Backwashing Precoat Filters:

After a filter has been precoated, it is put within service and kept on line until the pressure differential denotes in which the filter medium is becoming plugged. While this occurs, an old filter medium is erased and the filter is precoated again. Filters are commonly installed in pairs, so in which one filter could remain in service although the other is undergoing the filter backwashing and precoating procedure.

Because water pressure helps to hold the filter medium against the septums, a few of the old filter medium will fall off as soon as this pressure is removed. The Backwashing is used to remove the filter medium that does not fall off. It is usually done in one of two ways. With a few filters, demineralized water is pumped backwards through the center of the septums and a filter medium coating is knocked off by the water as it comes out through the septums.

Most filters use a multi-step backwashing procedure. First one, the inlet valve and the outlet valve are closed, and the drain valve and the top vent are opened to permit the water to drain. After that the drain valve and the vent are closed and the inlet water valve is opened to increase the water level. The filter is equipped along with a special high-domed top to trap and compress air. While the water inlet valve is closed and the drain valve is opened rapidly, the compressed air forces water down by the middle of the septums. That water knocks the filter medium off of the septums.

With both categories of backwashing, the filter medium coating which is erased is sluiced out by a drain line to a filter sludge tank, whereas it is stored for additional processing. A filter is then precoated again and put back within service.

Within precoat filters, the categories and quantity of filter medium is critical. Some of the finely divided crud in the water might get into the openings of the septums if too little material or too coarse a material is used. While the filter is backwashed, this crud is commonly not erased. It continues to build up during following use of the filter until the septums become so plugged within which they have to be replaced.

The layer that builds up on the septums will bridge the area between the septums if too much filter medium is used. While the filter is backwashed, these bridges are commonly not removed.

Thus the bridging continues, and the filter runs become progressively shorter. Eventually, a filter must be opened and the filter medium must be erased manually. Precoat filters are much more complicated than cartridge filters, and the equipment needed is much more expensive to install and manage. The main benefits of precoat filters are the remote operation that eliminates the physical handling of highly radioactive filter cartridges.