The correct answer is B. 30 days.
The acid regression stage of sludge digestion is the first stage of anaerobic digestion. It is a short stage that lasts for about 30 days at a temperature of 21°C. During this stage, the organic matter in the sludge is broken down by bacteria into simpler molecules, such as fatty acids and alcohols. These molecules are then further broken down into methane and carbon dioxide.
The acid regression stage is important because it helps to reduce the volume of sludge and produce methane, which can be used as a fuel. However, it is also a stage where the sludge can become acidic, which can damage the bacteria and reduce the efficiency of the digestion process. Therefore, it is important to monitor the pH of the sludge during this stage and to add lime or other alkaline materials if necessary to maintain a pH of 6.5-7.5.
The other options are incorrect because they are not the correct length of time for the acid regression stage of sludge digestion at a temperature of 21°C. Option A is 15 days, which is too short. Option C is 60 days, which is too long. Option D is 90 days, which is much too long.