Alaskan septic system benefits from MBR

InTech, Feb 2004

An Alaskan lodge on the Kenai Peninsula turned to a membrane bioreactor (MBR) technology-replacing its septic tank and a mound leach field system with a custom-designed MBR system.

The most important constraint was maintaining the option for subsurface discharge with the existing mound system at the increased wastewater flow rates anticipated for the expanded facility. The lodge needed to minimize the soluble organic and particulate load to the subsurface adsorption system. In addition to a high-guality effluent, they required a system that existing maintenance staff could operate with limited wastewater treatment training and experience. The system needed to operate throughout the peak and shoulder seasons and shut down during the off-season.

SYSTEM PERFORMANCE

The facility started with raw wastewater in May 2001 and operated for more than two years. The lodge generates a particularly strong waste stream with chemical oxygen demand (COD) values typically in excess of 1000 milligrams per liter. Effluent COD has been consistently below 60 milligrams per liter since start-up regardless of waste strength.

The lodge saw less consistent results for nitrogen removal. The 2001 season brought limited nitrification. Because the MBR started from only raw waste-water, it would take much of the first season to develop an active culture of nitrifiers. Also, the high waste strength the lodge generated exceeded the design waste strength. As a result, the initial aeration capacity was inadequate to consistently meet the oxygen demand. Based on the results of the first season of operation, the lodge installed additional blower capacity in 2002 and added an aeration header to one end of the anoxic tank to provide more aerobic detention time during peak months. These changes promoted better nitrification during the second and third years of operation. Since start-up, the lodge has measured consistently low levels of effluent nitrate.