Bacteria, viruses, and protozoa cause many human and animal diseases. Bacteria cause cholera, shigellosis, salmonella, and typhoid. Giardia and cryptosporidium are protozoa that cause dysentery. Hepatitis is caused by a viral contaminant. The comments listed below give some information about how biological contaminants are removed by on-site systems.
Bacteria are removed primarily by filtration, adsorption, and natural die-off. The biomat provides a barrier to the transport of many bacteria into the soil.
If the soil is unsaturated, bacteria are not usually transported more than three feet. If, however, saturated flow occuI'$, bacteria can move further.
Saturated flow, high wastewater effluent loading rates, shallow depth to soil wetness conditions, or fractured bedrock contribute to bacterial contamination from on-site systems.
Viruses, which are much smaller than bacteria, are removed by adsorption, filtration, precipitation, biological enzyme attack, and natural die-off.
Greater clay content, low soil pH, low soil moisture content, and low effluent loading rates are important factors that decrease the possibility of viral contamination to the ground water from on-site systems.
Wastewater also contains other micro-organisms, such as protozoa, which can cause disease. Unfortunately, with the exception of bacteria and viruses, little is known about the behavior of pathogens in on-site systems and in the soil. Because there have been few reported outbreaks of disease caused by microbes other than bacteria and viruses from subsurface wastewater disposal systems, it appears that these biologic agents are retained in the soil, probably because of their relatively large size.
Protozoa form cysts that can survive under a wide range of conditions and are very resistant to disinfectants usually employed in drinking water treatment. Protozoan cells and cysts are generally much larger than bacteria, which may mean that they can be filtered by the soil. Filtration by the soil of protozoan cysts has been shown in the case of Giardia lamblia cysts, as reported by Yates in 1987.
From the North Carolina Onsite Guidance Manual