The digestion of sludge is a biological wastewater treatment process that anaerobically decomposes the organic matter found in the sludge of municipal sewage under sufficient operational control conditions. The sludge digestion process is broken into three varying forms.

1. Digested sludge that’s stable humus such as solid matter with decreased moisture content.
2. Supernatant liquor that comprises liquefied as well as finely separated solid matter.

• Decomposition gases like carbon dioxide (CO₂), methane (CH₄), and so on, which are produced from microbial digestion.

The sludge that’sdigested gets de-watered, is dried up and is utilised as sewage sludge fertiliser whilst the gases that are produced are utilised as fuel or to drive gas engines. The supernatant liquid gets re-treated at the plant together with the raw sewage. The tanks inside which sludge digestion is accomplished are known as sludge digestion tanks.

Sludge digestion process

Three stages are known to take place in the biological action that’s involved in the sludge digestion process. These are acidification, liquefaction or an acid digestion period, and gasification or the conversion of acids into carbon dioxide and methane.

1. Acidification

Here, the new sewage-sludge starts to anaerobically decompose, bacteria attacks any food substances like carbohydrates (starches, sugars, and cellulose) and soluble nitrogenous compounds that are available, quite easily. The end products of decomposition comprise acid carbonates, and organic acids together with gases like hydrogen sulphide and carbon dioxide. The intensive production of acid decreases the pH value to below 6. Highly putrefactive odours are then evolved from the process. This is among reasons why you sometimes perceive odours while at a biological wastewater treatment plant.

2. Liquefaction

In this step, the nitrogenous compounds and organic acids of the first step are liquefied, i.e. transformed into either a finely dissolved or soluble form. The process is caused by hydrolysis using additional cellular enzymes. It’s during this period that fermentation’s intermediate products viz, ammonia compounds and acid carbonates accumulate and the resultant gasification into CO₂and H₂ is at its minimum. The pH value increases a bit to around 6.8, the odour is very offensive, and decomposing sludge entraps decomposition gases, turns into foam, and rises to form a scum on the surface. This step lasts quite longer than the acidification or processing step and hence is also called acid regression.

3. Sewage sludge gasification

This is the step in which more resistant materials such as organic acids and proteins are broken up. Large methane gas volumes of high calorific value, together with smaller carbon dioxide volumes are evolved. The pH value rises to the alkaline range, i.e. over 7 and tarry odour emerges. Gasification eventually becomes slow; the sludge turns truly adjusted and becomes stable enough to be disposed of. This step in sludge digestion is also known as alkaline fermentation.

In conclusion, these are the three basic steps of the biological wastewater treatment process that is known as sludge digestion. There are lots of trending technologies that make the process a lot easier and enable you to effectively treat wastewater in truly large quantities. The information offered here should give you a clear understanding of the process and help you choose the most appropriate technology.