Irradiation as a Means to Minimize Public Health Risks from Sludge-Borne Pathogens

Irradiation as a Means to Minimize Public Health Risks from Sludge-Borne Pathogens Author(s): J. Gary Yeager and R. T. O'Brien Source: Journal (Water Pollution Control Federation), Vol. 55, No. 7 (Jul., 1983), pp. 977-983 Published by: Water Environment Federation Stable URL: http://www.jstor.org/stable/25042005 Accessed: 29/11/2009 14:52 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=wef. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Water Environment Federation is collaborating with JSTOR to digitize, preserve and extend access to Journal (Water Pollution Control Federation). http://www.jstor.org
Irradiation as a means to minimize public health risks from sludge-borne pathogens J. Gary Yeager, R. T. O'Brien The purpose of the Federal Water Pollution Control Act of 1972 (PL 92-500) and later amendments was to improve the quality of U. S. surface and groundwater supplies, which had been seriously degraded through the years by the disposal of wastes into this convenient me dium. Industry is a major source of toxic chemicals that are waste products of many commercial processes. An other major source of surface water contamination is the discharge of toxic chemicals and nutrients from the wastewater treatment plants that serve growing popu lations often concentrated in metropolitan areas. The implementation of the Clean Water act has forced industry and municipalities to provide additional purifying treatments so that the effluents discharged to receiving waters would have much lower levels of un desirable contaminants. These additional treatments have dramatically helped to clean up municipal and in dustrial effluents. However, this success has proved a mixed blessing. As effluents have become cleaner, the volume of treatment by-products has increased in direct proportion. These by-products, known as sludge, are made up of the particulate material removed from liquid waste as it progresses through various treatment pro cesses. About 8.1 M dry metric tons of sludge annually, and greatly increased amounts by the end of the decade, create special disposal problems. The same (and similar) environmental legislation has limited the options for managing the increasing volume of sludge. Ocean dumping has been severely curtailed, and its ultimate fate is under review. The energy costs and air pollution associated with the incineration of sludge generally make this disposal option undesirable. Increasing population pressures on land area and soaring transportation costs have decreased the attractiveness of landfilling. The Resource Conservation and Recovery Act (PL 94-580) and a general national interest in resource re cycling and conservation have prompted a new per spective toward sludge management options. Sludge has been shown to be an excellent source of macronutrients and trace minerals for soils as well as of organic matter that can improve the physical characteristics of soil.1 The use of nutrients in undigested sludge as a source of animal feed has also been demonstrated in forward looking research at New Mexico State University in Las Cruces.2 Using municipal wastewater sludge as a resource for the beneficial purposes described above may be limited by the presence of heavy metals and toxic organic chem icals. These substances are often concentrated in sludges that result from wastewater with a heavy industrial in put. An additional limitation on the use of sludge in agriculture or in areas of unrestricted public access is the potential presence of pathogenic enteric microor ganisms. Toxic chemicals are subject to removal or re duction by strict source control, a tactic that forms the basis for the current U.S. Environmental Protection Agency (EPA) Industrial Pretreatment Program. How ever, pathogenic microorganisms originating in the in testinal tracts of infected individuals are normal con stituents of sludge and cannot be removed in the same way. Bacterial, parasitic, and fungal pathogens are effectively eliminated by 1 Mrad of gamma radiation, but viruses are somewhat more resistant. Pathogenic microorganisms in sludge can be removed or inactivated only by the various unit processes in sludge treatment. If toxic chemical species are at ac ceptably low levels, the resource potential of wastewater sludge can be realized if the indigenous pathogens are greatly reduced or eliminated before using sludge on foodchain crops or in areas of high public use and un limited access. EPA has issued regulations (40 CFR 257) that specify acceptable sludge treatments and associated public access restrictions that not only protect public health but also allow the large resource potential of sludge to be realized. EPA is currently drafting addi July 1983 977
