Cesspools and Seepage Pits; Effective Wastewater Treatment and Cesspools


The presence of improperly treated sewage is a threat to public health and the environment. Human exposure to sewage has resulted in disease outbreaks, severe illnesses, and in some instances death from the bacteria, viruses and parasites contained in the waste. Wastewater disposal systems that do not adequately treat wastewater also negatively affect our lakes, rivers, and groundwater by potentially introducing sediment, nutrients, and chemicals that result in contamination. 

There is also a safety concern with many of these systems because the tank lids may collapse resulting in an unsafe environment for people, animals and infrastructure.

What is proper wastewater treatment?

In Minnesota, we are concerned about all of our water resources for both beneficial use and recreation. One way to minimize damaging our waters is to ensure effective wastewater treatment is achieved across the State.

Effective wastewater treatment is simply the removal of solids, nutrients, bacteria and viruses from the wastewater and the predictable acceptance of the treated waste into the natural environment. In the case of individual sewage treatment systems (ISTSs), this level of treatment and acceptance is fundamental to our ISTS design requirements.

Specifically what is required for this level of treatment has been researched for over 100 years and remains true today – three vertical feet of dry, well-aerated soil with a wastewater distribution network sized based on use (e.g. single family home, day care facility, etc.) and soil properties. Often times our older ISTSs placed too much importance on wastewater going away (i.e. disposal), without adequate understanding or concern for treatment.

There are two primary reasons why seepage pits and cesspools do not provide adequate wastewater treatment; size and depth. Sewage is discharged into a small diameter pit and causes the wastewater to disperse under saturated, anaerobic conditions, limiting soil treatment. The small size of these systems also increases the likelihood of sewage back-up into the dwelling and surfacing sewage. Many cesspools and seepage pits were intentionally sited with the bottom of the pit in groundwater, as the natural water movement carried the sewage away. Raw or partially treated sewage should never reach groundwater, as the impacts to an aquifer are similar to the damages in a ditch, stream, or lake. There have been numerous studies documenting contamination of ground and surface water from wastewater systems in contact with groundwater (Allen and Morrison, 1973; Anan'ev and Demin, 1971; Crane and Moore, 1983; Kligler, 1921; Vaisman, 1964).

Do I have system with a cesspool or a system with a seepage pit, drywell or leaching pit?

A cesspool is an underground tank with holes in the side and/or bottom through which wastewater is discharged. The wastewater seeps into the surrounding soil through the bottom and openings in the side of the pit. Some designs may have a septic tank prior to the leaky tank and if so, it is considered a seepage pit, drywell or leaching pit.

Figure 1- A cesspool discharges raw sewage into the environment

diagram of a cess pit.

Figure 2 - A seepage pit discharges partially treated septic tank effluent into the environment
diagram of a seepage pit.

When and how do we fix noncompliant seepage pits and cesspools?

In Minnesota, noncompliant seepage pits and cesspools must be replaced with a compliant system. The time period for upgrade is based on local public health and environmental priorities and varies from location to location. Be sure to check with your local governmental unit (LGU). A list of MN LGUs can be found under "SSTS Local Units of Government" at: www.pca.state.mn.us

The noncompliant cesspool or seepage pit must be properly abandoned to eliminate the safety hazard and impact to public health and the environment. A licensed designer must be hired to evaluate site and soil conditions to determine the proper replacement ISTS to treat and disperse the wastewater at the site. This design is reviewed by a LGU to ensure it meets Minnesota Rules, Chapter 7080 and any additional LGU requirements. A list of licensed septic professionals can be found by contacting your LGU or under "SSTS Business Licensing, Individual Certification, and Enforcement" at: www.pca.state.mn.us

Referenced Research:

Allen, M.J. and S.M. Morrison. 1973. "Bacterial movement through fractured bedrock." Ground Water 11 (2): 6-10. 

Anan'ev, N.I. and N.D. Demin. 1971. "On the spread of pollutants in subsurface waters." Hygiene and Sanitation 36 (8):292-294. 

Crane, S.R. and J.A. Moore. 1984. "Bacterial pollution of groundwater: A review." Water, Air, and Soil Pollution. 22 (1): 67-83.

Kligler, I.J .1921. "Investigation on soil pollution and the relation of the various types of privies to the spread of intestinal infections." p. 1- 75 in ( ed.) Monograph of the Rockefeller Institute for Medical Research. Vol. No.15 .The Rockefeller Institute for Medical Research, New York. 

Vaisman, Y .I. 1964. Hygiene and Sanitation. 29:21.