A sewerage drain field and a septic tank make up a small scale sewage treatment system. A septic tank is basically a rectangular shaped underground receptacle for wastewater. It is watertight, made of concrete, and consists of tanks that are divided into chambers. The first chamber has twice the volume as the second and the majority of the organic solid matter, referred to as sludge, gets settled here. The second chamber further purifies the wastewater.

The septic tank collects wastewater from toilets, showers, sinks, etc. Due to the absence of a current the sludge quickly settles down to the tank floor. Anaerobic bacterial digestive activity on the sludge leads to the generation of methane and carbon dioxide; the sludge is stabilized and does not rot anymore. Particles of stabilized sludge stay settled, some may float to the top as scum.

A septic tank should be constructed such that there is maximum contact time with the microorganisms so as to enable swifter digestion of the sludge. This is achieved by placing the inlet, overflow, and outlet diagonal to each other and by making the streams flow in vertically placed pipes.

The semi-purified water moves to the second chamber via the overflows that are constructed such that the upward motion of water prevents larger particles from flowing into the second chamber. The feeding, digestion, and settlement of organic matter continues in the second chamber. The retention time in the second chamber is half that of the first chamber because of its size and the fact that the amount of organic matter to treat is less. The outlet is constructed in the diagonal corner to the overflow.

The treated wastewater is directed to a drain field, also known as a seepage field, where any impurities that still exist get decomposed naturally, the water is taken up by the plant root system or it may even become a part of the groundwater. The large sized drainage field is required if the soil is not very porous. A septic system can be made to work entirely by using gravitational forces; at some places, however, a pump may be required to overcome topographical limitations.

The rate of sludge generation in septic tanks is around 0.05 cubic yards every year for an individual. Septic tanks need to be emptied every few years so that there is enough reaction space for the microorganisms to attack the wastewater and for the sludge to settle.

The septic tank may be emptied every 5-7 years depending upon the amount of wastewater it processes. The tank is emptied using an exhauster truck manned by trained personnel. The sludge that is sucked into the exhauster truck is released into public waste water treatment plants, sanitary landfills, or sludge drying beds.

Properly digested sludge can be used as a natural fertilizer; this is possible only if no fresh wastewater is added to the tank for at least one week.

The design of the septic tank should be such that the wastewater always flows in its designated path and no short circuits in the water flow occur. T-pipe-connectors with inflow and outflow openings under the water level should be used for inlet, overflow, and outlet pipes. This prevents the floating scum from clogging the pipes. A grease trap should be provided for the wastewater inflow pipe.