The primary role of the gravel in the bio-sand filter is to prevent clogging of the PVC pipe with sand. For household intermittent sand filters, 2 layers of gravels are usually employed: on the bottom, a 5cm layer of 6 – 15mm gravel, followed by a second 5cm layer of 1 – 6mm coarse sand. The first layer is sufficiently deep to cover the inlet to the pipe and should be large enough to keep the openings in the filter bottom free for the flow of water out of the filter, and the upper layer fine enough that the overlying filter sand will not sink into its pores. Note that for drum filters, due to the configuration of the outlet pipe, the size of the gravel around the outlet needs to be greater than 15mm to prevent it clogging. Alternatively, mosquito mesh can be used.
In larger-scale slow sand filters, an underdrain of some kind is built into the base of the filter. To prevent the filter material from entering and blocking the underdrain a series (usually 3 or 4) of graded gravel layers is used. For stacked bricks with open joints 10mm wide, 4 layers are suggested, each 10cm thick with the coarsest layers at the bottom: 0.4 – 0.6mm, 1.5 – 2mm, 5 – 8mm, 15 – 25mm (IRC, 1981 [ref.01]Ref.01: IRC (1981). Small Community Water Supplies: Technology of Small Water Supply Systems in Developing Countries. Hofkes, E.H. (Ed.) Technical Paper Series 18. IRC, The Hague, p.259. Available here).
Ref 01: IRC (1981). Small Community Water Supplies: Technology of Small Water Supply Systems in Developing Countries. Hofkes, E.H. (Ed.) Technical Paper Series 18. IRC, The Hague, p.259. Available here.