Health of
and Hygiene Aspects Spray Irrigation
CHARLES A. SORBER, PhD, PE KURT J. GUTER, PhD
This study reviews the health and hygiene aspects of wastewater treatment or reclamation by spray irrigation.
Introduction Land disposal of domestic wastewater has 'been practiced on a large scale in Europe for several decades, mainly for the purpose of irrigating crops or pasture.' Recently, however, spray irrigation, one form of land disposal, has taken on new dimensions in this country as it is considered a means of disposal of wastewaters which heretofore have been discharged directly to streams or lakes. Small communities, mainly in arid regions, long have employed wastewater spray irrigation as a means of water conservation. More recently, this process of wastewater disposal has been recommended for use by larger metropolitan areas. Muskegon County, for example, has completed construction of a project which employs biological treatment, storage lagoons, disinfection, and spray irrigation for the disposal of the wastewater from the entire county.2 This system will initially serve a population of 170,000 but will have a design wastewater flow of 46 million gallons per day. The area to be irrigated will encompass approximately 6,000 acres. This paper is a review of the health and hygiene aspects of wastewater treatment or wastewater reclamation by spray irrigation. Although the discussion is limited to spray irrigation because of its current popularity among communities and sanitary engineers, many of the basic principles are applicable to the other methods of land disposal such as surface flooding and ridge and furrow irrigation. The health and hygiene aspects of wastewater treatment or wastewater reclamation by spray irrigation are Dr. Sorber is presently with the U.S. Army Medical Ft. Bioengineering Research and Development Laboratory,John Detrick, Frederick, Maryland, and Dr. Guter is with R. Snell Engineers, Inc., Lansing, Michigan.
dependent on a number of variables, the most important of which is the ultimate use of the contaminated wastewater. Possible uses for this water include: (1) discharge to a surface water body following percolation through soil permitting wastewater treatment to include nutrient removal through the filtering soil; (2) ground water reservoir recharge; (3) crop irrigation; (4) any combination of the first three. The degree of wastewater pretreatment before spray irrigation is intimately associated with the health and hygiene aspects of the problem. In fact, the effect of pretreatment on the physical, biological, and chemical characteristics of the wastewater will dictate to a large degree the ultimate use of the wastewater.
Physical Considerations Particular attention must be directed to the removal of suspended material which will ensure that both the spray distribution system and the soil treatment system will not clog.3'4 Spray distribution systems may be designed in various ways. The type of distribution system employed (solid set, mechanically moved, or fully portable) will determine the types of spray nozzles and their spacing. The spacing and type of nozzles used will significantly influence the aerosol formation from these spray distribution systems. Spray nozzles are manufactured that operate over a pressure range of 5 to 130 psi with increased pressure generally proportional to increases in aerosol formation.* Equally important is the type of soil available at the selected disposal site. Recently, Okums pointed out that the wide variations in soil characteristics, speciflcally the nonisotropism and heterogeneous character of most soils, * Keller, J., irrigation consultant, Utah State University. Personal communication, 1971.
SPRAY IRRIGATION
47
make it mandatory that each site be intensively studied before a spray irrigation site is selected. Hajek,6 performing a detailed investigation of the interaction of soil and wastewater, reported that the clays are chemically the most important size fraction of soil. He continued by stating that soils generally exhibit only cationic exchange. If the soil exchange capacity will be used in the wastewater land disposal plan, specific tests with the wastewater to be treated and the soil to be used should be conducted to determine the exchange capacity of the soil for the particular waste to be treated.
Biological Considerations Standards for Land Disposal and Microorganisms Present
California, with the Wastewater Reclamation and Reuse Law of 1967, has established standards which relate to the practical applications of reclaimed wastewater.7 These standards vary from the use of primary effluent without additional treatment for the irrigation of orchard crops which are processed before consumption, to the use of "adequately disinfected filtered wastewater" for irrigation of fodder, landscape in public areas, and filling impoundments for recreation. The California Standards define primary effluent as effluent from a sewage treatment process which provides partial removal of sewage solids by physical methods so that the settleable solids are reduced to less than 1 ml per liter. "Adequately disinfected filtered wastewater" is defined as water that has been oxidized, coagulated or filtered, and chlorinated to the extent that the 7-day mean residual coliform count does not exceed 2.2 or 23.0 organisms per 100 ml (depending on the specific application). In another section of the California Standards, "disinfected wastewater" is defined as water "in which pathogenic organisms have been destroyed." Obviously, there is a technical conflict in these statements as complete destruction of pathogenic organisms and the presence of as many as 23 organisms per 100 ml is inconsistent. Irrespective of this minor distinction, wastewater treated as specified in the California Standards approaches drinking water quality from a bacteriological standpoint.8 Arizona also has proposed standards restricting the use of domestic wastewater for irrigation. Domestic wastewater will undergo secondary treatment and disinfection for irrigation of crops which are processed before consumption and tertiary treatment and disinfection for use on crops intended for direct human consumption.9 Evidence has been collected demonstrating high levels of coliform bacteria on the surface of vegetables irrigated with raw sewage,* while other researchers have found that, if sewage irrigation is stopped 1 month before harvest, raw fruit would not be a likely vector for transmission of human bacterial enteric disease.' 0 These conflicting data substantiate the current requirements for treatment and * Franco, R., University of Texas, El Paso. Personal communication, 1971.
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disinfection of wastewater before use for irrigation of vegetables or any other crop intended for direct human consumption. Health problems that can arise from the use of either inadequately disinfected or nondisinfected treated wastewater become apparent when the presence of pathogenic bacteria in wastewater is evaluated. In a review of the work in this area, it was found that many pathogenic microorganisms pass through an activated sludge treatment process even though, in general, they are greatly reduced in number.1 1 Microorganisms found include: the typhoid group, Vibrio cholerae, Shigella, Mycobacterium tuberculosis; Coxsackie virus, poliovirus (I and II), and ECHO virus; Schistosoma, Ascaris, hookworm, and Trichuris. Even after chlorination many of these microorganisms were detected in effluents. Estimates and field studies have indicated enteric virus levels of domestic wastewater to range from up to 7,000 viruses per liter for raw sewage to about 50 viruses per liter for chlorinated secondary effluent.1 2-1 5Current chlorination practices for disinfection of domestic wastewater employ coliform organisms as a measure of the effectiveness of disinfection. Kruse and co-workers1 6 have demonstrated that in the presence of ammonia or amino acids, a zero coliform reading does not necessarily mean a virus kill. Therefore, the effectiveness of disinfection, as measured by residual coliform bacteria, does not by any measure ensure the destruction of viable enteric viruses. It is clear from the literature, therefore, that chlorination as practiced does not provide complete disinfection of pathogenic bacteria or viruses. As a consequence, wastewater spraying, with resultant biological aerosol formation, could disperse many of the pathogenic microorganisms found in wastewater. Other methods of disinfection, including the use of elemental iodine, bromine, or ozone, have been practicedon a limited scale. Of these methods, ozone is considered to be one of the leading materials which could replace chlorine in water or wastewater disinfection. Stumm,' 7 reporting on work done in Europe, indicates that the contact time necessary for 99 per cent destruction of Escherichia coli is 7 times smaller for ozone than with the same amount of hypochlorous acid. Indications are that the constituents of sewage cause a significant ozone consumption. A logical conclusion, therefore, is that there will be considerable interference in ozone disinfection of sewage and that suspended matter will protect embedded organisms against destruction. At present there are insufficient quantitative data in the literature to evaluate the effectiveness of ozone disinfection of domestic wastewaters. On the other hand, ozone is one of the most potent germicides considered for large scale disinfection and merits more research in the area of wastewater disinfection.
