Pressure sand filtration is the flow of water through a bed of granular media, normally following settling basins in conventional water treatment trains. The purpose of this filtration is to remove any particulate matter left over after flocculation and settling. The filter process operates based on two principles, mechanical straining and physical adsorption. Sand filtration is a "physical-chemical process for separating suspended and colloidal impurities from water by passage through a bed of granular material. Water fills the pores of the filter medium, and the impurities are adsorbed on the surface of the grains or trapped in the openings.
It is used for effective removal of color, odor, organic contamination (Loading upto 5 ppm in raw water) and chlorine in water. The activated carbon also removes certain chemicals that are dissolved in water passing through a filter containing GAC by trapping the chemical in the GAC. However, other chemicals, like sodium or nitrate, are not attracted to the carbon and are not removed
Softening by ion exchange resin is also known as Base Exchange softening. It is the most common and probably the easiest method of removing hardness like calcium & magnesium from water and renders the water suitable for utility purpose. As the name implies ion exchange is a process in which undesirable ions are exchanged for more desirable ions.
Mixed bed resins or mixed bed ion exchange resins are mainly used in the water purification industry for polishing process water to achieve demineralized water quality (such as after a reverse osmosis system). Mixed bed as the name states consists of strong acid cation exchange and strong base anion exchange resin.
Typical applications include
- Ultra pure water production
- Condensate polishing (boiler feed water)
- Micro-electronics cleaning
Anthracite/sand filter beds normally provide all of the advantages of single-media filtration but require less backwash water than sand or anthracite alone. Similar claims have been made for anthracite/sand/garnet mixed units. The major advantages of dual-media filtration are higher rates and longer runs. Anthracite/sand/garnet beds have operated at normal rates of approximately 5 gpm/ft² and peak rates as high as 8 gpm/ft² without loss of effluent quality.