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Five Residential Septic Tank and Drainfield Wastewater Treatment Tasks
What does a septic system actually do?
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This article explains the basic tasks of a residential septic tank and drainfield - the onsite wastewater treatment processes.
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.
Page top photo: during a septic loading and dye test conducted by the author this water seepage down rock confirmed that the septic system was built in shallow soil atop solid rock - it was not a functional septic system.
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Five Wastewater Treatment Tasks
- 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. - 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. - 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).
- 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 - 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 - 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).
...
Continue reading at FIVE WASTESATER TREATMENT PROCESSES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.
Or see these
Recommended Articles
- WASTEWATER TREATMENT BASICS
- FIVE WASTEWATER TREATMENT TASKS
- FIVE WASTESATER TREATMENT PROCESSES
- PERCENTAGES of WASTEWATER TREATMENT ACHIEVED
- SEPTIC TANK TREATMENT of WASTEWATER
- DRAINFIELD TREATMENT of WASTEWATER
- DRAINFIELD TREATMENT EFFECTIVENESS
- SEPTIC BACTERIA CONTAMINATION LEVEL
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FIVE WASTEWATER TREATMENT TASKS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.
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INDEX to RELATED ARTICLES: ARTICLE INDEX to SEPTIC SYSTEMS
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Citations & References
In addition to any citations in the article above, a full list is available on request.
- New York State Department of Health, APPENDIX 75-A WASTEWATER TREATMENT STANDARDS - INDIVIDUAL HOUSEHOLD SYSTEMS , [PDF] New York State Department of Health, 3 February 2010, retrieved 3/1/2010, original source: https://www.health.ny.gov/regulations/nycrr/title_10/part_75/appendix_75-a.htm
- 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]
- Onsite Wastewater Treatment Systems, Bennette D. Burks, Mary Margaret Minnis, Hogarth House 1994.
Septic System References & Books
- "International Private Sewage Disposal Code," 1995, BOCA-708-799-2300, ICBO-310-699-0541, SBCCI 205-591-1853, available from those code associations.
- "Manual of Policy, Procedures, and Guidelines for Onsite Sewage Systems," Ontario Reg. 374/81, Part VII of the Environmental Protection Act (Canada), ISBN 0-7743-7303-2, Ministry of the Environment,135 St. Clair Ave. West, Toronto Ontario M4V 1P5 Canada $24. CDN.
- US HEW, MANUAL of SEPTIC TANK PRACTICE [PDF] 1975, U.S. Department of Health, Education and Welfare, Public Health Servicve, original source: https://nepis.epa.gov/
- Manual of Septic Tank Practice, US Public Health Service's 1957, reprinted in 1963, 1967, 1969 and in a 1975 edition given just above.
- SEPTIC TANK CAPACITY VS USAGE in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
- SEPTIC TANK/SOIL-ABSORPTION SYSTEMS: HOW TO OPERATE & MAINTAIN [PDF] - , Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf.
- 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
- Septic Tank Pumping Guide: When, Why, How to pump the septic tank
- SEPTIC TANK & SYSTEM CARE BASICS An Engineer's View & Septic System FAQ's
- Advanced Onsite Wastewater Systems Technologies, Anish R. Jantrania, Mark A. Gross. Anish Jantrania, Ph.D., P.E., M.B.A., is a Consulting Engineer, in Mechanicsville VA, 804-550-0389 (2006), Advanced Onsite Wastewater Systems Technologies. Outstanding technical reference especially on alternative septic system design alternatives. Written for designers and engineers, this book is not at all easy going for homeowners but is a text we recommend for professionals--DF.
- In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested
CONTINUE READING or RECOMMENDED ARTICLES.
- Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: info@carsondunlop.com. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The HOME REFERENCE BOOK - the Encyclopedia of Homes and to use illustrations from The ILLUSTRATED HOME .
Carson Dunlop Associates provides extensive home inspection education and report writing material. In gratitude we provide links to tsome Carson Dunlop Associates products and services.
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