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• What are the Five Residential Septic Tank and Drainfield Wastewater Treatment Tasks
• 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 basic tasks of a residential septic tank and drainfield - the onsite wastewater treatment processes 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.

Five Wastewater Treatment Tasks

Burks and Minnis (on whose work this chapter is based) list five wastewater treatment processes [which I call "tasks" as these authors confuse the reader by also listing five functions of bacteria in wastewater treatment and five types of wastewater treatment processes], one or more of which may occur together simultaneously in a particular treatment system, and each of which can be accomplished by quite a variety of methods.

1. Mechanical filtration of solids: such as filtration that occurs in soil below a drainfield or sand bed. Because soil particles may also have a small electrical charge, some small (previously suspended) solids will also adhere to some charged soil particles.
   
   The biomat that forms below a conventional drainfield or on media in advanced systems has a key role in additional filtering as well as oxidation and disinfection discussed next. Filtration in advanced design septic systems occurs in the textile, foam, or other media. Some septic system install textile or media filters to further clean effluent before it is sent to the soil absorption system.
   
   Filtration also occurs, in a sense, in the septic tank by the settling of solids to the tank bottom (sludge) or the coagulation of some light solids and greases at the tank top (scum layer). The net free working volume of a septic tank and the sewage inflow rate determine the tank's retention time - a period necessary to allow sludge and scum formation.
2. Biological oxidation of organic material: such as may occur to a limited degree in a septic tank, or to an advanced degree by the introduction of additional oxygen in aerobic septic systems. This process is also referred to as microbial oxidation since other microbes besides bacteria are at work. Microbial oxidation is an "aerobic" process, meaning that oxygen is required for the process to complete. Biological oxidation is a two-step process.
   • Aerobic Processing of septic waste: First organic material in the wastewater is consumed by bacteria or other microbes (carbonaceous biochemical oxygen demand). Second the bodies of older dead bacterial cells are used to make new cells (endogenous respiration). Burke/Minnis explain that after these two steps, except for some un digestible bacteria hulls, all of the other products have been assimilated into new bacterial bodies.
   • Anaerobic Processing of septic waste: other bacteria can "digest" wastewater contents without requiring oxygen, breaking down organic matter ultimately into methane (which is explosive, explaining some of the exciting anecdotes I warn about in my septic safety web pages). Anaerobic processes break down organic waste first by acid fermentation, second by acid regression, and third by alkaline fermentation, in case you see these terms floating around.
   
   Keep in mind, however, that in a conventional septic system this process is quite incomplete in the septic tank (perhaps 45%) and that the remaining oxidation has to occur in the soil absorption system. Aerobic and other advanced systems achieve different levels of oxidation at different stages.
   
   Burks & Minnis ascribe five functions (these authors like fives) to bacterial processing of wastewater: removal of carbonaceous organic matter, nitrification, denitrification, phosphorous removal, and conversion of organic waste into new cell mass, CO₂, and water.
3. Disinfection: which may occur by the processing of pathogens by microorganisms in the septic system or by actual disinfection in some advanced systems which require the insertion of a disinfectant
4. Water disposal: such as by absorption into soils below a septic drainfield or soil absorption system, or by evaporation into the air in an evaporation/transpiration system
5. Byproduct disposal: such as the pump out of settled sludge and floating scum from a septic tank - (the septic pumper has to haul this septage to an approved dumping facility for ultimate disposal).

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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

• 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 SYSTEMS HOME
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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