Multimedia Filters

Sand and multimedia filters remove suspended solids, turbidity, and floc carried over from coagulation. With the right chemistry, they also remove iron, manganese and algae. They do not remove dissolved contaminants, and they are not an absolute barrier to Cryptosporidium.

Yes. Sand or multimedia filtration ahead of RO or ultrafiltration removes the bulk of the solids and lowers the silt density index, which protects the membranes from premature fouling. That reduces cleaning frequency and extends membrane life.

Well-operated rapid gravity and pressure filters deliver filtrate turbidity of 0.1 to 0.5 NTU, and consistently below 1 NTU. Slow sand filters produce similar turbidity with useful additional removal of bacteria and organics. None of them matches the sub 0.1 NTU consistency of a membrane barrier.

Sand and anthracite typically last 10 to 20 years. Media should be topped up as depth is lost through backwashing, and replaced in full when it becomes fouled, rounded or mud-balled. Regular media surveys show when intervention is due.

Rapid gravity filtration needs a large area of concrete structures. Pressure filters need far less, perhaps a third, because they are vertical and closed. Slow sand filters need the most land by a wide margin, often 20 to 30 times the area of a rapid gravity plant for the same output.

Yes, and it is a classic arrangement. Sand or multimedia filtration removes solids, then GAC polishes for pesticides, organics, taste and odour. Some sites cap a sand filter with carbon, although separate vessels give better control of contact time and changeout.