Energy Consumption Trends in BNR Mixing Technologies

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 densities. Typical energy inputs have ranged from 0.25 horsepower per 1,000 cubic feet (cf) to over 1 hp per 1,000 cf. Promising new technologies including hyperboloid mixers and large bubble mixing have resulted a reduction in mixing energy requirements to energy densities of 0.1 horsepower per 1,000 cf and below. Advances in the understanding and design of traditional mixing systems have also allowed for a reduction in energy input using conventional mixing. The results of this study compare energy inputs for different mixing technologies based on data from various pilot studies and papers. Furthermore, the results highlight the variability of energy consumption within one mixing technology and between the different types of mixing technologies.This paper will provide a historical overview of mixing technologies used in activated sludge treatment with an emphasis on the development and application of energy requirements. Energy efficient mixing design will be discussed. The evaluation of mixing technologies for the City of Greensboro's (North Carolina) T.Z. Osborne Water Reclamation Facility (TZO WRF) Expansion and BNR Upgrade Project will be presented as a case study to illustrate the technical, energy and cost considerations for selecting mixing technologies for anaerobic and anoxic zones. Energy consumption information and present worth costs that were developed for four mixing technologies (jet mix, vertical mixers, hyperboloid mixers and large bubble mixing) at the TZO WRF are presented.