Pressure sand filters / Multi Media Filter are used to filters the water to remove the turbidity & suspended solids in water.
Pressure sand filters / Multi Media Filter have commonly employed filtration units and largely found in the following sectors.
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Secondary filtration treatment in Raw water treatment unit |
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Post biological treatment and/ or secondary clarifier in Effluent treatment units |
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Pre-treatment of water in a De-mineralization water plant |
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Pre-treatment step to tertiary treatment of water like Ultra filtration or Reverse Osmosis unit. |
Pressure sand filters / Multi Media Filter are of steel construction with the system operating on the feed water pumping pressure. Filters are designed based on specific volumetric flow rate per unit cross sectional filtration area. Multiple sand filter vessels are installed to augment the treatment to the required capacity from a logistic point of view. The filter is complete with inlet distributors, bottom collector/ outlet pipe, vent arrangement, water inlet and outlet nozzles, drain and a pressure safety valve for vessel safety. The filter bed is made of selected layers of sand of various grain sizes.
The sand / multi media filter bed can either be supported by a bed plate with nozzle arrangement or simply filled over a support gravel media. Both models are provided and the selection is done on client’s requirement. Ergonomically designed nozzles of poly propylene nozzles are selected and arranged in a specific fashion for equal distribution and at low pressure drops across the filter nozzles for sand filters having bed plate support arrangement.
Multi layered gravel with different sizes are used as support media where the sand filters are designed with header–lateral arrangement. The water flow from the top inlet distributor, downwards through the filter bed & goes out from the bottom outlet .The filter pump is sized to have adequate head to discharge filtered water to the filtered water tank or further process units.
During the service cycle, the filter bed gets clogged with the inlet turbidity & suspended solids which increase the pressure drop across the filter bed. The sand bed also gets compacted during the continuous service cycle. Hence, it is necessary to periodically backwash the filter bed to remove the accumulated turbidity & suspended solids & also to loosen the filter bed. The frequency of backwash is dependent upon the inlet conditions.
The single media filters have the limitations to accept larger turbidity & suspended solids & clogs faster necessitating frequent backwash.
In such a situation dual media filters with sand at the bottom & layer of anthracite will extend the service cycle.
The backwashing of filters is done with the inlet water or with the filtered water. If it is to be done with the filtered water separate storage & backwash pumps of suitable capacity & head will be required.
To help effective backwashing air scouring is also used. The air scouring is done with low pressure air of 0.35 to 0.4 kg/cm2g & generally with 50 cum/m2 flow rate.
Various air scouring methods are adopted. For example air scouring first & followed by water backwash, simultaneous air & water wash. In the case of simultaneous backwash with air separate air grid is provided at the bottom. The selection of media depends on outlet quality required. The filters can be designed for manual or auto operation. In the case of auto operation, PLC can be programmed for full automation with the provision of necessary instruments & control.
