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.
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).
In a properly functioning tank, the grease and solids are retained and pumped out during regular maintenance of the the tank.
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 treated effluent is then absorbed into the soil below the leach field.
The leach field (also called a “drain field”) consists of rows of perforated pipes buried in gravel trenches 18 to 36 inches below grade, deep enough to avoid freezing, but close enough to the surface to allow air to reach the bacteria that further purify the effluent (see illustration).
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. Wastewater freezes at a much lower temperature than does pure water but freezing is still a concern in cold climates.
The remaining liquid, carrying small suspended solids, percolates though the native soil, where it 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 leach field must be properly sized for the soil type and volume of wastewater, typically based on the number of bedrooms. The soil, as measured by a perc test, must be porous enough to absorb the liquid, so the denser the soil, the larger the leach field required. For a three-bedroom house with normal soils, the total leach field area may vary from about 500 to 1,500 square feet.
It’s better to have excess capacity in your system than to cut it too close. Your family may grow or you may get an exceptionally wet spring with saturated soils and a higher-than-normal water table. If the ground cannot absorb the liquid, then sewage can back up into your home, pool on the surface, or pollute nearby groundwater.
Also, 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, causing backups.
But the soil can also be too porous. If the soil consists mostly of course sand and gravel, it can drain so fast that untreated sewage can pollute the aquifer or nearby bodies of water are polluted. System designers need to take all these variables into account.
Where a conventional leach field will not work due to poor 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.
Treat your system well and you will be rewarded with years of trouble-free service. Periodic maintenance, every 3-5 years depending on tank size and usage, is essential to pump out the solids (sludge and scum) from the septic tank. Otherwise, solids will eventually overflow the tank and spill out into the leach field, reducing its effectiveness and shortening its life. It is difficult or impossible to rehabilitate a clogged leach field, so regular pumping is critical! Failure to pump out the tank is the leading cause of premature failure of septic systems.
Cooking fats, grease, and solids can also wash into the leach field if the tank is too small for usage levels or is overused periodically. Fats and petroleum products that reach the leach field can clog the biological mat that forms where the leach trenches meet the soil and interfere with its job of purifying the effluent.
To avoid overloading the system, spread out large water loads and direct yard drainage away from the leach field. Don’t do a week’s worth of laundry for a family of five in one day. This will help keep the load manageable and prolong the life of your system. Follow these tips:
- Spread out laundry loads, and other heavy water usage, throughout the week
- Use low-flow appliances, faucets, and fixtures in kitchen and bath. In general, toilets generate the most water usage.
- Direct drainage from the yard, gutters, and basement sump pumps away from the leach field.
Also avoid putting solids, harsh chemicals, and just about anything else down the sink or toilet other than biological wastes and white toilet paper. Avoid the use of disposals. If you must have one, use it only for minor non-meat scraps. Avoid dumping chemicals or paints down the drain as as some chemicals can kill friendly bacteria or lead to water pollution nearby. Avoid putting down the drain:
Even with gentle use and regular maintenance, however, leach fields will not last forever. The soil will 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 the first field will eventually fail. A properly designed and maintained system should serve for 20 to 30 years or more.
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.
For more information, download the EPA Homeowner’s Guide to Septic Systems .
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