Multi Media Filtration
Multi-Media Filters (MMF)
MMFs (Multi-Media Filtration) remove impurities in the waste stream. They do it in a cost-effective way. As a primary treatment, MMFs filter streams of small amounts of total suspended solids (TSS). In some cases, MMFs are secondary treatments after primary treatments. They get rid of particles as small as 20 microns. To get an idea of the size of a micron, the width of a human hair is about 50 microns. With an addition of coagulant, they eliminate particles up to 10 microns, including clay, algae and other micro-organisms. Common MMFs include sand, anthracite and garnet.
Do we need MMFs?
When the Silt Density Index is greater than 3 or the turbidity is greater than 0.2 NTU, it is better to use a Multi-Media Filtration. There are no exact rules, but these guidelines allow prevention of premature fouling of reverse osmosis (RO) membrane whereby the membrane pores allow absorption of solutions or particles, thus degrading its performance.
How does Multi Media Filtration work?
There are three layers of media containing anthracite coal, sand and garnet, with a supporting layer of gravel at the base. The filter media arrangement is critical because it’s important to remove the dirt on the middle bed. For this to happen, the larger (but lighter) anthracite coal will be on top while the heavier (but smaller) garnet will stay on the bottom.
The waste stream enters the filter vessels at the top and flows down through the filter media, trapping the particles in the media bed. As the solids build, the pressure builds inside the filter. However, after reaching the loading capacity, the media will need to be backwashed at a specified pressure. Backwashing is when the flow of the water is reversed one filter at a time. During backwashing, the water enters from the bottom of the vessel expanding the filter media and releases the solids. When all the vessels are backwashed, the filters go back to original filtration capacity.
Components of Multimedia Filter
- Filter Tank: This houses the filtration media; it is either stainless steel, fiber-reinforced plastic FRP or epoxy coated steel. Metallic tanks can deal with higher temperatures and pressures.
- Media: This includes different layers of gravels, silica sand #20, garnet and anthracite depending on the quality of the filtered water that is needed. For better water quality, add a layer of garnet media.
- Internal upper and bottom distributors: The bottom distribution system prevents the media from escaping. The upper distribution system distributes the flow during the service cycle, also known as the normal softening cycle where the water flows normally through the valve at the top of tank and travels down to the lower collector.
- Valves: There are different types of valves, automatic or pneumatic for automatic water filters, or manual valves for manual filters.
- Controller: This controls the automation of the filter and usually based on the main control in the facility or building.
- Face piping: All valves are connected that control the different cycles. The material of the piping depends on the temperature, and operating pressure. The material also depends on if the application is indoors or outdoors.
- Flow controller: This controls the backwash flow rate and stops the media from escaping and is on the drain outle
