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 (curtain drains) around or uphill of the leach field area to lower the water table locally.

 

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Comments

  1. 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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