What is a multi-media filter?

A multi-media filter is generally one grade of crushed rock gravel, two grades of garnet and one grade of anthracite filtration media. The crushed rock is used for supporting the filter media and also to provide even distribution of flow during backwash. The course garnet is used as an interface media between the crushed rock and the finer garnet. The top layer (and coarsest) filtration media is the anthracite.

When is a multi-media filter required?

A multi-media filter is suggested 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 the above guidelines should be followed to prevent premature fouling of filtering membranes (reverse osmosis or ultrafiltration).

Clean effluent from a wastewater treatment plant may also be passed through a multi-media filter to remove any suspended solids.

How does the multi-media filter work?

The larger (but lighter) anthracite coal will be on top and the heavier (but smaller) garnet will remain on the bottom. The filter media arrangement allows the largest dirt particles to be removed near the top of the media bed with the smaller dirt particles being retained deeper and deeper in the media. This allows the entire bed to act as a filter allowing much longer filter run times between backwash and more efficient particulate removal. 

Media cleaning through automatic backwash.

The filter media should be cleaned on a routine basis. The length of the filtering cycle between cleaning sequences is dependent upon how dirty the source water is. Typical filtering cycles are in the 6 to 12-hour range. However, some applications allow for a much longer cycle, or in some cases, a shorter cycle.

The filtering cycle is determined by pressure differential between the influent and effluent gauges. Once the difference is approximately 10 PSID the media should be backwashed.

It is recommended that a filter be cleans at least once a day, regardless of application or differential pressure. The cleaning sequence of a filter system varies from one step (for simple systems) to as many as twenty (for more complex systems.) Regardless of the complex nature of the system, all filters are cleaned by reversing the water flow inside the filters. After backwash, the bed is allowed to settle and re-stratify before a final rinse and then it is placed back into service.

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A multi-media filter to remove suspended solids after wastewater treatment.