Characterization and evaluation of potential reuse options for wastewater sludge and combined sewer system sediments in Mexico
Jimenez, B., Mendez, J.M., Barrios, J.A., Salgado, G. and Sheinbaum, C. Institute of Engineering, National University of Mexico (UNAM) Apartado Postal 70472, Coyoacan 04510, Mexico, D. F. Mexico. [email protected]
Abstract. Combined sewer systems generate sediments that have characteristics similar to those of primary sludge. Mexico City has such a system composed of a network of pipes, regulation structures (dams, basins) and open channels. The annual generation of sediments is estimated in 2.8 Mm3, which includes 0.41 Mm3of sludge. As a result, the total capacity for transporting water is reduced considerably, making necessary to extract yearly an approximate 0.85 Mm3 of those materials and to send them to a final disposal site with a capacity that is being exhausted. As part of the local Governmental effort, this project evaluates the quality of sediments from 6 dams, 4 regulation basins, 2 open channels, and 3 transfer stations. Also, sludge from 20 wastewater treatment plants was sampled. The results showed an important presence of lead and hydrocarbons in some sediments, and some sludge samples contained arsenic and nickel above the limits. Moreover, microbial levels exceeded the limits in all the sediments and sludge samples. Erosion was linked to the generation of an important amount of sediments based on lead concentration. A classification was established to determine the degree of contamination of the sediments as well as the required treatment to allow their potential reuse. Key words: Combined sewer, drainage, sediments, wastewater sludge
Introduction In some countries, combined sewer systems receive and transport wastewater and rainwater. %s flow carries particles that sediment within the system, generating solids with characteristics similar to those of the primary sludge. Some countries, like the United Kingdom, partially characterise and clasify these materials according to their composition, origin and potential pollution to dispose them with no apparent reuse (Clegg et ul., 1992). On the other hand, France manages the sediments by including them within the Urban Plan in a program called Solids Trunsfered in Sewers (Chebbo et al., 1990). In both countries, some parameters have been considered as the basis for the characterisation of these sediments before their final disposal. They include solids content (total+volatile), chemical oxygen demand (COD), biochemical oxygen demand (BOD), organic matter, total nitrogen, metals (mainly lead) and hydrocarbons. These sediments are then sent to landfills and their management focuses on the protection of surface water since sewers discharge into them. In contrast, other countries have similar sewer systems but the information about the management of the sediments is limited. Such is the case of Mexico City’s sewer system that includes a complex network of pipes (0.30 to 3.05 m of diameter), complementary regulation structures, like dams and basins, as well as open channels. The system has an estimated capacity of 11.2 Mm3, and every year approximately 2.8 Mm3 of sediments are generated. From this amount, an estimated 15% is sludge that comes from the 27 wastewater treatment plants. This sludge is discharged to the sewer without any treatment and there is little information about its quality. In order for the sewer to operate adequately, every year 0.85 Mm3 of sediments are extracted from the system. These sediments are then disposed on a dedicated area near a landfill, without being mixed with solid waste. However, the capacity of this site is being exhausted and the authorities are looking for alternatives of disposal. So far, sediments have been used for levelling out the ground in areas where groundwater exploitation has made it sink, and also for growing trees for reforestation activities. Nonetheless, these practices have been limited and currently there is no regulation with respect to the use or disposal of this type of materials, which represents a potential risk to human health and the environment.
241 With respect to sludge management, the proposed Mexican Official Standard, NOM-004-ECOL- 2001, includes limits for faecal coliforms, Salmonella spp., helminth ova and heavy metals. Once the regulation is published, it would not be allowed to discharge the sludge to the sewer system and it should be treated adequately to reuse or dispose it. This would reduce the amount of sediments generated in the sewer system, but still the 85% of the current generation would need to be properly managed since these sediments are not included within the proposed regulation, even though their origin is similar to that of primary sludge. In an attempt to better know the composition and characteristics of the sediments generated in the sewer system and the sludge produced in Mexico City, the local Government decided to perform this study. The resulting information would help them to support the local regulation for the reuse or disposal of sewer system sediments and will propose complementary actions regarding their generation and management.
