How A Septic System Works

A septic system is a method of onsite wastewater treatment and disposal. Different states identify septic systems by various names - New Jersey, for example, refers to a septic system as an Individual Subsurface Sewage Disposal Systems (ISSDS). Pennsylvania Code refers to septic systems as Onlot Systems. The individual states regulate septic systems in unique ways; however, regardless of the name, a modern septic system generally includes a septic tank to separate liquids from solids and a disposal system to allow the liquid fraction to percolate into the soil.

Prior to the advent of indoor plumbing most rural and semi-rural properties had an outhouse or privy. As running water became the norm the sewage from the home was collected and directed through a building sewer to a cesspool. This is the simplest type of onsite disposal system and is common for homes built prior to 1930, although cesspools were still constructed up through the 1950's and even into the 1960's. The cesspool system consists of a single pit that has been constructed out of field stone or concrete block. Openings between the stones or blocks laid on their sides allow liquid to flow out of the pit. Solids accumulate in the bottom of the cesspool.

Sewage from the home enters the cesspool through the building sewer. The liquid level in the cesspool rises when sewage is added. The operating level drops as liquid flows out of the cesspool and into the surrounding soil. Cesspools are normally quite deep, and the pressure from the water column within the pit forces the liquids through the openings into the surrounding soil. Cesspools can and do work well in soils that are very permeable and do not have a high clay content. Unfortunately, even in these soils the solids in the bottom of the tank eventually seal the soil pores and the liquid operating level in the cesspool rises. Eventually the liquid level becomes high enough that a back-up into the home occurs.

Engineers came to realize that this situation could be eased by installing a closed receptacle (septic tank) between the house and the cesspool. If sized correctly this septic tank would capture the solids and allow only the liquids to flow into the cesspool tank. The soil pores would not become clogged and the life of the system would be prolonged. The former cesspool tank essentially became a seepage pit.

This concept has been refined over the years and the most of today's septic systems couple the septic tank with a disposal field. The disposal field can consist of a network of trenches or a single disposal bed. Either of these options provides a means of allowing the liquid fraction of the sewage (septic tank effluent) to percolate into the soil. Most disposal fields use a distribution box to spread the liquid out evenly over the surface of the field. Perforated lateral pipes extend like fingers from the distribution box and enable the effluent to flow into the field. A gravel envelope underlies the laterals to support the pipes and dissipate the flow across the full surface of the disposal trench or disposal bed.

To summarize, the modern septic system consists of a closed receptacle called a septic tank that traps solids and allows clarified effluent to flow to a disposal system that discharges this liquid to the soil. I will discuss different types of modern septic systems including sand mounds, peat moss systems, pump systems, and others in subsequent articles.

Here are some links that you may find useful:

http://www.nj.gov/dep/dwq/pdf/inspection_guidance.pdf

http://www.state.nj.us/dep/dwq/pdf/njac79a.pdf

http://www.pacode.com/secure/data/025/chapter73/chap73toc.html

http://www.psma.net/

http://www.co.hunterdon.nj.us/pdf/health/septicdocs/SepticRepairPolicy.pdf