Which of the following statement regarding Septic tank is true ?

SEPTIC TANK It is water-tight masonry tank into which household sewage is admitted for treatment. It is satisfactory means of disposing excreta & liquid wastes from individual dwellings, small groups of houses & institutions which have adequate water supplies but do not have access to a public sewerage system. Main design features of septic tank are: - Capacity : Depends on number of users. A capacity of 20-30 gallons or 2 and half to 5 c.ft. / person is recommended for household septic tanks. Minimum capacity of a septic tank= at least 500 gallons. - Length is usually twice the breadth. - Depth = 1.5 to 2 m (5-7 ft.). - Recommended liquid depth = 1.2 m (4 ft.). - There should be a minimum air space of 30 cm (12 in.) b/w level of liquid in tank & undersurface of cover. - There are submerged inlet & outlet pipes. - In some septic tanks, bottom is sloping towards inlet end, facilitating retention of solids as sludge. - Septic tank is covered by a concrete slab of suitable thickness & provided with manhole. - These are designed in this country to allow a retention period of 24 hours. Too long retention period will result in undue septicity of effluent. Too sho period gives insufficient treatment. - Sewage is purified by anaerobic digestion in tank f/b aerobic oxidation outside the tank. - De-sludging should be done atleast once/ year.