Passing floculated water through a rapid gravity sand filter strains out the floc and the particles trapped within it reducing numbers ofbacteria and removing most of the solids. The medium of the filter is sand of varying grades. Where taste and odour may be a problem (organoleptic impacts), the sand filter may include a layer of activated carbon to remove such taste and odour.
Sand filters become clogged with floc after a period in use and they are then backwashed or pressure washed to remove the floc. This backwash water is run into settling tanks so that the floc can settle out and it is then disposed of as waste material. The supernatant water is then run back into the treatment process or disposed of as a waste-water stream. In some countries the sludge may be used as a soil conditioner. Inadequate filter maintenance has been the cause of occasional drinking water contamination.
Sand filters are occasionally used in the treatment of sewage as a final polishing stage. In these filters the sand traps residual suspended material and bacteria and provides a physical matrix for bacterial decomposition of nitrogenous material, including ammonia and nitrates, into nitrogen gas.
All models are designed on brackish water(Max 10,000 ppm TDS) with velocity of 15 to 18 m/hr.
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Notable examples of plant resins include amber, Balm of Gilead, balsam, Canada balsam, Boswellia, copal from trees of Protium copal and Hymenaea courbaril, dammar gum from trees of the family Dipterocarpaceae, Dragon's blood from the dragon trees (Dracaena species), elemi, frankincense from Boswellia sacra, galbanum from Ferula gummosa, gum guaiacum from the lignum vitae trees of the genus Guaiacum, kauri gum from trees of Agathis australis, labdanum from mediterranean species of Cistus, mastic (plant resin) from the mastic tree Pistacia lentiscus, myrrh from shrubs of Commiphora, sandarac resin from Tetraclinis articulata, the national tree of Malta, styrax (a Benzoin resin from various Styrax species), Spinifex resin from Australian Spinifex grasses, and turpentine, distilled from pine resin. Amber is fossil resin (also called resinite) from coniferous and other tree species.
Copal, kauri gum, dammar and other resins may also be found as subfossil deposits. Subfossil copal can be distinguished from genuine fossil amber because it becomes tacky when a drop of a solvent such as acetone or chloroform is placed on it.