High-Flux Tubular MF Pretreatment for Industrial Wastewater Recycling
Despite its capability to produce high-quality re-usable water from wastewater, a large number of RO recycling installations in the field have experienced difficult operational problems. These problems include poor filtration rate, decrease in salt rejection, frequent membrane cleaning, and premature membrane failure. These problems result in high operating cost and in many cases shut down of production processes. The primary cause for such an undesired or unacceptable performance is that RO membrane has low tolerance for a broad range of incompatible components in water. These substances, if not removed, will cause scaling, fouling or permanent degradation of the RO membrane. Such issues coupled with the ever-increasing water recycle demand have driven the development of the next generation of high-flux tubular membrane microfiltration (MF) technology to augment the RO performance in the wastewater recycling process. Because of its versatility, robust nature and high filtrate quality, tubular microfiltrationcould well replace traditional equipment including gravity clarifiers, media filters, lime softeners and ion exchangers as RO pretreatment for removal of unacceptable substances.
The high-flux tubular MF filter fabrication process starts with preparation of a chemical solution consisted of a Polyvinylidene Fluoride (PVDF) polymer and other membrane formation enhancing chemicals. The unique solution formula and application procedures have effectively (1) eliminated all undesired large pores, (2) generated a much larger number of pores per square inch of membrane surface, and (3) formed strong interstitial anchorages between the PVDF membrane and polymeric tube surfaces. The first feature of uniform sub-micron pore size enables complete removal of the detrimental components and generates a flow stream with NTU (<1.0) and SDI (<3.0) values in full compliance with the RO feed water criteria. The second feature of increasing pore number explains the ability of the membrane to consistently operate at twice the operating flux of other similar MF membranes. This feature will allow the design to take one half as many membranes to filter the same number of gallons of waste water. The thirdfeature of forming strong surface bond produces a highly durable MF filter material that can extend a continuous operation for 5 to 10 years without membrane replacement. The extraordinary chemical resistant property of PVDF permits the use of a wide range of chemicals - acids, bases and solvents for effective cleaning of the persistent fouling substances which are very difficult, if not impossible, to be cleaned from the RO membrane if not removed.