| FILTRATION The water leaving the sedimentation tank still contains floc particles. In order to reduce turbidity, a filtration process is normally used. Water filtration is a process for separating suspended or colloidal impurities from water by passage through a porous medium, usually a bed of sand or other medium. Water fills the pores between the sand particles, and the impurities are left behind, either clogged ion the open spaces or attached to the sand itself. There are several methods of classifying filters. One way is to classify them according to the type of medium used, such as sand, coal, dual media or mixed media. Rapid sand filters are cleaned in place by forcing water backwards through the sand. This operation is called backwashing. The washwater flow rate is such that the sand is expanded and the filtered particles are removed from the bed. After backwashing, the sand settles back into place. The largest particles settle first, resulting in a fine sand layer on top and a coarse sand layer on the botton. Rapid sand filters are the most common type of filter used in water treatment today. DISINFECTION Disinfection is used in water treatment to reduce pathogens (disease-producing microorganisms) to an acceptable level. Disinfection is not the same as sterilization. Sterilization implies the destruction of all living organisms. Drinking water need not be sterile. Three categories of human enteric pathogens are normally of consequence: bacteria, viruses,and amebic cysts. Purposeful disinfection must be capable of destroying all three. To be of practical service, such water disinfectants must possess the following properties: 1. They must destroy the kinds and numbers of pathogens that may be introduced into water within a practicable period of time over an expected range in water temperature. 2. They must meet possible fluctuations in composition, concentration, and condition of the waters or wastewaters to be treated. 3. They must be neither toxic to humans and domestic animals nor unpalatable or otherwise objectionable in required concentrations. 4. They must be dispensable at reasonable cost and safe and easy to store, transport, handle, and apply. 5. Their strength or concentration in the treated water must be determined easily, quickly, and automatically. 6. They must persist within disinfected water in a sufficient concentration to provide reasonable residual protection against its possible recontamination before use, or--- because this is not a normally attainable property--- the disappearance of residuals must be a warning that recontamination may have taken place. Previous Next...... back to My Research |