With an old wastewater treatment facility and the need to remain compliant with its discharge permit, the North Salt Lake Refinery of Big West Oil LLC in North Salt Lake, Utah, needed to upgrade its plant. The refinery had increased its throughput by more than 100 percent over the last 20 years without any major additions to the wastewater system. The larger wastewater flow resulted in less holding time to respond to upsets and increased maintenance costs as a result of its existing circular dissolved air flotation (DAF) unit's age.

Refinery management decided to upgrade the 400 gal/min (91 m3/hr) treatment system. After an on-site pilot study, Siemens Water Technologies, Warrendale, Pa., designed a new rectangular folded flow DAF system that included an ancillary flocculation and flash mix tank.

The upgrade resulted in a smaller footprint that enhances separation and allows for better hydraulic loading at a higher rate for more effective removal of contaminants, say the companies. Siemens minimized the footprint by folding the effluent flow underneath the influent in the unit, allowing separation to occur over a surface area of 132 ft2 - less than a quarter of the surface area provided by the 27-ft dia circular tank. Dissolved air comes in with recycled water on the unit through a dispersion pipe that runs the full width of the tank, as opposed to being delivered in the middle of the tank with a dispersion tray as did the old circular DAF unit.

The new system requires less maintenance and less operator attention. It also has improved the consistency and reliability of treatment. Comparing discharge data six months before and after the project, biochemical oxygen demand levels in wastewater from the refinery fell by 67 percent, chemical oxygen demand levels dropped 60 percent, and ammonia, BTEX and sulfate/sulfide levels had minor reductions, according to Siemens.Siemens notes that the installation marked the first folded flow DAF unit to provide secondary oil/water separation in a U.S. refinery.