ANS:A - a sedimentation tank and an activated sludge treatment plant

For treating the sewage of a large city, a combination of treatment processes is typically required to effectively remove pollutants and ensure the quality of the treated effluent. The recommended treatment system depends on various factors including the characteristics of the sewage, the size of the city, environmental regulations, and budget constraints. Given the size of the city and the need for effective treatment, a combination of advanced treatment processes is often necessary. Here are the options provided and their considerations:

1. A sedimentation tank and an activated sludge treatment plant: This combination offers effective treatment for both suspended solids and organic matter. The sedimentation tank removes settleable solids, while the activated sludge process provides biological treatment, breaking down organic pollutants. This option is suitable for large cities where high levels of organic matter and suspended solids are present in the sewage.
2. A plant consisting of Imhoff tanks with low rate trickling filters: Imhoff tanks are primarily used for primary sedimentation to remove settleable solids, while low-rate trickling filters provide biological treatment by allowing microorganisms to degrade organic pollutants. This combination may be effective for smaller cities or areas with lower sewage loads, but it might not provide sufficient treatment for the sewage of a large city.
3. Sedimentation tanks with high rate trickling filters: Sedimentation tanks are effective for primary treatment to remove settleable solids, while high-rate trickling filters provide biological treatment. This combination is suitable for areas where space is limited, but it may not offer the same level of treatment as the activated sludge process.

Considering the need for effective treatment of sewage from a large city, the recommended option would be a sedimentation tank and an activated sludge treatment plant. This combination offers both physical and biological treatment processes, which are essential for handling the diverse pollutants present in sewage from a densely populated urban area.