An Installer’s Guide to Wastewater Pathogens

An Installer’s Guide to Wastewater Pathogens

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Removing pathogens is the most critical part of wastewater treatment. Pathogens are organisms that cause disease; they include viruses, protozoa and bacteria. Examples in wastewater include salmonella, Vibrio cholera, Entamoeba histolytica, and cryptosporidium, although almost all disease organisms could be present in wastewater.

Viruses are organisms too small to be seen by light microscopy. They are an obligate parasite, dependent on a host cell for its metabolic and reproductive needs. Pathogens may be found in wastewater generated anywhere in the house. Any human contact with water results in the potential to add pathogens to the environment. Because of their role in spreading disease, pathogens in wastewater make wastewater treatment a public health issue.

The removal of these organisms through the soil treatment process is the key design factor for systems. Since it is difficult to test directly for the presence of a large variety of pathogens, water is usually tested for coliforms and fecal streptococci instead. Sources of fecal contamination to surface waters include wastewater treatment plants, onsite septic systems, domestic and wild animal manure, and storm runoff.

Indicator organisms
Some of the microorganisms found in wastewater can cause disease, while others are harmless. It is nearly impossible to identify all the pathogenic organisms in wastewater. Since it is difficult, time-consuming, and expensive to test directly for the presence of a large variety of pathogens, water is usually tested for coliform bacteria. These coliforms are cultured in standard tests to indicate either contamination from sewage or the level of disinfection, and they are generally measured as number of colonies/100 mL or most probable number. Coliform bacteria are organisms that are present in the environment and in the feces of all warm-blooded animals and humans. Coliform bacteria will not likely cause illness. However, their presence in drinking water indicates that disease-causing organisms (pathogens) could be in the water system.

  1. Total coliforms are a group of bacteria that are widespread in nature. All members of the total coliform group can occur in human feces, but some can also be present in animal manure, soil, submerged wood, and in other places outside the human body.
  2. Fecal coliforms, a subset of total coliform bacteria, are more fecal-specific in origin. However, even this group contains a genus, Klebsiella, with species that are not necessarily fecal in origin. Klebsiella are commonly associated with textile, pulp, and paper mill wastes. Therefore, if these sources discharge to a body of water, monitoring more fecal and human-specific bacteria may be advised. FC is still the most common test for pathogens because it is a relatively easy and inexpensive test. Fecal coliform bacteria are easy to test for, and their presence is an indication that other pathogens, which are more difficult to isolate and identify, may also be present. An average most probable number for fecal coliform bacteria in septic tank effluent is 1 million cells per 100 milliliters.  
  1. E. coli is an example of one group of fecal coliform bacteria that is generally non-pathogenic. Most E. coli bacteria are harmless and found in great quantities in the intestines of people and warm-blooded animals. Some strains, however, can cause illness. The presence of E. coli in a drinking water sample almost always indicates recent fecal contamination, meaning there is a greater risk that pathogens are present. Some states have moved to using E.coli as the preferred indicator organism because of their known pathogenic effects, although it is important to note that most forms of E. coli are not pathogenic. The Environmental Protection Agency recommends E. coli as better indicators of health risk from water contact. E. coli outbreaks receive much media coverage. Most outbreaks have been caused by a specific strain of E. coli bacteria known as E. coli O157:H7.

Treatment for pathogens
The separation to the limiting condition is key to the removal of pathogens. The vertical separation is the vertical measurement of unsaturated soil or sand between the bottom of the distribution medium and the periodically saturated soil level or bedrock. For a septic system to properly treat wastewater, this zone of unsaturated soil must be present in order for beneficial bacteria and microbes in the soil to remove harmful bacteria and viruses from the wastewater. The periodically saturated soil level is commonly identified by the presence of redoximorphic features.

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation, and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to

This article is part of the series: An Installer's Guide to Wastewater Characteristics


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