FREQUENTLY ASKED QUESTIONS Index
| What is Ultrafiltration (UF)? | Go to Top | |
| Ultrafiltration is a pressure driven membrane separation operation in which particulates, colloids, emulsified oils, and macromolecules are separated from a liquid feed stream upon passage through a porous semi-permeable membrane. The separation is based primarily on the size of the species in the liquid relative to the size of the membrane pores. UF membranes can provide filtration in the 0.0015 to 0.1 micron (1000 MW to colloidal) range. As a consequence of high molecular weight of solutes, the osmotic pressures of the solutes are usually low. Thus the operating pressures of UF range from 0.5 to 7 bar only. | ||
| Which materials are used for UF membranes? | ||
| Poly-sulphone (PS) and it’s derivatives like poly-ether-sulphone (PES) are the widely used materials for UF. These materials can tolerate pH between 1 & 13, temperatures upto 75 deg C, and upto 50 ppm chlorine for long storage. These membranes can be cleaned with 0.1% NaOCl solution. | ||
| How Ultrafiltration (UF) Membranes are rated? | Go to Top | |
| UF are rated on their ability to remove certain sized molecules. The size of a molecule is roughly proportional to its weight, and therefore the MWCO specified for an UF indicates the minimum size (weight) of molecules that will be removed by a particular UF device. | ||
| When do I use an Ultrafiltration system for water treatment? | ||
| Ultrafiltration is a complimentary process to reverse osmosis where dissolved salts are not removed from water, which means there is no change in the chemical composition of the product water from an Ultrafiltration System. Large molecular weight organics (1000 daltons - 100,000 daltons) bacteria, colloids, viruses and other substances are removed from the water. This produces sparkling clear safe water. | ||
| The three main applications in water treatment are | ||
| • | Bottled water production where feed water TDS is within limits, but clarity and disinfection is required. | |
| • | Pretreatment to reverse osmosis systems to enhance life of the Reverse Osmosis membranes and protect them form fouling. It is a single stage step process which eliminates, settling, sand filtration and polishing cartridge filtration. | |
| • | Domestic water purifier for producing sparkling, crystal clear, safe drinking water without electricity, chemicals and Ultraviolet rays. | |
| What are the other applications of UF membrane? | Go to Top | |
| The solutes or the materials to be separated usually have molecular weights greater than 1000 such as: | ||
| a) | Macromolecules: proteins, polymers, starches, natural gums & enzymes etc. | |
| b) | Colloidal dispersions: clays, pigments, minerals, latex particles, and microorganisms. | |
| c) | Emulsions: grease-detergent, and oil-water emulsions. | |
| Current large scale applications include: | ||
| A. | Ultrapure water: Ultrapure water for production of IC/silicon wafers in the electronics industry. | |
| B. | Paint manufacturing industry waste: Cathodic electrocoat paint recovery. | |
| C. | Waste recovery: Handling and recovery of many aqueous wastes including: Dyes and pigments, Textile de-sizing water, Oil-water/latex/other emulsions & alkaline wash-streams etc. | |
| Besides these, wastewater reclamation and reuse has developed as a major application of UF in recent years. | ||
| D. | Clarification of newly fermented wine, beer & cider. | |
| E. | Pharmaceutical industry: Pyrogen removal in the production of Ultrapure water, cell harvesting, etc. | |
| F. | Concentration of proteins in foodstuffs such as whey. | |
| How does it work? | ||
| Untreated water is forced against a semi-permeable membrane. The membrane allows the water molecules to pass through and screens out the colloids much like a sieve. Excess water is used to wash the colloids off the surface and to a drain. Unlike most filtration, the sieve does not fill up with trapped material because the colloids are to large to fit into the opening of the sieve. A good analogy would be dumping a mixture of water and marbles (the colloids) over a window screen (the membrane). Most of the water would pass through the screen. The remaining water would help wash the marbles off the screen and would be sent to waste with the marbles. The screen would not become plugged because the marbles are too large to fit into the openings. | ||
| What can this be used for? | Go to Top | |
| We use this process as an alternative to flocculation and filtration. This technique is not always an appropriate alternative to flocculation and filtration. On site testing and lab testing can be done to determine how successful either of these techniques will be for a particular application. | ||
| What does a system consist of? | ||
| Because the UF filters are expensive, they are not usually sized large enough to handle peak flows. This requires the system to have several components to allow the UF to produce water during times of no use and store it for later peak demands. The three basic components of the system are the UF filter (filter, housing, controls pre-filter and solenoid), storage tank with control float and re-pressurization pump. The re-pressurization pump delivers water from the storage tank to the house. The re-pressurization pump will cycle on and off to deliver the water to the water fixtures. When the water level in the storage tank drops below the float, the solenoid valve will open and send water through the UF filter to the storage tank. | ||
| What type of membrane configurations are available? | Go to Top | |
| PS membranes are available in plate and frame, tubular, hollow fiber, and spiral-wound configurations. | ||
| What are the membrane ratings? | ||
| Membranes are rated in terms of molecular weight of the smallest molecule rejected by the membrane - called as Molecular weight cut-off (MWCO). MWCO ratings are given w.r.t. 0.5% solution of Poly-ethylene glycol (PEG). The typical ratings available are: 5000; 10,000; 30,000; 1,00,000; 5,00,000 etc. For example, for recycle of wastewater we use membranes with MWCO of 1,00,000. For pyrogen removal in production of Ultrapure water, membranes with MWCO of 10,000 are used whereas for bacteria removal membranes with MWCO = 85000 are used. | ||
| What are hollow fiber membranes? | Go to Top | |
| Hollow fiber membranes: The “Romicon” fibers utilize an internally skinned polysulfone UF membrane. The typical fiber dimensions are as follows: Bore diameter: 1 mm; Wall thickness: 0.5 mm; O.D. = 2 mm. | ||
| Pressurized feed passes through the fiber bore at comparatively high velocities. Reverse flow of permeate is also possible which allows back-flushing as a regular technique. The claim is made that back flushing provides a positive means of freeing the foulant from the membrane surface which increases the cleaning efficiency and extending run times and cartridge life. | ||
| Limitations: Due to high packing density and correspondingly fine channel interstices, they are more susceptible to fouling, and more difficult to clean. | ||
