Comparison of Ammonia and DO Aeration Control Strategies to Optimize Energy and Performance at Low Capital Cost: A Case Study

Wastewater treatment is an inherently energy-intensive process. Estimates of the energy consumption on a national scale have placed municipal water and wastewater treatment at as high as 5% of the national energy consumption. Furthermore, the rising costs of energy present an ever-increasing burden to each facility. In order to minimize the cost of energy, facilities have aimed to improve the efficiency of the most energy intensive processes. Energy audits performed by the authors at over 24 municipal wastewater treatment facilities in the Midwest have indicated that aeration systems can represent as much as 76% of a facility's energy bill. With permitted ammonia limits tightening, the cost of treatment is increasing. Thus, reducing the power required in the aeration process can produce significant energy savings for a treatment facility.Aeration systems are composed of three major parts: blowers, diffusers, and controls. An ideal aeration system matches the oxygen provided to the system with the real-time oxygen demand of the process. Utilities have realized significant savings by installing high-efficiency blowers and diffusers. However, equally significant savings can be realized by improving process controls. The system that controls the process can be responsible for reducing energy consumption by as much as 30% or more in the aeration system at low- to no-capital cost to the utility. The objective of this study is to determine the most operationally-effective and energy-efficient control strategy for aeration in activated sludge systems.Currently, most aeration systems that use real-time control are based on dissolved oxygen (D.O.) measurements in the aeration tanks. However, as technology is improving for instrumentation, new control strategies are being developed that optimize system performance and minimize power consumption. Of particular interest in this study is the use of predictive parameters that would allow control based on influent wastewater quality as opposed to the often slow-to-react D.O. parameter. While at the time of writing, the development of in-line BOD probes is not complete, real-time probes for measuring other parameters, such as ammonia, are ready for integration.Due to the comparatively slow metabolic rates of ammonia oxidizing bacteria (AOB) versus organic oxidizing bacteria, the latter outcompete AOBs. Ammonia is oxidized only when the BOD/COD load has been mostly consumed. The residual D.O. in tanks is indicative of the excess oxygen provided, and therefore considered a responsive parameter. If ammonia, on the other hand, is used to control airflow to the process, theoretically the absolute minimum oxygen can be provided while achieving process requirements and presumably resulting in lower oxygen supplied than would be used from a typical D.O. control strategy.Blower power is proportional to airflow and also related to pressure and site conditions of operation. Wheaton Sanitary District in Wheaton, Illinois operates a single-stage nitrification activated sludge system comprised of five equally-sized aeration tanks running in parallel with a normally equal upstream flow split. Given that the operating pressure and site conditions of this system are equal for each tank, the airflow to each tank can be used to estimate the relative power consumption of that tank. A common baseline was established using the design actual oxygen requirement (AOR) of the system. The AOR corresponds to the blower design point and therefore the blower output in a system with no blower control. In this study, full-scale control strategies were tested in the activated sludge system to judge the relative treatment efficiency and process stability as compared to the baseline, including the following:• Control of airflow to the tank based on a fixed airflow setpoint (previous mode of operation);• Control of airflow to the tank based on D.O. as measured at the downstream end of the tank, to maintain a D.O. setpoint of 2 mg/L;• Control of airflow to the tank based on upstream ammonia measurements such that the influent ammonia is used to predict the actual airflow required for oxidizing both organics as well as ammonia.The study demonstrated that while fixed airflow and D.O. control modes offer energy savings as compared to no control, neither allows the system to respond in time to dynamic loads. Ammonia feed-forward control, on the other hand, resulted in 17% energy savings compared to the baseline or 11% energy savings compared to D.O. control. The ammonia feed-forward system also responds faster to changes in process loadings, which can allow for tightened control of aeration zones for BNR processes. As compared to other control strategies, ammonia feed-forward is the only one to balance both energy savings and process reliability.By establishing a common baseline, the relative savings among control strategies can be used to estimate energy savings at other facilities for funding opportunities or to justify the purchase of instrumentation based on payback periods.