<P> Smaller sand grains provide more surface area and therefore a higher decontamination of the inlet water, but it also requires more pumping energy to drive the fluid through the bed . A compromise is that most rapid pressure sand bed filters use grains in the range 0.6 to 1.2 mm although for specialist applications other sizes may be specified . Larger feed particles (> 100 micrometres) will tend to block the pores of the bed and turn it into a surface filter that blinds rapidly . Larger sand grains can be used to overcome this problem, but if significant amounts of large solids are in the feed they need to be removed upstream of the sand bed filter by a process such as settling . </P> <P> The depth of the sand bed is recommended to be around 0.6 - 1.8 m (2--6 ft) regardless of the application . This is linked to the maximum throughput discussed below . </P> <P> Guidance on the design of rapid sand bed filters suggests that they should be operated with a maximum flow rate of 9 m / m / hr (220 US gal / ft / hr). Using the required throughput and the maximum flowrate, the required area of the bed can be calculated . </P> <P> The final key design point is to be sure that the fluid is properly distributed across the bed and that there are no preferred fluid paths where the sand may be washed away and the filter be compromised . </P>

Using proper chemistry terms how can you separate sand from water