探测原水中微生物粒径分布的有效工具
A. Chavez, B. Jimenez and C. Maya Engineering Institute. Group: Treatment and Reuse, UNAM. Ciudad Universitaria, P.O.Box 70-472, Coyoacan 04510 Mexico City, D.F. (E-mail:
[email protected]) Abstract Worldwide, raw or treated wastewater is used for irrigation. However, this practice implies that the microbial content must be controlled. Unfortunately, detection techniques for microorganisms are costly, time consuming, and require highly trained personnel. For these reasons, this study used particle size distribution to measure the microbial quality of wastewater through correlations between the number or volume of particles and the concentration of fecal coliforms, Salmonella spp. and helminth ova. Such correlations were obtained for both raw and chemically treated wastewater. The best fit was the one for helminth ova, which applies for both the influent and effluent and also for all the coagulants involved. This technique allows the on-line quantification of helminth ova at a cost of US$3 and it takes only 5 minutes, instead of the US$70 and 5 days for the standard technique. With respect to the coagulants applied, their behavior is different only for particles smaller than 8 µm, and thus this value is considered as the critical size for this particular treatment. The best coagulant was the aluminium polychloride. In addition, this work establishes the distribution of COD, TSS, nitrogen, and phosphorous for particles smaller and larger than 20 µm. TSS总固体悬浮物； Keywords Advanced primary treatment; agricultural irrigation; chemical enhanced primary treatment; helminth ova, particle size distribution
Introduction
Countries like Argentina, Brazil, Chile, India, Israel, Mexico, Saudi Arabia, and Tunisia use wastewater for agricultural irrigation, either with or without treatment (USEPA, 1992). However, there is a potential risk of transmitting gastrointestinal diseases, especially those caused by helminth ova (Cifuentes et al., 1993). For this reason, the World Health Organization (WHO, 1989) considers essential their detection and removal in irrigation water. The problem of gastrointestinal diseases is more severe in developing countries since the pathogenic levels in wastewater are very high as a result of public health problems. As an example, Jimenez et al. (2001) reported concentrations in wastewater of fecal coliforms, Salmonella spp. and helminth ova of 107–109 MPN/100 mL, 106–109 MPN/100 mL, and 6–93 ova/L, respectively. In contrast, values reported for the United States are 103 to 105 MPN/100 mL for fecal coliforms, 102 to 104 MPN/100 mL for Salmonella, and 1 to 8 ova/L for helminth ova (USEPA, 1992). In Mexico, it is estimated that at least 108 m3/s of wastewater are used in the irrigation of 254,000 ha. For that reason, there is a standard that limits the concentration of fecal coliforms to less than 1,000 MPN/100 mL in all the cases and, additionally, helminth ova must be less than 1 ova/L for crops eaten uncooked, and less than 5 ova/L for the rest of the crops. Nonetheless, to meet the standard it is required to have the analytical techniques for those two parameters. Commonly, these techniques are expensive, complex and time consuming, making harder to measure these parameters and, moreover, the control and optimization of the treatment processes becomes a difficult task. When measuring microorganisms, it has been forgotten that they are particles with different sizes. Fecal coliforms, traditional pollution indicators, have a size between 0.7 and 1.5 µm (Jawetz et al., 1995) while helminth ova range in size from 20 to 80 µm (Ayres,
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Conclusions
It is possible to correlate the particle size distribution with the microbial quality of the wastewater. Nonetheless, the obtained equations may be valid only for the waste water and effluent used (in this case, high content of helminth ova and fecal coliforms). Particle size distribution can be used to monitor helminth ova concentration at the influent and effluent on-line, in 5 minutes and at a cost of US$3 instead of using grab samples, 5 days and 70 USD. Fecal coliforms and Salmonella spp. can also be monitored using particle size distribution, but the equations presented only applied to certain concentrations (105 to 108). Last, it was found that the wastewater analyzed contained 1013 particles/m3 in a volume of 1,436 mL/m3. In number, 96% of the particles are smaller than 8 µm and basically include all the fecal coliforms and Salmonella spp. In fact, this may be considered as the critical value for all the coagulants since the difference in their performance is only appreciated for particles smaller than that value.
