ULTRAFILTRATION – Treatment System for Oily Wastewater

Volume 15, #9

ULTRAFILTRATION – Treatment System for Oily Wastewater

Environmental engineers at West Virginia University (WVU, College of Engineering & Mineral Resources, Dept. of Civil & Environmental Engineering, Box 6103, Morgantown, WV 26506-6103) are using a membrane-based treatment system to treat oily wastewaters. The leader of the research team, Brian E. Reed (Tel: 304/293-3031 Ext.613, Fax: 304/293/7109, E-mail: reed@cemr.wvu.edu ), associate professor at WVU, tells MSTN that in August 1993, a West Virginia company that produces a mixture of coolant and lubricant waste came under a consent order to upgrade their industrial wastewater treatment facilities.

Up until this time, the company collected a mixture of coolants and lubricants from their milling operations and mixed them with dilute waste in two lined ponds having a combined capacity of about five million gallons. A significant amount of the coolant/lubricant oils separated and were skimmed from the ponds. The resulting pond effluent, which contained about 0.5% oil/grease (O/G) and had a daily flow of 80,000 gallons, was then sprayed on a 36-acre site where the remaining O/G was degraded naturally. In 1993, the West Virginia Department of Environmental Protection issued new groundwater regulations that strongly discouraged land application of industrial wastes. Shortly thereafter, the company began to investigate alternative treatment methods.

WVU’s Dept. of Civil and Environmental Engineering (WVU-CEE) was asked to perform pilot scale studies for the company involving various possible treatment technologies, including oil/water separation, chemical addition-dissolved air flotation, tubular ultrafiltration (UF), biofiltration, constructed wetlands, and lined sprayfields. Based on data collected over a two-year period, WVU engineers proposed a treatment system that included an oil/water coalescer, followed by tubular UF, and finally constructed wetlands.

The polymeric membranes for the tubular UF systems had a molecular weight cutoff (MWCO) of 120,000 and carried a negative surface charge, which repelled the negatively charged oil droplets, resulting in a higher flux. The effluent from the tubular UF system had an O/G content that was consistently below 100 mg/l and had a very low solids content. After UF treatment, the plan called for the waste to be sent to a hybrid land application/constructed wetlands for polishing.

The residual from the tubular UF portion of the treatment system had an O/G content of about 5% and a production rate of about 2,600 gal/d. At this rate, the projected disposal costs represented a major portion of the operating costs, so several methods of reducing the volume of UF residuals were investigated. Acid cracking, while effective in bench scale tests, was eliminated from further consideration because of the associated health and safety hazards. Alternative forms of chemical cracking were dismissed because of high cost. Ultimately, a high shear rotary membrane system, manufactured by SpinTek (16421 Gothard St., Unit A, Huntington Beach, CA 92647, Tel: 714/848-3060, Fax: 714/848-3034) that improves upon conventional crossflow UF was chosen.

The proprietary SpinTek system uses a series of flat, round inorganic membrane disks set on a hollow rotating shaft inside a cylindrical housing. The fluid stream enters the membrane chamber under pressure and is distributed across the membrane surfaces. Permeate is forced through the membrane and is collected in the hollow shaft and discharged. The concentrate exits at the edge of the membrane packs. With time, the permeate rate decreases due to concentration polarization. To reduce the flux decline in conventional membrane systems, a large portion of the concentrate is recycled back to the membrane unit, producing large liquid velocities near the membrane surface. The large velocities increase turbulence, which reduces the thickness of the concentration polarization layer. In the SpinTek rotary system, the rotation of the membrane disk induces the turbulence required to minimize the thickness of the solute boundary layer. The SpinTek contact is William Greene, President.

In conventional membrane systems, maximum surface velocities of about 15 ft/s are possible. In SpinTek’s rotary systems, surface velocities of 60 ft/s are typical. As the concentrate thickens with treatment time, a conventional UF system is not able to maintain the high velocities because of the difficulty in pumping viscous material at large flow rates. Since the SpinTek system does not rely on pumping to produce the required membrane surface velocities, extremely concentrated wastes can be treated. In the pilot scale work conducted at WVU-CEE, wastes having an O/G content of about 5% were easily concentrated to over 50% oil. In several runs, the final O/G concentration approached 80%.

In the full scale system, tubular UF residual will be concentrated to 50% oil using the SpinTek rotary system and then transferred to storage tanks equipped with skimmers. At high oil contents, phase separation can occur without chemical addition, and the oil skimmed from the tanks will be disposed of by an oil recycler. WVU estimates the payback period for the rotary portion of the system at one to two years. The full scale system will be operational in fall of 1997. In addition to using the rotary system to treat UF tubular residuals, the treatment of coolants and lubricants directly (i.e., prior to entering the ponds) is possible. Having the option of separating the majority of the O/G from the waste prior to it entering the ponds would give the company flexibility in dealing with its waste management issues.