Water multimedia filter by far is the most common type of water filtration, they contain multi layers of media. Each layer is progressively sized in coarseness and layer depth. The media layers are stacked progressively with the coarsest and densest media, typically gravel, at the bottom and the lighter, finer media, at the top. A water multimedia filter is used to reduce the level of SDI (Silt Density Index), TSS (Total Suspended Solids) in the incoming feed water. Suspended solids consist of small particles such as silt, clay, grit, organic matter, algae and other microorganisms.
Incoming feed water that is high in suspended solids can cause a high pressure drop and reduce the effectiveness of downstream filtration equipment such as membranes, reverse osmosis membranes, electrodeionization EDI, UV sterilizer and ion exchange beds, if the multimedia filtration is not used as a pretreatment.
Because they can be easily cleaned, media filters are often used where there is a large amount of contamination, reducing the need for replacement filter cartridges or bags and operator effort. Media filters have the advantage over other types of filters in that they have the ability to be ‘backwashed’. Backwashing purges the filter and clean out the accumulated filtered particles and restores/extends the filters’ performance.
A multimedia filter is practically used when the Silt Density Index (SDI) value is greater than 3 or when the turbidity is greater than 0.2 NTU. There is no exact rule, but these guidelines should be followed to prevent premature fouling of RO or NF membranes. All major membranes’ manufacturers require the SDI to be treated to less than 3, otherwise the warrantee will not valid.
In a multimedia filter, there are multiple graded layers. The heavier layers become graded at the bottom and the lighter layers become graded at the top. Usually, the lighter layers are designed to have larger grains. This way larger contaminates are filtered out of the water before smaller contaminates, and the filtration efficiency for the volume of filter media is increased. The most common multimedia filter consists of sand and anthracite as the filtration media.
The sand has smaller grains and is heavier than the anthracite. This ensures that the sand layer settles beneath the anthracite and provides finer filtration. A well operated Multimedia Filter can remove particulates down to 20 microns. A Multimedia filter that uses a coagulant addition (which induces tiny particles to join together to form particles large enough to be filtered) can remove particulates down to 10 microns.