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With raw water, a filtrate quality is possible that has less than1 NTU,% removal of coliforms,% removal of Cryptosporidium and Giardia cysts,% removal of colour,% removal of Total Organic Content (WEDC, [ref_]). As a single process, slow sand filtration was ranked second most effective of all treatment processes, given a range of pathogenic, chemical and aesthetic factors.
Flow rates are usually around0.1 m/h but can increase up to0.4 m/h. Check out Flow Rates for more information.
Slow sand filters on the other hand, should ideally have an effective size of between0. –0.mm, and a Uniformity Coefficient of between1.5 –3, but preferably less than2.
Note that the unit metres/hour can be confusing, and is in fact a compaction of m3/m2/hour; that is, how much water volume can go through a certain area in one hour. Also sometimes this unit is expressed per day rather than per hour. For a fuller explanation of the relationship of water traveling through sand, check out Darcy’s Law explained.
