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Septic System Operation

Septic systems (also known as onsite sewage disposal systems or individual sewage disposal systems) are the primary method for treating and disposing sewage in rural areas where sewer systems are not available or too expensive to install. Septic systems are designed to provide partial treatment of the sewage, with disposal to the soil in such a manner that the sewage stays under the ground and is further treated by soil organisms so that contaminants do not reach groundwater or streams.

A septic system typically consists of a septic tank and a leaching device. The tank is usually 1000-2000 gallons in size and is designed to trap solids and grease and provide initial, primary treatment of the sewage. Treatment in the septic tank is anaerobic (without oxygen) and produces a fairly raw effluent that is still very high in bacteria and pathogens, dissolved solids and organics, ammonia, and organic nitrogen. The sewage then typically flows by gravity to the leaching device where the sewage soaks into the soil and most of the treatment takes place. Good treatment is primarily a biological process and it occurs most rapidly in upper soil layers that are rich in soil organisms (bugs) with plenty of oxygen to provide aerobic treatment.

Leaching devices typically consist of perforated pipe set along the top of one or more gravel-filled trenches (leachfields). The sides and bottom of the trench below the level of the perforated pipe provide the absorption area for the effluent to soak into the soil. The total amount of trench and absorption area needed is determined by the expected amount of sewage flow into the system and capabilities of the soil to absorb water. A sandy soil requires less absorption area than a clay soil. The effective depth of a trench is the depth of the trench below the distribution pipe. Because the pipe is covered with soil and typically laid 1-2 feet below the soil surface the total depth of the trench is usually 1-2 feet greater than the effective depth. See alternative leaching methods such as Geoflow (WEB- ).

Besides the basic tank and trench leaching device, an onsite sewage disposal system may include other Basic Components.

When Septic Systems Don't Work

A septic system is obviously not working properly if untreated sewage comes out on the surface of the ground or if it backs up into the house. This is known as a failure. Septic systems are also not working properly if contaminants such as bacteria or nitrate from those systems are reaching groundwater or surface water in excessive amounts. A system is close to failure if the leachfield inspection risers show that the leachfield is completely full of a sewage. This condition also limits adequate treatment.

Some of the factors that contribute to septic system failure or improper function:

  • Age - As a septic leachfield ages, the soil surfaces in the leaching device become clogged with an organic scum layer which limits the ability of the sewage to soak into the soil. The trench or pit fills up and untreated sewage comes out on the ground surface or backs up into the drains in the house. This clogging process usually takes place over a period of 10-40 years and works from the bottom of the trench up to the top. Standing effluent levels in the riser can give an indication of how much usable trench is left. The clogging process is speeded up by system overloading, saturation by groundwater, or lack of septic tank pumping which allows solids to enter the leaching device. Once a leaching device is clogged, it generally needs to be replaced, although some temporary or partial restoration may be gained by resting a leachfield or treating it with an oxidizing agent. In addition to the aging process, older septic systems tend to be undersized and too deep because they were installed prior to development of septic standards.
  • Poor Maintenance or Overloading - Inadequate pumping of the septic tank, use of damaging chemicals, or overloading the system by excessive wastewater discharge can inhibit the treatment process and/or contribute to premature system failure.
  • High Groundwater- High groundwater can cause septic failure or contamination of groundwater and surface water. In many parts of Santa Cruz County, groundwater levels rise significantly during the rainy season, often coming within 1-6 feet of the ground surface for at least several weeks. If a leaching device is flooded by groundwater, it greatly limits the ability of the sewage to soak into the ground and a failure may occur. If sewage enters groundwater before it has been adequately treated in the soil, bacteria, virus, and pathogens can contaminate wells or nearby streams. Studies in the San Lorenzo Valley have shown very high levels of bacteria within 25-50 feet of leachfields that penetrate groundwater. In some studies, pathogenic viruses have been shown to travel several hundred feet under high groundwater conditions. However, where soils are not saturated, it has been found that bacteria and virus are reduced to safe levels within only 2-5 feet of a leachfield.
  • Poor Surface Drainage - If roof runoff or surface runoff is allowed to flow into a leachfield area and soak into the ground, it can rapidly saturate the soil and cause the leachfield to fail.
  • Clay Soils- Clay soils cannot absorb sewage as rapidly as sandy or loamy soils and they become saturated more easily during winter months. Leachfields must be much larger in clay soils or they will become saturated, overloaded and fail.
  • Sandy Soils - Sandy soils absorb effluent more easily and are less prone to sewage failures. But the sewage moves through very rapidly and does not receive as much treatment. Studies in the San Lorenzo Valley have shown that septic systems in sandy soils contribute 10-15 times as much nitrate to groundwater as systems in other types of soils.
  • Steep Slopes and Cuts - Leachfields cannot be located too close to cuts or on steep slopes as there is a danger that the sewage can seep laterally out of the slope or cut before it has a chance to be fully treated. Septic systems can also cause slope failures if located in unstable slopes.