Slow sand filtration as a technique for the tertiary treatment of municipal sewages

The investigation was designed to demonstrate the viability, or otherwise, of slow sand filtration as a means of tertiary treatment for secondary effluents derived from conventional aerobic, biological treatment processes operating with municipal wastewaters. Secondary effluents derived from both an activated-sludge plant and from a percolating filtration plant were employed.The basic slow sand filtration unit used consisted of a 140 mm i.d. perspex cylinder, 2.65 m in height containing a 950 mm depth of fine sand. Treatment rates were either 3.5 or 7.0 m d⁻¹ and the sand used was of an effective size initially of 0.3 mm and then later of 0.6 mm.This investigation has demonstrated that a laboratory-scale slow sand filtration unit is capable of consistently removing at least 90% of the suspended solids, more than 65% of the remaining BOD and over 95% of the coliform organisms from the settled effluent from an operational percolating filter plant. The length of operational run averaged 20 days at 3.5 m d⁻¹ and 13 days at 7.0 m d⁻¹. Slightly inferior results were achieved when using the settled effluent from an operational activated sludge unit.Further investigation employing a horizontal-flow gravel pre-filter demonstrated that at flows of 2 m h⁻¹ with a contact time of 33 min up to 82% of the suspended solids in the secondary effluent could be removed prior even to slow sand filtration.