Surface and Ground Water Contamination If the applied wastewater must pass through 5 to 10 feet of continuous strata of fine soil before entering the ground water system, microbiological contamination of ground water should not occur under normal conditions. 8 Research has demonstrated that coliform penetration is
more a function of the soil type than of the hydraulic inflltration rate. It has also been demonstrated that once bacteria enter the ground water aquifer, they could travel only a few hundred feet horizontally.' 8,1 9 Exceptions to this statement include situations where coarse soils or solution channels are present. Under these conditions, bacteria and viruses may travel long distances underground. Viruses present in wastewater percolating through soil are primarily removed by adsorption. However, as pH increases the amphoteric protein viral shell will be more negatively charged, which will decrease the effective adsorption ability of a negatively charged soil.2 0 For example, in a soil from San Mateo County, California (50 per cent sand, 30 per cent silt, and 20 per cent clay by particle size distribution), the adsorption of the coliphage Ti was reduced by almost 40 per cent when the pH was increased from 6.8 to 8.0. Drewry and Eliassen suggest that insurance for virus removal can be achieved by following the normal public health practice of placing wells at least 100 feet from land disposal sites. Surface water supplies may be contaminated by overland flow of the sprayed wastewater, although this situation could be avoided by proper design of the spray system. Such a system design must prevent direct runoff to surface waters.
Contamination of the Air Aerosols have been defined as particles in the size range of 0.01 to 50 ,u suspended in air.21 Several studies regarding the emission of biological aerosols by spray irrigation of settled domestic wastewater have been described by Sepp.' On the other hand, no specific studies of biological aerosols emitted by spray irrigation of secondary wastewater effluents have been found in the literature. However, some preliminary work at the University of Utah* has demonstrated that the spraying of chlorinated effluent for landscape watering resulted in approximately 1.5 times the number of viable biological aerosols emitted by a trickling filter. Specific quantification of the work is difficult because of lack of experimental control, but it does demonstrate that spray irrigation of chlorinated effluent produces biological aerosols of the same order of magnitude as nonchlorinated wastewater applied to trickling filters. Investigations have been conducted on biological aerosols from trickling filter and activated sludge wastewater treatment plants.2 2-26 In general, bacterial aerosols remain viable and travel farther with increased wind velocity, increased relative humidity, lower temperature, and darkness.
Rapid desiccation of bacteria in flight appears to be a primary cause of bacterial die-off in biological aerosols.2 7 Studies of evaporation rates showed that a 50-,u water droplet will evaporate in 0.31 sec in air with 50 per cent relative humidity and a temperature of 220 C.23 These high rates of evaporation will result in nuclei of solid waste that * Goff, G. D., environmental microbiologist, University of Utah. Personal communication, 1971.
were originally dissolved or suspended in the biological aerosols.22 Atmospheric bacterial die-off is geometric in nature with the majority of the organisms dying within 3 sec. The remaining resistant bacteria, protected by chemical additives which inhibit evaporation, continue to die at a decreasing rate with time.2 6-2 8 This observation has a great effect on selection of the method of analysis and the specific bacteria that should be analyzed for in determining the pathogenesis of biological aerosols. Much of the reported work with biological aerosols has centered on the study of coliform bacteria, which have been the traditional indicators of domestic fecal water pollution. E. coli, probably the most frequently observed coliform bacteria in wastewater environments, generally have been shown to have an extremely short life span in aerosol form.29 Specific coliform bacteria of the genus Klebsiella, on the other hand, form a large capsule which apparently protects the organism from desiccation in flight.28 The most significant aspect of this observation is that all species of this genus are known pathogens of the respiratory tract. This differentiation demonstrates the need for specific testing for Klebsiella or other specific pathogens for biolgical aerosol wastewater studies in lieu of the more traditional general coliform group. A direct means of human infection by biological aerosols is by inhalation of these aerosols. The infectivity of an aerosol is further dependent on the depth of respiratory penetration. Biological aerosols in the 2- to 5-, size range are primarily captured in the upper respiratory tract. These particles are removed by cilia action and may pass into the digestive tract through the pharynx.26 If gastrointestinal pathogens are present in these aerosols, infection may result. However, a much higher incidence of infection results when respiratory pathogens are inhaled into the alveoli of the lung. The greatest alveolar deposition occurs in the 1- to 2-, range and then decreases to a minimum at approximately 0.25 ,. Below 0.25 ,u, alveolar deposition again increases due to Brownian movement.30 For comparative purposes, Fuchs3 1 observed that approximately 82 per cent of 1.0-,u particles, 28 per cent of particles in the 0.1- to 0.3-,u range, and 51 per cent of 0.03-, particles are deposited in the alveolar passages or the alveoli. This aspect of aerosol deposition becomes significant when consideration is given to viral size particles which range in size from about 0.01 to 0.1 ,u. Consideration must therefore be given to viral deposition with possible resultant infection. A summary of virus infections by the respiratory route demonstrates that extremely low relative doses of virus can cause infection in man.32 The minimal infective doses (TCD5o ) cited ranged from S1.0 to .790.0. Viruses included in these studies were influenza virus, coxsackie viruses (A21 and B4), adenovirus, rhinovirus, measles, and respiratory syncytial virus. Other workers have studied the inhalation of bacteria and determined that adjacent to an activated sludge aeration plant approximately 40 per cent of the biological aerosols will penetrate the lungs and approximately 6 per cent will penetrate the alveoli.28 These percentages increased to 60 and 13 per cent, respectively, 20 feet SPRAY IRRIGATION
49
downwind from the tank. Based on these figures they calculated that a man working within 5 feet on the downwind edge of an aeration tank would inhale a viable Klebsiella every two breaths. Wild Animals, Birds, and Mosquitoes Considerable interest is developing in the effect of spray irrigation on changes in both the population and the disease incidence among wild animals, birds, and mosquitoes. In a 3-year study by Parizek and co-workers,3 it was concluded that there were no measurable changes in population of -small animals and birds in areas subjected to spray irrigation. These workers, however, felt that the data served only as a basis for measuring future changes as the spray irrigation had not altered the vegetation in areas large enough to induce differences in bird and mammal populations. In a different but related study, E. coli serotypes were tracked from the wastewater before spraying, through the spray irrigation area, and ultimately to wild birds.33 It was concluded that the chlorinated effluent had no effect on the presence of the coliforn flora of the birds. Parizek and co-workers3 demonstrated that surface ponding developed on the spray irrigation site and that these ponds result in increased mosquito breeding. Since the mosquitoes will be breeding in wastewater, a tremendous potential for arthropod-bome disease transmission is present. These workers anticipated an increase in avian blood parasites due to the increase in numbers of mosquitoes. Through blood smears of birds on the irrigation site, a significant increase in Leucocytozoon was observed between the second and third year of the study. There are still conspicuous voids in the literature in this general area. The Institute for Research on Land and Water Resources of The Pennsylvania State University has proposed the study of faunal response to spray irrigation of chlorinated sewage effluent. The study, if initiated, will do much to provide answers relative to the long range effects of spray irrigation on population changes and disease in wild animals as well as the possibility of increasing the infection of other known vectors of disease.
