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Septic bacteria levels found in residential septic tanks and drainfields SepticAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• What are the levels of septic bacteria contamination found in home septic tanks and drainfield wastewater?
• An explanation of the basics of onsite residential wastewater treatment
• What portion of septic wastewater is treated in the septic tank or drain field?

This chapter explains the Septic bacteria contamination levels that occur in residential septic system wastewater and soil treatment systems. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References." This document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Septic bacteria contamination levels in wastewater and soil treatment systems

The soil environment is a hostile one for septic bacteria and for many viruses, so with adequate time and space for effluent treatment a drainfield can be successful. But some drainfields may not work this well (sandy soils, shallow bedrock, nearby waterways), in which case owners may elect for a disinfectant or other system to improve the treatment level. Jantrania/Gross point out that "Microorganisms in raw wastewater can be present in millions of counts per 100 ml, thus reducing them by 90% [presumably typical onsite treatment levels] will still leave a large quantity in the effluent before discharge." [- Jantrania. p.22]. Those authors conclude that if the target is a 10-fecal coli count in finished wastewater we need a 99.9999% reduction from the starting level of bacteria. Disinfection (such as a chlorine injection system, ozone, or a UV-light system) is used to reduce the levels of bacteria and viruses.

A Pollution Scale proposes a range from 0 (water) to 10 (sewage). (Other literature typically refers to simply primary, secondary, or tertiary levels of sewage treatment.) A typical septic tank achieves 45% treatment, or 5.5 on the 0-10 scale, and a properly working drainfield (see above for our doubts about this), achieves additional treatment from between 65% and about 91%, or treatment levels 3.5 down to about 0.9 on the 0-10 wastewater pollution scale. [We would not want to drink effluent that was 90% treated towards drinking water quality, but we might discharge this into groundwater.] [Op.cit. p48].

Advanced and alternative septic system designs change the ratio of wastewater treatment between in-tank and in-soil, or we should say "before soil" and in "in soil" since advanced treatment can take any of a variety of forms (aerobic systems, aboveground foam cube trickle-down systems, etc.). It should be apparent that improving wastewater treatment before it is discharged into the soil reduces the treatment load on the soil - that is the soil can either treat to a higher level or can achieve the prior level with less area. The rate of disposal of effluent by discharge into soil is probably unchanged by improving its treatment prior to disposal. In other words, you may still need a drainfield of the same size to adequately dispose of the volume of water being discharged.

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Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.

• Daniel Friedman - InspectAPedia.com ^TM Website Author/Editor
• Advanced Onsite Wastewwater Systems Technologies, Anish R. Jantrania, Mark A. Gross, Taylor & Francis 2006. Wastewater treatment levels are given for various system designs including conventional septic systems [this text p. 9] This text can be purchased directly online by clicking on the Septic Systems category in our InspectAPedia Bookstore
• Onsite Wastewater Treatment Systems, Bennette D. Burks, Mary Margaret Minnis, Hogarth House 1994. This text can be purchased directly online by clicking on the Septic Systems category in our InspectAPedia Bookstore
• Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

The Septic Systems Online Book - Where Are We?

• This Chapter: INTRODUCTION of the Septic Systems Online Book
• Next Chapter: SAFETY WARNINGS
• Contents: CONTENTS of the Septic Systems Online Book

More expert information on this topic

More Information on Building Diagnostic Inspections and Repairs

More Reading about septic system function and onsite wastewater treatment:
Biomats: Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
Buyers' Guide: Home Buyer's Detailed Guide to Septic Systems - Buying a Home With a Septic Tank
Components of a Septic System- the Basic Parts of a Conventional Septic Tank and Leachfield, a chapter in the Home Buyers Guide to Septic Systems
Design Alternatives: Advanced Onsite Wastewater Disposal Designs for Septic Systems - design engineers, consultants, products, books for special problem situations, difficult soils, old system repairs. Aerobic septic systems, chemical, composting, incinerating & waterless toilets, Evaporation-Transpiration (ET) Septic Systems, septic media filters, greywater systems, holding tank septic systems, mound septics, raised bed septics, pressure dosing septic systems, sand bed filters, peat beds, constructed wetlands, septic disinfection systems.
Retention Time & Net Free Area and effective septic tank working volume are discussed at "retention time" in our septic tank pumping guide.
Safety Septic System, Septic Tank, & Cesspool Safety Warnings for Septic Inspectors, Septic Pumpers, and Homeowners.
Sketches of the Septic System Components Private Sewage Disposal Systems - Septic Drawing Library
What is a Septic System An Engineer's View - Types of treatment tanks, adsorption systems, pumps, and other special equipment are discussed in some further detail in this text
Reference sources for this web page:
Onsite Wastewater Treatment Systems, Bennette D. Burks, Mary Margaret Minnis, Hogarth House 1994 - one of the best books around, small font, weak index
Anish Jantrania, Ph.D., P.E., M.B.A., Consulting Engineer, Mechanicsville VA, 804-550-0389 new (2006) book, Advanced Onsite Wastewater Systems Technologies can be ordered from his publisher. Outstanding technical reference especially on alternative septic system design alternatives, written for designers and engineers.
Septic System Owner's Manual, Lloyd Kahn, Blair Allen, Julie Jones, Shelter Publications, 2000 $14.95 U.S. - easy to understand, well illustrated, one of the best practical references around on septic design basics including some advanced systems; a little short on safety and maintenance. Buy this book here

Septic Systems Online Book

Basic references on septic systems

• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.

Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include

• Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
• Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
• Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)

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