ANS:D - 90 days

Extending the acid regression stage of sludge digestion to 90 days at a temperature of 21°C is less common and typically not desirable in practical wastewater treatment processes. During the acid regression stage, which usually lasts for 15 to 30 days, acidogenic bacteria break down complex organic compounds present in the sludge into simpler molecules such as volatile fatty acids (VFAs), alcohols, and hydrogen gas. This stage is crucial for preparing the sludge for further digestion processes. Extending the acid regression stage to 90 days may indicate an inefficient or problematic digestion process. Prolonging this stage excessively may lead to several issues:

1. Incomplete digestion: While acidogenesis is necessary to break down complex organic matter, excessively long acid regression stages may result in incomplete digestion, leaving behind organic compounds that are difficult to degrade further.
2. Increased volatile solids: Extended acid regression can lead to an accumulation of volatile solids in the digester, which may cause stability issues and inhibit the performance of subsequent digestion stages.
3. Biogas production inefficiency: Prolonged acidogenesis may affect the efficiency of biogas production in the subsequent methanogenesis stage. Methanogenic bacteria rely on the products of acidogenesis for their metabolism, and if the acid regression stage is excessively long, it can limit the availability of these substrates for methane production.
4. Operational inefficiency: Operating the digestion process for an extended period without progressing to the next stages can be costly in terms of energy, resources, and time.

Therefore, while it's theoretically possible for the acid regression stage to extend to 90 days, it's not typically recommended or practiced in efficient sludge digestion processes. Monitoring and optimizing the digestion process to ensure timely progression through each stage is essential for effective wastewater treatment.