Chemical Considerations The health and hygiene aspects of spray irrigation must also include a consideration of organic and inorganic chemical movement in the soil. This consideration could be extremely broad in scope if industrial wastewaters are considered for land disposal by spray irrigation either separately or combined with domestic wastewaters. Dissolved chemical substances normally found in water have been reported to generally increase inconcentration when used by man. Water of five Ohio municipalities in one cycle of water use (i.e., raw water source to secondary waste treatment plant effluent) experienced an average increase in total dissolved solids (TDS) concentration of 50
AJPH JANUARY, 1975, Vol. 65, No. 1
approximately 300 mg per liter.34 This water, when treated by land disposal techniques, may experience larger increases in TDS level if passed through a soil filter system. An extreme example of this would be the Imperial California Irrigation District, where inflow irrigation water had a TDS concentration of 13.14 mEq per liter (1,242 mg per liter) and the outflow contained 45.97 mEq per liter (3,844 mg per liter).35 Therefore, when spray irrigation results in recharge of ground water sources used for human consumption, TDS concentrations may exceed the levels recommended in the U.S,P.H.S. Drinking Water Standards.8 More important than the esthetically objectionable TDS level, the associated high sodium content may be harmful to persons suffering with cardiac, renal, and circulatory disease.36 Heavy Metal and Trace Organic Compounds Heavy metals or toxic organic compounds are two groups of materials that would require special consideration for wastewater disposal by spray irrigation. Heavy metals, including chromium, copper, lead, manganese, and zinc, have increased in concentration in soil where digested sludge was being wasted.37 Of these metals, zinc and manganese have increased in concentration in the leachates. The mechanism for heavy metal ion removal in the soil system has not yet been well defined. McCarty and King studied the movement of pesticides in soils and have concluded, as have other researchers, that organic phosphorous pesticide removal by the soil can be correlated with the clay content of the oil.38,39 Microbiological acclimation to all of the applied pesticides occurred with the exception of ethion. This acclimation will result in destruction of some of the absorbed pesticide. The movement of dissolved chemicals with percolating water is primarily dependent on the nature of the filtering soil. There have been many instances, especially in Germany, of ground water contamination by soluble industrial wastes.40 In one case, picric acid traveled several miles and caused abandonment of ground water supply. Chemical elements are primarily removed in soil media by the process of ion exchange. Therefore, the ion exchange capacity of soil is proportional to its chemical clarification ability and clay soils, because of their large surface area, have the largest exchange capacities. Cationic exchange, one possible mechanism for the removal of heavy metal ions and ionized organic compounds, will occur until the exchange capacity of the soil is exceeded. If cation removal is the objective of a land disposal system, consideration must be given to the ion exchange exhaustion of the soil and the possible toxicity to vegetation cover. Nitrogen Compounds
Biologically treated domestic wastewater will contain from 5 to 30 mg per liter of total nitrogen. The primary source of this nitrogen is the nitrogen metabolism of man.4 ' The dominant form that the nitrogen assumes will depend on the specific type of pretreatment process, but it
will usually be in the ammonia or nitrate form. If the ammonium ion is the dominant nitrogen form, it will be adsorbed by the soil and eventually will be used in plant growth or it will be biologically oxidized to nitratenitrogen. Therefore, if nitrate-nitrogen is not applied initially, it may be biologically formed on or near the soil surface. Nitrate ions are not adsorbed by the soil and will eventually get into the ground water system if not utilized immediately by plants. Nitrate-nitrogen has been demonstrated to be the causative agent of methemoglobin in children. Ingested nitrate is biologically reduced to nitrite in the digestive tract. The nitrites are then adsorbed into the blood stream ultimately causing suffociation of the child by reducing the ability of the blood to carry oxygen.42 The U.S.P.H.S. Drinking Water Standards8 recommend a concentration of 10 mg per liter for nitrate-nitrogen; however, these values have been exceeded in a number of areas in ground water supplies.4 3 Nitrate-nitrogen levels exceeding 10 mg per liter may develop in areas where the purpose of the spray irrigation is ground water recharge and where the recharged ground water is cycled back through the water supply system. Studies at the Pennsylvania State University have demonstrated that, by growing and harvesting corn from the land under irrigation, up to 65 per cent of the applied nitrogen may be removed by the crop.3 More recently, Kardos reported that reed canarygrass is the most effective plant for removing nitrate-nitrogen.44
Conclusions The following conclusions relative to the health and hygiene effects of spray irrigation land disposal of wastewater can be drawn: 1. Many of the detrimental health and hygiene aspects of land disposal should be significantly reduced by proper wastewater pretreatment including secondary treatment, filtration, and complete disinfection. 2. By choosing a land disposal site that has from 5 to 10 feet of continuous fine soil, biological contamination of ground water can be avoided. 3. The probability of inhaling pathogenic aerosols near a spray irrigation site may be significant. 4. If ponding results in spray irrigation areas, mosquito breeding is enhanced. 5. In areas where land disposal is the first step in a water recycle program, total dissolved solids, sodium, and nitrate-nitrogen buildup in the ground water supply can be a problem.
