Sand Filtration System

Deionised water is obtained from raw water through a process of filtration, reverse osmosis, and ion exchange. In the process, the raw water is first passed through a coarse filtration stage to remove big particles. Sand filtration is often used because it can be easily cleaned by backwashing of the filter media. Furthermore, the filter media need not be changed very often.

The common filter media are:

  • Gravity filter – removal of suspended solids and turbidity
  • Multi-media filter – removal of suspended solids and turbidity
  • Activated Carbon filter – removal of free residual chlorine
  • Greensand filter – removal of iron and manganese
  • Cartridge filter – removal of suspended solids

Multimedia filter uses two or more filtration media. Anthracite, the largest particle is the lightest and settles at the top. It traps the larger particles. Sand is intermediate in size and density. It settles as the middle layer. Garnet, the smallest particle is also the heaviest. It settles out as the bottom layer. The multi-media filter needs fewer backwashes for a given capacity compared to a single media bed.

Greensand is taken from natural zeolite media. The ferrous and manganese ions react with the permanganate coating on the greensand and oxidize to ferric and manganic forms. Eventually, the greensand is exhausted and must be regenerated with potassium permanganate. Generally, potassium permanganate is added continuously to the influent to regenerate the greensand.

Gravity filters are so named because water flows through the filter bed by gravity instead of through a pressure pump. They are mostly used for municipal drinking water applications.

Multi-media filter vessels are usually lined with a spray-on epoxy polyamide lining that prevents both corrosion as well as contamination of the water by dissolved iron. In cases where dissolved iron is of no concern, the lining can be omitted. In installations where the spray-on lining might get scratched, a sheet type of rubber lining is used.