shows interesting perspectives for a wide range of applications. Complemented with proper post-treatment methods, anaerobic wastewater treatment produces both treated water and nutrients for further agricultural usage. Local institutions and engineers are needed to further develop and adapt the treatment systems to the local (climate) conditions.

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