Ultrafiltration belongs to membrane processes. It is a method of water filtration, involving the membrane as a filter in separation of the particles from a liquid medium. Membranes utilized in the ultrafiltration process have the porosity of 0,1 to 0,01 µm. The driving force is the hydrostatic pressure difference. During the process of ultrafiltration, the inlet water is divided into a retentate, which remains on the inlet side, and the permeate, which passes through the membrane.
In the case of the ultrafiltration, there are two dominant mechanisms of particle separation:
- particles larger than the membrane pore diameter are captured on the surface of the partition (membrane)
- particles smaller than the membrane pore diameter penetrate the pores and are adsorbed on their surface. The adsorption (physical and chemisorption) occurs also outside the pores on the membrane surface.
Depending on which mechanism prevails, a specific method of membrane cleaning is used. In the first case it is sufficient to remove the collected particulate matter simply by backflush. In case when the second mechanism prevails, there is usually a need of adding the cleaning chemicals in the flushing water. Then we talk about so-called chemically reinforced flushing. The use of the cleansing chemicals is also necessary when anorganic substances are precipitating on membranes, or when membranes are clogged with organic substances. In both cases, the cause is the poorly pretreated inlet water. From the facts mentioned above it is clear, that the ideal state is the one where the inlet water coming to the membrane already contains only particles which are aggregately stable, with a diameter greater than the pore diameter. For this reason, according to the nature of the inlet water resource, raw water is usualy pretreated before entering the ultrafiltration unit.
Most of the membranes are currently produced from polymers rather than from marginally used ceramic materials. Operation of the ultrafiltration membranes is cyclic, so after the filtration phase comes the wash phase - exactly backwashing.
Ultrafiltration is a compact alternative to traditional filtration technologies for water treatment. However, due to a small pore diameter, membranes allow the removal of particles which are several orders smaller than particles removed by e.g. sand filtration. This means that membranes separate the bacteria, viruses and macromolecular organic compounds. The process can easily be automated, but it will require daily supervision service, especially raw water quality monitoring, together with the possible response to its changes. Like all modular systems, ultrafiltration technology can be relatively easily expanded according to the changes of the production capacity.