<prompt> All are features of septic tank except:Ideal retention period-48 hoursMinimum capacity-500 gallonsAerobic oxidation takes place outsideSludge is solids setting down  <response>: SEPTIC TANK: Is a water-tight masonary tank into which household sewage is admitted for treatment Is a satisfactory method of disposing liquid and excreta wastes from individuals dwellings, small groups of houses or institutions which have ;adequate water supply but donot have access to a public sewerage system; Design features of a septic tank: Ideal retention period: 24 hours Steps of purification in a septic tank: Anaerobic digestion: Takes place in septic tank proper Aerobic oxidation: Takes place in sub-soil outside septic tank Ref: Park 25th edition ; Pgno: 797 