Declining rate rapid sand filters start at ________ gpm/ft2 and decline to about ________ gpm/ft2.

Declining rate rapid sand filters are designed to initially operate at a higher flow rate to effectively handle the large volume of water and contaminants entering the filtration system. Typically, they start at around 3 gallons per minute per square foot (gpm/ft²). This higher initial flow helps to efficiently remove a greater amount of solids and turbidity from the water.

As the filter continues to operate, it becomes increasingly clogged with particulates, which reduces its efficiency. To maintain optimal performance and prevent breakthrough of contaminants, the flow rate is gradually reduced down to about 1 gpm/ft². This decline in flow rate allows the filter to continue operating effectively while minimizing the risk of clogging and ensuring that the water leaving the filter meets required quality standards.

Therefore, the statement that declining rate rapid sand filters start at 3 gpm/ft² and decline to about 1 gpm/ft² accurately reflects the operational parameters commonly used in water treatment facilities, making it the correct answer.