The use of air scouring (purging to assist the cleaning action) during backwashing is almost universal practice (see Figure 7.33.7). With mono-media filters, wastewater treatment facilities can use high air and backwash water rates without disrupting the bed layers or carrying the lighter components of the bed out of the filter.
Downflow filters eliminate the tendency of upflow units to fluidize or break through, especially near the end of the filtration cycle when pressure drop across the filter is substantial. Both gravity and pressure-motivated filters are used in waste treatment, but pressure units have deeper beds, higher capacities, and the ability to be completely
DROP (FT. H-O) CONCENTRATION
, OF FILTRATE
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