Recommendations for Future Study There are, however, several areas where additional information is required in order to make more definitive and reliable statements relative to the health and hygiene aspects of spray irrigation. Areas requiring additional study include: 1. Determination of which type of spray distribution
system, nozzles, and associated operating pressures minimize the health hazard from biological aerosol formation. 2. Determination of residual chlorine levels and the method of chlorination required to achieve complete disinfection. Complete disinfection will be considered to be the elimination of all viable pathogenic microorganisms. 3. Determination of the viability of human pathogenic bacteria and virus in biological aerosols emitted from spray irrigation of domestic wastewater. 4. Determination of the infectivity rate for aerosol inhalation of viable Klebsiella pneumonia and other pathogenic bacteria or viruses. 5. Determination of the long range effects of spray irrigation on wild animal populations and incidence of diseases in the animals. 6. Determination of the effects of spray irrigation on the incidence of infection of mosquitoes and other disease vectors. 7. Development of a standardized procedure for evaluating the ability of: (1) soil systems to remove trace organics, (2) microorganisms to acclimate to particular trace organics for biological destruction. 8. Determination of the extent of heavy metals transport in various soils, their toxicity to vegetative cover, and the probability of their buildup in a ground water system. References 1. Sepp, E. The Use of Sewage for Irrigation-A Literature Review. Bureau of Sanitary Engineering, State of California, 1971. 2. Federal Water Quality Administration. Engineering Feasibility Demonstration Study for Muskegon County, Michigan, Wastewater Treatment-Irrigation System, 1970. 3. Parizek, R. R., Kardos, L. T., Sopper, W. E., Myers, E. A., Davis, D. E., Farrell, M. A., and Nesbitt, J. B. The Pennsylvania State University Studies No. 23, Waste Water Renovation and Conservation. Pennsylvania State,University, University Park, PA, 1967. 4. Anderson, D., Bishop, W., and Ludwig, H. Percolation of Citrus Wastes through Soil. Proceedings of the 21st Purdue Industrial Waste Conference, pp. 892-901, 1966. 5. Okum, D. A. New Directions for Wastewater Collection and Disposal. J. Water Pollut. Control Fed. 43:2171-2180, 1971. 6. Hajek, B. F. Chemical Interactions of Wastewater in a Soil Environment. J. Water Pollut. Control Fed. 41:1775-1786, 1969. 7. Foster, H. B., and Jopling, W. F. Rationale of Standards for Use of Reclaimed Water. J. Sanit. Eng. Div. Proc. Am. Soc. Civil Eng. 95, SA3:503-514,
1969. 8. United States Public Health Service Drinking Water Standards. U.S. Department of Health, Education, and Welfare, Washington, DC, revised 1962. 9. Arizona State Department of Health, Draft-Rules and Regulations for Reclaimed Waters. Division of Water Pollution Control, Environmental Science Services, Phoenix, AZ, 1971. 10. Rudolfs, W., Falk, L., and Ragotzkie, R. Contamination of Vegetables Grown in Polluted Soil. I. Bacterial Contamination. Sewage Ind. Wastes 23:253-268, 1951. SPRAY IRRIGATION
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11. Kabler, P. Removal of Pathogenic Microorganisms by Sewage Treatment Processes. Sewage Ind. Wastes 31:1373-1382, 1959. 12. Clarke, N., Berg, G., Kabler, P., and Chang, S. Advances in Water Pollution Research, Vol. II. Macmillan, New York, 1964. 13. Sproul, O., Larochelle, L., Wentworth, D., and Thorup, R. Viruses Removal in Water Reuse Treating Processes. Water Reuse Chem. Eng. Progr. Symp. Ser. 63, 78:130-136, 1967. 14. Grinstein, S., Melnick, J., and Wallis, C. Virus Isolations from Sewage and from a Stream Receiving Effluent of Sewage Treatment Plants. Bull. 42:291-296, 1970. 15. Lund, E., and Hedstrom, C-E. Recovery of Viruses from a Sewage Treatment Plant. Symposium held at R. A. Taft Sanitary Engineering Center, Transmission of Viruses by the Water Route. Interscience Publishers, New York, 1965. 16. Kruse, C., Hsu, Y. C., Griffiths, A., and Stringer, R. Halogen Action on Bacteria, Viruses, and Protozoa. Proceedings of the National Specialty Conference on Disinfection, University of Massachusetts, Amherst, MA, 1970. 17. Stumm, W. Ozone as a Disinfectant for Water and Sewage. Boston Soc. Civil Eng. J. 45:68-69, 1958. 18. Butler, R. G., Orlob, G. T., and McGauhey, P. H. Underground Movement of Bacterial and Chemical Pollutants. J. Am. Water Works Assoc. 46:97-111, 1954. 19. Bouwer, H. Ground Water Recharge Design for Renovating Waste Water. J. Sanit. Eng. Div. Proc. Am. Soc. Civil Eng. 96, SAl:59-74, 1970. 20. Drewry, W., and Eliassen, R. Virus Movement in Groundwater. J. Water Pollut. Control Fed. 40:(Part 2)R257-R271, 1968. 21. Magill, P., Holden, F., and Ackley, C. Air Pollution Handbook. McGraw-Hill, New York, 1956. 22. Ledbetter, J. Air Pollution from Aerobic Waste Treatment. Water Sewage Works 111:62-63, 1964. 23. Glaser, J., and Ledbetter, J. Sizes and Numbers of Aerosols Generated by Activated Sludge Aeration. Water Sewage Works 114:219-221, 1967. 24. Ledbetter, J., and Randall, C. Bacterial Emissions from Activated Sludge Units. Ind. Med. Surg. 34:130-133, 1965. 25. Napolitano, P., and Rowe, D. Microbial Content of Air Near Sewage treatment Plants. Water Sewage Works 113:480-483, 1966. 26. Adams, A. P., and Spendlove, J. C. Coliform Aerosols Emitted by Sewage treatment Plants. Science 169:1218-1220, 1970. 27. Poon, C. Viability of Long-Storaged Airborne Bacterial Aerosols. J. Sanit. Eng. Div. Proc. Am. Soc. Civil Eng. 94, SA6:1137-1146, 1968. 28. Randall, C. W., and Ledbetter, J. 0. Bacterial Air Pollution from Activated Sludge Units. Am. Ind. Hyg. Assoc. J. 27:506-519, 1966.
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29. Poon, C. Studies on the Instantaneous Death of Airborne Escherichia Coli. Am. J. Epidemiol. 84:1-9, 1966. 30. Brown, J., Cook, K., Ney, F., and Hatch, T. Influence of Particle Size upon the Retention of Particulate Matter in the Human Lung. Am. J. Public Health 40:450-458, 1950. 31. Fuchs, N. A. The Mechanics of Aerosols, Revised Ed. Pergamon Press, New York, 1964. 32. Plotkin, S., and Katz, M. Minimal Infective Doses of Viruses for Man by the oral Route. Symposium held at the R. A. Taft Sanitary Engineering Center, Transmission of Viruses by the Water Route. Interscience Publishers, New York, 1965. 33. Glantz, P., and Jacks, T. Significance of Escherichia Coli Serotypes in Wastewater Effluent. J. Water Pollut. Control Fed. 39:1918-1921, 1967. 34. Bunch, R., and Ettinger, M. Water Quality Depreciation by Municipal Use. J. Water Pollut. Control Fed. 36:1411-1414, 1964. 35. Bishop, A., and Peterson, H. Characteristics and Pollution Problems of Irrigation Return Flow, p. 38. U.S. Department of the Interior, Federal Water Pollution Control Administration, Robert S. Kerr Water Research Center, Ada, OK, 1969. 36. McKee, J., and Wolf, H. Water Quality Criteria, Ed. 2. Resources Agency of California, State Water Quality Control Board, Publication No. 3-A, 1963. 37. Hinesly, T., and Sosewitz, G. Digested Sludge Disposal on Crop Land. J. Water Pollut. Control Fed. 41:822-839, 1969. 38. McCarty, P., and King, P. The Movement of Pesticides in Soils. Proceedings of the 21st Purdue Industrial Waste Conference, pp. 156-171, 1966. 39. Huang, J., and Liao, C. Adsorption of Pesticides by Clay Minerals. J. Sanit. Eng. Div. Am. Soc. Civil Eng. 96, SA5:1057-1078, 1970. 40. Lang, A., and Bruns, H. On the Pollution of Ground Water by Chemicals. J. Am. Water Works Assoc. (Abst.) 33:2075, 1941. 41. Narkis, N. A Literature Survey, The Fate of Nitrogenous Compounds through Sewage Treatment Plants. Soil and Aquatic Task Force, Center for the Biology of Natural Systems, Washington University, St. Louis, MO, 1970. 42. Gelperin, A. The Health Effects of Nitrates in Water. Nitrate and Water Supply: Source and Control. Proceedings of the 12th Sanitary Engineering Conference, University of Illinois, pp. 51-52, 1970. 43. Ward, P. C. Existing Levels of Nitrates in Waters-The California Situation. Nitrate and Water Supply: Source and Control. Proceedings of the 12th Sanitary Engineering Conference, University of Illinois, pp. 14-27, 1970. 44. Kardos, L. T. Using Cropland for Sewage Wastewater and Sludge Disposal. Notice of Research Project, Science Information Exchange, STE No. GY-5440-1, 1970.
and Hygiene Aspects Spray Irrigation
CHARLES A. SORBER, PhD, PE KURT J. GUTER, PhD
This study reviews the health and hygiene aspects of wastewater treatment or reclamation by spray irrigation.
