Which of the following is NOT a characteristic of rapid sand filters?

Rapid sand filters are primarily designed for the mechanical filtration of water, effectively removing larger particles and suspended solids. They operate through a bed of sand that captures these particulates as water flows through, making option C about removing dissolved pollutants the correct answer because rapid sand filters do not remove dissolved substances. While they are excellent for turbidity and large particle removal, dissolved contaminants often require additional treatment methods, such as chemical coagulation or adsorption.

Regular backwashing is crucial to maintain the effectiveness of rapid sand filters since the accumulation of trapped solids can lead to increased head loss and reduced flow. This ensures the longevity and efficiency of the filter. Additionally, these filters typically operate using gravity flow, relying on the weight of the water column above the filter to create pressure for filtration. Thus, options A, B, and D accurately describe key characteristics of rapid sand filters, while the inability to effectively remove dissolved pollutants differentiates option C.