The Process and Water Quality Specialists

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Granular Media Filtration

Conventional filtration involves passing the treatment flow through a bed of media that is typically made of sand (mono-media) or sand and anthracite (dual-media), where suspended particles are physically removed by attachment to the media or previously removed particles. A granular media filter can only be operated for a certain amount of time before the removed solids result in enough head loss that the filter media requires cleaning. The cleaning process is referred to as backwashing and involves flushing water (and possibly air) backwards through the filter at a high rate. Depending on the particle loading, conventional filters are typically backwashed roughly every 0.5 to 2 days.

Targeted particles for removal from the water and the filter media itself are both typically negatively charged and thus are naturally repulsed from each other. Filter pretreatment is commonly required through the addition of a coagulant to achieve good particle attachment efficiency and thus effective filtration. For different water qualities and applications, the coagulant addition and pretreatment process will differ and ranges from (a) only coagulant addition (i.e. inline filtration), to (b) coagulation and flocculation (i.e. direct filtration), and (c) coagulation, flocculation and sedimentation.