Backwashing (water treatment)

In terms of water treatment, including water purification and sewage treatment, backwashing refers to pumping water backwards through the filters media, sometimes including intermittent use of compressed air during the process. Backwashing is a form of preventive maintenance so that the filter media can be reused. In water treatment plants, backwashing can be an automated process that is run by local programmable logic controllers (PLCs). The backwash cycle is triggered after a set time interval, when the filter effluent turbidity is greater than a treatment guideline or when the differential pressure (head loss) across the filter exceeds a set value.

Water treatment filters that can be backwashed include rapid sand filters, water softening systems, pressure filters and granular activated carbon (GAC) filters. Diatomaceous earth filters are backwashed according to the proprietary arrangement of pumps, valves and filters associated with the filtration system.^[2] Slow sand filters and self-cleaning screen filters employ mechanisms other than backwashing to remove trapped particles. To keep water treatment filters functional, they have to be cleaned periodically to remove particulates. Ineffective backwashing is one of the main reasons that water treatment filters fail.