Ultrafiltration (UF) is a filtration technique whereby a fluid diffuses through a semi-permeable membrane. The purpose of ultrafiltration is the removal of suspended solids, bacteria, macromolecules, viruses and colloidal particles. Hardness, pesticides and salts are only removed when using a RO unit.
A UF unit operates according to the dead-end principle: the entire feed flow rate becomes permeate, whereby filtration takes place from the inside to the outside (or reverse) and the dirt is left behind in (or on the outside) of the membrane fibers. As filtration time progresses, this dirt accumulates, thereby increasing the pressure drop over the membrane and decreasing the yield.
Ultrafiltration processes are preferred over traditional treatment methods because no chemicals are required and a constant product quality can be guaranteed, regardless of feed quality (achieving between 90% and 100% pathogen removal).
In osmosis, a semi-permeable membrane allows a spontaneous diffusion of solvent from a diluted solution or pure solvent phase to a more concentrated solution. In this case, semi-permeable means that only the solvent can pass through the membrane. This solvent flux occurs spontaneously.
By increasing the difference in pressure even further, a net flow in the direction of the least concentrated solution occurs. This is referred to as reverse osmosis (RO).
Electrodeionization (EDI) is a technique whereby a combination of electrodialysis and ion-exchange with resins is used to prepare ultrapure water. By way of electrodialysis, ions are separated from water, macromolecules and uncharged solutes. The ions move through a membrane under the influence of a potential difference. The membranes are selected so as to improve the separation.
The EDI module consists of a number of compartments applied between the anode and the cathode. The compartment through which the power supply flows, contains a membrane on both sides. An anion membrane (AEM) is fitted directed towards the anode (only allows anions through), while there is a cation membrane (CEM) on the side facing towards the cathode (only allows cations through).
Anions present in the supply can therefore migrate towards the anode through the anion membrane, while cations move through the cation membrane in the direction of the cathode. These ions thus end up in so-called reject compartments, constructed as such that the ions cannot migrate further: the anions are stopped by a cation membrane, while the cations are stopped by an anion membrane.
The latter also forms the partition walls with two other supply compartments along which cations and anions are transferred. Net, both anions and cations end up in every reject compartment, so this solution becomes rich in salts.
Both the supply and reject compartments are filled with anion and cation exchange resins (mixed-bed). These resins speed up the transport of ions from the supply to the reject compartment but also ensures water is split under the influence of the applied electric current. The resulting H+ and OH- ions regenerate the cation and anion resins, resulting in a fully continuous desalting system.