Yeager & O'Brien tional regulations concerning the sale and giveaway of municipal sludge destined for the "home gardener" market. As defined in the Appendix of 40 CFR 257, there are two categories of pathogen-reducing sludge treatments that have different restrictions for use and public access. In the first category are Processes to Significantly Reduce Pathogens (PSRP), which include aerobic digestion, air drying, anaerobic digestion, low-temperature compost ing, lime stabilization, or other techniques giving equiv alent pathogen reduction. If PSRP-treated sludge is ap plied to land, food crops that would likely contact the sludge cannot be grown for 18 months. Also, animals whose products are to be consumed by humans cannot graze for 1 month, and public access is restricted for at least 12 months. These restrictions are eliminated if the sludge is treated by the second category of treatments known as Processes to Further Reduce Pathogens (PFRP). These include gamma or beta irradiation to an absorbed dose of 1 Mrad, pasteurization, or methods giving equivalent pathogen reduction given after a PSRP treatment. A PSRP is required first because these treat ments do not stabilize sludge by reducing volatile solids. Other PFRPs that do not require a prior PSRP are high temperature composting, heat drying, or thermophilic aerobic digestion. The mission of the U. S. Department of Energy (DOE) Beneficial Uses Program is to identify and develop ways in which radioactive by-products of U. S. defense nu clear programs can be used to provide alternative so lutions to major national problems. About 7 years ago, after PL 92-500 was passed, scientists began work on a biological research and engineering program to deter mine the feasibility of using the isotope cesium-137, a nuclear fuel-cycle by-product, to disinfect municipal wastewater sludge. The researchers realized that vastly increased amounts of sludge, a resource too valuable to justify disposal, would be generated on implementation of the act. This project has resulted in numerous sci entific publications on the efficacy of pathogen reduction by gamma irradiation and the development of a sludge irradiation technology. A 7.3-metric tons/d (8-ton/day) sludge irradiator has been operated since 1979 at a site within SNL. This effort was an integral part of the de cision by EPA to make irradiation to a 1-Mrad dose an approved PFRP. PATHOGEN DESTRUCTION BY GAMMA IRRADIATION Research derived from radiation treatment of foods and from sterilization of heat labile medical materials has shown that the penetrating gamma radiation emitted by cesium-137 and certain other gamma emitting ra dioisotopes (primarily cobalt-60) is effective in destroy ing pathogenic and saprophytic microorganisms in a wide range of media from plastics and food to municipal wastewater sludge. Radiation destroys pathogen in sev eral ways. First, the energy in the photon of gamma radiation may interact directly with a sensitive site in the organ ism. This site is usually the deoxyribonucleic acid (DNA) that directs cellular reproduction and synthesis of cell components, rather than relatively radiation-resistant constituents such as protein and lipids. This type of ra diation damage is termed a "direct effect" of ionizing radiation.3 In addition, the energy in the gamma photon can be deposited in molecules in the medium causing the for mation of toxic products that subsequently damage the microorganism. This damage is termed an "indirect ef fect" of radiation. In dilute aqueous environments most radiation dam age is thought to be caused by "indirect effects." In the presence of high concentrations of organic substances, microbe damage is mainly attributed to "direct effects." In sludge, an extremely complex, concentrated organic environment, radiation inactivation of microorganisms by ionizing radiation is probably almost exclusively caused by direct effects. Recent research by Ward4 dem onstrated that in a complex, organic environment fa voring direct radiation effects, two-thirds of poliovirus inactivation with gamma radiation was caused by dam age to the ribonucleic acid (RNA) genome of the virus. One-third of the damage was caused by alterations of the protein coat of the virus. Under conditions pro moting indirect effects, about one-fourth of the virus inactivation was caused by RNA damage. Three-fourths was attributed to alterations in the protein coat. The sensitivity of a microorganism to ionizing radia tion is described by its Di0 value (the absorbed radiation dose required to reduce a population of microbes by one order of magnitude or 90%). An organism with a high D10 value is resistant to radiation. A low D10 value in dicates sensitivity to radiation. PATHOGENS IN MUNICIPAL SLUDGE AND THEIR INACTIVATION BY IONIZING RADIATION Enteric pathogens are discharged to municipal waste water in the feces of infected individuals who may or may not exhibit signs of disease. These pathogens be come associated with sludge during wastewater treat ment because they remain in their original solids-bound state, or because suspended pathogens have an affinity for the particulate fraction of wastewater. The concen tration of a given enteric pathogen in sludge varies with the disease morbidity in the area served by the treatment plant. Some pathogens are found in sludge at fairly con stant levels throughout the year. Others vary on a sea sonal basis. 978 Journal WPCF, Volume 55, Number 7
