Posted by on Wednesday, January 18, 2012
Backwashing is the process of cleaning the filters. Consider that the job of the filters is to trap as much of the various types of pollution as possible and you will appreciate that over time, more and more of this pollution will build up within the filter media. At regular intervals, this accumulated pollution needs to be gotten rid of. The method is to reverse the flow of water through the filter, so instead of water going in at the top and coming out at the bottom, the water goes in at the bottom and out at the top. As it does this, the pressure of the water moving up through the sand in the filter causes the sand to break up and eventually 'fluidise'. When this happens, the pollution is forced out of the sand and through the top of the filter,and from there it is directed to the drains (NOT back into the swimming pool!).

It is a fairly simple and straight-forward process, and there should be a step-by-step guide contained within your normals operating procedures or systems of work etc. to guide you through it. For the pool plant operator, it is recommended that they get to know their pool plant system well enough that, eventually, they will not need to refer to a guidance document in order to properly perform a backwash. A very simplified, generic backwash description is given below:

1. Switch off circulation and chemical dosing.
2. Adjust valves so that the pool water goes into the filter at bottom and out at top, and from there to drains.
3. Switch circulation back on and leave running until pool water is visibly clear.
4. Switch circulation off.
5. Adjust valves so that the pool water goes in at the top and out at the bottom, but still runs to drains.
6. Switch circulation back on and leave running for a couple of minutes (this is called the 'rinse'.
7. Switch circulation off.
8. Adjust valves so that the pool water goes in at the top and out at the bottom, but this time returns to the pool.
9. Switch the circulation back on.
10. Open the air release valve to purge the system of trapped air.

As previously mentioned, this is a very watered-down (excuse the pun) description of the process. Other factors need to be considered such as air scouring, backwash velocity and filter pressure differential.

Tags: backwash rinse "air scouring" "backwash velocity" "filter pressure differential" 

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