Sampling of sediments and wastewater sludge Sewer system sediments were obtained from five dams, four regulation basins, two open channels and three transfer stations. These transfer stations collect sediments extracted from sewer pipes with Vactoa trucks. All the sites were selected considering the type of wastewater that they receive as well as the socio-economic level of the population of each area. It should be mentioned that not all the existing sites were sampled. On the other hand, wastewater sludge was sampled from 20 treatment plants which operate activated sludge (1 8) and physicochemical (2) processes. Based on the operating conditions, sludge samples consisted of a mixture of primary and secondary, waste activated only, or thickened sludge. Sediments were collected during the dry season and sludge was obtained during dry and rainy seasons. Table 1 shows the parameters analysed on sediments and sludge. The total number of samples was 112, including 20 from the wastewater treatment plants.
Table 1. Parameters used to characterise sewer system sediments and wastewater sludge. Physical Chemical Microbial Humidity* PH* Faecal coliforms* Density Ammonia Nitrogen* Salmonella spp.* Helminth Conductivity* Total Nitrogen* ova (total and viable)* Total solids* Total P/Orthophosphates* Volatile solids* Organic Matter Cation Exchange Capacity Total Petroleum Hydrocarbons (TPH’s) CRETI test Benzene, Toluene, Xilene (BTX) Heavy Metals* * Parameters analysed also on sediments and wastewater sludge
Classification of sewer system sediments and wastewater sludge. Considering the lack of information with respect to the quality of these types of sediments, a classification was proposed. This classification takes into account grouped parameters that indicate the level and type of pollution of these materials (Table 2). Since there is no regulation concerning these sediments, the design is based on the compliance with recommended standards for biosolids and soils. Microbial and heavy metal limits are taken from the proposed sludge regulation for class B biosolids (NOM-004-ECOL-2001). Limits for organics are based on the Mexican standard for cleanup of hydrocarbon-contaminated soils (NOM-EM- 138-ECOL-2002). In addition, the CRETI test evaluates the characteristics of wastes to determine if they are hazardous. The advantage of this approach is that all the parameters included are measurable and not subjective. This approach intends to help authorities to take decisions with respect to their use or disposal with a reasonable risk, reducing the potential impact on pubIic health and the environment.
242 Table 2. Groups of parameters used to classify sewer system Sediments. Group Parameter Units Analytical technique (‘1 Regulation limit Faecal coliforms MPNlg TS 9221E SM < 2X1O6 Microbial Salmonella spp. MPN/g TS 9221 B SM < 300 Total helminth ova- ovdg TS NOM-004-ECOL-200 1 < 35 TPH’s EPA 418.1 M 1000 Benzene EPA 8260B M4 20 Organics Toluene EPA 8260B M4 40 Xilene EPA 8260B M4 40 Arsenic EPA 60 10 75 Cadmium EPA 60 10 85 Copper EPA 60 10 4300 Heavy Chromium EPA 60 10 1300 metals Lead mgfl
Table 3 proposes a classification with five levels that indicate the degree of pollution of sediments and their potential management. Those levels consider the groups presented in Table 2. Level 1 indicates that the sediments may be reused with no Mertreatment since they meet the limits described on Table 2. Sediments may be used for levelling out the ground, soil amendment, and even brick construction. Levels 2 and 3 are proposed for those materials that require a basic or advanced treatment as they do not meet the limits for microbiaVBTX or TPH’s, respectively. In the first case, lime stabilisation may be used to reduce pathogens and parasites, while for TPH’s bioremediation techniques should be employed. The selection of those processes needs to take into account issues like the volume of sediments, pollutant concentrations, land available, and cost. I When sediments contain heavy metals above the limits (Level IV), they should be sent to a final disposal site since processes that reduce these pollutants in soils and sludge, like chemical extraction or ion exchange, may result prohibitive in terms of cost. With respect to Level V, current Mexican standards consider that if a waste is hazardous by the CRETI test, it must be sent to a dedicated hazardous waste disposal site. Wastewater sludge was classified according to the proposed sludge regulation (NOM-004- ECOL-2001). This regulation is partially based on US EPA’s with slight modifications with respect to microbial limits: class B biosolids include a limit for Salmonella spp. and helminth ova (<300 MPN/g TS and < 35 ovdg TS, respectively). These values consider the high microbial content in Mexican sludge as stated by Jimenez et al. (2002).
