HOW A SEPTIC SYSTEM WORKS

In This Article
Septic Tank & Leach Field
System Maintenance
Performance Problems                    View all SEPTIC SYSTEM articles

In areas without municipal sewage systems, each home must treat its sewage on its own land using an “on-site sewage disposal system,” more commonly called a septic system. A typical system consists of a waste pipe from the house, a large concrete, fiberglass, or plastic septic tank, and a leach field. The most common type of leach field consists of a series of perforated distribution pipes, each set in a gravel-filled absorption trench. Sometimes a small group of homes share a larger community septic system that performs much the same way as a single-house system.

SEPTIC TANK & LEACH FIELD

The leach field typically consists of a row of perforated pipes buried about 2 feet below grade, deep enough to avoid freezing , but close enough to the surface to allow air to reach the bacteria, that purifies the effluent.

The leach field (also called a “drain field) typically consists of a row of perforated pipes buried about 2 feet below grade, deep enough to avoid freezing , but close enough to the surface to allow air to reach the bacteria, that purifies the effluent (see Illustration).

Where a conventional leach field will not work due to soil conditions or a high water table, an alternative system may be allowed. These often cost two or more times the price of a conventional system and require greater maintenance. Special systems may also be required  near flood plains, bodies of water, or other environmentally sensitive areas.

 

Wastewater from the house collects in the septic tank, where it separates and begins to break down before flowing to the leach field.    Click to enlarge.

After passing from the house, the wastewater collects in the tank, giving it time for the solids to sink to the bottom as sludge and for a host of bacteria and other microorganisms to begin breaking down the materials in an anaerobic (without oxygen) process. Water and grease float to the top of the tank as scum (see illustration). As new wastewater flows into the tank from the house, the partially treated water, or effluent, in the tank flows into the leach field, where most of the sewage treatment takes place.

The leach field typically consists of a row of perforated pipes buried about two feet below grade, deep enough to avoid freezing (since wastewater freezes at a much lower temperature than does pure water) but close enough to the surface to allow air to reach the aerobic bacteria, which take over to further purify the effluent. The effluent then passes into gravel placed around the pipes and on to the native soil. Where the gravel meets the soil a bacteria-rich slime mat grows and does the heavy lifting of water purification. All elements carrying liquid must be buried deep enough to prevent freezing, as this will stop the bacterial action that the system depends on.

The remaining liquid, carrying small suspended solids, percolates though the native soil, where receives its final treatment by bacteria and other tiny critters in the soil. These convert the remaining pathogens into important plant nutrients, while sand, gravel, and soil filter our any remaining solids. Some of the liquid evaporates into the air, and some is absorbed by plant roots. At this point, if the system is working properly, the filtered wastewater returns to the aquifer as naturally pristine water, fit for human consumption. It’s a pretty neat trick that takes perfect advantage of nature’s highly effective process for recycling organic waste.

For the system to work well, the soil must be able to absorb the liquid and the leach field properly sized for the soil type and volume of wastewater, typically based on the number of bedrooms. If there is not enough depth of good soil before reaching rock, impervious hardpan, or the water table, then the system will not work correctly. Dense clay soils will not absorb the liquid fast enough and the sewage will back up the house plumbing or pool on the surface of the leach field. If the soil consists mostly of course sand and gravel, it can drain too fast and possibly pollute the aquifer or nearby bodies of water. System designers need to take all these variables into account.

SEPTIC SYSTEM MAINTENANCE REQUIRED
Periodic maintenance, every 3-6 years depending on tank size and usage, is essential to remove the solids (sludge and scum) from the septic tank. Otherwise, solids will eventually fill the tank and wash out into the leach field, reducing its effectiveness and shortening its life. Even with regular maintenance, however, the soil may eventually become clogged from dissolved materials in the wastewater, and the soil will be unable to absorb the incoming water. An odorous wet area over the leach field or plumbing backups in the house are often the first signs that things are amiss. Many areas require septic system designs to include a second “reserve drain field” under the assumption that sooner or later the first field will fail. A properly designed and maintained system should serve for 20 to 30 years or more.

