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Improvement of water quality
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With pre-treated raw water, a filtrate quality of less than 1 NTU, 90 percent coliform removal, 50–90 percent Cryptosporidium and Giardia cyst removal, 10% colour removal, and 5% Total Organic Content reduction is attainable.
Given a variety of pathogenic, chemical, and aesthetic criteria, quick sand filtering is judged third most effective of all treatment techniques as a single step.
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With raw water, a filtrate with less than 1 NTU, 95 percent coliform removal, 99 percent Cryptosporidium and Giardia cyst removal, 75 percent colour removal, and 10% Total Organic Content removal is possible.
Given a variety of pathogenic, chemical, and aesthetic criteria, slow sand filtration is judged second most effective of all treatment techniques as a single step.
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Rate of flow
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A rapid sand filter may produce a flow of between 4 and 21 m/h, which is 20 to 50 times faster than slow sand filtration.
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Flow rates are typically around 0.1 m/h, but they can reach 0.4 m/h.
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Filter media
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Rapid sand filters are manufactured with graded sand and, in some cases, a coarser layer of material on top of the sand to improve flow rate (for example, anthracite), in which case they are referred to as dual-media filters. Rapid filters typically have an effective size of greater than 0.55mm and a Uniformity Coefficient of less than 1.5.
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Slow sand filters, on the other hand, should have an effective size of 0.15–0.35mm and a Uniformity Coefficient of 1.5–3, with a preference for less than 2.
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Penetration of solid matter
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Rapid sand filters, which are usually cleaned by backwashing, have a deeper penetration of suspended particles into the sand bed.
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Solids only penetrate slow sand filter beds by 0.5 to 2 cm on average, allowing for more hand cleaning procedures.
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Pre-treatment
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For rapid sand filtration, pre-treatment is frequently required. Coagulation and flocculation, followed by sedimentation, could be used in this treatment.
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Raw waters with turbidity’s less than 50 NTU normally don't require any pre-treatment. Coagulation and flocculation are not recommended as pre-treatments for slow sand filters due to the risk of floc carryover.
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