methods are commonly used to do this. In the first, the top few millimetres of fine sand is scraped off to expose a new layer of clean sand. Water is then decanted back into the filter and re-circulated for a few hours to allow a new biofilm to develop. The filter is then filled to full volume and brought back into service. The second method, sometimes called wet harrowing, involves lowering the water level to just above the hypogeal layer, stirring the sand; thus precipitating any solids held in that layer and allowing the remaining water to wash through the sand. The filter column is then filled to full capacity and brought back into service. Wet harrowing can allow the filter to be brought back into service more quickly. Features Slow sand filters have a number of unique qualities: 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 under water, 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 bioclogging 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. While many municipal water treatment works will have 12 or more beds in use at any one time, smaller communities or households may only have one or two filter beds. In the base of each bed is a series of herringbone drains that are covered with a layer of pebbles which in turn