The first stage of natural process of sludge digestion is 

Explanation:

Sludge digestion process:-

• The sludge withdrawn from the sedimentation basins contains a lot of putrescible organic matter and if disposed of without any treatment, the organic matter may decompose, producing foul gases and a lot of nuisance, pollution, and health hazardous.
• In order to avoid such pollutions, the sludge is, first of all, stabilized by decomposing the organic matter under controlled anaerobic conditions and then disposed of suitably after drying on drying beds.
• The process of stabilization is called sludge digestion, and the tank where the process is carried out is called the sludge digestion tank.

Stages of sludge digestion process: 

The stages are:

1. Acid fermentation stage or acid production stage:

• In this stage of sludge digestion, the fresh sewage sludge begins to be acted upon by anaerobic and facultative bacteria, called acid formers.
• Intensive acid production makes the sludge highly acidic and lowers the PH value to less than 6.
• BOD of the sludge increases to some extent, during this stage.

2. Acid regression stage:

• In this intermediate stage, the volatile organic acids and nitrogenous compounds of the first stage are attacked by the bacteria, so as to form acid carbonates and ammonia compounds.
• The decomposed sludge has a very offensive odor, and its pH value rises up to 6.8.
• This stage continues for a period of about 3 months
• BOD of the sludge remains high even during this stage.

3. Alkaline fermentation stage:

• During this stage, the liquid separates out from solids, and the digested sludge is formed. This sludge is granular and stable and does not give offensive odors. The digested sludge is collected at the bottom of the digestion tank and is also called ripened sludge.
• Digested sludge is alkaline in nature and pH is slightly above 7.
• The BOD of the sludge also rapidly falls down during this stage.
• Large volumes of methane gas along with a small amount of carbon dioxide and nitrogen are evolved during this stage.