Introduction Land disposal of domestic wastewater has 'been practiced on a large scale in Europe for several decades, mainly for the purpose of irrigating crops or pasture.' Recently, however, spray irrigation, one form of land disposal, has taken on new dimensions in this country as it is considered a means of disposal of wastewaters which heretofore have been discharged directly to streams or lakes. Small communities, mainly in arid regions, long have employed wastewater spray irrigation as a means of water conservation. More recently, this process of wastewater disposal has been recommended for use by larger metropolitan areas. Muskegon County, for example, has completed construction of a project which employs biological treatment, storage lagoons, disinfection, and spray irrigation for the disposal of the wastewater from the entire county.2 This system will initially serve a population of 170,000 but will have a design wastewater flow of 46 million gallons per day. The area to be irrigated will encompass approximately 6,000 acres. This paper is a review of the health and hygiene aspects of wastewater treatment or wastewater reclamation by spray irrigation. Although the discussion is limited to spray irrigation because of its current popularity among communities and sanitary engineers, many of the basic principles are applicable to the other methods of land disposal such as surface flooding and ridge and furrow irrigation. The health and hygiene aspects of wastewater treatment or wastewater reclamation by spray irrigation are Dr. Sorber is presently with the U.S. Army Medical Ft. Bioengineering Research and Development Laboratory,John Detrick, Frederick, Maryland, and Dr. Guter is with R. Snell Engineers, Inc., Lansing, Michigan.
dependent on a number of variables, the most important of which is the ultimate use of the contaminated wastewater. Possible uses for this water include: (1) discharge to a surface water body following percolation through soil permitting wastewater treatment to include nutrient removal through the filtering soil; (2) ground water reservoir recharge; (3) crop irrigation; (4) any combination of the first three. The degree of wastewater pretreatment before spray irrigation is intimately associated with the health and hygiene aspects of the problem. In fact, the effect of pretreatment on the physical, biological, and chemical characteristics of the wastewater will dictate to a large degree the ultimate use of the wastewater.
Physical Considerations Particular attention must be directed to the removal of suspended material which will ensure that both the spray distribution system and the soil treatment system will not clog.3'4 Spray distribution systems may be designed in various ways. The type of distribution system employed (solid set, mechanically moved, or fully portable) will determine the types of spray nozzles and their spacing. The spacing and type of nozzles used will significantly influence the aerosol formation from these spray distribution systems. Spray nozzles are manufactured that operate over a pressure range of 5 to 130 psi with increased pressure generally proportional to increases in aerosol formation.* Equally important is the type of soil available at the selected disposal site. Recently, Okums pointed out that the wide variations in soil characteristics, speciflcally the nonisotropism and heterogeneous character of most soils, * Keller, J., irrigation consultant, Utah State University. Personal communication, 1971.
SPRAY IRRIGATION
47
make it mandatory that each site be intensively studied before a spray irrigation site is selected. Hajek,6 performing a detailed investigation of the interaction of soil and wastewater, reported that the clays are chemically the most important size fraction of soil. He continued by stating that soils generally exhibit only cationic exchange. If the soil exchange capacity will be used in the wastewater land disposal plan, specific tests with the wastewater to be treated and the soil to be used should be conducted to determine the exchange capacity of the soil for the particular waste to be treated.
Biological Considerations Standards for Land Disposal and Microorganisms Present
California, with the Wastewater Reclamation and Reuse Law of 1967, has established standards which relate to the practical applications of reclaimed wastewater.7 These standards vary from the use of primary effluent without additional treatment for the irrigation of orchard crops which are processed before consumption, to the use of "adequately disinfected filtered wastewater" for irrigation of fodder, landscape in public areas, and filling impoundments for recreation. The California Standards define primary effluent as effluent from a sewage treatment process which provides partial removal of sewage solids by physical methods so that the settleable solids are reduced to less than 1 ml per liter. "Adequately disinfected filtered wastewater" is defined as water that has been oxidized, coagulated or filtered, and chlorinated to the extent that the 7-day mean residual coliform count does not exceed 2.2 or 23.0 organisms per 100 ml (depending on the specific application). In another section of the California Standards, "disinfected wastewater" is defined as water "in which pathogenic organisms have been destroyed." Obviously, there is a technical conflict in these statements as complete destruction of pathogenic organisms and the presence of as many as 23 organisms per 100 ml is inconsistent. Irrespective of this minor distinction, wastewater treated as specified in the California Standards approaches drinking water quality from a bacteriological standpoint.8 Arizona also has proposed standards restricting the use of domestic wastewater for irrigation. Domestic wastewater will undergo secondary treatment and disinfection for irrigation of crops which are processed before consumption and tertiary treatment and disinfection for use on crops intended for direct human consumption.9 Evidence has been collected demonstrating high levels of coliform bacteria on the surface of vegetables irrigated with raw sewage,* while other researchers have found that, if sewage irrigation is stopped 1 month before harvest, raw fruit would not be a likely vector for transmission of human bacterial enteric disease.' 0 These conflicting data substantiate the current requirements for treatment and * Franco, R., University of Texas, El Paso. Personal communication, 1971.
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disinfection of wastewater before use for irrigation of vegetables or any other crop intended for direct human consumption. Health problems that can arise from the use of either inadequately disinfected or nondisinfected treated wastewater become apparent when the presence of pathogenic bacteria in wastewater is evaluated. In a review of the work in this area, it was found that many pathogenic microorganisms pass through an activated sludge treatment process even though, in general, they are greatly reduced in number.1 1 Microorganisms found include: the typhoid group, Vibrio cholerae, Shigella, Mycobacterium tuberculosis; Coxsackie virus, poliovirus (I and II), and ECHO virus; Schistosoma, Ascaris, hookworm, and Trichuris. Even after chlorination many of these microorganisms were detected in effluents. Estimates and field studies have indicated enteric virus levels of domestic wastewater to range from up to 7,000 viruses per liter for raw sewage to about 50 viruses per liter for chlorinated secondary effluent.1 2-1 5Current chlorination practices for disinfection of domestic wastewater employ coliform organisms as a measure of the effectiveness of disinfection. Kruse and co-workers1 6 have demonstrated that in the presence of ammonia or amino acids, a zero coliform reading does not necessarily mean a virus kill. Therefore, the effectiveness of disinfection, as measured by residual coliform bacteria, does not by any measure ensure the destruction of viable enteric viruses. It is clear from the literature, therefore, that chlorination as practiced does not provide complete disinfection of pathogenic bacteria or viruses. As a consequence, wastewater spraying, with resultant biological aerosol formation, could disperse many of the pathogenic microorganisms found in wastewater. Other methods of disinfection, including the use of elemental iodine, bromine, or ozone, have been practicedon a limited scale. Of these methods, ozone is considered to be one of the leading materials which could replace chlorine in water or wastewater disinfection. Stumm,' 7 reporting on work done in Europe, indicates that the contact time necessary for 99 per cent destruction of Escherichia coli is 7 times smaller for ozone than with the same amount of hypochlorous acid. Indications are that the constituents of sewage cause a significant ozone consumption. A logical conclusion, therefore, is that there will be considerable interference in ozone disinfection of sewage and that suspended matter will protect embedded organisms against destruction. At present there are insufficient quantitative data in the literature to evaluate the effectiveness of ozone disinfection of domestic wastewaters. On the other hand, ozone is one of the most potent germicides considered for large scale disinfection and merits more research in the area of wastewater disinfection.
