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Six Years Operating Experience with a Membrane System for Pretreatment of Pcb-Laden Oily Wastewater

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A major power utility generates wastewater by cleaning utility manholes with high-pressure water and detergent. The wastewater primarily contains emulsified oil and grease, polychlorinated biphenyls (PCBs), suspended solids, and dissolved metals. A wastewater treatment facility (WWTF) was constructed in 1998 to treat the wastewater and allow permitted discharge to the sanitary sewer, substantially reducing the volume of waste that requires disposal as hazardous waste.

The WWTF was designed to primarily remove PCBs and oil from the collected wastewater. The removal of metals occurs as incidental treatment in the WWTF. The treatment train consists of oil/water separation, ultrafiltration, and liquid-phase granular activated carbon adsorption. The WWTF was designed to treat 2.1 million gallons per year at a flowrate of 20 gpm over an average 16 hour/day, five days per week. Currently, the WWTF is operating at 25 gpm, which is 25% greater than design, and the system performance for PCB and oil removal exceeds design.

The permitted discharge parameters include PCBs, metals (antimony, barium, cadmium, chromium, cobalt, copper, lead, mercury, molybdenum, nickel, tin, and zinc), volatile organic compounds, semi-volatile organics compounds, hydrogen sulfide, and pH. Historically, the WWTF had generally been in compliance with the discharge limits. However, when a new limit for lead was instituted in October 2002, the facility experienced several lead exceedances in late 2002 and early 2003. The allowable concentration of lead in the effluent was reduced from 2 mg/L to 0.172 mg/L (based on a 30-day average value). The installation of an ion exchange unit downstream of the carbon adsorption units was done to assure compliance with the new lead limit.

During normal operation, the ultrafiltration membranes require cleaning. At this WWTF, the membrane system is cleaned daily. Hot water and either detergent or citric acid are circulated through the filter system to remove organic or inorganic fouling, respectively. The spent cleaning solutions are sent to the head of the plant for treatment and disposal. It is believed that one of the cleaning agents used for this purpose affected the WWTF's ability to remove lead and contributed to the presence of hydrogen sulfide in the system. The WWTF has instituted the use of a new cleaning agent in an attempt to suppress these effects.

This paper discusses the long-term operating performance of the WWTF, specifically the membrane unit, and the modifications to the treatment train that were made to respond to tighter effluent limitations.
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Document Type: Research Article

Publication date: 2005-01-01

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