8.7.5 Efficiency or Performance of Rapid Sand Filter.

1] colour : Rapid sand filter is efficient in removal of colour. The intensity of colour is reduced below three on cobalt scole. Colourless can be produced if poly-electrolytes are added to water before filtration.

2] Taste and Odours: Taste and odours are not removed in rapid sand filters.

3] Turbidity : Filters can reduce the turbidity to the extent of 1 N.T.U. if the turbidity of applied raw water is less than 35 to 40 $\mathrm{mg} / /$ which is possible due to chemically assisted sedimentation.

4] Iron and Manganese: Oxidised or oxidising iron is removed but it is less efficient in removing manganese.

5] Bacteria Removal Efficiency: In cases of low bacterial loadings, it removes bacteria to the extent of $90-99 \%$ . In comparison to slow sand filters, rapid filters are less efficient. Chlorination is recommended after filtration.

6] Wash Water Consumption: The filter is said to be efficient if wash water consumption does not exceed 2$\%$ of the filtered quantity.