Sand for pools and filters: grading, backwashing and handling

Why does a swimming pool filter use a bed of sand, and is that sand the same as any other? A sand filter traps particles as water passes through a deep layer of graded grains, then cleans itself when the flow is reversed. This page explains that process, the grain-size figures that public-health and water-treatment bodies describe, what backwashing is, what the sources say about replacing media in general terms, and how to think about dust. It is an explainer. It does not recommend products, and the rules for any actual pool come from its manufacturer and local authority.

01How a sand filter works

The University of Memphis civil engineering department describes filtration as separating solids from water by passing it through a porous medium, most commonly a bed of granular material. In a typical gravity filter the water enters above the media, passes downward through the granular bed and a supporting layer of gravel, and is collected in an underdrain below. Particles are removed by several mechanisms, including straining, where a particle is simply larger than the gap between grains, and by sedimentation or sticking to the surface of the grains as the water winds through.

Swimming pools use a pressurised version of the same idea. The World Health Organization (WHO) describes medium-rate sand filters in its Guidelines for Safe Recreational Water Environments as comparatively large-diameter pressure vessels, in a horizontal or vertical format. It says they can nominally filter down to about 7 micrometres (millionths of a metre) in size when a suitable coagulant is added. A coagulant is a chemical that helps fine particles clump together so the sand can catch them. The WHO also lists other filter types, including cartridge filters, which work differently.

The Memphis notes also distinguish slow and rapid sand filtration. Slow sand filters run at much lower flow rates and build up a layer of filtered material on the surface that becomes part of the filtering, while rapid filters filter through the depth of the bed. Water treatment is covered here only as background for the pool case.

02What grain sizes the bodies describe

Filter media are described by two figures. Effective size is the grain diameter that 10 per cent of the material by weight is smaller than. The uniformity coefficient is the ratio of the grain size at the 60th percentile to the effective size, so a lower number means the grains are more alike in size. The Memphis notes give these definitions, and they explain why the figures matter: if grading is too wide, effective filtering is confined to the upper few inches of sand because the finest grains collect at the top after backwashing.

For pools, the Council for the Model Aquatic Health Code (CMAHC) annex to the Model Aquatic Health Code (MAHC) is detailed. The MAHC is guidance from the US Centers for Disease Control and Prevention (CDC) for public pools, hot tubs and splash pads; CDC says it is not a federal law and only becomes law if a local, state, territorial or tribal jurisdiction adopts it. The annex says filter sand should pass through a number 20 US standard sieve, so that all grains are smaller than about 0.85 millimetres. It adds that sand can instead be specified by an effective size of 0.45 millimetres with a uniformity coefficient of 1.45 or less, which it says is roughly equivalent to 20/40 mesh sand, meaning grains that pass a number 20 sieve and are retained on a number 40 sieve (about 0.42 millimetres). Sieves and mesh numbers are explained in aggregate sizes and grading.

Filter sand figures described by two sources

Source	Context	Effective size	Uniformity coefficient	Bed depth
CMAHC annex to the MAHC	Swimming pool filters	0.45 mm	1.45 or less	24 inches or more above the underdrains (recommended)
University of Memphis, civil engineering	Conventional water treatment sand	0.45 to 0.55 mm	less than 1.65	24 to 30 inches

The two lines are not in conflict. They describe different applications, and both come from documents that explain their own context. The CMAHC annex also notes that the commonly quoted "number 20 silica sand" wording in pool manuals does not appear to represent the actual sand that might be installed, and it describes the specifications of pool filter sand, or the lack of them, as something that can lead to media that cannot be cleaned effectively.

03Backwashing

Backwashing is how a granular filter is cleaned. The WHO describes it as reversing the flow, fluidising the filter material and passing pool water back through the filter to waste. Fluidising means the upward flow lifts and loosens the bed so trapped material can escape. The WHO says backwashing should be done as the filter manufacturer recommends, when the allowable turbidity (cloudiness) value has been exceeded, when a certain time has passed without backwashing, or when a pressure difference is observed.

The MAHC annex adds numbers for public venues. It gives the rationale for requiring a backwash rate of at least 15 gallons per minute per square foot (37 metres per hour) of bed area for all swimming pool sand filters, and recommends that the expanded bed during backwashing be at least 20 per cent deeper than the settled bed. It also warns that sand that cannot be properly cleaned can lead to filter failures or biofilms in the bottom of a filter.

04Replacing media, in general terms

Sand does not last forever, though no single lifespan applies to every pool. The WHO guidelines say that for indoor heated pools the sand medium typically has a life of between five and seven years. How and when to replace media in a particular filter is a question for the filter manufacturer's instructions and, for public pools, the local health authority, which can apply different rules from the MAHC.

05Handling dust

Crystalline silica is a mineral found in sand and stone (our silica page cites OSHA for this), and the agencies' concern is respirable dust, the very fine particles made when rock and similar materials are cut, drilled, ground or crushed. What those agencies say about the health effects, the controls they describe and the exposure limits regulators set is gathered in silica dust and quarry safety.

NIOSH's workplace guidance on silica advises avoiding dry sweeping or compressed air during housekeeping and notes that wet methods can suppress dust. Beyond that, we do not give handling instructions. The safety information supplied with a material, the filter manufacturer, and the workplace-safety regulator in your country are the right sources for how to deal with dust from a specific task.

06How filter sand differs from other sands

In general terms, filter sand is selected for its grading and for being cleanable, as the figures above show, whereas sands for other jobs are selected for other properties. The range of sands and their typical uses is described in sand types and what each is used for. Sand for children's play areas is a separate subject with its own safety questions, covered in sand for play areas and sandboxes.

Because the same word, sand, covers many materials, it is unwise to assume one sand can stand in for another. Pool and filter makers publish media specifications, and local pool codes may add requirements. Rules and practice differ by country and by site, and the local authority, the manufacturer or a qualified pool or water-treatment professional has the final word.

07The short version

A sand filter traps particles in a graded bed and is cleaned by backwashing. Bodies such as the CMAHC and water-treatment educators describe media by effective size, uniformity and depth, and the WHO notes a typical life of five to seven years in indoor heated pools. Dust questions are covered by the silica guidance linked above, and local rules decide what applies to a real pool.