Before putting a filter into service, what is the minimum required backwashing expansion of sand?

The correct answer, which indicates a 3% minimum required backwashing expansion of sand before putting a filter into service, is based on the need to ensure effective and efficient operation of sand filters in water treatment processes.

During backwashing, the flow of water reverses through the filter media, which causes the sand grains to expand and agitate. This expansion is necessary to dislodge accumulated particles and debris from the sand granules, allowing them to be flushed out of the filter. A minimum of 3% expansion is crucial because it provides sufficient space for the media to maintain effective cleaning action and prevents channeling, which can lead to poor filtration performance.

If the expansion is less than the specified amount, the sand may not effectively clean itself, resulting in a buildup of contaminants in the media. This can ultimately lead to diminished filtration capacity and higher operational costs due to the need for more frequent cleaning or premature replacement of the filter media.

Moreover, while higher expansion rates could theoretically enhance the cleaning process, 3% is considered a balanced rate that ensures both effective filter cleaning and operational efficiency, aligning with industry standards for maintaining optimal water quality.