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Basic Information of Septic Tank Systems

What is a Septic Tank

A septic tank is a buried, watertight receptacle designed and constructed to receive wastewater from a home, to separate the solids from the liquid, to provide limited digestion of organic matter, to store solids and aloe the clarified liquid to discharge for further disposal and treatment. Settleable solids and particularly decomposed sludge settle to the bottom of the tank and gradually build up. A scum of lightweight material including fats and greases rises to top. The partially treated effluent is allowed to flow through an outlet structure just below the floating scum layer. This partially decomposed liquid can be disposed through soil absorption systems, soil mounds, evaporation beds or anaerobic filters depending upon the site conditions. The essential components of a septic tank systems are Compartment baffle, inlet, outlet baffle, outlet.

Process in a Septic Tank

Although a septic tank is simply sedimentation basin with no external or internal moving parts or added chemical, the natural processes that take place in the tank are complex and interact with each other. The most important processes that take place in the tank – include separation of suspends solids, digestion of sludge and scum stabilization of liquids and growth of microorganism.

Separation of suspended solids is a mechanical process which results in the formation of three distinct layers in the septic tank – a layer of sludge at the bottom, a floating layer of scum on the top and a relatively clear layer of liquid in the middle.

Anaerobic bacteria degrade the organic matter in the sludge as well as in the scum and as a result of this bacterial action, volatile acids are formed at the first instance and eventually are converted mostly to water, carbon dioxide and methane. The formation of gases in the sludge layer causes irregular floatation of sludge flocs that resettle after the release of gas at the surface.

Organic materials in the liquids are also stabilized by anaerobic bacteria, which break down complex substances into simpler ones in a process similar to the one that take place in the sludge layer.

Large varieties of micro-organisms grow, reproduce and die during the biodegradation processes that take place in the tank. Most of them are attached to organic matter and are separated out with the solids. Although there is an overall reduction in the number of micro-organisms, a large number of bacteria, virus, protozoa and helminthes survive and remain active in the effluent, the sludge and the scum.

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