Sludge Digestion

• Sludge with drawn from the sedimentation tank must be decomposed in a sludge digestion process.
• Decomposing: In the decomposition process, 40 to 60% of organic solids are converted by bacteria into CO2 , CH4

Sludge digestion process: Sludge gets broken up Into three following forms:

• Digested Sludge : Stable humans like solid matter with tary black in colour free of pathogenic bacteria may contain cysts, eggs of bacteria etc.

Vol = 1/3 vol. of undigested sludge Dried up and used as a fertilizer

• Supernatant liquor: Finely divided solid matter and liquid with BOD about 3000ppm Retreated in treatment plant along with raw sewage
• Gases of decomposition:Methane (65 to 70%)  Co2(30%), other gases like nitrogen, H2S are evolved methane gas is produced .

Stages of decomposition :

1. Acid fermentation
2. Acid regression
3. Alkaline fermentation .

Acid fermentation (or) Acid production stage: Acidic in nature.

Acid regression: BOD remains high during this stage

It becomes foamy and scum forms at top due to gases trapped.

Alkaline fermentation: 

• During this stage, liquid, digested solids and gases get separated.
• Alkaline in nature
• BOD falls rapidly
• Large volume of Methane gas along with small amount of other gases is evolved.

Factors affecting sludge digestion:

Temperature:

Rate of digestion is more at higher temperature Optimum: 

• Period: 30 days
• Temp: 29°C (Mesophilic bacteria will act)

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.

Seeding wish digested sludge: Proper seeding helps in balance condition.

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 x 1/2 months. However if the factor are controlled, duration can be shortened. 

Sludge and its moisture content(m.c):

• Sludge from sed.tank — 95% m.c
• Secondary sludge from trickling filter-96 to 98% m.c
• Secondary sludge from A.S.P —98 to 99%
• 95% mc means 5 lit solid matter in 100 lit sludge
• 90% means 10 lit solid matter in 100 lit sludge
• The sludge with 90% m.c. will be half the qnty as compared to that of sludge with 95% m.c.       
• V1 (100 - p) = V2(100 - p2)

Where 

V1 = volume of sludge at moisture content of p1%

V2 = volume of sludge at moisture contant of p2%

SLUDGE DIGESTION TANK

Detention time : Generally,30 days

Volume of sludge disaster: 

Vf = Volume of fresh sludge added/day

Vd= Volume of digested sludge added/day

T = digesting period

M = M of solids in sludge in kg/day