243 Table 3. Classification of sewer system sediments according to the groups of pollutants. Level Color Description Potential management I Green Meets all limits May be reused 11 Blue Exceeds microbial and/or BTX limits Requires basic treatment I11 Yellow Exceeds TPH’s limits Requires advanced treatment IV Brown Exceeds heavy metal limits Landfilling V Red Fails the CRETI test (Hazardous waste) Disposal in a hazardous waste site
Results
Sewer system sediments. Microbial quality. Considering that sediments come from a combined sewer system, there is a direct influence of domestic discharges in microbial quality. Table 4 presents a summary of the microbial quality in all samples taken. Even though the average concentration of faecal coliforms is slightly lower than the proposed limit (6.3 logs), their levels indicate an important contribution of faecal material and in some cases they need to be treated to meet the class B sludge limit. Overall, concentrations ranged ffom 2.8 to 7 logs. In contrast, densities of Salmonella spp. exceeded the limit for class B sludge in all the cases with the exception of two samples taken in a regulation basin that receives an important amount of sediments ffom erosion and limited wastewater discharges. The concentration of the rest of the samples taken fkom that site indicates that Salmonella levels should be reduced less than one log to meet the limit. Overall, the concentration of Salmonella in sediments needs to be reduced an average of 2 logs to meet the class B limit.
Table 4. Microbial content of sewer system sediments from Mexico City.
~ Faecal coliforms Salmonella spp. Helminth ova, ovdg TS Site Value log (NMP /g ST) log (NMPIg ST) Viable Total Mean 5.8f0.30 4.6 f 0.50 17f 12 20h 13 Dams Max 6.8 5.8 54 59 Min 4.8 3.6 6 6 Mean 4.9f 0.70 3.9 f 1.0 8f2 8*2 Regulation Max 6.0 5.4 12 12 basins Min 2.8 2.3 4 4 Mean 4.7 1.7 9*6 10*7 Open 5.8h0.80 f Max 7.0 6.8 21 23 channels Min 4.8 3.5 5 6 Mean 6.0* 0.1 4.2 0.30 9f3 12*2 Transfer h Max 6.4 4.8 12 14 stations Min 5.6 3.8 6 10 Proposed limits NOM-004-ECOL-2001 Class B < 6.30 < 2.48 NA < 35 NA: Not applicable
On the other hand, viable helminth ova are present in relatively low concentrations, ranging from 4 to 54 ovdg TS. These levels are slightly lower than those reported for sludge generated in Mexico City (73-177 viable ovdg TS; Jimenez et al., 2002). According to Nelson and Jimenez (2000), an increase in sludge age (in this case the age of the sediments) reduces the number of helminth ova. It should be mentioned that an important amount of these sediments spend at least a couple of years before they are extracted ffom the sewer system, which may reduce the density of these parasites. In
244 addition, soil particles that enter the system and become sediments cause a dilution effect in their concentration. Nonetheless, wastewater contribution is important as the faecal coliforms and Salmonella spp. levels indicate on Table 4.
Organic compounds. BTX compounds presented values that ranged fiom 0.006 to 17 mgkg which are well below the limits of 20, 40 and 40 mgkg for benzene, toluene and xilene, respectively. According to this study, these compounds do not present any risk of pollution if these materials are reused. In contrast, average total petroleum hydrocarbons were higher than the limit for agricultural, forest, recreational or commercial soils (1000 mgkg) established by the Official Mexican Standard NOM-EM- 138-ECOL-2002. While average concentrations reported on Table 5 suggest that all the sediments have TPH’s concentrations above the limit, there were some sites that did not exceed such value. However, the high TPH’s concentration, especially in sediments from transfer stations, reflects the large amount of hydrocarbons entering the sewer system. Even though the concentration of TPH’s in dams, regulation basins and open channels is lower than that of transfer stations, the levels of these compounds requires that a large proportion of the sediments should be treated to meet the limit. This difference in concentrations may be due to the length of time that the sediments spend in those sites. This means that sediments in regulation I structures are affected by weathering, while transfer stations receive “fresher” sediments.
