septic tank

A covered, horizontal flow tank (Figure S.3) that involves a series of treatments and is used for isolated dwellings which are uneconomic to connect to a wastewater treatment system. In the UK, the minimum volume of a septic tank in litres is 180 N + 2000, N being the population served. Where garbage grinders are used, the factor 180 should become 250, and for schools or other intermittent uses it should be 90. A rectangular septic tank is normally at least 1 m wide, with a length:width ratio of 3:1 to 2:1 and a minimum depth of 1.5 m. The tank is often divided into two sections, the first being twice as long as the second. Circular pre-formed GRP tanks are available. The solids settle to the floor and undergo anaerobic digestion. Some of the gas evolved lifts the sludge and forms a thick scum which traps other floating solids grease and fats. The inlet and outlet weirs should be below the scum level. The effluent from a septic tank is very variable, with a BOD ₅ of 100 to 300 mg/l and suspended solids from 70 to 150 mg/l. In general, the effluent has about 50% less BOD and 70% less suspended solids than the wastewater influent. The poor BOD reductions are partly caused by hydrolysis of the sludge particles. Further treatment by soakaway or trickling filter may be needed. Septic tanks should be desludged once every...