High Rate Sand Filter Maintenance and Repair
The typical high rate sand filter is normally a round pressure vessel with a special effluent baffle in the top of the tank, a bed of filter sand and mechanical under-drain system to collect the filtered water and direct it to the return piping system.
The under-drain is often of a design which serves as a barrier to the migration of the sand bed, and as a backwash water distributor without the requirement of the stratified gravel bed which was traditionally used in the rapid sand systems.
High rate system differs from rapid sand filter substantially in its total plant size and hydraulic characteristics. Instead of operating at the traditionally flow rate of 200 litres/min/m square of filter surface area, it is operated at flow varying between 400 and 600 litres/min/m square of filter surface area.
Since the total flow requirements of a given pool can thus be satisfied through a sand bed of considerably smaller surface area than in the case of rapid sand systems, the physical plant at the high rate filter is much smaller as well. It is therefore possible, for example, to adequately filter 1680 meter cubic pool through a single high rate filter tank of 2.4m diameter X 4 meter long.
Higher flow rates drive the dirt load deeper into the filter bed, thus increasing the dirt holding capacity of the medium. It is normally backwashed at or near filter flow rate of 600 litres/min/m square.
The plastic under-drain system consists of pipes with very fine slots to prevent sand from passing through into the pool.
The filter medium is one single grade of silica sand, sized 0.4mm to 0.9mm.
Most filters are equipped with multi-port valves for services drain and backwashing.