Ultrafiltration and Microfiltration are both pressure driven membrane separation processes that separate particulate matter from soluble components in the carrier fluid (such as water). UF membranes typically have pore sizes in the range of 0.01 – 0.10 µm and have a high removal cabability for bacteria and most viruses, colloids and silt (SDI). A typical microfiltration membrane pore size range is 0.1 to 10 micrometres. The smaller the nominal pore size, the higher the removal cabability. Membrane filters are widely used in biotechnology and food and beverage applications where sterile product is required.
Increasingly used in drinking water treatment, it effectively removes major pathogens and contaminants such as Giardia lamblia cysts, Cryptosporidium oocysts, and large bacteria. For this application the filter has to be rated for 0.2 micrometres or less.
Most materials that are used in ultra- and micro-filtration are polymeric and are naturally hydrophobic. Common polymeric materials used in UF include: Polysulfone (PS), Polyethersulfone (PES), Polypropylene (PP), or Polyvinylidenefluoride (PVDF). For mineral and drinking water bottlers, the most commonly used format is pleated cartridges usually made from Polyethersulfone(PES) media. This media is asymmetric with larger pores being on the outside and smaller pores being on the inside of the filter media.
DAICO uses a DOW™ Ultrafiltration module which utilizes a double-walled hollow fiber (capillary) PVDF membrane, with a very small nominal pore diameter for PVDF material that allows for the removal of all particulate matter, bacteria and most viruses and colloids. Despite the small pore diameter, the membrane has a very high porosity resulting in a flux similar to that of micro-filtration (MF) and can effectively replace MF in most cases.
Systems designed with DOW™ Ultrafiltration use an outside-in flow configuration which allows for less plugging, higher solids loading, higher flow area and easy cleaning. The primary flow design is dead-end filtration but the module can be operated using a concentrate bleed. Dead-end filtration uses less energy and has a lower operating pressure than the concentrate bleed, therefore reducing operating costs.
Typically, DOW™ Ultrafiltration is operated at a constant permeate flow. The transmembrane pressure (TMP) will naturally increase over time and the module can be cleaned periodically by back flushing and air scouring to remove the fouling layer. Disinfectants and other cleaning agents can be used to fully remove and prevent performance loss due to biological growth as well as other foulants.