AFM Grade 1, Activated Filter Media, 25 kg

Sand filters grow biofilm. Within months the grains are coated, the bed channels, and bacteria colonise the filter that was meant to remove them. Activated filter media is made from recycled glass with a surface that resists biofilm and carries a slight charge, so it filters finer than sand (about 4 micron without a coagulant), never needs replacing, and backwashes with a third less water because it is lighter and rinses clean.

Grade 2 forms the coarse support layer around the bottom strainer; grade 1 does the filtering above it.

Weekly, or when the pressure across the vessel rises 0.5 bar, at about 40 metres an hour: 50 litres a minute in our 75 litre vessel. The bed should lift 20 to 40%, which is why the vessel is filled to 75 litres and not to the neck. Twelve to fifteen minutes, then a two-minute rinse.

Ten years or more. It does not wear, dissolve or rot; the only reason to take it out is to move the vessel.