_Process Research Table 1?Typical bacterial pathogens found in waste water sludge. Organism Disease Salmonella sp. Gastroenteritis, enteric fever Shi gel la sp. Gastroenteritis Escherichia coli Gastroenteritis Mycobacterium sp. Tuberculosis and enteritis Leptospira sp. Leptospirosis Bacterial pathogens. Bacterial pathogens, including the most extensively studied Salmonella sp., enter a wastewater treatment plant and become associated with sludge during primary clarification. During this sedi mentation process about 80 to 90% of salmonellae (and presumably other bacterial pathogens) become asso ciated with the settled sludge.5 A recent study detected 2400 salmonellae/kg of raw sludge.6 A concentration of 5000 colony-forming units (cfu)/L of raw wastewater is a commonly used average.7 Table 1 lists some typical enteric bacterial pathogens that may be found in sludge depending on local morbidity. Because irradiation of sludge must be preceded by a PSRP treatment, pathogen populations will be reduced by the combined effects of irradiation and the PSRP process. Two commonly used PSRPs that would prob ably precede gamma irradiation in a typical sludge-treat ment train are anaerobic digestion and sand-bed drying. Anaerobic digestion generally reduces bacterial patho gens by one to two orders of magnitude.7 Bacterial reduction during evaporative drying has not been studied widely, but recent studies (Figure 1) dem onstrated that evaporative drying to about 95% solids reduced seeded populations of Salmonella typhimurium more than one order of magnitude.8 Figure 1 shows that the reduction in Salmonella typhimurium and other en teric organisms studied was generally proportional to the reduction in the water content of the sludge. In sludge dried to over 95% solids (a level achievable in the arid Southwest), reductions ranged from one-half order of magnitude to about four logs with Proteus mirabilis. Figure 2 shows that similar results were obtained when various classes of indigenous microflora were observed during drying.8 Mechanical sludge dewatering (for ex ample, centrifugation, belt press) generally gives a solids content of about 20% and would not be expected to reduce significantly bacterial pathogens. After an anaerobically digested sludge is dewatered, the population of the gram-negative enteric pathogens of greatest concern (Salmonella sp., Shigella sp., and enterotoxigenic Escherichia coli) would be reduced by two to three orders of magnitude. The asporogenic, gram-negative bacterial pathogens are generally very sensitive to inactivation by ionizing radiation. When the radiation sensitivities of the gram 10' ,0L 10 Iff1 Iff2' Iff3' S.FAECALIS ?LCOU oKLEBSEUASP. a BfTBW?ACTBISP 20 40 60 % Solids 100 Figure 1 ?Reduction of Salmonella typhimurium and other enteric microorganisms as a result of drying of sludge. negative enteric bacteria used in the previous study were determined, the D10 values in 50% solids sludge averaged 50 krad.8 Table 2 summarizes the observed D10 values % solids Figure 2?Reduction of indigenous microflora as a result of drying of sludge. July 1983 979
Yeager & O'Brien Table 2?Inactivation of seeded enteric bacteria in raw sludge by gamma radiation.8 D10 value (krad) Sludge moisture Salmonella Proteus Streptococcus (% Solids) E. co// Klebsiella sp. Enterobacter sp. typhimurium mirabilis faecalis Liquid (5%) 22 48 36 <54 <24 130 Dewatered (42 to 57%) 22 76 42 <60 <22 130 Dried (94 to 95%) 22 41 50 120 <50 160 a Pure cultures of bacteria were grown to saturation density in sterilized sludge. The sludge was dried by evaporation, and samples taken at the indicated moisture levels were irradiated. for the six enteric bacteria in liquid sludge (5% solids), moderately dry sludge (about 50% solids), and air-dried sludge (about 95% solids). Table 2 shows that the radiation resistance of the E. coli used in this study was not altered by evaporative dewatering. The Klebsiella sp. was protected in partially dewatered but not fully dewatered sludge, and the En tewbacter sp. was slightly protected against radiation by dewatering. The protective effect of drying noted with Enterobac ter sp. was more pronounced with S. typhimurium, where the D10 value observed in liquid sludge more than doubled in dried sludge. P. mirabilis, the other hydrogen sulfide (H2S) producer studied, was extremely sensitive to ionizing radiation at all moisture levels. Finally, S. faecalis, a nonpathogenic indicator bacterium, was found