Ultrafiltration (UF) involves pressure-driven separation of materials from a feed solution. The technology is used to remove particulate and microbial contaminants, but it does not remove ions and small molecules. Pressure drives the process, which typically operates with a feed pressure of 4 to 100 psig. UF plants are automated and have low operational labor requirements. These systems, however, can require frequent cleaning. UF membranes have a service life of three to five years or longer, which is comparable to reverse osmosis membranes. UF modules are commercially available in tubular, hollow-fiber, plate and frame, and spiral wound configurations.
UF membranes reject solutes ranging in size from 0.03 microns and larger. Figure 1 provides a guide to the relationship between common materials, separation processes, and pore size measurements. The UF membrane process separates molecules in solution on the basis of size. The pore size and molecular weight cut-off (MWCO) are often used to characterize a membrane. The pore size is the nominal diameter of the openings or micropores in the membrane expressed in microns. The MWCO is the molecular mass or weight of a solute that rejects greater than 90 percent. The unit of measurement for MWCO is the Dalton (D).
Different membrane materials with the same nominal MWCO may have differing solute rejection. Pore size distribution and uniformity rather than the chemical nature of the membrane material may cause this effect. Because factors other than pore size or MWCO affect the performance of membranes, challenge studies are used to demonstrate membrane performance and benchmark different membranes.
MEMBRANE FILTRATION SPECTRUM
Figure: Material Size and Membrane Process Guide
DESCRIPTION OF ULTRAFILTRATION MODULE
Ultrafiltration membranes are designed for particulate removal. Wastewater is pressurized through the membrane and particulates are left at the membrane surface. Due to the small pore size of the membrane, effectively all suspended solids including microorganisms are rejected. These particulates build up in concentration at the membrane surface, thus the Wastewater flow direction is periodically reversed to remove particulate matter (backwash). The representative removal capability of the module is summarized in below Table.
Table . Removal capability of UF
Ultrafiltration System Removal
The Ultrafiltration modules are made from high strength, hollow fiber membranes that offer the following features:
• 0.03 μm nominal pore diameter for removal of bacteria, viruses, and particulates including colloids
• PVDF polymeric hollow fibers for high strength and chemical resistance
• Hydrophilic PVDF fibers for easy cleaning and wettability that help maintain long term performance
• Outside-In flow configuration for high tolerance to feed solids and the use of air scour cleaning
• U-PVC housings eliminate the need for pressure vessels and are resistant to UV light
The outside-In flow configuration is tolerant of wide ranging feed water qualities and allows air scour cleaning. The dead-end flow offers higher recovery and energy savings. The pressurized vertical shell-and-tube design eliminates the need for separate pressure vessels and allows easy removal of air from cleaning and integrity testing steps.
The hollow fiber membranes are 1.3 mm outside diameter and 0.7 mm inside diameter and are made from PVDF polymer. The fibers are strong because of a combination of the PVDF polymer, asymmetric dense spongy layer, and skins formed on each side of the fiber. The PVDF membranes offer high chemical resistance and are tolerant to temperatures of 40ºC. The hydrophilicity of the PVDF fibers is increased by using a proprietary treatment during manufacturing.
The 0.03μm nominal pore size combines high filtration performance and high flux. The smaller pore size provides stabile long term filtration performance compared to microfiltation.
Guidelines for Acceptable UF Feed Water
Parameter Unit Design Basis Maximum Allowable
Depending on application, a safety screen of 150 – 300 microns is recommended on the feed to the UF system. A variety of technologies can be used such as self-cleaning screens and bag, cartridge, or disc filters. Depending on the type of water or range of feed water parameters other pretreatment technologies such as oxidation, coagulation, sedimentation and media filtration may also be recommended.
ULTRAFILTRATION PROCESS DESCRIPTION
The UF elements are connected together in a parallel array of skids. Each elements will process Wastewater, backwash, and undergo an integrity test as a single entity. Each Skid contains UF modules,automatic valves, instrumentation, structural supports and piping manifolds.
The basic operating conditions for the DOW Ultrafiltration modules are shown in below Table. The process operating parameters for the cleaning steps in provided in the cleaning section.
Normal operation refers to the routine operating sequence of a system using the Ultrafiltration module and includes the operating and backwash steps. At initial start up the modules are flushed using a “forward flush” to remove any residual chemicals or trapped air from the module. The flush occurs on the outside of the fibers and does not filter the feed water to make filtrate. After the forward flush is discontinued the modules can be placed in the operating mode. An operating cycle ranges from 20 to 60 minutes. While operating 100% of the feed water is converted to filtrate. As contaminants are removed during the operating step the transmembrane pressure will rise. At the end of the preset operating cycle time, a backwash sequence is triggered.
The backwash mode occurs automatically and may include an air scour, but always includes draining, backwash through the top drain, backwash through the bottom drain, and a forward flush. The air scour step, when included, is used to loosen particulates deposited on the outside of the membrane surface. Air is introduced on the outside of the fibers and displaced feed flow/concentrate is allowed to discharge through the top of the module for disposal. After 20 to 30 seconds of air scour the module is drained by gravity to remove dislodged particulates. If the air flush is not included, the backwash sequence is started with simply draining the module by gravity to remove the concentrated feed water before starting any backwashing.