SEPTIC SYSTEM PERFORMANCE PROBLEMS
Most septic system problems are a result of poor initial design, misuse, or physical damage, such as driving heavy vehicles over the leach field. Common conditions that can cause a septic system to perform poorly include:

House plumbing. clogged or inadequate plumbing vents, blockage between the house and septic tank, or inadequate pitch in sewer pipe from house

• Septic tank to leach field. Blockage between the septic tank and leach field caused by a plugged or broken tank outlet, or a plugged line to the leach field cause by tree roots, or by solids that overflowed from the tank

Leach field piping. Driving or parking heavy vehicles over the leach field can break the perforated leach pipe, especially if it is not uniformly supported by the gravel bed. Usually tree roots do not penetrate through the gravel bed into the perforated piping.

Leach field sizing: Drain field may be too small for current flow levels out of the home. Reducing flows though use of flow restricters, and low-flow faucets and fixtures might help.

• High water table. A seasonal high water table can saturate the soil around the trenches impairing the soil’s ability to absorb wastewater. This is sometimes an issue on relatively flat building sites with poor surface drainage. This can often be fixed by installing subsurface drains or curtain drains to intercept the water flow toward the leach field area and to lower the water table locally.

 

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Comments

  1. otis bivens says:

    Septic System for Storage Building?

    Can you have storage buildings on lot without a septic system?

    • buildingadvisor says:

      In general, any residential dwelling unit or commercial building with a kitchen and/or bathroom would require a septic system. If you are using a lot for storage buildings only without any running water, then you shouldn’t need any sort of on-site sewage system.

      If you have utility sinks or other water uses in a commercial building, but no bathroom, you will probably need at a minimum some type of wastewater holding tank. The type of system will depend on the type of wastewater being disposed.

      Since these regulations vary from town to town and state to state, you need to check in with your local department of health or building department to find out what rules would apply to your intended uses.

  2. Multiple Septic Systems on One Lot?

    I am looking at purchasing 5-10 acres with the hopes of placing a 3-bedroom home and possibly two or three “Tiny Homes” to use as rental units on the same property. I have seen several properties that perc in numerous places on the property. Does this mean that there could be more than one septic system and leach field if the different locations are far enough apart?

    • buildingadvisor says:

      In general, the answer is yes, it’s possible to have multiple septic systems on one property, but it may be more economical to tie into one large system. At most, you would want two systems – one for the main house and one for the Tiny Houses. These issues are heavily regulated by the local health department, so you need to check with them to see what is allowable.

      The distance from one leach field to the another is generally not a concern. Of greater concern to regulators – and to you – is the distance from the leach field to a well, house foundation, potable water piping, and open water (lakes, ponds, streams). Also, you need to consider what is at the ground surface. In general, you don’t want trees near the piping or drain field or roads running over any of the components, unless they are specifically designed to handle traffic overhead.

      If the buildings are spread out, having more than one system may make sense. Typically, the distance from the house to the septic tank is in the range of 10 to 20 feet, but could go farther. The distance from the tank to the leach field can be much greater as the effluent at this point is clear liquid. If you cannot drain by gravity to the leach filed, a pump is needed which increases cost, complexity, and maintenance.

      Talk to your town or county health department about your ideas and see what options are acceptable in your jurisdiction. Best of luck with your project.

  3. How To Tell If Drainfield Exhausted?

    How do you know when a drain field is “used up”? We are looking at a home with a 26 year old drain field. It held a family of six for the first 10 years and then dwindled to the last two lovely owners who hold grand parties of 40-60 guest two to three times each year, the tank is pumped every five years presently.

    The home in in South Carolina and has two large creeks on one side of the home, a flood plain area, and an acre pond below the drain field and house.

    General information online says a field will last 15-30 years. How do you know when it is at the end? Is there a way to test and have a better understanding of the potential longevity of an old field?

