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The Basics of Onsite Septic Wastewater Treatment SepticAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• 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 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.

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 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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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).

Five Types of Wastewater Treatment Processes

1. Aerobic wastewater treatment: use microbes (bacteria, fungi which require oxygen) to treat septic effluent. There are four aerobic processes available:
1. Activated sludge - in which "activated" or "pepped up" bacteria treat the wastewater
2. Trickling filters - effluent is trickled (by gravity) over a filter media containing the bacteria
3. Treatment lagoons - not found in a single-family residential system
4. Soil beds - such as a conventional soil absorption or "leach field" or perhaps a sand bed filter system
2. Anoxic wastewater treatment: these processes work where oxygen is not available and remove nitrogen (denitrification using heterotrophic bacteria) and phosphorous from wastewater (also using bacteria).
3. Anaerobic wastewater treatment used to treat septic sludge by heterotrophic bacterial processes requiring little or no oxygen.
4. Combined wastewater treatment processes which make use of more than one of the methods listed
5. Pond wastewater treatment processes by using natural-forming bacteria or algae in (usually) site-built ponds, possibly supplemented by an air pump or spray system to add oxygen to the treatment water and treatment process. Sludge settled to the bottom of a pond forms an anaerobic treatment zone and waste in the upper level of the pond is treated aerobically (facultative ponds - [Burks/Minnis].

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

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 where we describe 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%.]

What Factors Determine Drainfield Effectiveness in Treating Septic Wastewater or Effluent?

The treatment of wastewater achieved by soils in a properly working and properly designed drainfield (for example at proper flow rate, soil characteristics, total area and treatment time, and proper depth to achieve both aerobic and anerobic processes) can achieve onsite treatment level 2 (OTL-2). While texts commonly cite soil depth to limiting conditions (such as bedrock or imperableclay) and soil percolation rate (water flow rate through the soil), I add these additional practical considerations that are also determining influences on the ability of a drainfield to treat wastewater to make the following list:

• Septic Drainfield size in relation to total wastewater load - the total square feet of area available (and presuming that piping is properly installed parallel to slope lines etc.) See Septic Systems - Design, ... for a discussion of how to determine the necessary septic field size.
• Extraneous Drainfield Water Loads: Septic Drainfield exposure to additional water saturation from groundwater, surface, or building runoff
• Drainfield Piping: Condition of drainfield piping: tipped D-boxes, broken connections, clogs forcing effluent into only a portion of the system
• Soil percolation rate or soil porosity, influenced by both starting soil conditions and age of the drainfield as it may become clogged by grease, solids, or salts which have escaped the septic tank. See Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs.
• Soil depth to limiting conditions such as bedrock, imperable clay, etc, and soil depths which may limit the presence of aerobic bacteria (this is why cesspools probably do not treat efflent - only anerobic bacteria are likely to be active at the soil depths equal to the typical bottom depth of a cesspool.)
• Drainfield loading rate: how often and in what quantities does wastewater arrive at the drainfield - how much time does it have to process effluent. This factor is in essence a re-statement of the percolation rate factor to add consideration of the level of usage of the system.
• Soil compaction: the extent to which drainfield soils have been compacted by vehicle or other traffic. [See Causes of Septic System Drainfield Failure
• Plantings over septic fields: the extent to which certain plants have been installed or permitted to grow over a drainfield can affect its ability to dispose of water by evaporation, or the extent of root clogging. [See Planting Over Septic System Components.

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.

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.

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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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