<P> Slow sand filters have a number of unique qualities: </P> <Ol> <Li> 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 . </Li> <Li> 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 under water, and the water used for cleaning is drained to waste . </Li> <Li> 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 . </Li> <Li> 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 bioclogging occurs more quickly and pre-treatment is recommended . </Li> <Li> 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 . </Li> </Ol> <Li> 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 . </Li> <Li> 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 under water, and the water used for cleaning is drained to waste . </Li>

The period of cleaning of a slow sand filter is usually