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

Wastewater treatment:

This article series explains the basics of onsite wastewater treatment processes - what is an onsite or private septic system & how does it work?

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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- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

The Basics of Onsite Wastewater Treatment

Illustration of a conventional septic system including tank, D-box, and distribution field.Onsite Wastewater Treatment Systems - Burks & Minnis

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:

Step1: Waste flows from House Toilets & Other Fixtures to Septic Tank

Septic tank schematic (C) Carson Dunlop Associates

Wastewater 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.

That wastewater will include waste from a flushed toilet, containing excrement and urine, and in many buildings, also wastewater from showers, tubs, and sinks.

Step 2: What Happens in the septic tank: Treatment + solids retention

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.

So baffles in the septic tank keep floating scum, solids, greases and oils from flowing out of the septic tank and into the drainfield or soakaway bed. Other solid waste material settles to and collects on the bottom of the septic tank as sludge.

Periodically the solid sludge from the tank bottom and floating scum from the tank top must be removed by a septic pumping contractor.

If the septic tank baffles are removed or omitted, or if the solids and scum in the septic tank become too thick, one or two bad things happen:

the septic tank may accepting wastewater - causing toilets to overflow or sewage to back up into the building

or

solid waste may be forced out of the septic tank, flowing into the drainfield, clogging and destroying that component and also ultimately leading either to sewage backup in the building or sewage flowing to the ground surface.

Any of those conditions lead to both health hazards and significant repair costs.

Step 3: Septic Tank Effluent Flows out of the Septic Tank & Into the Drainfield

Illustration of a conventional septic system drain field

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.

Depending on where you live a "drainfield" might be called a leachfield, soakaway bed, seepage bed.

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.

In the drainfield, small suspended solids

 (tiny bits of floating sewage debris) which flowed out of the septic tank n the effluent, are filtered by the soils of the drainfield and captured there.

Step 4: Septic Drainfield to Nature

Around the drainfield trenches 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.

At our Continue Reading lilnk below we expand and add detail to the "how a septic tank works" just described, above, beginning with explaining the terms that septic designers, engineers, and other experts use to describe typical as well as alternative methods of treatment of wastewater.

All of these septic 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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Continue reading  at FIVE WASTEWATER TREATMENT TASKS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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