How do septic systems work?

Every day, people who live in rural areas rely on their septic systems to process their waste. Even rural businesses have on-site wastewater treatment systems. We don’t often think of how they work until something goes wrong.

Septic systems clean waste with the help of soils and the roots, microbes, and other life living in soil. Illustration by Jake Mowrer

People who live in cities or the suburbs don’t really have to pay any attention to the waste they put in their sinks and toilets. They can rely on their city to take the waste from their drains and into the wastewater system. Of course, there are city and county fees to pay which support this immense infrastructure.

Rural-dwellers are cautioned to be more careful with what they put down their drains. Septic tanks are something that everyone has heard about, but that very few people think carefully about. They are a form of what is called on-site wastewater treatment systems. In fact, these systems do our very dirty work for us underground all day long so we don’t have to be confronted with the details. Well, that is until they fail, and confrontation can be thrust very unpleasantly upon the owner of the system.

The soil-based septic system

In this post, I want to focus on how septic systems function, rather than how they fail. To put it simply, they function because of soil, our greatest national resource. And I think that whether you live in the city or the country, this poorly understood microcosm of our American lives provides a fascinating and (dare I say) revelatory insight into the value of soil as an integral component in our global eco-functionality (I may have made this last word up).

Septic systems rely upon natural physical and biogeochemical processes that occur in soil. They absorb and eliminate what might otherwise be health-averse environmental pollutants. Soils are relatively simple, inexpensive, and if properly constructed and maintained, highly effective. Soils have an amazing capacity to assimilate and transform organic matter, nutrients, and pathogenic bacteria. Soil performs crucial functions for us 24/7 without credit or complaint, even as we kick it about and treat it like dirt.

Nuts and Bolts 

In a soil-based septic system, household wastewater leaves the home through a drainage pipe and enters a tank buried under ground. The tank should detain wastewater long enough to allow the solids to settle to the bottom. Oils and grease float to the top. The liquid fraction of the wastewater is allowed to exit the tank into a drain field. Solids remaining in the tank (sludge) are broken down by micro-organisms more slowly and must be removed periodically to maintain septic system functionality.

The drain field is composed of a series of porous pipes leading from the tank to the surrounding subsurface area. Here, wastewater percolates into the soil. The size of the drain field may be a few hundred square feet to an entire acre or more. The size depends upon the capacity of the soil to treat wastewater and the amount of wastewater to be treated.

The soil in the drain field is almost always the soil that was there before the system was installed. Soils are a matrix of irregularly shaped particles arranged in an irregular way so that there is space in between all these particles for air and water. Both air and water occupy and flow through these voids all the time. Plant roots and other living organisms, including worms, insects, and microorganisms, also occupy these voids.

In fact, soils contain a host of micro-organisms that begin immediately to work on cleaning your wastewater:

• heterotrophs to break down organic molecules;
• nitrifiers to convert ammonia to nitrate;
• denitrifiers to convert nitrate to atmospheric nitrogen (N₂ gas); and
• general predators to consume the coli and other pathogens that cause human and animal diseases.

After several days’ or weeks’ exposure to the soil, what was once wastewater becomes environmentally safe water, and clean enough to percolate into groundwater and/or evaporate from the soil surface to the atmosphere. Viola!

Helping your Septic System Help You

Efficient function of a soil-based septic system requires occasional attention and maintenance.  Neglect can lead to some pretty unpleasant outcomes. However, following a few recommended practices helps safeguard your health, your property values, and your neighbors’ opinion of you.

• Do not ask septic systems to handle too much organic matter from oils and greases, or from in-sink garbage disposals.
• Do not flush or sink large amounts of chlorine or other disinfection products. They will sterilize the micro-organisms you rely on to process wastewater and solids. Cleaning products may be disposed of with water in moderation without affecting septic system function.
• Have the system cleaned out (pumped) on a regular schedule as per your installer or local authority’s guidelines.
• There are no recommended ‘wonder’ products that work better to promote the activity of soil microorganisms than the normal stuff you flush.
• Avoid excessive wastewater flows that may overwhelm the system.
• Verify that the system is not located near a wellhead to avoid high potential for contamination of your drinking water.

Finally, from the “files of the weird and unsupported,” we have to share some of the claims we have heard from septic system owners over the years for things you should flush to make it work better. By the way, none of these “non-traditional additives” do anything to improve your system, and some may hurt it:

• a pound of raw ground meat
• red wine
• beer
• oatmeal
• a packet of yeast
• ½ a raw chicken (picture that goin’ down the commode)

So next time you walk on some soil, stop and think about all the things it does for us thanklessly. In addition to growing our plants, soil harbors a staggeringly diverse and important ecosystem of macro and microorganisms. It supports and shelters above-ground wildlife. It filters water from rain and snow. It minimizes atmospheric carbon dioxide levels. It transforms organic and mineral materials into usable nutrients and energy.

Without pay, soil does some of our dirtiest jobs for us every day right in our own backyards.

Answered by Jake Mowrer and Ward Ling, Texas A&M

Interested in more? Read research about septic tanks’ potential impact on groundwater.

To watch a video about how soils support buildings and infrastructure (including septic systems), visit SSSA’s YouTube video: https://www.youtube.com/watch?v=zJi-73qeE-0

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