What is Multimedia Filtration (MMF) and What is its Purpose?

A multimedia filter is used to reduce the level of suspended solids (turbidity) in the incoming feed water. These suspended solids consist of small particles such as silt, clay, gravel, organic matter, algae, and other microorganisms. High levels of suspended solids in the feed water can cause a significant pressure drop, reducing the effectiveness of downstream filtration equipment, such as reverse osmosis membranes and ion exchange layers.

When is it best to use a multimedia filter?

A multimedia filter is recommended when the Silt Density Index (SDI) is greater than 3 or when the turbidity is greater than 0.2 BTU. There is no hard and fast rule.

How does a multimedia filter work?

A multimedia filter typically consists of three layers of media, made up of anthracite, sand, and garnet, with a supporting (unfiltered) layer of gravel at the bottom. These media are preferred due to their size and density differences. The larger (but lighter) anthracite coal is at the top, while the heavier (but lighter) gannet remains at the bottom. The arrangement of the filter media allows larger dirt particles to be removed near the top of the media layer, while smaller dirt particles remain deeper and deeper within the media. This allows the entire layer to act as a filter, enabling much longer run times between backwashes and more efficient particle removal.

An efficient multi-media filter can remove particles as small as 15–20 microns. A multi-media filter that uses a coagulant (which encourages small particles to clump together into particles large enough for filtration) can remove particles as small as 5–10 microns. To put this in perspective, a human hair is about 50 microns wide.

Maintenance

As the multi-media filter removes turbidity from the incoming feedwater, it will eventually exhibit a significant pressure drop across the layer and/or increased turbidity levels coming from the multi-media filter. As a result, the multi-media filter will eventually require backwashing to clean the layer.

Its role in backwashing

Backwashing should be performed when the pressure differential (Delta-P) reaches 10 psi (above the cleanliness level) across the filter media or when the turbidity of the wastewater increases by 10%. The normal pressure drop across a “clean” multimedia filter ranges from 3 to 7 psi, so this should be considered before starting backwashing.

Backwashing is performed by reversing the flow of water across the multimedia filter media to remove suspended solids. During backwashing, the entire multimedia filter media relaxes and rises, allowing suspended solids to be flushed out of the top of the filter along with the backwash water. The supporting grit does not rise, which helps distribute the backwash flow evenly throughout the media.

The ideal backwash rate is 12–15 gallons per minute/square foot, which is sufficient to raise the media media adequately without forcing any material out of the top of the filter. Most filters are equipped with a flow restrictor at the backwash outlet to maintain this flow rate. This is important with seasonal fluctuations in water temperature, as colder water is more viscous and raises the filter media to the top with less flow, potentially causing material loss from the top during backwashing.

After backwashing, the filter is left to settle and re-form its layers before a final rinse, then it is restarted. The edges of the sand and other media may become rounded over time, reducing their filtration capacity after many years of use, and they must be replaced. A trained maintenance technician should refill the media, determine the appropriate dosage of coagulant to optimize filter performance, and perform any necessary maintenance and troubleshooting.

Advantages of Multimedia Filtration Compared to Traditional Sand Filters

Unlike traditional sand filters, multimedia water filters consist of three filter media layers arranged in order of decreasing porosity.

• Thanks to their multi-layered design, multimedia water filters can capture a significantly larger number of particles than traditional sand filters before requiring backwashing.
• The retention of sediment and particles throughout the entire depth of the filter layer allows multimedia water filters to operate for much longer periods than traditional sand filters.
• Multimedia filtration produces high-quality, filtered water at significantly faster flow rates than traditional sand filtration.

Features and Benefits of FRP Media Tanks

Standard construction of chemical-resistant polyester or vinyl ester; easy to handle and install; rust-proof and requires no painting; tested to withstand up to 100,000 cycles without leakage; most vessels are tested and certified by NSF International in accordance with NSF/ANSI 61 standards for material and structural safety requirements.

• Excellent compatibility with RO seawater
• Superior corrosion resistance in many applications
• Low cost compared to epoxy-coated carbon steel
• Stainless steel is lightweight – a quarter the weight of steel – and therefore easier to handle
• More slip-resistant than metal
• Non-conductive (prevents short circuits in electrical equipment)
• Good insulating material with reasonable thermal expansion properties

Features and benefits of epoxy-coated steel or stainless steel media filter tanks

• Heavier than FRP
• Can handle higher pressure and temperature
• Customizable by adding handholes, drain holes, and various outlets
• Suitable for outdoor use
• Directly mounted to a platform
• Suitable for the pharmaceutical and beverage industries
• Understands vacuum differently than glass fiber reinforced plastic (FRP) tanks
• The head design is engineered according to ASME standards to ensure durability and safety.

How to choose a multimedia filter?

The selection of a water media filter depends on several factors that must be considered to achieve good filtration results: The maximum required flow rate, the nature of the suspended solids or turbidity (colloidal or non-colloidal), feed water analysis, the desired quality of the treated water, and the availability of a sufficient water supply for backwashing.

When do we need a multimedia filter?

• Due to their ease of cleaning, multimedia filters are often used in areas with high levels of contamination. This reduces the need to replace filter cartridges or bags and minimizes operator effort.
• Media filters differ from other filter types in their ability to be backwashed. Backwashing purifies the filter, removes accumulated particles that have been filtered out, and restores/enhances filter performance.

Components of a multimedia filter:

1. Filter tank: This component contains the filter media, which is made of either stainless steel, fiberglass-reinforced plastic, or epoxy-coated steel. Metal tanks withstand higher temperatures and pressures.
2. Media: The filter media includes various layers of gravel, No. 20 silica sand, pomegranate, and anthracite. This depends on the desired filtered water quality. To improve water quality, adding a layer of pomegranate media is recommended.
3. Internal upper and lower distributors: The lower distribution system prevents media leakage, while the upper distribution system distributes the flow evenly during the service cycle. Construction materials can be either Type 80 PVC or stainless steel. If your application requires high-temperature water, we recommend using stainless steel for the internal parts, tanks, and front piping.
4. Valves: The valves open and close according to different cycles. They can be electric or pneumatic automatic valves for automatic water filters, or manual valves for manual filters. For seawater, non-metallic valves are recommended. Some industries do not allow the use of electric valves.
5. Control unit: This unit controls the automation of your filter. The control unit can be a programmable logic controller (PLC), a digital regulator, or an electromechanical timer. This typically depends on the main control unit in the facility or building.
6. Facing pipes: Facing pipes connect all the valves that control the different cycles. They can be made of Type 80 PVC, stainless steel, or epoxy-coated carbon steel. The pipe material depends on the operating temperature or pressure, and whether the application is indoors or outdoors.
7. Flow control unit: This component is installed at the drain outlet to control the backwash flow rate and prevent media from escaping into the drain.

Choosing the right partner

Once we understand the importance of multi-media filtration (MMF) as a fundamental and essential step in water treatment. Choosing a partner who efficiently designs, supplies, and operates these systems becomes crucial to ensuring water quality and business continuity. In this context, Water Care stands out as a leading provider of integrated water treatment solutions. From designing highly efficient MMF filters tailored to your operating environment (whether you need lightweight FRP tanks or high-pressure epoxy-coated steel tanks). Providing advanced maintenance and technical support services, why is CareWater the best water treatment company?