Multi Media Filters

Pressure sand filters are also referred as "Dual Media / Multimedia filters". They are used to filters the water to remove the turbidity & suspended solids in water. They have commonly employed filtration units and largely found in the following sectors.

1. Secondary filtration treatment in Raw water treatment unit
2. Post biological treatment and/ or secondary clarifier in Effluent treatment units
3. Pre-treatment of water in a De-mineralization water plant
4. Pre-treatment step to tertiary treatment of water like Ultra filtration or Reverse Osmosis unit

Dual Media sand filters 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 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.