The correct answer is A. (i) and (ii).
Slow sand filters are a type of water filter that uses a bed of sand to remove suspended particles and bacteria from water. They are typically used in small communities or individual homes. Slow sand filters have a slower filtration rate than rapid sand filters, but they are more efficient in removing bacteria.
Slow sand filters work by passing water through a bed of sand. The sand traps suspended particles and bacteria, and the water slowly percolates through the bed. The sand is periodically cleaned by scraping off the top layer, which is called the schmutzdecke. The schmutzdecke is a biofilm that forms on the surface of the sand and helps to remove bacteria from the water.
Slow sand filters are a simple and effective way to filter water. They are relatively low-maintenance and do not require any chemicals. However, they do have a slower filtration rate than rapid sand filters.
Rapid sand filters are a type of water filter that uses a bed of sand to remove suspended particles and bacteria from water. They are typically used in large communities or industrial settings. Rapid sand filters have a higher filtration rate than slow sand filters, but they are not as efficient in removing bacteria.
Rapid sand filters work by passing water through a bed of sand. The sand traps suspended particles and bacteria, and the water quickly percolates through the bed. The sand is periodically cleaned by backwashing, which is a process of reversing the flow of water through the filter.
Rapid sand filters are a more efficient way to filter water than slow sand filters. They have a higher filtration rate and can remove more suspended particles and bacteria from water. However, they are more complex and require more maintenance than slow sand filters.