Soil is captured in the sand filter bed by combination of two processes. First, solid particles lodge in the extremely small spaces and voids between the particles.  Second, gelatinous, and mucous-like substances and oils tend to cling to the grains of filter sand.  The latter, then, enhances the medical effectiveness of the former, as “soil entraps soil” and the filter efficacy actually improves over time and usage.  (In high-rate sand filters, this phenomenon works its way deep into the sand bed rather than existing only on the surface – “three-dimensional” filtration.)

Crushed, angular silica sand works well while rounded beach sand is very poor for entrapment. The sharper the sand, the more likely particles will be held while allowing the water to pass.

The finer the medium, the more successful the filtering process.

If the sand is too coarse, the voids between the particles are too large to efficiently stop the passage of fine solids. If it’s too fine, the bed will be too dense and very little space is left for dirt to accumulate between the sand grains.

The sand particle size best for pool filters has been established in a range of 0.4 to 0.6 millimeters diameter, or, in grade, number 20. Number 30, a finer sand grade, is sometimes recommended for indoor pools when the soil volume is inadequate to effectively load the more common grade.

Uniformity of coefficient should be less than 1.53.

Specific gravity should be 2.5.

The pH of the sand should be 7.0.

  • Zeolite
    • Zeolite (Clinopure – Neptune-Benson trade name) comprises 20% of the filter bed. Filters out chloramines and removes organics.
    • Superchlorination of the filter bed is required every 6 months.
    • Zeolite has a life cycle of 4-5 years before it has to be replaced in the filter bed.
    • Zeolite is NSF-rated not to exceed 12 GPM/ft2.
    • Neptune Benson, Astral and Paddock currently have NSF approval.

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