Preferable because of both cost and space requirements.
Finer sands are used along with a controlled-distribution flow pattern.
Freeboard is typically 50% of the sand depth.
The backwash ratio is 1:1 (as opposed to rapid sand where it is 1:4 or 1:5).
High rate sand filters work on the principle of both surface and depth entrapment of solids.
The minimum diameter of horizontal fiberglass tanks is 42”. 36” diameter tanks are discouraged because of short fitter runs due to shallow bed depths. Use 36” diameter tanks only on retrofit work where getting the filter through doorways and halls are issues.
Working pressure of filters must be no less than 50 PSI.
While many manufacturers rate their system at 20 GPM/ft2, field experience has shown that the lower flow rate results in better water quality. As a result, most health departments require a maximum of 15 GPM/ft2 of filter area. The system should be designed to completely turn over the respective pool volume as per code.
Backwash rate must be 15 GPM/ft2.
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