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• An explanation of the basics of onsite residential wastewater treatment
• What are the jobs of the septic tank and drainfield?
• An explanation of the five basic processes in wastewater treatment: mechanical filtration, biological oxidation, disinfection, waster disposal, and byproduct or solid waste disposal
• What portion of septic wastewater is treated in the septic tank or drain field?

This chapter explains the basics of onsite wastewater treatment processes - an explanation of the basics. 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.]

This page is a supplement to the introduction to our online book: "Septic Systems - Design, Inspection, Testing, & Maintenance" whose chapters are shown at the left of this page. 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 2009 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.

The Basics of Onsite Wastewater Treatment

Our photograph at page top shows a private septic system which demonstrated failure even before we began our septic loading and dye test. A too-small septic tank installed at the top of a steep hill emptied into a too-small gravel pit buried in less than 36" of soil which in turn sat upon solid rock at the top of this steep site.

Effluent could be seen running down the face of nearby rocks whenever the septic system was in use. In the upper portion of the photo (above at page top) you can see a overflow pipe protruding from the septic tank itself, and also hay which the owner or contractor had piled atop of this system in anticipation that we would not notice its unfortunate condition. Of course by simply taking a few steps down the hillside and looking up, these regrettable conditions were quite visible. Effluent simply ran down the hillside and into a nearby stream.

The Basics of Wastewater Treatment in a Private Septic System

"Treatment" of wastewater (sewage that flows out of building toilets, sinks, showers) means making sure that the wastewater released to the environment is acceptably clean. In a most-conventional private home septic system, this is about what happens:

1. House Toilet to Septic Tank: Waste flows from building toilets (and sinks, showers, etc.) through building drains, usually by gravity, out to a septic tank, buried in the ground, often close (10 ft.) to the house. The job of the septic tank is to retain solid waste, oils, and grease in the tank, and to perform limited (perhaps 45%) treatment of the sewage by bacterial and other microbial action which digest organic waste and pathogens. Periodically the solid sludge from the tank bottom and floating scum from the tank top must be removed by a septic pumping contractor.
2. Septic Tank to Drainfield: Liquid from the septic tank (clarified effluent) flows out of the septic tank and into the drainfield - a network of perforated pipes buried in the ground, often in gravel-filled trenches, perhaps 12" to 24" below ground level. In the drainfield, also called leach field, seepage bed, or soil absorption system, the effluent seeps out of perforations in the pipes, through gravel in the trench, and into the soils below the trench system. Suspended solids (tiny bits of floating debris) which escaped the septic tank by flowing in the effluent, are filtered by the soils of the drainfield and captured there.
3. Drainfield to Nature: a biomat of bacteria and other microorganisms grows naturally in the soil below and around the drainfield. These microorganisms further digest or break down both the captured suspended solids filtered by the soil, and other pathogens and organics and nitrites and nitrates which are present in the septic effluent. As effluent passes through and is processed by this biomat, it is eventually released to local soils where it joins ground water already present in the area (disposal).

For difficult home sites where there is limited space, rocky ground, or wet soils, a variety of "advanced" wastewater treatment systems are available to successfully handle the process I've just described.

What follows this very very basic description of onsite wastewater treatment is simply the same scenario just described, but with the introduction of the terms which septic designers and other experts use to describe various alternative methods of treatment of wastewater. All of these systems have the same objective: when septic effluent is finally released to the environment it must be as clean or cleaner than the natural groundwater which is already there, and must be clean enough so as not to be a health hazard or harmful to the environment.

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WASTEWATER TREATMENT BASICS
  The Basics of Septic Waste Treatment
  Five Wastewater Treatment Tasks
  Five Wastewater Treatment Processes
  Percentages of Wastewater Treatment Achieved
  Septic tank treatment of wastewater
  Drainfield treatment of wastewater
  Drainfield Effectiveness
  Septic bacteria contamination levels

• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• Advanced Onsite Wastewater 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

SEPTIC INFO ARTICLES
HOME BUYERS GUIDE to SEPTIC SYSTEMS
SEPTIC INSPECT & TEST
SEPTIC SYSTEMS ONLINE BOOK
SEPTIC SYSTEM SAFETY WARNINGS
SEPTIC SYSTEM DESIGN BASICS
SEPTIC DESIGN ALTERNATIVES
SEPTIC INSPECTIONS
SEPTIC SYSTEM DEFINITIONS
SEPTIC FAILURE LAWSUIT
SEPTIC REFERENCES
SEPTIC CONSULTANTS
SEPTIC AUTHORITIES
SEPTIC SYSTEM BOOKS REFS CODES

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
SEPTIC SYSTEM 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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