Filtration Methods

Filtration is the oldest water treatment method and is still employed worldwide from advanced filtration processes using nano-materials to villagers in India using multi-layer silk fabric filters. Although originally used to remove particulate matter, all filtration devices can reduce pathogens if associated with small particles or if the pores are small enough to physically remove them from the treated water. Therefore, the performance of filters is primarily related to the pore size. Granular filtration employing rapid or slow sand filtration can only remove particles larger than 10 μm and so are only fully effective against protozoan cysts and oocysts when combined with coagulation-sedimentation beforehand. The use of a disinfection step is required to remove any remaining bacterial or viral pathogens. Membrane technology in the drinking water sector is only 25 years old and is still very much in its developmental stage. There are four main types of membrane filter used in water treatment microfiltration (MF – employing 0.1 μm pores), ultrafiltration (UF – 0.01 μm pores), nanofiltration (NF – 0.001 μm pores) and reverse osmosis (RO – <0.001 μm), which uses a non-porous membrane. Removal of microorganisms is done primarily using MF for protozoa and bacteria, although UF is required for the removal of viruses. In these processes the pathogenic microorganisms are not destroyed or inactivated but separated and removed becoming concentrated in the waste stream which can then pose a disposal problem. MF and UF are now mainstream unit processes in water treatment and will inevitably replace granular media filtration as they do not require coagulation, flocculation and sedimentation. Process technology and the removal efficiency of filters are described as are their application. The development of larger membranes for large-scale plants, the ability to backwash membranes other than hollow fibre such as spiral wound or cartridge configurations, easier integrity testing, reduction in energy usage, reduction in fouling by membranes are all currently under development. Membrane filtration is also increasingly being used as a point-of-use treatment system in households, businesses, schools and hospitals.