to be the most radiation resistant of the six enteric bacteria studied at all moisture levels, and was slightly protected in dewatered sludge. S. faecalis and other fecal streptococci have been consistently shown to be more resistant to radiation than most bacteria associated with wastewater.9 Similar results were seen when the classes of indige nous sludge organisms described in Figure 2 were irra Table 3?Inactivation of several classes of indigenous sludge bacteria by gamma radiation.8 D10 value (krad) Sludge - moisture Lactose H2S Non-lactose (% Solids) S. faecalis fermenters producers fermenters Liquid (3.5) 177 30 60 <40 Dewatered (44) 160 30 40 <30 Dried (95) 95 50 <85 <60 a Raw sludge was dried by evaporation. Samples taken at the indicated moisture levels were irradiated and assayed for the various classes of organisms. diated under the same conditions. The results in Table 3 indicate that the lactose-fermenting bacteria recovered on Hektoen enteric agar had D10 values of less than 50 krad at the three moisture levels. This category of or ganisms includes E. coli, Klebsiella sp., and Enterobacter sp., which were used in the previous experiment with seeded organisms. H2S producers, a category that usually includes the salmonellae, were present at low initial numbers, and the D10 values were similar to those noted for seeded S. typhimurium. The indigenous S. faecalis also had radiation sensitivities similar to those observed in the seeded experiments. However, no protective effect was noted in the 95% solids samples. These data generally agree with earlier studies.10,11 Extrapolation of these results to full-scale operations suggests that bacterial pathogens in sludge would be re duced to less than detectable levels by the combination of a PSRP and the 1-Mrad PFRP dose of gamma ra diation. Parasite pathogens. The ova and cysts of protozoan and helminthic parasites sometimes found in wastewater are generally resistant to inactivation by physical and chemical processes.12 These parasites are not common to all U.S. sludges, but they may be found where local morbidity is sufficient to provide a source to the waste water treatment system. The resistant ova and cysts are far heavier than bacteria or viruses, and almost all will settle into sludge during primary clarification. These ova and cysts must be removed or killed by later sludge treat ments if sludge is to be used without risk to individuals exposed to it. Some human and animal parasites that might be found in wastewater sludge are shown in Table 4. The cysts of the protozoan parasites Giardia lamblia and Entamoeba histolytica are destroyed by anaerobic digestion, but the resistant stages of the other parasites in Table 4 are not and must be eliminated by a PFRP treatment before unrestricted use of sludge. The highly resistant ova of Ascaris have been used in most studies of parasite inactivation in sludge. These ova are resistant to chemicals such as chlorine, and are 980 Journal WPCF, Volume 55, Number 7
_ Process Research Table 4?Parasites of medical or veterinary importance that may be found in wastewater sludge. Parasite Disease Ascaris sp. Ascariasis Trichuris sp. Whipworm infestation Toxocara sp. Roundworm infestation Taenia sp. Taeniasis Echinococcus sp. Hydatid disease Entamoeba histolytica Amoebic dysentery Giardia lamblia Dysentery undamaged by physical stresses such as pH extremes and moderate temperatures. Ascaris ova are generally found in higher concentrations in sludge than are the resistant stages of other parasites.12 Ascaris ova are un touched by anaerobic digestion, but recent research in dicates that the ova may be inactivated when sludge is dewatered to moisture levels around 20%.12 Research indicated that Ascaris ova are sensitive to gamma radiation with D10 values in liquid and com posted sludge of about 50 krad.13 Because of recent con cern that ova removed from the uterus of adult ascarids may be more sensitive to irradiation than are ova hard ened by transit through the intestines, a double-blind study was initiated in which three types of Ascaris ova were seeded into liquid sludge: Ova recovered from pig feces; Ova recovered from pig feces with the outer layer removed by chemical treatment; and Ova recovered from adult female Ascaris and treated to remove the outer layer. The sludge containing the added ova was irradiated to several dose levels determined for ovum viability. Results of these experiments (Table 5) show that the D10 values for the three types of ova are identical and agree with earlier results.13 These and previous studies indicate that the 1-Mrad PFRP dose will eliminate the parasite hazard from mu nicipal sludges. Virus pathogens. Up to 1 X 106 enteric viruses/g are reportedly excreted in the feces of infected individuals, and over 1 X 105/L have been recovered from raw waste water.14 The actual number of indigenous, and poten tially pathogenic, viruses