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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?