Disadvantages of using rapid sand filters
What are the disadvantages of using rapid sand filters as a process of water treatment compared to roughing filters followed by slow sand filters.
Engineering in Emergencies page 329 compares rapid and slow sand filters. Both types of filter are designed for the treatment of low turbidity water, so if the water to be treated has say turbidity > 20 NTU, pre-treatment such as roughing filters will be required independent of whether slow or rapid filtration is used. Basic Water Treatment, 5th edition by Chris Binnie and Martin Kimber, Atkins Water, UK gives greater detail.
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.
Nothing in life is free. Decreasing the footprint (area requirement) of most water treatment processes (hence increasing the rate) increases the energy requirement, complexity and potential for things to go wrong.
Another thing that you miss when going from slow sand to rapid sand filters is the schmutzdecke - a biological surface layer that you get with slow sand filtration. This can take out many contaminants that rapid sand filters can't.
That said I wouldn't quite agree that there is no contest. The massive benefit of rapid sand filters is the reduction in footprint per m3 of water treated. In high population density areas this often makes rapid sand filtration the only conventional choice.
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