## 2 Materials and Methods  
For this study, an ABAC system was installed at the Inland Empire Utility Agency's Regional Water Recycling Plant No. 1 (RP-1), located in Ontario, California. The Inland Empire Utilities Agency (IEUA) is a regional wastewater treatment and water agency providing sewage treatment, biosolids handling and recycled water to portions of the San Bernardino County in the state of California. RP-1 is currently the largest treatment plant within IEUA's service area with a design capacity of 44 million gallons per day (MGD) [5]. The RP-1 treatment process includes three activated sludge systems consisting of two aeration trains each for a total of six trains. A combined flow of primary effluent and return activated sludge is diverted by influent gates to each train. Each train contains three basins that function as conventional hardenpho treatment system: the first basin mixes flow and provides anoxic treatment, the next three basins can add air through fine bubble diffusion system supplied by four large blowers to provide aerobic treatment. In 2015, a pilot study was initiated at RP-1 to test the compatibility of an ABAC system. In 2019, the ABAC unit was purchased and installed in the aeration system for further testing. The metered energy consumption (kWh) of the aeration four-blower system was reviewed from 2018 to present day. Working with the IEUA Planning Department, a cost of $0.10 per kWh was used for the cost analysis; this cost-per-kWh accounts for the multiple energy power agreements honored in the facility. The energy consumption data were normalized by what were determined to be the largest contributing factor: contaminant (i.e., ammonia and total organic carbon (TOC)) mass loading and air transfer ratio. The ammonia mass loading normalizing ratio was calculated by averaging the daily samples of influent ammonia concentration as well as the metered influent plant rate and converting to mass basis, as shown in Equation (1) [6]. \[\text{M}_{N\!H\!4}=\text{C}_{N\!H\!4}*\text{V}_{\!inf}*8.34 \tag{1}\]  
where M\({}_{N\!H\!4}\) is the ammonia mass loading in units of pounds per day (lbs/d), C\({}_{N\!H\!4}\) is the daily average ammonia concentration sampled and tested in units of milligrams per liter (mg/L), V\({}_{\!inf}\) is the daily average plant flow rate in MGD, and 8.43 is a constant used for unit conversion. The air transfer ratio (ATR), which is a measure of the cubic feet of air needed to transfer a pound of dissolved oxygen within a treatment train, was calculated using the metered air flow rate to the system, the metered total influent rate to the treatment basin, and the measured DO concentration in the system. Equation (2) below describes the calculation further [6]. \[\text{ATR}=(\text{A}_{\textit{Train}}*24)/([(\text{V}_{\textit{Inf-Train}}+ \text{V}_{\textit{RAS-Train}})/2)]*\text{C}_{\textit{DO-Train}}*8.34) \tag{2}\]  
In the above, ATR is the air transfer ratio for a given treatment train in units of cubic feet per pound of dissolved oxygen (CF/lbs DO), A\({}_{\textit{Train}}\) is the air flow rate in units of standard cubic feet per hour (SCFH), V\({}_{\textit{Inf-Train}}\) and V\({}_{\textit{RAS-Train}}\) are the volume flow rates from the influent pump station and returned activated sludge pump station, respectively, in units of MGD, C\({}_{\textit{DO-Train}}\) is the dissolved oxygen concentration in units of milligrams per liter (mg/L), and 8.34 is a constant used for unit conversion. Normalizing the data by these factors adjusted the metered energy consumption to account for the changes in contaminant loading as well as the change in air transfer ratio since the change of aeration diffuser panels within the facility in early 2018. Metered data were collected by a variety of existing probes and sensors located in the treatment process: DO probes, ammonia analyzer, suspended solids sensor, UV nitrate sensor. The real-time controller module and digital controller were the main control modules of the ABAC system installed.