Unlike other filtration methods, slow sand filters use biological processes to clean the water and are non-pressurized systems. Slow sand filters do not require chemicals or electricity to operate. Cleaning is traditionally done by use of a mechanical scraper, which is usually driven into the filter bed once the bed has been dried out. However, some slow sand filter operators use a method called “wet harrowing”, where the sand is scraped while still underwater, and the water used for cleaning is drained to waste. For municipal systems, there usually is a certain degree of redundancy, since it is desirable for the maximum required throughput of water to be achievable with one or more beds out of service. Slow sand filters require relatively low turbidity levels to operate efficiently. In summer conditions with high microbial activity and in conditions when the raw water is turbid, blinding of the filters due to bio-clogging occurs more quickly and pre-treatment is recommended. Unlike other water filtration technologies that produce water on demand, slow sand filters produce water at a slow, constant flow rate and are usually used in conjunction with a storage tank for peak usage. This slow rate is necessary for healthy development of the biological processes in the filter.
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