Filter Sand or Filter Gravel? The Layer Logic

Should a water filter use filter sand or filter gravel? This is not really an either-or question, because in a well-designed filter bed both are present, performing different tasks. Filter sand is the main layer that strains the water, while filter gravel is the support layer that carries the sand and protects the drainage system beneath it. In this article we explain the role of each, the layer logic and how to choose correctly.

What Do Filter Sand and Filter Gravel Do?

Two different granular materials work together in a sand filter:

• Filter sand: the main filtration layer that physically strains the water. Thanks to its fine, controlled grain size, it captures suspended solids, sediment and turbidity.
• Filter gravel: the coarser support layer beneath the sand. It carries the sand, prevents fine sand from escaping into the underdrain system below, and distributes water evenly across the bed during backwashing.

The Layer Logic: Why Both Together?

A filter bed is designed in graded stages from top to bottom: fine filter sand at the top, with progressively coarser layers of filter gravel below. This graded structure serves three important purposes. First, it prevents the fine sand from passing into the drainage system and being lost. Second, during backwashing it distributes water evenly across the base of the bed so that the whole bed is cleaned properly. Third, it provides a firm, stable foundation for the sand layer. Without gravel, the sand both escapes and backwashes unevenly.

Critical Parameters for Filter Sand

Two key parameters determine the performance of filter sand: the effective grain size (d₁₀) and the uniformity coefficient. The effective grain size is the grain diameter below which 10% of the material by mass falls; in water treatment it is typically in the 0.45–0.55 mm range. The uniformity coefficient (d₆₀/d₁₀) indicates how narrow the grain size distribution is and is generally required to be below 1.65. The sand must also be washed and low in fines, so that it does not add turbidity to the water.

Critical Parameters for Filter Gravel

Filter gravel is laid in graded layers (coarse at the base, becoming finer towards the top). The aim is to provide a smooth transition between the sand and the drainage system; each layer retains the finer material above it. The total depth of the support layer is generally in the order of 30–40 cm, and the coarsest gravel layer is selected according to the opening size of the underdrain system used.

The Right Choice: You Need Both

In conclusion, filter sand and filter gravel are not alternatives but complements. A properly designed system uses both together, in the correct gradation. Which sand size and which gravel grades are required depends on the filter type, the flow rate and the drainage system. For higher performance, a second medium such as anthracite can be added on top of the sand; we cover this in Sand or Anthracite for Water Treatment?. Water treatment sand in detail →

Conclusion

In summary, in a sand filter the filter sand strains the water while the filter gravel carries and protects that layer; together they form a graded bed. To determine the right gradation for your plant, please get in touch with us.