Slow sand filtration involves removing material in suspension and/or dissolved in water by percolation at slow speed. In principle, a slow filter comprises a certain volume of areal surface, with or without construction of artificial containment, in which filtration sand is placed at a sufficient depth to allow free flow of water through the bed. When the available head loss reaches a limit of approximately 1 m, the filter must be pulled out of service, drained, and cleaned. The thickness of the usual sand layer is approximately of 1 to 1.50 m, but the formation of biochemically active deposits and clogging of the filter beds takes place in the few topmost centimeters of the bed. The filter mass is pored onto gravels of increasing permeability with each layer having a thickness of approximately 10 to 25 cm. The lower-permeability layer can reach a total thickness of 50 to 60 cm. So-called gravels 18 to 36 cm in size are used and their dimensions are gradually diminished to sizes of 10 to 12 cm or less for the upper support layer. The sand filter must be cleaned by removal of a few centimeters of the clogged layer. This layer is washed in a separate installation. Hie removal of the sand can be done manually or by mechanical means. The removed sand may not be replaced entirely by fresh sand. Placing preconditioned and washed sand is recommended as this takes into account the biochemical aspects involved in slow filtration. An alternative to manual or mechanical removal involves cleaning using a hydraulic system. Sometimes slow filtration is used without previous coagulation. This is generally practiced with water that does not contain much suspended matter. If the water is loaded (periodically or permanently) with clay particles in suspension, pretreatment by coagulation-flocculation is necessary. Previous adequate oxidation of the water, in this case preozonization producing biodegradable and metabolizable organic derivatives issuing from dissolved substances, can be favorable because of the biochemical activity in slow filters. There are several disadvantages to the use of slow filters. They may require a significant surface area and volume, and may therefore involve high investment costs. They are also not flexibile — mainly during the winter, when the open surface of the water can freeze. During the summer, if the filters are placed in the open air, algae may develop, leading to rapid clogging during a generally critical period of use. Algae often cause taste and odor problems in the filter effluent. Additional construction costs to cover slow filters are often necessary.
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