Filtration is the physical removal of suspended matter through a mechanical or chemical reaction. This is one of the most important steps in the planning/design of a water treatment plant and can mean the success or failure of a plant or project.
Sand and other types of specific media filtration are used for the removal of suspended matter, as well as floating and sinkable particles. The wastewater flows vertically through a fine bed of sand and/or gravel. Particles are removed by way of absorption or physical encapsulation. If there is excessive pressure loss on the filter, it must be rinsed.
Micro-filtration is the filtration of a liquid suspension through a membrane with pores of approximately 0.1-10 um in diameter. It is a pressure-driven process used to retain microorganisms and other suspended solid particles from a process liquid while transmitting solutes that are smaller than the selective membrane pore size.
Ultra-filtration (UF) is a pressure-driven membrane processes. The ultra-filtration process uses a membrane which, only allows particles smaller than 20 nm to pass through it. The pore size varies between 20 nm and 0.1 microns.
Nano-filtration (NF) lies between ultra-filtration and reverse osmosis, with typical pore size cut-off values in the range of 150-500 Dalton, depending on the molecular structure. A nano-filtration membrane is also ion-selective. This is the ability to distinguish various ions from one another, which makes it ideal for specialized industrial effluent treatment.
Reverse Osmosis (RO) is a pressure-driven membrane process with a separation range between 0.1 and 1 nm. Thus reverse osmosis membranes have high retention for bacteria, viruses and micro particles. Bivalent and some univalent ions are also blocked by the membrane.