Fate and Transport of Pathogens In Onsite Wastewater Treatment Systems.

Pathogens potentially present in domestic wastewater include bacteria, viruses, protozoa, and helminths (parasitic worms).  In onsite wastewater treatment systems (OWTS), several physical, chemical, and biological processes in a septic tank and drain field are relied upon to remove and/or inactivate wastewater-borne pathogens.  About 15 million US households get their drinking water from private groundwater wells that are likely located near OWTS drain fields.  We conducted a literature review to ascertain common pathogens found in domestic wastewater and their fate and transport in OWTS.  This indicated that the most common bacteria and viruses in wastewater are those that can cause the diseases gastroenteritis, dysentery, salmonella, cholera, typhoid, hepatitis, rotavirus, and poliovirus.  The most common protozoa and helmiths present in wastewater can cause giardia, cryptosporidium, and roundworm infections.  Because of their larger sizes, protozoa and helminth eggs usually settle out of wastewater in the septic tank, with reports of up to 99.9% removal.  However, bacteria and viruses are smaller, and are less likely to settle out in the septic tank.  Therefore, two soil processes (physical straining, adsorption) in the drain field are the most important means of bacteria and viruses removal to protect groundwater and well waters.  While OWTS are highly effective at removing most pathogens, this primarily depends on soil properties particularly soil texture especially when OWTS are sited in unsaturated soils and with proper setbacks from surface waters.  We present a discussion of soil properties that influence pathogen fate and transport in OWTS and a review of several case studies of instances when disease outbreaks were linked to poorly sited or poorly functioning OWTS, with recommendations for improving OWTS performance for public health protection.