- Underdrain system - The underdrain system has 3 main purposes:
- to support the filter media
- to collect the filtered water
- to distribute water evenly over the entire filter floor during a backwash.
- Two types of underdrains
- Gravel supporting
- immediately above the underdrain are layers of gravel of decreasing size, i.e. large on bottom progressing to small pea gravel on top
- Gravel less
- no gravel - layer above underdrains would be coarse sand
- Gravel subfill
- Functions
- Support - smaller filtering media which is above gravel
- Distribution - aids in even distribution of backwash water
- Depth - 0-400 mm
- Particle size - 1-50 mm
- Sp. gr. - 2-6
- Hardness - 7
- Filter media
- Depth - sand 300mm and anthracite 450 mm
- Types - sand and carbon
- Wash water troughs
- located above top layer of media
- purpose - to collect backwash water and convey it to a pipe or channel for disposal
- Surface wash / air scour
- Backwash
- purpose - to reverse the flow of water through the filter by using a pump or other device at sufficient pressure to fluidize the sand and anthracite and remove accumulated particles
- filter expansion varies between 25 to 40%
- clean water is used
- best backwash rate depends on the size, specific gravity, and shape of the filter media
- flow per unit area: 40-45 m/h
- Filter to waste
- After a filter has been backwashed, the filtered water is filtered to waste for either a period of time or until the turbidity drops to a pre test level. Then the water being filtered goes to process.
- Piping and Valves
- Piping
- influent
- effluent
- backwash
- surface wash or air scour
- filter to waste
- Valves
- influent
- effluent
- backwash
- surface wash
- air scour
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