In modern sewage works, the sludge digestion tanks are circular in plan with flat or hopper-shaped bottom. These are usually covered from the top to retain the heat and odours and for collecting the sludge gas. If the sludge contains some grit the bottom is given a slope of 1 in 2 (vertical to horizontal).

Sufficient space is provided in the tanks for the complete digestion of the sludge. For taking out sludge liquor two or more outlets are provided at different levels. One pipe is provided to collect the sludge gas from the tank.

Sludge outlets are provided in the bottom to collect the digested sludge for its disposal. If the sludge digestion is to be done at different temperatures, then hot water coils are placed inside the tanks for heating the sludge.

In some tanks scum breaker arms are provided which break the scum. Most of the sludge digestion tanks are provided with stirring equipment for thorough mixing of the sludge. These stirs also serve the purpose of scraper and scrape the well digested materials and collect it near the digested sludge outlet pipe.


Some firms manufacture sludge digestion tank with floating covers. The floating cover permits the withdrawal of the supernatant liquor after or before the fresh sewage enters the tanks, so that it may not disturb the incoming sewage. Stirring, heating and seeding or recirculation are done by a pump in the tank.

Figs. 18.3, 18.4 and 18.5 show the details of various types of sludge digestion tanks, which are commonly used in the sewage treatment works.

Two Storey Ettling and Sludge Digestion Tank

Sludge Digestion Tank with Floating Cover

Sludge Digestion Tank with Scum Breaker, Heating Coils and Scraper