Energy Consumption Trends in BNR Mixing Technologies

摘要

Secondary treatment is typically the largest energy consumer at municipal wastewater treatment plants (WWTPs), with aeration representing most of this demand. The energy used in the activated sludge process generally increases with enhanced levels of treatment. As WWTPs face more stringent nitrogen and phosphorus effluent limits, energy requirements for pumping and mixing will continue to increase relative to aeration. Although much of the energy reduction efforts in activated sludge treatment have been focused on reducing aeration energy and demand, comparatively, limited attention has been focused on reducing the energy associated with recycle pumping and mixing. Biological nutrient removal (BNR) processes generally include multiple unaerated anoxic and/or anaerobic zones to remove nitrogen and phosphorus, respectively. Greater unaerated tankage requirements are associated with increased levels of treatment; for example five-stage BNR processes require anaerobic, pre-anoxic and post-anoxic zones. These unaerated zones need to be mixed in order to maintain solids in suspension and provide efficient treatment. Unaerated tankage may account for more than 50% of the total activated sludge volume in a five-stage BNR process, and mixing requirements for these zones can exceed aeration requirements, particularly in underloaded systems. For example, Figure 1 illustrates a case in which the energy consumed by the mixing process was significant in comparison to the aeration process. The Figure illustrates the breakdown of energy consumption in the biological treatment system at a 20 million gallon per day (mgd) 5-stage BNR plant located in North Carolina. At this facility, the jet aeration mixing system consumed 32 % of the total energy consumption. Submersible mixers, vertical turbine mixers and jet mixing systems have provided decades of reliable mixing of unaerated zones at municipal WWTPs. These technologies have typically been applied at a range of mixing energy densit