BOD stands for Biological Oxygen Demand, which is a measure of the amount of dissolved oxygen required by microorganisms to decompose the organic matter present in a sample of water or wastewater. In the context of a sludge digestion process, the BOD refers to the amount of organic matter present in the supernatant liquor.

The options given are:

a) 3000 ppm
b) 0 ppm
c) 1000 ppm
d) 250 ppm

To determine the correct answer, we need to consider the characteristics of the sludge digestion process and the expected concentration of organic matter in the supernatant liquor.

Sludge digestion is a biological process that converts organic matter in sludge into methane gas and stabilized solids. During this process, microorganisms break down organic compounds through anaerobic digestion, which occurs in the absence of oxygen. As a result, the organic matter in the sludge is reduced, and the BOD of the supernatant liquor should be significantly lower than the initial concentration.

Considering this information, we can eliminate options c) and d) as they suggest a higher BOD concentration, which is inconsistent with the sludge digestion process.

Option b) suggests a BOD concentration of 0 ppm, which is theoretically possible if all the organic matter has been completely digested by microorganisms. However, it is unlikely to achieve complete digestion in practice, so a BOD concentration of 0 ppm is not a realistic expectation.

Therefore, the correct answer is option a) 3000 ppm. This suggests that there is still a significant amount of organic matter present in the supernatant liquor after the sludge digestion process. The high BOD concentration indicates that further treatment or stabilization of the supernatant liquor may be required before it can be discharged or reused.

In summary, the BOD of the supernatant liquor in a sludge digestion process is expected to be 3000 ppm, indicating a significant concentration of organic matter that may require additional treatment.