| This is the process of decomposing organic matter | | | | hydrogen sulfide. Intensive acid production lowers pH |
| of municipal sewage sludge anaerobically under | | | | value to less than 6. Highly putrefactive odors are |
| conditions of adequate operational control. During the | | | | evolved. |
| digestion of sludge, it is broken up into three | | | | 2. Liquefaction |
| different forms: | | | | In this stage, the organic acids and nitrogenous |
| (i) digested sludge which is a stable humus like solid | | | | compounds of the first stage are liquefied i.e., |
| matter with reduced moisture content | | | | transformed from large solid particles to either a |
| (ii) supernatant liquor which includes liquefied and | | | | soluble or finely dissolved form. The process is |
| finely divided solid matter, and | | | | brought about by hydrolysis using extra cellular |
| (iii) gases of decomposition like methane (CH4), | | | | enzymes. It is during this period, that the |
| carbon dioxide (CO2), nitrogen (N2) etc produced by | | | | intermediate products of fermentation viz., acid |
| microbial digestion. | | | | carbonates and ammonia compounds accumulate and |
| The digested sludge is de-watered, dried up and used | | | | the resulting gasification into H2 and CO2 is at a |
| as sewage sludge fertilizer while the gases produced | | | | minimum. The pH value rises a little to about 6.8, odor |
| are used as fuel or for driving gas engines. The | | | | is extremely offensive and the decomposing sludge |
| supernatant liquor is re-treated at the treatment plant | | | | entraps gases of decomposition, becomes foam and |
| along with the raw sewage. The tanks in which | | | | rises to the surface to form scum. This stage is |
| sludge digestion is carried out are called sludge | | | | known to last much longer than the proceeding stage |
| digestion tanks. | | | | of acidification and hence also termed as acid |
| Process Of Digestion Of Sludge | | | | regression. |
| Three stages are known to occur in the biological | | | | 3. Sewage Sludge Gasification |
| action involved in the process of digestion of sludge. | | | | It is the stage when more resistant materials like |
| These are (1) acidification (2) liquefaction or a period | | | | proteins and organic acids are broken up. Large |
| of acid digestion and (3) gasification or conversion of | | | | volumes of methane gas of high calorific value, along |
| acids into methane and carbon dioxide. | | | | with comparatively smaller volumes of carbon dioxide |
| 1. Acidification | | | | are evolved. The pH value goes to the alkaline range |
| As the fresh sewage-sludge begins to decompose | | | | i.e., above 7 and tarry odor appears. Gasification |
| anaerobically, bacteria attacks easily available food | | | | finally becomes very slow; the sludge becomes well |
| substances such as carbohydrates (sugars, starches, | | | | adjusted and is stable enough for disposal. This stage |
| and cellulose) and soluble nitrogenous compounds. | | | | in the digestion of sludge is also termed as alkaline |
| The products of decomposition are acid carbonates, | | | | fermentation. |
| organic acids with gases as carbon dioxide and | | | | |