ULTRAFILTRATION – Oily Wastes Head for Spin Cycle
Conventional crossflow membrane technology is not effective in treating concentrated oily wastes because of membrane fouling/concentration polarization and the resultant decrease in permeate flux. One solution to this problem is to provide additional shearing energy at the membrane surface. Environmental engineers at West Virginia University (WVU, College of Engineering & Mineral Resources, Dept. of Civil & Environmental Engineering, Box 6103, Morgantown, WV 26506-6103) are testing a centrifugal membrane (CM) process by SpinTek (16421 Gothard St. Unit A, Huntington Beach, CA 92647; Tel: 714-848-3060). The SpinTek contact is William Greene, President.
Ultrafiltration (UF) is a pressure driver membrane technique that uses porous membranes to clean a stream of material in the 1 nanometer (nm) to <10 micron size range or of compounds with molecular weights in excess of 5,000. Colloidal material, macromolecules, and micelles are examples of items that can be fractionated. Permeate is forced through the porous membrane, while the waste (the concentrate) is retained by the membrane. In all membrane-based processes, there is a buildup of the solute at the membrane surface due to convective mass transport.
This buildup, or “concentration polarization,” is one reason why the permeate flux for a waste is greater than the clean water flux. The buildup of the solute at the membrane surface is reduced by back diffusion and turbulence. If the solute concentration at the membrane is high enough, a gel layer can form.
The proprietary SpinTek CM process uses a series of flat, round membrane disks set on a hollow rotating shaft inside a cylindrical housing. The leader of the research team, Brian E. Reed (Tel: 304/293-3031 Ext. 613, Fax: 304/293-7109, email: reed@faculty.coe.wvu.edu), Associate professor at WVU, tells MSTN that feed enters the membrane chamber under pressure. Permeate is forced through the membrane and discharged via the hollow shaft. Concentrate exits at the edges of the membrane packs.
In conventional UF systems, about 98% of the concentrate is recycled back to the membrane in order to produce high liquid velocities near the membrane surface. The high velocities produce turbulence which reduces the thickness of the concentration polarization layer. In the CM system, membrane disk rotation produces the required turbulence and cleaning action.
Conventional systems can produce maximum liquid velocities of about 15 feet per second (ft/s), but CM liquid velocities of 60 ft/s are typical. As the concentrate thickens with time, a conventional systems is not able to maintain high velocities because of the difficulty in pumping viscous material. Because the CM system does not rely on pumping to produce the required membrane surface velocities, extremely concentrated wastes can be treated.
Reed, whose research team included Wei Lin, Roger Viadero, Jr., and Joseph Young, provided MSTN with the details of WVU pilot-scale work in which aluminum rolling mill coolants/lubricants containing 5% oily materials were treated. Two types of membranes, ceramic and polymeric, were evaluated. System performance was measured with respect to permeate flux, permeate quality; cleaning effectiveness, and membrane durability.
Six experimental runs were made, three for each membrane material. The polymeric membrane was a polyvinyliden fluoride with a molecular weight cutoff (MWCO) of 100,000. The Ti02-Al203 ceramic membrane had a mean pore size of 0.1 micron. Both types of membranes removed at least 97% of the oil and grease, and the ceramic version removed better than 99%. With respect to total suspended solids (TSS), both types of membranes removed in excess of 99%, but the ceramic-based system was slightly superior.
Based on the data generated in the test work, Reed’s team concluded that ceramic membrane-based CM systems are capable of directly treating aluminum rolling mill coolants and lubricants. The superiority of the ceramic membrane was the result of its larger flux, higher operational temperature, and absence of permanent membrane fouling. In a full-scale system, the maximum rotational speed possible should be used. Decreasing the rotational speed decreases the permeate flux, and the flux did not recover when maximum rotational speed was resumed. Reed theorizes that when the rotational speed was decreased, a gel formed that was not easily removed when the rotational speed was increased.
Separated oil, says Reed, can be reused or sold to an oil recycler. In addition, the CM system can replace conventional chemical treatment/evaporation processes commonly used to treat residuals from conventional UF systems, as well as other oily wastes.