I Table 5. Total petroleum hydrocarbons (TPH’s) in sewer system sediments from Mexico City. Total petroleum hydrocarbons (mgkg TS) Value Regulation Transfer Dm Open channels basins stations Mean 1419 f 992 6666 f 9383 4119 f 2746 28810 f 29095 MaX 4347 32545 7780 86776 Min 89 139 636 7955 Limit NOM-EM-1 38-ECOL-2002 Agricultural,forest, recreational or commercial 1000 (mgkg) soils
Heavy metals. The results from the analyses of heavy metals showed that the concentration of lead exceeded the limit in two dams, one regulation basin, and two transfer stations. Average lead content was 1289 and 2137 mgkg in the two contaminated dams; 4423 mgkg in the regulation basin; and 1629 and 4322 mgkg in two transfer stations. Sediments from these sites can not be reused and they must be landfilled. According to Morton-Bermea et al. (2002), the presence of lead in soils from Mexico City comes from the use of this metal in gasoline additives during several decades. They found concentrations of up to 1570 mgkg in certain soils and the erosion of them may transport this metal to the sewer system. The rest of the regulated heavy metals were below the limits.
CRETZ Test. According to the Official Mexican Standard, NOM-052-ECOL-1993, this test determines if a waste is hazardous in terms of corrosivity, reactivity, explosivity, toxicity or flammability. Based on the results, the sediments from one of the regulation basins and the three transfer stations failed the reactivity test because the released sulphides in an excess of 500 mgkg. The high concentrations are attributable to the decomposition of sulphates under anoxic conditions generally present in sewers and they are generated by sediment deposition in pipes (Schmitt and Seyfiied, 1992).
245 Wastewater sludge characterisation.
Microbial quality. Table 6 presents the results of the microbial analyses. With respect to faecal coliforms, their concentration ranged from 2.4~10~MPN/g TS up to 1. lx109 MPN/g TS. Only two plants generated sludge that met the limit for this parameter. Even though the concentration found in these samples is higher than that of sediments, there is an influence of wastewater and sludge discharges in the sewer system since the average concentration in those materials is within the range found in sludge. With respect to Salmonella spp., all the sludge sampled exceeded the limit of < 300 MPNIg TS, with concentrations that ranged from 7.9~10’to 2.5~10’MPN/g TS. In this case, their concentration should be reduced by an average of 3 logs and a maximum of 4.9 logs to comply with the standard. Regarding helminth ova concentration, only 3 plants met the limit of 35 ovdg TS. The rest of them showed concentrations that ranged from 38 to 168 total ovdg TS. These values reflect the level of parasitic diseases caused by helminths in the population of the City, which in the year 2000 accounted for more than 62,000 reported cases (SSA, 2001). Also, according to the World Health Organization 10% of the world’s population is infected by these parasites (WHO, 2002). Moreover, the most common helminth found in sludge was Ascaris which is one of the most resistant helminths to conventional stabilisation processes (Carrington and Harman, 1984), unless heat or acidalkaline chemicals are applied.
Table 6. Microbial content in wastewater sludge generated in Mexico City. Faecal coliforms log Salmonella spp. Helminth ova, ovdg TS Value (MPN/g TS) log (MPN/g TS) Viable Total Mean 7.6 f 1.0 5.2 f 1.3 72 f 38 77 f 43 Max 9.0 7.4 160 178 Min 5.4 2.9 4 4 Proposed limits NOM-004-ECOL-200 1 Class B sludge < 6.30 < 2.48 NA < 35
Heavy metals. The analysis of heavy metals showed that 3 plants exceeded the limit for arsenic (135, 158 and 224 mgkg vs. 75 mgkg) which represents 0.41 Mm3/year. In addition, one of those plants also produced sludge with 477 mgkg of nickel, value slightly higher than the limit of 420 mgkg. Arsenic comes from different sources that include oils, pesticides, plastics, pharmaceutical products, inks, hospitals and laboratories, which makes difficult to track it to its origin. Other metals detected in low concentrations were zinc, copper, selenium and barium but they were below the maximum allowed. As a result, the sludge from those 3 treatment plants needs to be disposed in a landfill or a dedicated site according to the proposed regulation. The rest of the plants may reuse their sludge once they treat it to comply with the microbial limits.
