Following factors influence to a great extent the above mentioned three stages of sludge digestion: 1. Temperature 2. Sludge Seeding 3. Mixing 4. pH- Value 5. Miscellaneous Conditions.

Factor # 1. Temperature:

An aerobic decomposition of organic matters in sewage is greatly influenced by temperature at which it takes place. The above mentioned three stages are based at a temperature of about 21 °C.

There are two distinct temperature zones in sludge digestion:

(a) Zone of thermophilic digestion


(b) Zone of mesophilic digestion

Fig. 18.2 shows the effect of temperature on the digestion period. In zone of thermophilic digestion, thermophilic organisms are more active and it ranges from 35°C to 60°C. The optimum temperature in this zone is about 54°C and at this temperature, the digestion period can be brought down to about 7 to 10 days only.

Effect of Temperature on the Digestion Period

In zone of mesohilic digestion, common or mesophilic organisms are more active and its practical range is 26°C to 35°C. The optimum temperature in this zone is about 29°C and at this temperature the digestion period can be brought down to about 30 days only.


In general, it may be stated that higher the temperatures lower is the period of digestion. The production of gas is also more at high temperature. In practice, mesophilic digestion of sludge is preferred to thermophilic digestion.

Factor # 2. Sludge Seeding:

Process of introducing fresh sludge into previously well digested sludge is known as ‘sludge seeding’ and it should be done in such a way that alkaline condition may prevail in the tank. Usually about 2.5% to 3.5% by weight of fresh, dry solids in dry digested sludge could be added per day so as to maintain the pH- value of about 7.1 % so.

The lime may be used for the purpose of attaining the desired pH- value, and in that case the above percentage can still be increased to about 5.5. Proper seeding of sludge develops favourable conditions for m.o. and digestion of sludge takes place in a short period.

Factor # 3. Mixing:

Incoming fresh raw sludge should be thoroughly mixed with digested sludge by some effective method of mechanical agitation. The proper stirring results in even distribution of the incoming sludge, maintenance of uniform temp, in the tank, reduction of scum and increase in the production of gas.

Factor # 4. pH- Value:


Desired pH range for the efficient digestion of sludge is 6.8 to 7.2 and to maintain the pH- value within this range, the following remedies may be applied:

(a) Adding of lime, or

(b) Introducing the required quantity of raw sludge, or

(c) Controlling the removal of digested sludge.

Factor # 5. Miscellaneous Conditions:


The other factors which influence the process of sludge digestion are as follows:

(a) Quality of public water supply.

(b) Presence of copper, fluorides, radioactive substances etc.