WATER TREATMENT: RAPID SAND FILTERS
Rapid sand filters work at much higher rates of flow (up to 20 meters per hour) and essentially rely on physical removal of suspended solids, including any floc carried over from the settlers. Although rapid sand filters achieve some reduction in microbial populations in water as it removes particles to which bacteria are attached, it is not a biological treatment and the use of a terminal disinfectant is vital to ensure that bacteria in the water have been inactivated. Rapid sand filters require frequent cleaning (daily or twice daily) which is achieved through backwashing filters with clean water to re-suspended the sediment. Cleaning takes relatively little time and the filters can be put back into operation immediately.
Rapid sand filters are far smaller than slow sand filters and are commonly employed in
'batteries'. The rapid flow rate through these filters means that demand can be more easily met from smaller plants. Rapid sand filters do not require low influent turbidities, as they are essentially a physical treatment process, although higher suspended solids loads will result in more frequent cleaning. Backwashing is usually rapid and filters are not out of commission for mare than a matter of minutes. Cleaning and operation can be largely mechanised and air scour is commonly employed to make backwashing more effective. With the small land requirement, several rapid sand filters can be accommodated in small area and thus it is easy to maintain capacity to meet demand when filters are being cleaned.
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