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Photograph of a failed septic system on bedrock on a hill (C)2006 Daniel FriedmanSeptic Tank and Drainfields
What Level of Sewage Treatment is Achieved ?

This article describes the level of residential septic wastewater treatment that is achieved in the septic tank and drainfield. We explain that in a conventional septic tank and soakbed or drainfield, only about 40% of effluent or sewage treatment occurs in the septic tank while we depend on the soakbed or drainfield for the remainder of treatment as well as effluent disposal.

Other types of onsite wastewater treatment systems such as aerobic systems can achieve much higher levels of treatment in the septic tank but also require additional equipment and maintenance.

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What are the Percentages of Total Wastewater Treatment Achieved by Each Major Septic System Component?

Wastewater treatment is "The process of removing pollutants and pathogens from wastewater, discharging the water to the environment where it is recycled, and disposing of the byproducts of the treatment process." [Onsite Wastewater Treatment Systems, Burks & Minnis, cited below.] Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted.

By "wastewater treatment" we refer to the removal of the contents of septic systems that would be considered pollutants to groundwater or pollutants to nearby wells, streams, lakes, or other bodies of water. The chief pollutants of concern in septic wastewater are biological organisms (fecal coliform bacteria), and chemical (nitrates and nitrites).

Sewage may contain a wide variety of other pollutants which we elaborate in detail at Residential Septic Tank Sludge and Scum - what pathogens and contaminants do they contain?.

We provide a brief description of different types of septic systems and thus septic treatment systems at Alternative Septic System Designs This article describes the typical wastewater treatment levels achieved by each approach. What follows are the most basic numbers for conventional septic tank and drainfield systems.

Septic tank treatment of wastewater

Typically a conventional septic tank and drainfield is used to treat raw sewage or "blackwater" discharged from a home into an on-site wastewater system. The septic tank itself accomplishes about 45% of the onsite wastewater treatment level to be achieved. [Ref: Jantrania/Gross].

This same treatment level in septic tanks is cited by Burks/Minnis. This achieves what Jantrania/Gross refer to as OTL-1 or the "first level" of wastewater treatment and alone this would be considered insufficient by any septic system expert.

Drainfield treatment of wastewater

The septic system drainfield, also commonly referred to as leach field or leaching bed, accomplishes the remaining 55% of the onsite wastewater treatment level to be achieved. [Op.Cit. & by simple calculation from septic tank treatment levels of 45%.]

This article series is a supplement to the introduction (SEPTIC SYSTEM BASICS) to our online book SEPTIC INSPECTION & TEST GUIDE


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