The correct answer is C. 30 days.
Sludge digestion is a process of anaerobic decomposition of organic matter in sludge. The process is carried out by a consortium of microorganisms, including bacteria, fungi, and protozoa. The microorganisms break down the organic matter into simpler compounds, such as carbon dioxide, methane, and water.
The rate of sludge digestion depends on a number of factors, including the temperature, pH, and the type of sludge. In normal conditions, the period for sludge digestion is 30 days. However, the period may be shorter or longer depending on the factors mentioned above.
Sludge digestion is an important process in wastewater treatment. It helps to reduce the volume of sludge and to produce a stabilized product that can be safely disposed of.
Option A is incorrect because the period for sludge digestion is typically longer than 10 days.
Option B is incorrect because the period for sludge digestion is typically longer than 20 days.
Option D is incorrect because the period for sludge digestion is typically shorter than 60 days.