Multimedia Filters

Multimedia Filters and Depth Filters: Produce filtered water and remove sediment efficiently.

Traditional filter systems use graded silica sand filter media. Since the sand grains all have about the same density, larger grains lay toward the bottom of the filter bed and finer grains lay at the top of the filter bed. As a result, filtration occurs only within the first few inches of the finer grains at the top of the bed. A depth filter has four layers of filtration media, each of different size and density which allows it to filter water more efficiently. It also decreases the chance of fouling – called “mud-balling” – within the filter.

How depth filter systems work.

A depth filter has four layers of filtration media, each of different size and density. Light, coarse material lays at the top of the filter bed. The media become progressively finer and denser in the lower layers. Larger suspended particles are removed by the upper layers while smaller particles are removed in the lower layers. Particles are trapped throughout the bed filter inlet and outlet increases by 5 – 10 psi (34 to 68 kPa) from the beginning of the cycle, the filter should be reconditioned. Operating beyond this pressure drop increases the chance of fouling – called “mud-balling” – within the filter.

The reconditioning cycle consists of an upflow backwash followed by a downflow rinse. Backwash is an upflow operation, at about 14 gpm per square foot (34 m/hr) of filter bed area that lasts about 10 minutes. Turbidity washes out of the filter bed as the filter media particles scour one another. The downflow rinse settles the bed before the filter returns to service. Fast rinse lasts about 5 to 10 minutes.

Chemical pretreatment is often used to enhance filter performance, particularly when turbidity includes fine colloidal particles. Suspended particles are usually electrically charged. Feeding chemicals such as alum (aluminum sulfate), ferric chloride, or a cationic polymer neutralizes the charge, allowing the particles to cling to one another and to the filter media. Chemical pretreatment may increase filtered water clarity, measured in NTU, by 90% compared with filtration alone. If an operator is present to make adjustments for variations in the raw water, filtered water clarity improvements in the range of 93 to 95% are achievable.

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