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Based on these results, by separating all the particles larger than 20 µm, all the helminth ova, and up to 43% of COD, 60% of ammonia nitrogen, and 10% of the phosphorus will also be removed. To achieve this, more than 71% of the suspended solids must be eliminated. This supports the importance of the coagulation–flocculation in the treatment of wastewater used for agricultural irrigation, since, by separating mostly solids and not the soluble fraction or very small particles, the concentration of pathogens is reduced and organic matter and nutrients are kept in the water.
References American Public Health Association, Water Works Association and Water Environment Federation (1995). Standard Methods for the Examination of Water and Wastewater. 19th edn. Washington D.C. Allen, T. (1997). Particle size mesurement. Vol 1. Fifth edition. ISBN 0 412 72950-4 Chapman & Hall 525 p. Ayres, R. (1989). The enumeration of human intestinal nematode ova in raw and treated wastewater. ODA Research Scheme R. 4336. Final report. University of Leeds, Department of Civil Engineering. Leed, U.K. 639–646. Broadwell, M. (2001). A Practical Guide to Particle Counting for Drinking Water Treatment. Lewis Publishers, ISBN 1-56670-306-9. 222 p. Chavez, A. and Jimenez, B. (2000). Particle size distribution (PSD) obtained in effluent from an advanced primary treatment process using different coagulant. Chemical Water and Wastewater Treatment VI: Proceedings of the 9th International Gothenburg Symposium, Editorial Springer-Verlag, ISBN3-54067574-4, pp. 257–268. Berlin. Cifuentes, E., Blumenthal, J., Ruiz-Palacios, G. and Beneth, S. (1993). Health impact evaluation of wastewater in Mexico Public Health Review, 19, 243–250. Harleman, D. and Murcott, S. (1999). The role of physical-chemical wastewater treatment in the mega-cities of the developing world. Wat. Sci. Tech., 40(4–5), 75–80. Jawetz, Melnick & Adelberg’s Medical Microbiology (1995). 20th edition, Appleton & Lange. ISBN 0 8385-6243-4. 807 P. Jimenez, B. and Chavez, A. (1997). Treatment of Mexico City wastewater for irrigation purposes. Environmental Technology, 18, 721–730. Jimenez, B., Chavez, A, Maya, C. and Jardines, L. (2001). Removal of microorganisms in different stages of wastewater treatment for Mexico City. Wat. Sci. Tech., 43(10), 155–162. Lartiges, B., Deneux-Mustin, S., Villemin, G., Mustin, C., Barrés, O. Chamerois, M., Gerard, B. and Babut, M. (2001). Composition, structure and size distribution of suspended particulates from the Rhine River. Water Research, 35(3), 808–816. Levine, A. (1991). Size distribution of particulate contaminants in wastewater and their impact on treatability. Water Research, 25(8), 911–922. Marquet, R. (1999). Characterization of trickling filter effluent by particle size distribution and high performance size exclusion chromatography. Water Research, 33(6), 1415–1424. 185
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Neis, U. and Tiehm (1997). Particle size analysis in primary and secondary wastewater effluents. Wat. Sci. Tech., 36(4), 151–158. Ødegaard, H. (1992). Norwegian experiences with chemical treatment of taw wastewater. Wat. Sci. Tech., 25(12), 255–264. Official Mexican Standard. NOM-001-ECOL/1996 (Establishes the maximum permitted limits of pollutants in the wastewater discharges into water and onto national property). The Official Federal Gazette. January 6, 1997, 67–81. Thiem, A., Herwing, V. and Neis, U. (1999) Particle size analysis for improved sedimentation and filtration in wastewater treatment. Wat. Sci. Tech., 39(8), 99–106. Ullmann’s Encyclopaedia of Industrial Chemistry (1999). Sixth Edition, Completely Revised Edition. Volume B2: Unit Operation I. Editor Wolfgang Gerhartz, pp. 2–2 to 2–30. US Environmental Protection Agency (1992). Control of pathogens and vector attraction in sewage sludge. Report EPA/625/R-92/013. Environmental Regulations and Technology, Washington, DC. World Health Organization (1989). Health guidelines for the use of wastewater in agriculture and aquaculture, Technical Report Series No. 778. WHO, Geneva, 63 p.