in wastewater sludge is difficult to determine because not all enteric viruses are recov ered or detected by any given procedure. This can occur because the viruses are incompletely recovered from their sludge-associated state, or because the selected tis sue-culture assay system does not support visible repli cation of the recovered viruses. A series of indigenous virus recovery experiments was recently done to help provide a standard method for virus monitoring at sites where full-scale irradiators will be constructed. A modification of a common virus Table 5?Inactivation of Ascaris Ova in sludge by gamma irradiation.8 D10 value Ovum source/treatment (krad) Pig feces/untreated 62 Pig feces/decoated 45 Ascaris uterus/decoated 45 a Intact ova from pig feces, decoated ova from pig feces, and decoated ova from female, adult ascarids were added to liquid sludge and irradi ated. Viability of ova was determined by observing the ova for embry onation. purification procedure was used. A sample of chilled, liquid raw sludge (generally 100 mL) was mixed in equal proportions with chilled Genetron {trichloro trifluoro ethane) and 10% (V/V) calf serum. The mixture was blended and centrifuged to separate the Genetron and aqueous phases. The aqueous phase containing viruses was saved, and the Genetron phase was re-extracted as before with phosphate buffered saline. The aqueous phases were pooled and ultracentrifuged to pellet the extracted viruses. The pellet was resuspended and treated with ether to reduce bacterial contamination. The final concentrate was then applied to monolayers of five different tissue culture cell lines commonly used to assay enteric viruses. Vero, HeLa, and RD (rhabdo sarcoma) cells are commonly used to recover entero viruses (for example, poliovirus) while L and MDBK cells are used to detect reoviruses. The results of these experiments are shown in Table 6. Under these exper imental conditions, HeLa and MDBK cells provided the greatest recovery of indigenous viruses from raw sludge. If it is assumed that the HeLa and MDBK detected dif ferent viral populations, the sludge used in these studies contained about 1.7 X 104 pfu of recoverable enteric viruses/L. This figure is consistent with results cited ear lier. Table 7 lists some enteric viruses that may be found in wastewater and sludge. Hepatitis A virus (the caus ative agent of infectious hepatitis) and rota viruses (which are responsible for high infant mortality and adult gas Table 6?Recovery of indigenous viruses from raw sludge with selected tissue culture lines.8 Enteric virus recovery Cell line (pfu/L) MDBK 1.3X 104 L 5 X 102 RD 7 X 102 HeLa 4.2 X 103 Vero 4 X 102 a Raw sludge from the Albuquerque Wastewater Treatment Plant was extracted with Genetron as described in the text. Duplicate monolayers of the various cell lines were infected, overlaid with soft agar, and in cubated at 37?C until stained and counted. July 1983 981
Yeager & O'Brien_ Table 7?Representative enteric viruses that may be found in wastewater sludge. Virus Disease Enteroviruses Gastroenteritis.meningitis, cardiac conditions, Poliovirus central-nervous-system involvement Coxsackievirus Rotavirus Gastroenteritis; infant diarrhea Hepatitis A virus Infectious hepatitis Adenovirus Conjunctivitis; respiratory infections Reovirus Respiratory infections troenteritis around the world) are of major concern. Because these viruses cannot be cultured in the labo ratory, most experiments with sludge use enteric viruses such as reovirus and poliovirus that are easily assayed in the laboratory. Because of the affinity of viruses for solids in waste water, it is expected that most will be associated with sludge during primary clarification. About 60% of waste water solids settle into primary sludge, and a corre sponding percentage of viruses are probably partitioned into the sludge. These enteric viruses have largely been physically re moved with the sludge rather than inactivated, and sub sequent sludge treatment processes must be relied on to reduce these pathogens to acceptable levels. Experiments with indigenous and seeded viruses in sludge have shown that anaerobic digestion removes about 90% of the de tectable viruses from sludge.15 Studies by Ward and Ashley16 have shown that polioviruses are irreversibly inactivated by the process, largely because of the am monia produced during anaerobic digestion. Other stud ies by Ward and Ashley17,18 have shown that detergents, which are normal constituents of sludge, can alter the sensitivity of viruses in sludge. Additional studies indi cated that air drying effectively reduces the population of enteric viruses in sludge.19 Recent studies using the indigenous virus extraction technique already described, confirmed that reduction Table 8?Recovery of indigenous viruses from liquid raw and digested sludge and from dried digested sludge.8 Enteric virus recovery (pfu/L) Sludge - (% solids) MDBK HeLa Liquid, raw (5) 8.7 X 104 8.2 X 103 Liquid, digested (5) 2.2 X 103 1.1 X 103 Dried, digested (95) 6 X 102 6 X 102 a Sludges from the Albuquerque Wastewater Treatment Plant were extracted and concentrated as described in the text. Dried sludges were adjusted to 5% solids before extraction. Samples were assayed as de scribed in the legend for Table 6. of viruses occurs during anaerobic digestion and evap orative drying to about 95% solids. Although the sludge samples were not temporally matched, Table 8 shows that anaerobic digestion reduced the indigenous virus content by about one order of magnitude for viruses detected on HeLa cells, and more than one order of magnitude for viruses detected on MDBK cells. In both cases, drying the digested sludge to 95% solids reduced the indigenous viruses below detectable limits. Because of their small size and relatively simple chem ical structure, viruses are more resistant to radiation than bacteria or parasites. Considerable research has been done on the inactivation of viruses in aqueous media, sludges, and food by ionizing radiation. Studies at Sandia have shown D10 values of 330 krad for enteroviruses in sludge.20 Studies on beta-ray inac tivation of enteric viruses in liquid sludge gave D10 val ues of from 300 to 400 krad.21,22 Experiments on the reduction of viruses in other materials have generally yielded D10 values of 150 to 500 krad. These combined results indicate that viruses in sludge would be reduced about three orders of magnitude by the 1-Mrad PFRP dose. Although this in itself is a sub stantial reduction, the radiation treatment coupled with a PSRP should ensure that viruses in sludge are reduced to below detectable limits and that any potential virus hazard in sludge is eliminated. Fungal pathogens. Little is known about the numbers and fate of pathogenic fungi during sludge treatment. One opportunistic fungal pathogen, Aspergillus fumi gatus, has been shown to be present in composting sludge. The airborne spores of this fungus, if inhaled, can cause respiratory disease in individuals whose health is already compromised. Earlier studies demonstrated that the D10 value for A. fumigatus spores in dried sludge ranged from 50 to 60 krad.23 This value is comparable to the Djo values for enteric bacteria and parasites. Thus, any fungal spores of A. fumigatus and other spores of similar radiation sensitivity would be effectively elimi nated by the EPA-prescribed 1-Mrad dose. SUMMARY Table 9 summarizes the previous discussions on path ogen inactivation by gamma irradiation. Bacterial, par Table 9?Summary of pathogen reduction in sludge with PFRP gamma irradiation. Pathogen type Radiation effectiveness Bacteria 1 -Mrad effective Parasites 1 -Mrad effective Viruses 1 -Mrad effective in conjunction with other virucidal effects in wastewater treatment Fungi 1 -Mrad effective for those studied 982 Journal WPCF, Volume 55, Number 7
_ Process Research asitic, and fungal pathogens should be eliminated from sludge by the 1-Mrad PFRP dose required by EPA reg ulations. While viruses are much more radiation resis tant than the other pathogen types, the 1-Mrad PFRP dose of gamma radiation given in conjunction with a required PSRP should effectively eliminate the health hazard posed by viral pathogens in sludge. ACKNOWLEDGMENTS Credits. This work was supported by the Division of Advanced Nuclear Systems and Projects, U. S. Depart ment of Energy, Washington, D. C, and Municipal Environmental Research Laboratory, U.S. Environ mental Protection Agency, Cincinnati, Ohio, Inter agency Agreement E (29-2)-3536/EPA-IAG-D6-0675. This paper was presented at the 54th Annual Conference of the Water Pollution Control Federation, Detroit, Mich. Authors. J. Gary Yeager is a member of technical staff, Environmental Research Division, Sandia Na tional Laboratories, Albuquerque, N. M. R. T. O'Brien is a professor, and chairman of the Department of Bi ology, New Mexico State University, Las Cruces. Our Correspondence should be addressed to J. Gary Yeager, Environmental Research Division 4774, Sandia Na tional Laboratories, Albuquerque, NM 87185. REFERENCES 1. McCaslin, B. D., et al, "Aspects of Land Application of Sewage Solids and Gamma-Irradiated Dried Sewage Solids on Calcareous Soils." In Proc. of a National Symposium on the Use of Cesium-137 to Process Sludge for Further Reduction of Pathogens, SAND80-2744, Sandia National Laboratories, Albuquerque, NM, 91 (1980). 2. Smith, G. S., et al, "Recycling Sewage Solids as Feedstuff's for Livestock." In Proc. of the Third National Conf. on Sludge Management Disposal and Utilization, Informa tion Transfer, Inc., Rockville, Md, 119 (1977). 3. Luria, S. E., and Exner, F. M., "The Inactivation of Bac teriophages by X-Rays-Influence of the Medium." Proc. Nat! Acad. Sei. U. S A., 27, 370 (1941). 4. Ward, R. 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