Ultra filtration is a pressure-driven purification process in which water and low molecular weight substances permeate a membrane while particles, colloids, and macromolecules are retained. The primary removal mechanism is size exclusion, although the electrical charge and surface chemistry of the particles or membrane may affect the purification efficiency. Ultra filtration pore ratings range from approximately 1,000 to 500,000 Daltons, thereby making UF more permeable than nano-filtration (200 — 1.000 Daltons).
UF membranes are composed of a polymer, such as polysulfone or polyamide, that is usually extruded into flat sheets or hollow fibers or cut into disks as required by the specific application. A small disk of UF membrane may be subject to rapid fouling and produce a low flow rate for many processes.
As a result, UF membranes are typically arranged in a configuration which maximizes surface area and reduces fouling by using a tangential flow design to reduce solute accumulation at the membrane surface. Tangential flow UF devices may be spiral-wound cartridges containing several square feet of membrane wrapped onto a central core tube or hollow-fiber cartridges containing dozens of thin UF membrane fibers.
Typically, the process is suitable for retaining bio molecules, bacteria, viruses, polymers, colloidal particles and sugar molecules.
Ultra filtration membranes are defined by their nominal molecular weight cut-off (MWCO). The MWCO generally represents the smallest molecular weight for which the membrane has a retention value of more than 90%. In many cases, however, separation efficiency is not only influenced by cut-off but also by interaction between the membrane and the raw solution. The operating pressure for ultra filtration is usually between 0.1 and 1 MPa.
The following operations are typical for the application of ultra filtration:
- Filtration of raw solutions
- Concentration of substances
- Fractionating of substances
In the fractionating of dissolved substances or the separation of dissolved substances and solvents, a sufficient degree of separation is reached when the size of the particles differs by the factor 10. Nowadays, different applications for ultra filtration can be found in nearly all industrial sectors:
- Sterile filtration of drinking and beverage water
- Treatment of surface water
- Recovery of filter back flushing water
- Separation of oil/water emulsions
- Recovery of electro deposition paint
- Removal of metal hydroxides in wastewater treatment
- Separation of biomass in biotechnology
- Wastewater treatment and re-use
- Membrane bioreactors
Great significance is given to the use of ultra filtration for sterile filtration because only a suitable ultra filtration process can ensure a retention of >log 4 for bacteria, viruses, Legionella and possibly even endotoxins. Ultra filtration is, therefore, the choice process for point-of-use filtration for the safe production of drinking water.
One of the most sophisticated uses of ultra filtration lies in the application of membrane bioreactors (MBR) for wastewater. The ultra filtration can operate in the normal way on a cross-flow by-pass system, or submerged in the bioreactor vessel by means of vacuum suction at low trans-membrane pressures.
The combination of activated sludge with membrane separation in the MBR results in efficiencies of footprint, effluent quality and residuals production that cannot be attained when these same processes are operated in sequence.
- No chemicals required (aside from cleaning)
- Constant product quality regardless of feed quality
- Compact plant size
- Capable of exceeding regulatory standards of water quality, achieving 90-100% pathogen removal.
UF processes are currently limited by the high cost incurred due to membrane fouling and replacement. Additional pretreatment of feed water is required to prevent excessive damage to the membrane units.
In many cases UF is used for pre filtration in reverse osmosis (RO) plants to protect the RO membranes.