Multi media filters typically utilize three or four layers of media for multimedia filtration: anthracite, sand, garnet, and gravel. These media are often chosen for use in multimedia filters due to the distinct differences in their densities. Anthracite is the lightest filtration media per unit volume, followed by sand, and then garnet. . A very high degree of clarity is achieved in the filtered water on account of finer particles of garnet at the bottom which trap finer turbidity particles. and garnet - the heaviest, finest media at the bottom. Turbidity is trapped and held throughout the entire bed depth, rather than the top 1 or 2 in.

Multi media filter is used to reduce the level of suspended solids (turbidity) in process water. Suspended solids can consist of small particles such as silt, clay, grit, organic matter, larvae, zooplankton, algae, and other microorganisms. The filtration degree of a multi-media filter depends heavily on the filter media and flow (velocity) through the filter.

Multimedia filtration which utilizes three or more different media as opposed to a “sand filter” that typically uses one grade of sand alone as the filtration media. In a single media filter, The filter run times are thus very short before the filter “blinds” or develops so much head pressure that it must be backwashed.  In a single media filter, to avoid seriously impeding or stopping the flow during the “settling” cycle, the finest or smallest media particles remain on top of the media bed while the larger, and heavier particles, stratify proportional to their mass lower in the filter. This results in very limited use of the media depth since virtually all filterable particles are trapped at the very top of the filter bed or within 1-2 inches of the top where the filter media particles have the least space between them.

Anthracite/sand filter beds normally provide all of the advantages of single-media filtration but require less backwash water than sand or anthracite alone. Similar claims have been made for anthracite/sand/garnet mixed units. The major advantages of dual-media filtration are higher rates and longer runs. Anthracite/sand/garnet beds have operated at normal rates of approximately 5 GPM/ft² and peak rates as high as 8 GPM/ft² without loss of effluent quality.

The idea behind using media with differing masses is that during backwashing the lightest media with the largest particles (anthracite) will naturally stratify at the top of the filter, while the intermediate sized media (sand) will settle in the middle, and the heaviest media with the smallest particles (garnet) will settle to the bottom. The filter is filled with media such as garnet, sand of varying sizes, and anthracite. These filters could be dual-media or multimedia based on the raw water characteristics.

This layering of the filtration bed encourages the very largest contaminants to become trapped in the first layer of the filter, with smaller particulates sifting farther down into the lower layers. Trapping contaminants in this manner allows for more efficient turbidity removal and for longer run times between backwash cycles. A simple sand filter can be expected to eliminate particles down to 25-50 microns in size, as compared to a multimedia filter that can remove particles down to 10-25 microns.

Multi media water filters utilize anthracite, sand, gravel, and garnet with a gravel under bed, chosen for their distinct density and size differences. Anthracite - the lightest, most course media at the top, sand in the middle, and garnet - the heaviest, finest media at the bottom. A typical multi media filter setup contains three layers of filling. Those are anthracite, sand, and garnet. less dense particles are at the top of the filter bed, and fine, more dense particles are at the bottom. Downflow filtration allows deep, uniform penetration by particulate matter and permits high filtration rates and long service runs. Because small particles at the bottom are also denser (less space between particles), they remain at the bottom. Even after high-rate backwashing, the layers remain in their proper location in the mixed media filter bed :

This filtration layering allows the largest contaminants to be trapped in the top filter layer, with smaller particulates sifting farther down into the lower layers, providing more efficient turbidity removal and longer run times between backwash cycles :

• The anthracite traps the courser dirt particles to prevent the formation of a layer cake on top of the filter bed.
• The smaller particles get trapped in the sand layer. With a low fluid velocity, particles will adhere to the sand with a cohesive bonding.
• The gravel allows the water to flow evenly to the mushroom diffusers and out of the filter media.

This filter package allows the entire filter bed to function longer between backwash. A well operated filter removes particles down to 15-20 microns. With the use of a coagulant addition it can filter particles down to 5-10 microns. 

Multimedia Filter Backwash :

The time for the cleaning is determined by one of the following criteria:

1. Pressure drop across the filter
2. Operating time

When the filters are loaded with particles the filter will show a high-pressure drop across the bed. Thus, it requires a backwash to clean the bed. the backwash has to be performed once the pressure drop across the bed drops below a target threshold. A standard pressure drop ranges between 0.2 to 0.5 BAR (3 to 7 PSI), so it has to be considered before initiating a backwash., the flow direction is reversed and the flow is increased to clean the filter again. This step is called a backwash. A backwash should be performed each time the media reaches capacity which should be about once a week for a properly designed system.A multi media filter has a dirt holding capacity of 3 to 6 kg TSS / m2 of the sand surface.

Over time the pressure drop over the filter rises while the flow remains the same, due to the adhered suspended solids. Eventually, the filter media needs to be cleaned, a process called backwashing. The use of too many different media layers can cause severe backwashing difficulties. For example, if all four materials (Anthracite, Sand, Garnet, and Magnetite) a wash rate high enough to expand the magnetite layer might wash the anthracite from the filter. High wash water requirements would also result.

Multimedia Sand filters are employed as primary treatment in the water/wastewater industry. Raw water with various particle sizes of colloidal needs to be coagulated/flocculated with coagulants and flocculants.

Multimedia Filter Applications:

• Pre-filtration of Reverse osmosis systems
• Separation processes in which flocculants are used
• Water treatment plants
• Suitable for various filter media (active carbon, neutralization media)

• Treatment of wastewater

• Filtration of grey or surface water

• Removal of iron 

• Preparation of cooling water

• Filtration in swimming pools

• Pre-filtration of disinfection equipment

• Preparation for micro and ultrafiltration

• Pre-filtration for membrane systems

• Recirculation systems

• Cooling water processes

• Irrigation systems
• Swimming-pool filtration
• Basin water filtration