A pressure filter is an upright, closed cylinder containing a filter bed of layered sand and gravel on top of a collection system. Water under pressure passes through the filter and then continues on through the water treatment system.
As the water passes through the filter, oxidized minerals (such as iron and manganese) and foreign matter collect in the top portion of the sand. Continued build-up of these particles tends to clog the filter. The clogged filter requires more force or pressure to pass water through and filter at the same rate as an unclogged filter.
A head loss gauge measures the pressure of the water leaving the filter. By comparing the pressure of the water leaving the filter to the pressure of the water entering the filter, you can determine how much pressure was lost due to the water passing through a clogged filter - a measurement also known as head loss.
When the head loss gauge shows that there is excessive build-up on the filter, then the filter must be washed. Excessive build-up can cause clogging, restricted flow, pressure build-ups, and possible breakthroughs. A breakthrough is a crack or break in a filter bed which allows the water to pass through without contacting the filter and being cleaned. In addition to damaging the filter bed and piping, a clogged filter can allow poor quality water to go into the system, causing problems and complaints.
Filters are cleaned by backwashing. The influent valve is closed and a waste line is opened. Treated water from the system is pumped upward through the filter bed. The water pumped upward has the velocity and volume to agitate the sand and carry away the foreign matter that has collected there.
Backwashing normally takes about 10 minutes, though the time varies depending on the length of the filter run and the quantity of material to be removed. Filters should be backwashed until the backwash water is clean.
The filter rate can be increased to 3 gpm (gallons per minute) per square foot of filter and the backwash rate may be reduced to 12 gpm per square foot of filter if this provides proper washing.