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Diatomite Filters for Private Swimming Pools

Diatomite or diatomaceous earth filters were first developed for swimming pools after 1945. Although they are widely used in private swimming pools, their application to public pools in Australia is only just beginning.

The Diatomite filter is designed to receive a charge of diatomite powder on a porous septum. The filter itself does not clarify the water this operation is carried out by the filter aid (diatomaceous earth). The design of the diatomite filter is very important because it is essential that the filter aid be applied in a uniform thickness over all the septa. This diatomite or diatomaceous silica is composed of the fossil remains of tiny aquatic plants called diatoms. Under the microscope, the fossils are revealed as unbelievably thin, transparent valves or shells of diverse shapes with ornate and symmetrical markings. The average diatom is so small that more than a thousand would have to be laid end to end to equal one inch.

Under favourable conditions of light, temperature and nutrition, such plants grow in great profusion and, during the geological past, many deposits of these plant skeletons were built up in different areas of the earth. Each skeleton is an extremely porous framework of nearly pure silica.

Diatomaceous Earth Filter Construction

The type of filter developed for large swimming pools consists of a vertical, cylindrical tank in which a number of hollow tubular filter elements are suspended. The pool water entering the tank is filtered through the diatomite cake on the outside of the elements, flows inside the tubes to the outlet and then back to the pool.

The elements themselves are finished in a variety of construction materials and designs. The most common is porous aluminium oxide or porous metal cylinders, helically wound wire on plastic or metal cores, and metal or plastic cores covered with either metal or synthetic fibre filter cloth. Each design has its own particular advantages.

Diatomite Filter Operation

In the operation of the filter a layer of diatomite, called the precoat, is formed on a filter cloth or screen. The filtration is performed by this fine porous layer of diatomite, not by the filter septum itself. The main purpose of the filter septum is to form a framework on which to build the filter cake that actually does the work. The porous precoat of diatomite allows free flow of the water but traps the particles of dirt as the water flows through.

Only after the precoat is in place is the swimming pool water circulated through the filter. As the water passes through the precoat, suspended solids are screened out. usually, there are enough suspended solids in the water to plug the tiny openings in the surface of the precoat, causing too rapid pressure increases and decreasing flow rates. For this reason, most pools can be operated more satisfactorily by intermittently or continuously feeding a small amount of additional diatomite. This operation is call the body or slurry feed. In this procedure, fine porous, freely filtering cake is built up. The precoat feeder must be large enough to deliver an initial charge of two ounces of diatomite per square foot of filter are. The body feeder must have the capacity to deliver 1 - 4 ounces per 1,000 gallons of water recirculated per day.

Although high filtration rates can be achieved with these filters, the higher rates give short filter cycles. The compromise in the cost of filter units with that of power, labour and filter aids is a filtration rate of 11/2 to maximum of three gallons per square foot per minute, with backwash rates of 10-12 gallons per minute per square foot.

The circulating water cannot be shut off and on indiscriminately on these filters otherwise the filter cake may fall off the septum and when started up again some of the filter aid and dirt may pass trough before the water pressure binds the filter aid to the septum again.