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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.