PATHOGENS IN RECLAIMED WATER
M. V. Yates
University of California Riverside
The need to conserve water has resulted in an increase in the use of treated sewage effluent, or reclaimed water, for many non-potable purposes. However, reclaimed water may contain potentially harmful contaminants with which the user must be familiar in order to minimize detrimental environmental or human health effects. The focus of this paper is on human pathogenic (disease-causing) microorganisms that may be present in reclaimed water.
Bacteria are microscopic organisms, ranging from approximately 0.2 to 10mm in length. They are distributed ubiquitously in nature and have a wide variety of nutritional requirements. Many types of harmless bacteria colonize the human intestinal tract, and are routinely shed in the feces. One group of intestinal bacteria, the coliform bacteria, has historically been used as an indication that an environment has been contaminated by human sewage. In addition, pathogenic bacteria, such as Salmonella and Shigella, are present in the feces of infected individuals. Thus, a wide variety of bacteria may be present in domestic wastewater (Table 1).
Table 1. Bacterial Pathogens in Wastewater
Viruses are obligate intracellular parasites; that is, they are incapable of replication outside of a host organism. They are very small, ranging in size from approximately 20 to 200 nm. Viruses that replicate in the intestinal tract of man are referred to as human enteric viruses. These viruses are shed in the fecal material of individuals who are infected either purposely (i.e., by vaccination) or inadvertently by consumption of contaminated food or water, swimming in contaminated water, or person to person contact with an infected individual. More than one hundred different enteric viruses may be excreted in human fecal material; as many as 106 plaque-forming units (pfu) of enteroviruses (a subgroup of the enteric viruses) per gram and 1010 rotaviruses per gram may be present in the feces of an infected individual. A few of the viruses that have been detected in wastewater are listed in Table 2.
Table 2. Viral Pathogens in Wastewater
A third group of microorganisms of concern in domestic sewage is the parasites (Table 3). In general, parasite cysts (the resting stage of the organism which is found in sewage) are larger than bacteria, although they can range in size from 2 j£m to over 60 pm. Parasites are present in the feces of infected persons; however, they may also be excreted by healthy carriers. Cysts are similar to viruses in that they do not reproduce in the environment, but are capable of surviving in the soil for months or even years, depending on environmental conditions.
Table 3. Parasitic Pathogens in Wastewater
Treatment of wastewater can effect from 50% to almost 100% pathogen removal, depending on the treatment processes used. A summary of average pathogen concentrations reported to be present after several stages of sewage treatment is presented in Table 4. It can be seen that even tertiary treatment (consisting of primary sedimentation, trickling filter/activated sludge, disinfection, coagulation, direct filtration, and chlorination) does not remove all pathogens. It is important to consider the infective dose of the organism in relation to the final concentration when assessing the potential public health risk associated with use of reclaimed water. It is relatively unlikely that the two Salmonella organisms would cause disease, considering that the infective dose is more than 1000 organisms. On the other hand, the final concentrations of viruses and Giardia are sufficiently high to cause several people to become ill if they ingested the water.
Table 4. Pathogen Removal by Wastewater Treatment
There are several ways in which an individual can acquire disease from wastewater use. Direct ingestion of the wastewater or aerosols created during spray irrigation may result in infection. In addition, infection may occur from ingestion of pathogens on contaminated vegetation or other surfaces. Another potential route of exposure is from the ingestion of ground water that has been contaminated by pathogens in irrigation water. Indeed, viruses have been detected in ground water located 27.5m below a site irrigating crops with reclaimed water.
In order for infection or disease to result from exposure to reclaimed wastewater, however, several conditions must be met. In addition to surviving the sewage treatment process (Table 4), the pathogens must also survive in the environment for a sufficient period of time to be exposed to a susceptible host. Table 5 lists the results of several studies that have investigated the length of time various pathogens could be detected in an infective form on the surface of several crops. In all cases, the experiments were performed by adding the pathogens to crops growing in the field.
Table 5. Pathogen Survival on Crops
The survival of one specific virus, poliovirus, was studied as a part of a five-year study on the feasibility of using reclaimed water for crop production in California. Results of this study are shown in Table 6. Caution must be taken in extrapolating these data to other pathogens, however, as poliovirus has been found to be a poor model for the behavior of other pathogens in the environment. For example, hepatitis A virus has been found to survive longer under other adverse environmental conditions than poliovirus; however, few if any studies of its survival on crops have been performed to date.
Table 6. Poliovirus Survival on Vegetables in situ
Generally, it has been found that bacteria are inactivated more rapidly than either viruses, and that parasites are able to survive for longer than viruses or bacteria. However, these are only generalizations; the actual length of survival of pathogens is very dependent of the pathogen in question, the type of crop, and the environmental conditions. For example, pathogen survival has been found to be longer on root crops and leafy vegetables than on other types of crops due to the protective environment. Temperature is one environmental conditions that has a major impact on the length of time many pathogens remain infective, with relatively shorter survival times observed at high temperatures.
Evidence supporting the spread of disease through irrigation with reclaimed water is scarce. Table 7 lists outbreaks known to be associated with reclaimed water irrigation. There have been other outbreaks associated with the consumption of contaminated fruits and vegetables, however, the source of the wastewater was not specified in those cases. In addition, there have undoubtedly been outbreaks that have gone unrecognized and unreported. The mildness of disease associated with many viral infections (i.e., short-lived gastroenteritis) which do not require medical intervention could result in an outbreak being unrecognized as a true outbreak.
Table 7. Disease Outbreaks Associated with Sewage-Contaminated Plants
In conclusion, reclaimed water is likely to contain pathogenic microorganisms. Depending on the treatment processes used, the pathogens may be present in high enough concentrations to pose a potential threat to human health. Adverse impacts can be minimized by careful management of the irrigation process to minimize public and worker exposure to the reclaimed water and aerosols. Choosing plant materials that will not be consumed (e.g., turf and landscape materials), or crops that will be processed prior to consumption will also minimize the potential for human exposure. The site should be assessed carefully to determine the potential for contamination of surface water by runoff or ground water by leaching, so that the possibility of waterborne transmission is minimized.
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