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PIPING FOR SAND FILTER SYSTEMS The piping arrangements possible when using sand filters are extremely flexible. This can be very helpful when laying out a new or retrofit system since the influent, effluent and backwash connections can be configured to match the way the pipes come from and return to the pool.

The most critical factor in the filter piping is that part of the system that handles the filter backwash. Sand filters are backwashed at a rate of 15 gpm per square foot of filter surface. The flow rate in the filter mode can be considerably less than this. A system with one filter and one pump can be designed so that the filter pump will also provide the required water flow on backwash. This can be accomplished because the system resistance to backwash water is usually low. Typically this resistance consists of the losses through the filter bed plus a short run of pipe to the backwash receiving tank. Most pumps which are selected for a high efficiency operating point (typically mid-range on the pump curve) will be capable of producing the required backwash flow when running out on the curve at the lower head required for backwash. Systems which have a long run of backwash piping and/or an elevation change need to be examined to make sure the head the pump encounters is not so high as to preclude reaching proper backwash flow.

Two-tank systems may be handled the same as the one-tank system as long as the flow pattern of the backwash water is perfectly symmetrical. Refer to our Double Tanks Front In/Front Out drawing. You will see that if the water is diverted down the vertical tee it faces the same resistance going to the left or right tank and both tanks will backwash evenly. The next arrangement Double Tanks Front In/Side out does not offer a symmetrical path. It is a longer route for the water to go through the left tank and out the top-right backwash point. Consequently, less water will flow to the left and eventually the left tank will not backwash properly. Rate control is required and each tank must be backwashed separately.

Multiple tank systems present a different problem. Attempts to backwash all tanks simultaneously will result in improper backwash of the outboard tanks. This is because the first tank to receive water will clean first, allowing more water to flow through this tank than the following tanks. Thus, the early tanks will get very clean (possibly to the point of blowing out sand) while the end tank or tanks will get less and less. Ultimately the end tank will not get clean at all and will stop filtering. Continuation of this procedure over time will then cause the next-to-last filter to stop functioning and so forth up the line.

The only way to avoid this and properly backwash a group of filters is to provide some form of flow limiter in the backwash piping to limit the flow to that required for one tank, and backwash one tank at a time. This control may be accomplished by a Griswold flow limiting valve set for the proper flow (our preference) or by using a flow meter in the backwash pipe. A modification of the flow meter arrangement is to fix a valve in the backwash line once the backwash rate has been confirmed. Once fixed, this valve should limit the flow to that required for each tank.