Surface and Ground Water Contamination If the applied wastewater must pass through 5 to 10 feet of continuous strata of fine soil before entering the ground water system, microbiological contamination of ground water should not occur under normal conditions. 8 Research has demonstrated that coliform penetration is
more a function of the soil type than of the hydraulic inflltration rate. It has also been demonstrated that once bacteria enter the ground water aquifer, they could travel only a few hundred feet horizontally.' 8,1 9 Exceptions to this statement include situations where coarse soils or solution channels are present. Under these conditions, bacteria and viruses may travel long distances underground. Viruses present in wastewater percolating through soil are primarily removed by adsorption. However, as pH increases the amphoteric protein viral shell will be more negatively charged, which will decrease the effective adsorption ability of a negatively charged soil.2 0 For example, in a soil from San Mateo County, California (50 per cent sand, 30 per cent silt, and 20 per cent clay by particle size distribution), the adsorption of the coliphage Ti was reduced by almost 40 per cent when the pH was increased from 6.8 to 8.0. Drewry and Eliassen suggest that insurance for virus removal can be achieved by following the normal public health practice of placing wells at least 100 feet from land disposal sites. Surface water supplies may be contaminated by overland flow of the sprayed wastewater, although this situation could be avoided by proper design of the spray system. Such a system design must prevent direct runoff to surface waters.
Contamination of the Air Aerosols have been defined as particles in the size range of 0.01 to 50 ,u suspended in air.21 Several studies regarding the emission of biological aerosols by spray irrigation of settled domestic wastewater have been described by Sepp.' On the other hand, no specific studies of biological aerosols emitted by spray irrigation of secondary wastewater effluents have been found in the literature. However, some preliminary work at the University of Utah* has demonstrated that the spraying of chlorinated effluent for landscape watering resulted in approximately 1.5 times the number of viable biological aerosols emitted by a trickling filter. Specific quantification of the work is difficult because of lack of experimental control, but it does demonstrate that spray irrigation of chlorinated effluent produces biological aerosols of the same order of magnitude as nonchlorinated wastewater applied to trickling filters. Investigations have been conducted on biological aerosols from trickling filter and activated sludge wastewater treatment plants.2 2-26 In general, bacterial aerosols remain viable and travel farther with increased wind velocity, increased relative humidity, lower temperature, and darkness.
Rapid desiccation of bacteria in flight appears to be a primary cause of bacterial die-off in biological aerosols.2 7 Studies of evaporation rates showed that a 50-,u water droplet will evaporate in 0.31 sec in air with 50 per cent relative humidity and a temperature of 220 C.23 These high rates of evaporation will result in nuclei of solid waste that * Goff, G. D., environmental microbiologist, University of Utah. Personal communication, 1971.
were originally dissolved or suspended in the biological aerosols.22 Atmospheric bacterial die-off is geometric in nature with the majority of the organisms dying within 3 sec. The remaining resistant bacteria, protected by chemical additives which inhibit evaporation, continue to die at a decreasing rate with time.2 6-2 8 This observation has a great effect on selection of the method of analysis and the specific bacteria that should be analyzed for in determining the pathogenesis of biological aerosols. Much of the reported work with biological aerosols has centered on the study of coliform bacteria, which have been the traditional indicators of domestic fecal water pollution. E. coli, probably the most frequently observed coliform bacteria in wastewater environments, generally have been shown to have an extremely short life span in aerosol form.29 Specific coliform bacteria of the genus Klebsiella, on the other hand, form a large capsule which apparently protects the organism from desiccation in flight.28 The most significant aspect of this observation is that all species of this genus are known pathogens of the respiratory tract. This differentiation demonstrates the need for specific testing for Klebsiella or other specific pathogens for biolgical aerosol wastewater studies in lieu of the more traditional general coliform group. A direct means of human infection by biological aerosols is by inhalation of these aerosols. The infectivity of an aerosol is further dependent on the depth of respiratory penetration. Biological aerosols in the 2- to 5-, size range are primarily captured in the upper respiratory tract. These particles are removed by cilia action and may pass into the digestive tract through the pharynx.26 If gastrointestinal pathogens are present in these aerosols, infection may result. However, a much higher incidence of infection results when respiratory pathogens are inhaled into the alveoli of the lung. The greatest alveolar deposition occurs in the 1- to 2-, range and then decreases to a minimum at approximately 0.25 ,. Below 0.25 ,u, alveolar deposition again increases due to Brownian movement.30 For comparative purposes, Fuchs3 1 observed that approximately 82 per cent of 1.0-,u particles, 28 per cent of particles in the 0.1- to 0.3-,u range, and 51 per cent of 0.03-, particles are deposited in the alveolar passages or the alveoli. This aspect of aerosol deposition becomes significant when consideration is given to viral size particles which range in size from about 0.01 to 0.1 ,u. Consideration must therefore be given to viral deposition with possible resultant infection. A summary of virus infections by the respiratory route demonstrates that extremely low relative doses of virus can cause infection in man.32 The minimal infective doses (TCD5o ) cited ranged from S1.0 to .790.0. Viruses included in these studies were influenza virus, coxsackie viruses (A21 and B4), adenovirus, rhinovirus, measles, and respiratory syncytial virus. Other workers have studied the inhalation of bacteria and determined that adjacent to an activated sludge aeration plant approximately 40 per cent of the biological aerosols will penetrate the lungs and approximately 6 per cent will penetrate the alveoli.28 These percentages increased to 60 and 13 per cent, respectively, 20 feet SPRAY IRRIGATION
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downwind from the tank. Based on these figures they calculated that a man working within 5 feet on the downwind edge of an aeration tank would inhale a viable Klebsiella every two breaths. Wild Animals, Birds, and Mosquitoes Considerable interest is developing in the effect of spray irrigation on changes in both the population and the disease incidence among wild animals, birds, and mosquitoes. In a 3-year study by Parizek and co-workers,3 it was concluded that there were no measurable changes in population of -small animals and birds in areas subjected to spray irrigation. These workers, however, felt that the data served only as a basis for measuring future changes as the spray irrigation had not altered the vegetation in areas large enough to induce differences in bird and mammal populations. In a different but related study, E. coli serotypes were tracked from the wastewater before spraying, through the spray irrigation area, and ultimately to wild birds.33 It was concluded that the chlorinated effluent had no effect on the presence of the coliforn flora of the birds. Parizek and co-workers3 demonstrated that surface ponding developed on the spray irrigation site and that these ponds result in increased mosquito breeding. Since the mosquitoes will be breeding in wastewater, a tremendous potential for arthropod-bome disease transmission is present. These workers anticipated an increase in avian blood parasites due to the increase in numbers of mosquitoes. Through blood smears of birds on the irrigation site, a significant increase in Leucocytozoon was observed between the second and third year of the study. There are still conspicuous voids in the literature in this general area. The Institute for Research on Land and Water Resources of The Pennsylvania State University has proposed the study of faunal response to spray irrigation of chlorinated sewage effluent. The study, if initiated, will do much to provide answers relative to the long range effects of spray irrigation on population changes and disease in wild animals as well as the possibility of increasing the infection of other known vectors of disease.
