Sludge Digestion Process in Waste Water Treatment Process

Sludge withdrawn from the sedimentation tank must be decomposed in a sludge digestion process. In the decomposition process, 40% to 60% of organic solids are converted by bacteria into CO2, CH4 ( anaerobically)

1. Sludge digestion process

In sludge digestion process, the sludge gets broken up into three following forms.

a. Digested sludge

Stable humans like solid matter with vary black in colour free from pathogenic bacteria may contain cysts, eggs of bacteria etc..

Volume= 1/3 volume of undigested sludge

Dried up and used as a fertilizer

b. Supernatant liquor

Supernatant liquor is a finely divided solid matter and liquid with BOD about 3000 ppm. Supernatant liquor is retreated in the treatment plant along with raw sewage.

c. Gases of decomposition

Methane ( 65 to 70%), CO2 (30%), Other gases like nitrogen, H2S are evolved methane gas is produced.

2. Stages of decomposition process

a) Acid fermentation or Acid production stage.

Acid fermentation stage is acidic in nature.

b)  Acid regression

In acid regression, BOD remains high during this stage. It becomes foamy and scum forms at top due to gases trapped.

c) Alkaline fermentation

In alkaline fermentation stage, liquid, digested solids and gases get separated.

Alkaline in nature and BOD falls rapidly

A large volume of methane gas along with small amount of other gases is evolved.

Sludge digestion process

3. Factors affecting sludge digestion process

a) Temperature

Rate of digestion is more at higher temperature


a. Period : 30 days

b. Temperature: 29-degree Celcius.

b) pH value

Alkaline conditions must prevail ( Optimum pH = 7.2 TO 7.4 ) to encourage bacterial action.

To increase the pH, hydrated lime is added

c) Seeding with digested sludge

Proper seeding helps in balance condition

d) Mixing and stirring of raw sludge with digested sludge

Bacterial enzymes present in digested sludge should be mixed thoroughly for better decomposition.

Under uncontrolled naturally condition, sludge digestion takes about 4.5 months. However, if the factors are controlled, the duration can be shortened.

4. Sludge and its moisture content (m.c)

a) Sludge from sedimentation tank- 95% m.c (moisture content)

b) The secondary sludge from trickling filter– 96 to 98% of m.c

c) Secondary sludge from A.S.P ( Activated sludge process)- 98 to 99%. 95% ms means 5-litre solid matter in 100-litre sludge.

90% m.c ( moisture content) means 10-litre solid matter in 100-litre sludge.

Therefore the sludge with 90% m.c will be half the quantity as compared to that of sludge with 95% m.c

Sludge digestion process


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