Classification of sediments and sludge and evaluation of their potential reuse
Sewer system sediments The classification of sewer system sediments indicates that all the sites sampled generate materials that need to be treated before they may be reused, since all of them exceeded at least one of the criteria set in Table 2. One dam and two regulation basins are Level I1 as they did not meet the microbial limits. These three sites generate 0.28 Mm3/year which corresponds to 20.7% of the total generated. A similar percentage (21.2%) is classified as Level I11 due to the presence of TPH’s. Heavy metal content resulted in 3.2% of the sediments being Level IVYwhich need to be
246 disposed. The concentration of released sulphides made that the sediments fiom 5 sites were hazardous waste (Level V) and, thus, they need to be treated as such. These materials represent more than half of the generation in the sites analysed. On this respect, authorities need to evaluate if the concentration of released sulphides is really a problem, especially considering the origin of these sediments.
Table 7. Classification of sewer system sediments. Classification in number of sites (generation, Mm3) Sites I I1 I11 Iv V Dams 0 (0) 1 (0.02) 2 (0.08) 2 (0.03) 0 (0.0) Regulation basins 0 (0) 2 (0.26) 0 (0.0) l(O.01) l(0.69) Open channels 0 (0) 0 (0.0) 1 (0.21) 0 (0.0) 1 (0.04) Transfer stations 0 (0) 0 (0.0) 0 (0.0) 0 (0.0) 3 (0.01) Total 0 (0) 3 (0.28) 3 (0.29) 3 (0.04) 5 (0.74) % from total 0 (0) 21 (20.7) 21 (21.5) 21 (3.0) 37 (54.8)
Classification of wastewater sludge Based on the sludge characterisation, all the treatment plants need to treat their sludge to comply with the microbial limits of the proposed regulation (Table 8). Moreover, 12% of the sludge sampled exceeds the limits for heavy metals and need to be disposed in a landfill or a similar site. To solve this problem, pre-treatment of industrial discharges needs to be enforced to reduce the amount of metals that reach the treatment plants. It is important to consider that removal of heavy metals fiom sludge is not an easy task and only certain processes are able to reduce them but their application has been limited (Marchioretto et al., 2002).
Table 8. Classification of WWTP sludge. No. of WWTP that meet the. No. of WWTP that meet the limits No of Generation microbial limits for heavy metals WWTP Mm3/year (generation, Mm3/year) (generation, Mm3/year) 20 0.41 0 (0) 17 (0.36)
Conclusions While this study is an initial approach for the management of sewer system sediments and wastewater sludge generated in combined sewer systems, monitoring of the quality of these materials is an activity that should be performed on a regular basis. In addition, the rest of the sites have to be monitored to determine the classification of their sediments. The enforcement of discharge pre-treatment programs as well as the adequate management of WWTP sludge should improve the quality of the sediments that need to be disposed every year. Moreover, sludge treatment and reuse would reduce the generation of sediments by an estimated 15% and may provide farmers with a potential soil amendment. Also, considering that the erosion in 22% of the total surface of the city is high (500-3500 todha; CORENADER, 2002), the implementation of reforestation programs to reduce soil erosion will reduce the amount of these materials as well as the costs associated to their extraction and disposal. The sediments fiom combined sewer systems have a direct influence of domestic discharges as microbial levels indicate. The results obtained in this study showed that the levels of faecal coliforms and Salmonella are similar in the sediments and the sludge. However, helminth ova are present in lower concentrations in sediments which suggests that the age of these materials has
241 influence on the viability of these parasites and also that the contribution of soil particles dilutes their density. These may suggest that the treatment required to reduce them to levels that meet the limits may be cheaper than for sludge. According to the proposed classification, 0.28 and 0.29 Mm3/year are level I1 and level 111, respectively. These materials require basic and advanced treatment to reduce the microbial content and the total petroleum hydrocarbons. The rest of the sediments (0.78 Mm3/year) can not be treated and most of them are considered hazardous waste because of the released sulphides. For this reason, it is necessary to determine the feasibility of excluding this characteristic from the classification as these compounds are generated under anaerobic conditions in sewer systems and regulatiodtransport structures. Bearing in mind that sewer system sediments are similar to primary sludge, there are several alternatives for them to be reused. These alternatives should be evaluated on a case-by-case basis to determine the feasibility of implementing them.
Acknowledgements. The authors wish to thank Gobierno del Distn'to Federal, especially Secretaria del Medio Ambiente, for their support for this project.
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