The correct answer is A. A sedimentation tank and an activated sludge treatment plant.
A sedimentation tank is a type of wastewater treatment process that uses gravity to separate solids from liquids. The solids settle to the bottom of the tank, while the liquids flow out of the top. An activated sludge treatment plant is a type of wastewater treatment process that uses bacteria to break down organic matter. The bacteria are suspended in a mixture of wastewater and recycled sludge, which is called activated sludge. The activated sludge is then aerated, which helps the bacteria to grow and multiply. The bacteria then break down the organic matter in the wastewater, and the resulting sludge is separated from the treated wastewater.
A plant consisting of Imhoff tanks with low rate trickling filters is not a good option for treating the sewage of a large city. Imhoff tanks are a type of primary treatment process that uses sedimentation to remove solids from wastewater. Low rate trickling filters are a type of secondary treatment process that uses a bed of rocks or plastic media to filter out suspended solids and bacteria from wastewater. However, Imhoff tanks and low rate trickling filters are not effective at removing dissolved organic matter from wastewater. Therefore, they are not suitable for treating the sewage of a large city.
Sedimentation tanks with high rate trickling filters are a type of secondary treatment process that uses a bed of rocks or plastic media to filter out suspended solids and bacteria from wastewater. However, high rate trickling filters are not effective at removing dissolved organic matter from wastewater. Therefore, they are not suitable for treating the sewage of a large city.
In conclusion, the correct answer is A. A sedimentation tank and an activated sludge treatment plant.