Filtration process removes particulate matter by passing the water through porous media. It consists of different types of media which is usually made up of sand, gravel and charcoal. There are two types of sand filtration, slow sand filtration and rapid sand filtration. Slow sand filtration is a biological treatment process which uses bacteria to treat the wastewater. Bacteria from a top layer of sand clean the water by digesting the contaminants in the water. The disadvantage of using a slow sand filtration process is that it requires large area for the system to operate as the flow rate of water must be maintained between 0.1 to 0.3 metre per hour. As a result, new technology was developed known as rapid sand filtration. Rapid sand filtration has an advantage over slow sand filtration that it can handle higher flow rate.
Types of filter
- Dual Media Filter (DMF)
- Activated Carbon Filter (ACF)
- Pressure Sand Filter (PSF)
- Multi Grade Filter(MGF)
Dual media filter (DMF)
- Treated water or effluent from the treated water tank is feed to DMF.
- Dual Media Filter (DMF) is ideal for filtration of water having very fine suspended matter like mud, rust particles and biological growth.
- DMF is a vessel constructed of welded mild steel and provided with manhole with cover/top and bottom flanged covers, supports, raw water distributor, underdrain collection and backwash water jet system.
- Treated water flows downwards through the filter bed, and the turbidity and suspended matter are retained on the sand surface.
- Filtered water is collected by an underdrain system in the bottom of the vessel and flows through the outlet to service.
- Resistance is experienced when a water of normal flow rate passes through the passage.
- Cleaning of filter bed is effected by passing a reverse upward flow of water through the filter for approximately 3 to 5 minutes.
Activated carbon filter (ACF)
- Treated water will be transferred to the activated carbon filter.
- Activated carbon filter consists of a vertical pressure vessel fitted with a set of frontal pipe and valves, different type of filtration media will be supported by layers of a bed consisting of pebbles and gravels with a distributor at top to distribute the incoming water across the filter and an underdrain system for the collection of filtered water.
- ACF is a pressurized filter with backwash arrangement.
Pressure sand filter (PSF)
- The Pressure Sand Filter consists of a multiple layer of sand with a variety in size and specific gravity.
- These Filters are designed to remove turbidity and suspended particles present in the feed water with minimum pressure drop.
- The Pressure sand filter consists of a pressure vessel either vertical or horizontal, with a set of pipe work and valves, graded silica quartz sand supported by layers of graded under bed consisting of pebbles and gravels, and a top distributor to distribute the incoming water uniformly throughout the cross section of the filter, also an under drain system to uniformly collect the filtered water.
Multi Grade Filter (MGF)
- Multi grade filter is a depth filter that makes use of coarse and fine media mixed together in a fixed proportion.
- The arrangement produces a filter bed with adequate pore dimensions for retaining both large and small suspended particles. The filter performs at a substantially higher specific flow rate than conventional filters.