This is the process of decomposing organic matter of municipal sewage sludge anaerobically under conditions of adequate operational control. During the digestion of sludge, it is broken up into three different forms:

(i) digested sludge which is a stable humus like solid matter with reduced moisture content

(ii) supernatant liquor which includes liquefied and finely divided solid matter, and

(iii) gases of decomposition like methane (CH4), carbon dioxide (CO2), nitrogen (N2) etc produced by microbial digestion.

The digested sludge is de-watered, dried up and used as sewage sludge fertilizer while the gases produced are used as fuel or for driving gas engines. The supernatant liquor is re-treated at the treatment plant along with the raw sewage. The tanks in which sludge digestion is carried out are called sludge digestion tanks.

Process Of Digestion Of Sludge

Three stages are known to occur in the biological action involved in the process of digestion of sludge. These are (1) acidification (2) liquefaction or a period of acid digestion and (3) gasification or conversion of acids into methane and carbon dioxide.

1. Acidification

As the fresh sewage-sludge begins to decompose anaerobically, bacteria attacks easily available food substances such as carbohydrates (sugars, starches, and cellulose) and soluble nitrogenous compounds. The products of decomposition are acid carbonates, organic acids with gases as carbon dioxide and hydrogen sulfide. Intensive acid production lowers pH value to less than 6. Highly putrefactive odors are evolved.

2. Liquefaction

In this stage, the organic acids and nitrogenous compounds of the first stage are liquefied i.e., transformed from large solid particles to either a soluble or finely dissolved form. The process is brought about by hydrolysis using extra cellular enzymes. It is during this period, that the intermediate products of fermentation viz., acid carbonates and ammonia compounds accumulate and the resulting gasification into H2 and CO2 is at a minimum. The pH value rises a little to about 6.8, odor is extremely offensive and the decomposing sludge entraps gases of decomposition, becomes foam and rises to the surface to form scum. This stage is known to last much longer than the proceeding stage of acidification and hence also termed as acid regression.

3. Sewage Sludge Gasification

It is the stage when more resistant materials like proteins and organic acids are broken up. Large volumes of methane gas of high calorific value, along with comparatively smaller volumes of carbon dioxide are evolved. The pH value goes to the alkaline range i.e., above 7 and tarry odor appears. Gasification finally becomes very slow; the sludge becomes well adjusted and is stable enough for disposal. This stage in the digestion of sludge is also termed as alkaline fermentation.