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The_Specialist gravatar image
RedR

Rapid gravity sand filters are difficult to operate and maintain and usually required mechanical plant for backwashing and preferably air scouring as well. They do no produce a very good microbiological quality in the treated filtrate (about 1 log reduction in indicator organisms) and they usually require coagulation, which requires a reliable supply of chemicals.

In contrast roughing filters followed by slow sand filters have no moving parts (well mechanically driven ones anyway), require no chemicals and produce a much better treated water quality. Although not complicated to operate and maintain, they do need careful management to maintain the high quality of treatment though.

No contest?