TCEQ Class C Surface Water License Practice Exam

Under normal conditions a rapid sand filter operates at 2 gpm/ft square. During the backwash cycle, the backwash rate should be _______________ per ft square.

15 gpm

20 gpm

35 gpm

16 gpm

The correct backwash rate for a rapid sand filter during its backwash cycle is typically set to ensure effective cleaning of the filter media. A rate of 16 gallons per minute per square foot is sufficient to fluidize the sand, dislodge trapped particles, and wash away any accumulated impurities.

During backwashing, the flow rate is significantly higher than the normal operating rate to achieve the desired turbulence within the filter bed. Excess water flows through the filter in reverse, effectively lifting and expanding the sand media, which allows for the removal of contaminants that have built up during the filtration process.

Choosing a backwash rate within the commonly accepted range for rapid sand filters typically falls between 10 to 20 gpm per square foot, so the selected value of 16 gpm aligns with standard practices for maintaining filter efficiency while avoiding potential damage to the filter media. Rates that exceed this could lead to issues such as sand loss or disruption of the filter bed structure.