J.H. Cockerell Consulting Engineers

Pool Water Filtration

Good pool water filtration will reduce consumption of pool water chemicals and also the risk of bather infection from bacteria concealed within dirt particles suspended in the pool water. Our pool condition reports regularly identify substandard performance of pool filters, with associated potential health risks for pool users.

Sand Filters

Sand filters have been used over a very long period of time to cost effectively filter water for both drinking and use in pools. Recent independent research into the efficiency of sand filters has found that high rate sand filters with a small depth of sand (e.g. 300mm) can have efficiencies that are as low as 10%. Of course it is also important to ensure that when backwashing sand filters, the sand bed is cleaned. In our experience, reporting on the condition of large pools, filters are often not adequately backwashed. For the sand bend of a filter to be adequately backwashed, the sand bed must uniformly expand by approximately 20% to allow particles trapped in the top of the sand bed to be discharged to waste. Inadequate backwashing usually passes unnoticed due to current widespread use of pressurised sand filters, constructed without sight glasses built into the filter’s shell, to allow visual confirmation of sand bed expansion.

Gravity Sand Filters

Early sand filters allowed water to flow through the sand bed at a slow rate which was determined by the depth of water above the sand bed. Such filters were referred to as gravity sand filters. If well designed, constructed, operated and maintained, they are very efficient filters. With the top of the filter open, they allow confirmation of sand bed expansion during filter backwashing. Unfortunately gravity sand filters are very expensive to construct and are more complicated to operate and maintain than most pressurised sand filters. Incorrect operation and/or maintenance can adversely affect the efficiency of gravity sand filters. As a result, sand filters used on most pools are now closed pressure vessels.

Pressure Sand Filters

If well designed and constructed, pressurised sand filters can have the efficiency of a gravity sand filter and also be cheaper to manufacture and easier to operate. The efficiency of a sand filter is not easily determined. As a result, over the last 10 to 20 years in Australia, the flow rates through pressurised sand filters used on large pools have continued to increase and sand bed depths have continued to decrease, making pool water filtration cheaper, but significantly less efficient.

Diatomaceous Earth (DE) Filters

Diatomaceous earth (DE) filters have the ability to trap particles as small as 3 to 4 microns in size, which is significantly less than the 10 to 12 micron capacity of sand filters when they are used without a flocculant. DE filters have the ability to capture cryptosporidium oocyst and provide improved water clarity. DE filters are typically more expensive to install, run and maintain than sand filters.