    • buildingadvisor says:

      The numbers I usually hear are 20 to 30 years for a properly designed and maintained drain field. A lightly used and well-maintained field can certainly last longer. The longevity depends heavily on the original design, usage level, and maintenance. Certain chemicals, grease, animal fats, disposals, and solid materials disposed into a drain field can also shorten its life. Assuming that the original design is adequate and usage moderate, the most important factor is regular pumping, which it sounds like the previous owners have done.

      A septic system contractor can tell you if you have a failed system, but I’m not aware of a test to determine how long a functioning system has left. The typical signs of a failed system are slow drainage or backups, and a soggy, smelly mess at the surface of the ground over the leach field. If you have slow drainage or backups, you will need to determine if these are caused by problems with the tank and other components or by a failed leach field. When they pump the system, the pumping contractor can usually identify problems. If necessary, a dye test can be used to confirm that the field has failed.

      Being surrounded by water and a flood plain could also be a concern. A water table that is too high can impair the drain field and also lead to contamination of the aquifer or pond. Modern codes call for the drain field to be a minimum distance above the seasonal high water table and may require a raised drain field (mound system) or other alternatives in that case. Pollution of the aquifer would show up as an elevated nitrate level in your well or pond, so you might consider a water test, which can usually be arranged through your town or county health department for a nominal fee.

      I recommend visiting our sister site, Inspectapedia.com, which has excellent information on SEPTIC SYSTEM TESTING and related topics. Dan Friedman, of Inspectapedia, recommends the following.

      • Find out what sort of tank is installed. Steel with rusted baffles is bad. Concrete in good condition with a safe cover is good.

      • Try asking the septic pumper if, the last time they pumped the tank, they saw trouble signs like backflow into the tank from the drain field or damaged baffles, or evidence of a damaged tank. Low sewage levels indicates tank leaks. Unless the pumpout was recent,however, they may be reluctant to provide information.

      • We don’t test the leach field biomat itself. But you could have a septic loading and dye test done. Read more at SEPTIC LOADING & DYE TEST.

      That can at least tell you if the system has already failed. You can also tell this from on-site inspection: look for odors, liquid “breakout” on the surface, or funny grass.

  4. Same Trenches for Leach Field and Geothermal Heating?

    I am in the early planning stages of construction in the Teton Valley of Idaho. I would like to do a geothermal loop and I am wondering if I could use the same trenches for the leach field. Since the GT lines are approximately 6-7 feet in the ground I could cover with a few feet of dirt and add the drain pipe, then cover the remaining 3-4 feet. Thoughts?

    • buildingadvisor says:

      Interesting idea and could save you some money, but I’m not sure it will fly. First off, I would make sure it passes muster with both the septic system designer/installer and geothermal contractor – as well as the local code officials, assuming you have building codes in your area.

      A couple of key issues occur to me:

      1. The soil around the geothermal loop gets chilled all winter and could freeze in your area over the course of the winter (which is why most ground loops use antifreeze). Since freezing can be an issue with septic systems in cold climates, it’s possible that combining the two systems would increase the likelihood of septic system freeze-ups.

      2. Leach fields are generally placed on well-drained, undisturbed soil. The percolation rate of the soil (along with the usage) determines the size of the leach field. If you are building your leach field on backfill, you will need to take that into account and use suitable fill. In general, geothermal systems like dense and moist soil, while septic systems like dry and granular soils, so finding a soils type that works for both could be tricky.

      Anyone else have experience with similar installations?

      You can read an interesting discussion on this topic at:
      https://www.geoexchange.org/forum/threads/installing-a-horizontal-loop-under-a-septic-field.710/

  5. Nice article – helpful and clear.

    A common problem on older sites is that there is or was a septic tank or cesspool and drainfield but no one is quite sure where. It’s often possible to make an intelligent guess by examining the lay of the land, slopes, location of mature trees, etc. asking “where would an excavator have considered it possible to *put* a septic system tank and drainfield?

    I’d add a safety warning to beware of abanodoned or old, poor condition septic tanks, cesspools, and drywells; if a cover is unsafe or if someone falls into one of these it can be fatal very quickly.

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