Anthracite

Backwashing a drinking water system filter means reversing and increasing the water’s flow to flush out accumulated debris and particles. Backwashing is not only vital to the life of a filter, it is fundamental to the quality of water coming out of the filter. Sooner or later, all filters need to be backwashed or replaced. What is filter backwashing? Most drinking water systems use filters to collect, catch, or gather particles from an incoming flow. When the filter’s pores become clogged, they need to be cleaned. One of the best ways to clean a drinking water system’s filter is to backwash it, meaning reversing the flow and increasing the velocity at which water passes back through the filter. This, in effect, blasts the clogged particles off of the filter. Although every filter is unique, the principles of backwashing are similar for all of them. One key ingredient to a good filter backwash is using clean water, usually out of the clear well, first storage tank, or distribution system. This Tech Brief examines the most common filters: conventional and direct filtration. These filters use either pressure or rapid rate gravity processes. When to backwash? The cleanliness or cloudiness (turbidity) of the water coming out of the filter just before it goes into the clear well is the best way to determine when to backwash. A good rule of thumb is 0.1 nephelometric turbidity units (NTU) on each individual filter’s effluent. This may sound a little extreme when the combined filter effluent (CFE) for conventional and direct filtration systems is 0.3 NTU. But 0.1 NTU gives the operator time to react to any problems within the treatment system. This photo shows a cross-section structure of a typical rapid sand filter