Energy Audits and Benchmarking of Activated Sludge Aeration Systems

Since 2008, the authors have performed energy audits of aeration systems at over twenty wastewater treatment plants (WWTPs). Despite the impacts of the current economic downturn on municipal budgets, the audit results have already contributed to motivating several WWTP owners into proceeding with aeration energy efficiency improvement projects. One of these projects, a simple replacement of a positive displacement blower with a high-speed direct-drive turbo with air bearings, has been operational for over two years and has achieved the desired energy use reduction. A more comprehensive aeration system retrofit, involving blowers, diffusers, and controls, has been in successful operation since before the audits started. An even more comprehensive retrofit, additionally including a complete replacement of all air mains and installation of complex controls, is now in substantial completion of construction and expected to be operational in time for the October 2011 conference. Still more projects by Baxter & Woodman varying in complexity and delivery method are currently in planning, design, or construction, and include one of the first installations in the U.S. of rotary screw compressors, as part of a project to convert from anaerobic to aerobic digestion.The presentation will summarize the audit results and their use for screening of potential activated sludge and aerobic digestion aeration system improvement projects for probable cost-effectiveness and swift payback. The plant dataset is a modest size. Consequently, it requires a focus on plant and aeration fundamentals rather than on statistical analyses. The results and the discussion should prove useful to WWTP owners by illustrating the screening of potential improvements for inclusion in more detailed studies or implementation designs.For example, Facility A, an activated sludge facility with primary clarifiers was audited just two weeks before Facility B, which uses the same treatment process. The comparison of power consumption at these remarkably similar facilities and only two weeks apart showed activated sludge energy consumption at Facility B of 2,140 kilowatt-hours per million gallons treated (kWh/MG) and 1,428 kilowatt-hours per thousand pounds BOD treated (kWh/1,000 lb BOD). In contrast, previously completed comprehensive upgrades at Facility A lowered the energy consumption to 226 kWh/MG and 257 kWh/1,000 lb BOD. This comparison was used to justify a comprehensive aeration system upgrade at Facility B, which project is now well into construction and should produce power use results in time for the conference.Repeated audits at a facility offer additional insights. Comparison of the results for Facility F, before and after the replacement of a positive displacement blower with a turbo, indicates a roughly 35% reduction in aeration energy use.The use of the audit information coupled with pollutant loading on the treatment system also allows the contribution of individual improvements to energy use reduction to be identified. A comparison of energy reduction cost savings to capital costs allow the payback period to be indicated as well.