The fundamentals of working with sand filters

By Brian Lilleberg

The three keys to a clean and healthy pool are circulation, chemistry, and filtration. The latter is commonly overlooked when problems arise, however. Circulation usually is not a problem since most pools are designed with properly sized pumps, filters, plumbing (pipes), and proper placement of skimmers, and returns. In most cases, water chemistry is the initial cause of problems, such as green or cloudy pool water, but it can be easily remedied. After the water is properly balanced, water clarity should improve within a day or two. However, if the water does not clear up there may be filter issues.

The sands of time

As a service professional, this author’s favourite type of filter is the sand filter. Of all the other types of filters (e.g. cartridge, diatomaceous earth [DE]), sand filters are efficient with a minimum amount of maintenance, but as with all equipment, there can be issues.

For those new to the pool industry, the following is a quick explanation of how sand filters work (see Figure 1). The pump draws water from the pool and pushes it to the filter. The water enters via the dial valve, travels through the diffuser, then through the sand bed. As the water passes through the sand, the debris is caught between the sand particles. The filtered water then enters the filter lateral assembly at the bottom of the filter, travels up the stand pipe and back out the filter to be returned to the pool.

This is just a warning

Common warning signs of potential filter issues are the need for frequent backwashing, sand returning to the pool, low or high filter pressures, and water leaking at the dial head through the backwash line. The most common problem, however, is lack of proper maintenance. Quite often it is the result of overlooking an annual chemical cleaning of the sand, or it could be because the filter is due for a sand change.

Backwashing

Filters should be cleaned by backwashing when the pressure increases by 34.5 to 68.9 kPa (5 to 10 psi), or weekly by a service professional hired to maintain a swimming pool. A yearly filter cleaning with a chemical cleaner will also help to remove scale, greases, and oils from the filter sand.

Replacing sand

Changing the sand in the filter should be done every six to seven years. This author has seen filters that are still working with the original sand after 20 years, but at a loss of efficiency. What is even more concerning in these cases, however, is once there is a problem it usually requires a full replacement of the filter system. Sand is more efficient at collecting debris when it is new and the particles have course edges. Over time with water passing through the sand, the edges slowly begin to wear down and the grains of sand become round. Once this occurs, fine debris is allowed to pass through the sand, which will lead to cloudy water.