What is a Septic Tank
Most of us have seen septic tanks in our daily life. But we mostly don’t know how do septic tanks work exactly. 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 to allow the clarified liquid to discharge for further treatment and disposal.
Settleable solids and partially decomposed sludge settle to the bottom of the tank and gradually build up. A scum of lightweight material including fats and greases rises to the 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 of through soil absorption systems, soil mounds, evaporation or anaerobic filters depending upon the site conditions.
How Septic Tanks Work
Although a septic tank is simply a sedimentation basin with no external or internal moving parts or added chemicals, the natural processes that take place within the tank are complex and interact with each other. The most important process that takes place within the septic tanks includes separation of suspended solids, digestion of sludge and scum, stabilization of the liquid and growth of micro-organisms.
Separation of suspended solids is a mechanical process that 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. Produced gases within the sludge layer cause irregular flotation of sludge flocks that resettle after the release of the gas at the surface.
Organic materials in the liquid are also stabilized by anaerobic bacteria which break down complex substances into simpler ones in a process similar to the one that takes place in the sludge layer.
Large varieties of micro-organisms grow, reproduce and die during the bio-degradation process that takes place in the septic 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, viruses, protozoa and helminths survive the processes in the tanks and remain active in the affluent, the sludge, and the scum. Check my other post on septic tank size and dimension. If you have any questions on how a septic tank works you can ask in the comments.