Short note: Need of sludge treatment

Sludge is the semi-solid material (95-99% water) that settles out in the primary and secondary sedimentation tanks, or is removed from screens and filters, in a sewage treatment plant. It is putrescible and contains pathogens, so it needs treatment before disposal.

Need of sludge treatment

1. Control of odour and nuisance: fresh sludge from a primary tank putrefies within hours, giving hydrogen sulphide and other foul gases and attracting flies. Example: untreated sludge dumped near a settlement gives a smell nuisance; digestion makes it inoffensive.
2. Pathogen destruction: raw sludge from domestic sewage carries bacteria, helminth eggs and viruses. Example: using raw sludge as manure on vegetables can transmit worm infection; digestion and drying kill most of them.
3. Volume reduction: the sludge from a plant is about 0.25-0.5% of the sewage volume but contains 95% water. Thickening and dewatering reduce the volume from, say, 100 m³ at 5% solids to 25 m³ at 20%, which lowers the cost of transport.
4. Stabilisation of organic matter: anaerobic digestion converts about half of the volatile solids to gas, leaving inert humus-like matter.
5. Energy and resource recovery: digester gas (about 65% methane) is used for heating and power; dried sludge is used as soil conditioner.
6. Environmental protection: untreated sludge discharged into a river creates sludge banks and oxygen depletion.