Ultrafiltration or UF is a pressure driven membrane separation process that separates 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). The smaller the nominal pore size, the higher the removal cabability. Most materials that are used in UF are polymeric and are naturally hydrophobic. Common polymeric materials used in UF include: Polysulfone (PS), Polyethersulfone (PES), Polypropylene (PP), or Polyvinylidenefluoride (PVDF). Although these materials can be blended with hydrophilic agents, they can reduce the membranes ability to be cleaned with high strength disinfectants such as hypochlorite that impacts removal of bacterial growth.
The DOW™ Ultrafiltration module utilizes a double-walled hollow fiber (capillary) PVDF membrane which has 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.
|Fiber Physical Properties||Operating Conditions||SI Units||US Units|
|Configuration (fluid flow)||Hollow Fiber (Outside-In)||Feed Pressure, Maximum at Inlet||3 bar||45 psi|
|Base Polymer||H-PVDF||Operating TMP (Maximum)||2.1 bar||30 psi|
|Nominal Pore Diameter||0.03 μm||Backwash Pressure (Maximum)||2.5 bar||36 psi|
|Hollow Fiber ID||0.70 mm (0.028″)||Filtrate Flux @ 25°C||40-120 L/m2/hr||24-70 gfd|
|Hollow Fiber OD||1.3 mm (0.051″)||Backwash Flux||100-150 L/m2 /hr||59-88 gfd|
|Temperature (UPVC limited)||1-40°C||34-104°F|
|Operating pH Range||2-11|
|NaOCl, Cleaning Maximum||2,000 ppm|
|Turbidity, NTU||< 50||300|
|TSS, mg/L||< 50||100|
|Particle Size, μ||< 150||300|
|TOC, mg/L||< 10||40|
|Oil/Grease, mg/L||0||< 2|
|Cl2 Continuous, mg/L||0.5||200|
|Cl2 Cleaning, mg/L||2000||5000|
|Backwash Frequency||Once every 20-60 minutes|
|Backwash Duration||40-120 seconds|
|Chemically Enhanced Backwash Frequency||As needed with NaOCl|
|Typical Cleaning Frequency||1-3 months|
|Cleaning Chemicals||NaOCl, NaOH, HCl, Citric Acid, Oxalic Acid|
|Air Scouring Frequency||0 times per day to backwash frequency|
|SI units||US units||SI units||US units||SI units||US units|
|Length – L||1860 mm||73.2″||2360 mm||92.9″||1860 mm||73.2″|
|Length – L1||1500 mm||59.1″||2000 mm||78.7″||1500 mm||59.1″|
|Length – L2||1610 mm||63.4″||2110 mm||83.1″||1630 mm||64.2″|
|Length – L3||1710 mm||67.3″||2210 mm||87.0″||1820 mm||71.7″|
|Diameter – D||165 mm||6.5″||165 mm||6.5″||225 mm||8.9″|
|Width – W||250 mm||9.8″||250 mm||9.8″||342 mm||13.5″|
|Module Surface Area||33 m2||355 ft2||44 m2||474 ft2||51 m2||549 ft2|
|Volume per Module||16 L||4.2 gal||20 L||5.3 gal||35 L||9.3 gal|
|Weight (water filled)||41 kg||90 lbs||53 kg||117 lbs||83 kg||183 lbs|
|Shipping Weight (w/o packaging)||25 kg||55 lbs||33 kg||73 lbs||48 kg||106 lbs|
|Fibers per Module||5760||5760||9000|
‘X’ in SFX = Application (P = Pretreatment, D = Drinking Water)
* MWCO = molecular weight cut-off
There are 4 membranes geometries:
- Spiral wound module: this design tries to maximize surface area in a minimum amount of space. It is the less expensive but more sensitive to pollution due to its manufacturing process. It consists of consecutive layers of large membrane and support material in an envelope type design rolled up around a perforated steel tube.
- Plate and frame module: it is normally used for bad quality water. They are set up with a stack of membranes and support plates.
- Tubular membrane: Generally used for viscous or bad quality fluids. These modules do not need a preliminary pre-treatment of the water. As the feed solution flows through the membrane core, the permeate passes through the membrane and is collected in the tubular housing.
The main drawback is that the system is not very compact and has a high cost per m2 installed and it is not very compact. Diameter’s tube is generally between 4 and 25mm.
- Hollow fiber membrane: The modules contain several small (0.6 to 2 mm diameter) tubes or fibers. As the feed solution flows through the open cores of the fibers, the permeate is collected in the cartridge area surrounding the fibers. It can carry out the filtration in two ways, either “inside-out” or “outside-in”
Comparison between the different types of geometry (Source: Water Treatment Handbook, Degrémont Suez, 7th edition):
|Tubular||Hollow fiber||Plate and frame||Spiral wound|
|Reverse Osmosis, Nanofiltration||yes||yes||yes||>95%|
|Cost per m2||–||+++||+||+++|
+++: obvious advantage
DOW™ Ultrafiltration Literature
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