Chemical Considerations The health and hygiene aspects of spray irrigation must also include a consideration of organic and inorganic chemical movement in the soil. This consideration could be extremely broad in scope if industrial wastewaters are considered for land disposal by spray irrigation either separately or combined with domestic wastewaters. Dissolved chemical substances normally found in water have been reported to generally increase inconcentration when used by man. Water of five Ohio municipalities in one cycle of water use (i.e., raw water source to secondary waste treatment plant effluent) experienced an average increase in total dissolved solids (TDS) concentration of 50
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approximately 300 mg per liter.34 This water, when treated by land disposal techniques, may experience larger increases in TDS level if passed through a soil filter system. An extreme example of this would be the Imperial California Irrigation District, where inflow irrigation water had a TDS concentration of 13.14 mEq per liter (1,242 mg per liter) and the outflow contained 45.97 mEq per liter (3,844 mg per liter).35 Therefore, when spray irrigation results in recharge of ground water sources used for human consumption, TDS concentrations may exceed the levels recommended in the U.S,P.H.S. Drinking Water Standards.8 More important than the esthetically objectionable TDS level, the associated high sodium content may be harmful to persons suffering with cardiac, renal, and circulatory disease.36 Heavy Metal and Trace Organic Compounds Heavy metals or toxic organic compounds are two groups of materials that would require special consideration for wastewater disposal by spray irrigation. Heavy metals, including chromium, copper, lead, manganese, and zinc, have increased in concentration in soil where digested sludge was being wasted.37 Of these metals, zinc and manganese have increased in concentration in the leachates. The mechanism for heavy metal ion removal in the soil system has not yet been well defined. McCarty and King studied the movement of pesticides in soils and have concluded, as have other researchers, that organic phosphorous pesticide removal by the soil can be correlated with the clay content of the oil.38,39 Microbiological acclimation to all of the applied pesticides occurred with the exception of ethion. This acclimation will result in destruction of some of the absorbed pesticide. The movement of dissolved chemicals with percolating water is primarily dependent on the nature of the filtering soil. There have been many instances, especially in Germany, of ground water contamination by soluble industrial wastes.40 In one case, picric acid traveled several miles and caused abandonment of ground water supply. Chemical elements are primarily removed in soil media by the process of ion exchange. Therefore, the ion exchange capacity of soil is proportional to its chemical clarification ability and clay soils, because of their large surface area, have the largest exchange capacities. Cationic exchange, one possible mechanism for the removal of heavy metal ions and ionized organic compounds, will occur until the exchange capacity of the soil is exceeded. If cation removal is the objective of a land disposal system, consideration must be given to the ion exchange exhaustion of the soil and the possible toxicity to vegetation cover. Nitrogen Compounds
Biologically treated domestic wastewater will contain from 5 to 30 mg per liter of total nitrogen. The primary source of this nitrogen is the nitrogen metabolism of man.4 ' The dominant form that the nitrogen assumes will depend on the specific type of pretreatment process, but it
will usually be in the ammonia or nitrate form. If the ammonium ion is the dominant nitrogen form, it will be adsorbed by the soil and eventually will be used in plant growth or it will be biologically oxidized to nitratenitrogen. Therefore, if nitrate-nitrogen is not applied initially, it may be biologically formed on or near the soil surface. Nitrate ions are not adsorbed by the soil and will eventually get into the ground water system if not utilized immediately by plants. Nitrate-nitrogen has been demonstrated to be the causative agent of methemoglobin in children. Ingested nitrate is biologically reduced to nitrite in the digestive tract. The nitrites are then adsorbed into the blood stream ultimately causing suffociation of the child by reducing the ability of the blood to carry oxygen.42 The U.S.P.H.S. Drinking Water Standards8 recommend a concentration of 10 mg per liter for nitrate-nitrogen; however, these values have been exceeded in a number of areas in ground water supplies.4 3 Nitrate-nitrogen levels exceeding 10 mg per liter may develop in areas where the purpose of the spray irrigation is ground water recharge and where the recharged ground water is cycled back through the water supply system. Studies at the Pennsylvania State University have demonstrated that, by growing and harvesting corn from the land under irrigation, up to 65 per cent of the applied nitrogen may be removed by the crop.3 More recently, Kardos reported that reed canarygrass is the most effective plant for removing nitrate-nitrogen.44
Conclusions The following conclusions relative to the health and hygiene effects of spray irrigation land disposal of wastewater can be drawn: 1. Many of the detrimental health and hygiene aspects of land disposal should be significantly reduced by proper wastewater pretreatment including secondary treatment, filtration, and complete disinfection. 2. By choosing a land disposal site that has from 5 to 10 feet of continuous fine soil, biological contamination of ground water can be avoided. 3. The probability of inhaling pathogenic aerosols near a spray irrigation site may be significant. 4. If ponding results in spray irrigation areas, mosquito breeding is enhanced. 5. In areas where land disposal is the first step in a water recycle program, total dissolved solids, sodium, and nitrate-nitrogen buildup in the ground water supply can be a problem.
