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How does my septic system work?
Design & Function - Septic Tanks - Septic Tank Cross Section - Cesspool - Drainfield
DESIGN AND FUNCTION To work properly, a septic system should be designed and sized correctly for the household or business it serves. The absorption area should be installed in a location where soil type will allow the proper rate of drainage.
Septic Tanks should have at least 250 gallons of capacity for each person in the house. Standard sizes are 750, 1000, 1200, and 1500 gallons. They can be constructed of precast concrete, plastic or fiberglass. Older tanks may be made of steel, which often corrode over time, or they may be built in place of block construction. Larger tanks are often divided into two chambers to improve solids separation. Manholes and inspection ports are located in the cover for service and inspections.
Inside the tank, bacteria will reproduce in the floating scum mat and bottom sludge layer. By a process called anaerobic (without oxygen) digestion. Most solid matter will be converted to water, sewer gas and a small volume of indigestible sludge which must eventually be pumped out. The rate and degree of liquefaction is determined by various factors. Included are: temperature, pH, bacterial efficiency, water usage, amount and types of waste and amounts of household cleaners, bleach, drain openers, and detergents added to the system.
In each case the perforated portions are surrounded by a gravel bed. The large surface area within the bed supports a population of microbes which will aerobically (using oxygen) digest most of the remaining organics in the waste stream. Air is supplied by natural diffusion down through the soil to the gravel bed. For this reason, a drainage area should never be paved over, compacted or covered. A bio-mat of microorganisms forms a filtering layer at the soil/gravel edge and removes other contaminants including viruses as the water passes through and into the soil. Excessive amounts of organics escaping from the septic tank can result in a bio-mat growing so thick it will actually prevent adequate water flow into the soil.
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Both the inlet and outlet are located near the top of the tank, so the tank will fill to capacity before anything exits to the drainage area. Effluent slows on entering the tank, allowing lighter solids to float and heavier solids to sink, providing good separation from the water phase. A baffle or a "T" section pipe is also part of the inlet, to direct solids towards the bottom of the tank. On the outlet side they prevent solids from escaping to the drainage area. Only relatively clear water, which contains small amounts of suspended or dissolved organics should leave the tank and enter the drainage area. A tank which is too small, overfilled with solids or receives a high volume of water in a short period of time will allow solids to pass through and contaminate the drain field.
A cesspool incorporates both functions of a septic system in one structure. It consists of a large perforated tank in which digestion takes place, surrounded by an absorption bed where suspended and dissolved solids undergo final digestion and water is filtered. Cesspools are not as efficient as other systems, more prone to failure, and difficult to restore to operation.
Drainage Areas may be in the form of a large perforated vessel called a seepage or leach tank, a single perforated pipe (lateral) or a number of laterals extending from a distribution box (D-Box) or boxes. Variations include pressure fed systems and systems using chambers in place of laterals.
