Back Wash

The most important challenge to prove is the ability to clean the stacked rapid sand filter by the back wash method. In order to back wash the filter, we must prove that the individual filter layers can be fluidized in sequence, and thus result in a fully fluidized filter. Once fully fluidized, the filter can then be back washed, and required significantly less water than a comparable conventional single layer filter.

Method of Back wash

Step 1: Close all influent and effluent valves

Step 2: Open the valve to the uppermost filter layer

Step 3: Increase the flow rate to 4800 mL/min

Allow enough time for this layer to fluidize (~30 seconds). It is easy to test is a sand bed is fluidized-- if you try to insert a stick into an unfluidized bed of sand, it will not easily go through it. A stick inserted into a fluidized sand bed will easily slide through.

Step 4: Simultaneously close the uppermost valve and open the next lower valve (middle)

Allow enough time for this layer to now fluidize (~30 seconds).

Step 5: Simultaneously close the middle valve and open the lowermost valve

Once the bottom layer of the filter is fully fluidized, back wash can begin.

Step 6: Wait

The filter must now be allowed to back wash. The water level inside the filter column will rise until it flows over the effluent weir. The sand will remain at ~30% expansion, which is lower than the effluent weir height. The sand will remain in the filter column while the trapped particles are pushed out over the effluent weir. Allow the filter to backwash until effluent water appears clear of particles (~7 minutes).

Return filter to filtration mode

Step 7: Lower the flow rate to 2700 mL/min

Step 8: Open all influent and effluent valves

Step 9: Close valve to distribution tank, open an effluent waste valve

It is recommended that in an full scale filter design, the first 30 seconds of water produced by the effluent should be wasted. During back wash, particles can become trapped inside the effluent pipes prior to the valve, and result in a very high turbidity water that would be dumped into the clean distribution tank.

Step 10: Open valve to distribution tank

Produce clean water!