Photograph of a failed septic system on bedrock on a hill (C)2006 Daniel Friedman The Basics of Onsite Septic Wastewater Treatment

  • WASTEWATER TREATMENT BASICS - CONTENTS: 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?
  • POST a QUESTION or READ FAQs about residential onsite wastewater or septage or sewage treatment

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Wastewater treatment:

This article series 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."

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The Basics of Onsite Wastewater Treatment

Onsite Wastewater Treatment Systems - Burks & Minnis

Article Series Contents:

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:

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.

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 sewage debris) which escaped the septic tank by flowing in the effluent, are filtered by the soils of the drainfield and captured there.

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.

This article series is a supplement to the introduction (SEPTIC SYSTEM BASICS) to our online book SEPTIC INSPECTION & TEST GUIDE


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