WATER TREATMENT: PREFILTRATION
As many secondary filtration processes, and in particular slow sand filtration, require low influent turbidities, some form of pretreatment to reduce suspended solids load is required. One way to achieve this is by using prefiltration of water through coarse media, usually gravel or coarse sand. Prefilters can have many different configurations: horizontal; vertical upflow; and vertical upflow-downflow. Vertical prefilters have become increasingly popular as they require far less land than horizontal prefilters and can take faster flow runs through them. An alternative are pressure filters, through which water is pumped at pressure to remove the suspended solids
Prefilters have an advantage in that they do not require chemicals, have limited working parts and are robust. They do however, require frequent cleaning and maintenance and are ineffective in removing fine particles, thus where the suspended solid load is primarily made up of silt and clay particles prefiltration is ineffective. Prefiltration is a physical process designed to remove suspended solids and therefore it's efficiency in removal of microorganisms is a function of the microbes associated with particles. Virus removal is poor and prefiltration is not effective in the removal of cysts or bacteria associated with fine particles.
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