Recommendations for Future Study There are, however, several areas where additional information is required in order to make more definitive and reliable statements relative to the health and hygiene aspects of spray irrigation. Areas requiring additional study include: 1. Determination of which type of spray distribution
system, nozzles, and associated operating pressures minimize the health hazard from biological aerosol formation. 2. Determination of residual chlorine levels and the method of chlorination required to achieve complete disinfection. Complete disinfection will be considered to be the elimination of all viable pathogenic microorganisms. 3. Determination of the viability of human pathogenic bacteria and virus in biological aerosols emitted from spray irrigation of domestic wastewater. 4. Determination of the infectivity rate for aerosol inhalation of viable Klebsiella pneumonia and other pathogenic bacteria or viruses. 5. Determination of the long range effects of spray irrigation on wild animal populations and incidence of diseases in the animals. 6. Determination of the effects of spray irrigation on the incidence of infection of mosquitoes and other disease vectors. 7. Development of a standardized procedure for evaluating the ability of: (1) soil systems to remove trace organics, (2) microorganisms to acclimate to particular trace organics for biological destruction. 8. Determination of the extent of heavy metals transport in various soils, their toxicity to vegetative cover, and the probability of their buildup in a ground water system. References 1. Sepp, E. The Use of Sewage for Irrigation-A Literature Review. Bureau of Sanitary Engineering, State of California, 1971. 2. Federal Water Quality Administration. Engineering Feasibility Demonstration Study for Muskegon County, Michigan, Wastewater Treatment-Irrigation System, 1970. 3. Parizek, R. R., Kardos, L. T., Sopper, W. E., Myers, E. A., Davis, D. E., Farrell, M. A., and Nesbitt, J. B. The Pennsylvania State University Studies No. 23, Waste Water Renovation and Conservation. Pennsylvania State,University, University Park, PA, 1967. 4. Anderson, D., Bishop, W., and Ludwig, H. Percolation of Citrus Wastes through Soil. Proceedings of the 21st Purdue Industrial Waste Conference, pp. 892-901, 1966. 5. Okum, D. A. New Directions for Wastewater Collection and Disposal. J. Water Pollut. Control Fed. 43:2171-2180, 1971. 6. Hajek, B. F. Chemical Interactions of Wastewater in a Soil Environment. J. Water Pollut. Control Fed. 41:1775-1786, 1969. 7. Foster, H. B., and Jopling, W. F. Rationale of Standards for Use of Reclaimed Water. J. Sanit. Eng. Div. Proc. Am. Soc. Civil Eng. 95, SA3:503-514,
1969. 8. United States Public Health Service Drinking Water Standards. U.S. Department of Health, Education, and Welfare, Washington, DC, revised 1962. 9. Arizona State Department of Health, Draft-Rules and Regulations for Reclaimed Waters. Division of Water Pollution Control, Environmental Science Services, Phoenix, AZ, 1971. 10. Rudolfs, W., Falk, L., and Ragotzkie, R. Contamination of Vegetables Grown in Polluted Soil. I. Bacterial Contamination. Sewage Ind. Wastes 23:253-268, 1951. SPRAY IRRIGATION
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11. Kabler, P. Removal of Pathogenic Microorganisms by Sewage Treatment Processes. Sewage Ind. Wastes 31:1373-1382, 1959. 12. Clarke, N., Berg, G., Kabler, P., and Chang, S. Advances in Water Pollution Research, Vol. II. Macmillan, New York, 1964. 13. Sproul, O., Larochelle, L., Wentworth, D., and Thorup, R. Viruses Removal in Water Reuse Treating Processes. Water Reuse Chem. Eng. Progr. Symp. Ser. 63, 78:130-136, 1967. 14. Grinstein, S., Melnick, J., and Wallis, C. Virus Isolations from Sewage and from a Stream Receiving Effluent of Sewage Treatment Plants. Bull. 42:291-296, 1970. 15. Lund, E., and Hedstrom, C-E. Recovery of Viruses from a Sewage Treatment Plant. Symposium held at R. A. Taft Sanitary Engineering Center, Transmission of Viruses by the Water Route. Interscience Publishers, New York, 1965. 16. Kruse, C., Hsu, Y. C., Griffiths, A., and Stringer, R. Halogen Action on Bacteria, Viruses, and Protozoa. Proceedings of the National Specialty Conference on Disinfection, University of Massachusetts, Amherst, MA, 1970. 17. Stumm, W. Ozone as a Disinfectant for Water and Sewage. Boston Soc. Civil Eng. J. 45:68-69, 1958. 18. Butler, R. G., Orlob, G. T., and McGauhey, P. H. Underground Movement of Bacterial and Chemical Pollutants. J. Am. Water Works Assoc. 46:97-111, 1954. 19. Bouwer, H. Ground Water Recharge Design for Renovating Waste Water. J. Sanit. Eng. Div. Proc. Am. Soc. Civil Eng. 96, SAl:59-74, 1970. 20. Drewry, W., and Eliassen, R. Virus Movement in Groundwater. J. Water Pollut. Control Fed. 40:(Part 2)R257-R271, 1968. 21. Magill, P., Holden, F., and Ackley, C. Air Pollution Handbook. McGraw-Hill, New York, 1956. 22. Ledbetter, J. Air Pollution from Aerobic Waste Treatment. Water Sewage Works 111:62-63, 1964. 23. Glaser, J., and Ledbetter, J. Sizes and Numbers of Aerosols Generated by Activated Sludge Aeration. Water Sewage Works 114:219-221, 1967. 24. Ledbetter, J., and Randall, C. Bacterial Emissions from Activated Sludge Units. Ind. Med. Surg. 34:130-133, 1965. 25. Napolitano, P., and Rowe, D. Microbial Content of Air Near Sewage treatment Plants. Water Sewage Works 113:480-483, 1966. 26. Adams, A. P., and Spendlove, J. C. Coliform Aerosols Emitted by Sewage treatment Plants. Science 169:1218-1220, 1970. 27. Poon, C. Viability of Long-Storaged Airborne Bacterial Aerosols. J. Sanit. Eng. Div. Proc. Am. Soc. Civil Eng. 94, SA6:1137-1146, 1968. 28. Randall, C. W., and Ledbetter, J. 0. Bacterial Air Pollution from Activated Sludge Units. Am. Ind. Hyg. Assoc. J. 27:506-519, 1966.
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29. Poon, C. Studies on the Instantaneous Death of Airborne Escherichia Coli. Am. J. Epidemiol. 84:1-9, 1966. 30. Brown, J., Cook, K., Ney, F., and Hatch, T. Influence of Particle Size upon the Retention of Particulate Matter in the Human Lung. Am. J. Public Health 40:450-458, 1950. 31. Fuchs, N. A. The Mechanics of Aerosols, Revised Ed. Pergamon Press, New York, 1964. 32. Plotkin, S., and Katz, M. Minimal Infective Doses of Viruses for Man by the oral Route. Symposium held at the R. A. Taft Sanitary Engineering Center, Transmission of Viruses by the Water Route. Interscience Publishers, New York, 1965. 33. Glantz, P., and Jacks, T. Significance of Escherichia Coli Serotypes in Wastewater Effluent. J. Water Pollut. Control Fed. 39:1918-1921, 1967. 34. Bunch, R., and Ettinger, M. Water Quality Depreciation by Municipal Use. J. Water Pollut. Control Fed. 36:1411-1414, 1964. 35. Bishop, A., and Peterson, H. Characteristics and Pollution Problems of Irrigation Return Flow, p. 38. U.S. Department of the Interior, Federal Water Pollution Control Administration, Robert S. Kerr Water Research Center, Ada, OK, 1969. 36. McKee, J., and Wolf, H. Water Quality Criteria, Ed. 2. Resources Agency of California, State Water Quality Control Board, Publication No. 3-A, 1963. 37. Hinesly, T., and Sosewitz, G. Digested Sludge Disposal on Crop Land. J. Water Pollut. Control Fed. 41:822-839, 1969. 38. McCarty, P., and King, P. The Movement of Pesticides in Soils. Proceedings of the 21st Purdue Industrial Waste Conference, pp. 156-171, 1966. 39. Huang, J., and Liao, C. Adsorption of Pesticides by Clay Minerals. J. Sanit. Eng. Div. Am. Soc. Civil Eng. 96, SA5:1057-1078, 1970. 40. Lang, A., and Bruns, H. On the Pollution of Ground Water by Chemicals. J. Am. Water Works Assoc. (Abst.) 33:2075, 1941. 41. Narkis, N. A Literature Survey, The Fate of Nitrogenous Compounds through Sewage Treatment Plants. Soil and Aquatic Task Force, Center for the Biology of Natural Systems, Washington University, St. Louis, MO, 1970. 42. Gelperin, A. The Health Effects of Nitrates in Water. Nitrate and Water Supply: Source and Control. Proceedings of the 12th Sanitary Engineering Conference, University of Illinois, pp. 51-52, 1970. 43. Ward, P. C. Existing Levels of Nitrates in Waters-The California Situation. Nitrate and Water Supply: Source and Control. Proceedings of the 12th Sanitary Engineering Conference, University of Illinois, pp. 14-27, 1970. 44. Kardos, L. T. Using Cropland for Sewage Wastewater and Sludge Disposal. Notice of Research Project, Science Information Exchange, STE No. GY-5440-1, 1970.
