Energy and Other Efficiencies Gained from Influent Flow Equalization

Introduction Clean Water Services (CWS) is a wastewater, stormwater, and watershed management utility serving the populous of Washington County, Oregon USA through implementation of a watershed based NPDES permit. The Rock Creek AWRRF recovers water resources from an average dry weather flow of 25 MGD wet weather flow of 80 MGD and peak hours of 150 MGD. The resource stream begins the recovery process at the influent pump station (IPS) where it is lifted approximately 65 feet by 7 influent pumps. The influent wet well includes two discharge headers, two flow meters and, in total, five 900 hp pumps and two 450 hp pumps (Figure 1). The idea has long existed at CWS to increase the level setpoint, decreasing the total discharge head on the pumps to gain energy efficiency. In 2018, catalyzed by the WEF/WRF/LIFT Intelligent Water Systems (IWS) Challenge, CWS undertook a 6-week test to increase the level setpoint by 16 feet to measure energy savings and the effects on the conveyance system by leveraging available data. Although successful, the energy savings was doubtfully worth the potential for long term degradation to the collection system. However, without catastrophic failures the door was opened to pursue other ideas. Further data analysis of diurnal flow patterns revealed that up to 70% flow equalization (FEQ) could be achieved on a daily basis. During FEQ control the IPS wet well level varies linearly from 103' to 118' at 10:00 and 01:00 respectively. The first test was performed in the summer of 2020 for a two-week period. Subsequent analysis showed efficiency gains not only at the IPS but in downstream processes. The concept was refined and in 2021 the facility began operating full time in FEQ control. Since then FEQ control has been the default operating mode except during storm events when the wet well level is lowered to standard mode. This has offered CWS the opportunity to examine and attempt to quantify the efficiency gains from FEQ on a whole plant basis. Methods Data analysis was performed on hourly average values extracted from the SCADA historian. Energy savings were calculated from motor and VFD efficiency and pump data, at an average level of 110 feet, and compared over 7 periods of FEQ control and 6 periods of standard control from 2/18/2022-6/9/2022. Equipment runtime was compared in terms of number of hours of lag unit runtime per total hours during either operating mode. It was estimated that the facility would operate in FEQ control 98.3% of the time, returning to standard mode only for wet weather events. This data analysis was done in partnership with Energy Trust of Oregon (ETO), which provided rebates for realized energy savings. Results Energy Efficiency The following plant processes were evaluated for energy savings:

*IPS

*IPS Odor Control

*Primary Effluent Pump Stations (PEPS)

*Blowers

*RAS Pumps

*Tertiary Pump Stations

*Air Compressors Energy savings were realized in the IPS, IPS odor control, PEPS, and RAS pumps. The IPS odor control fan speed is reduced at wet well levels over 105 feet. Influent FEQ transfers downstream to equalize flow in the PEPS and the RAS pumps. Energy savings in these systems were calculated from measured motor speed and equipment data as the difference between FEQ and non-FEQ periods. Total energy savings were estimated at 445,851 kWh/yr. The breakdown of energy savings per system is given in Table 1. Lag Unit Runtime The IPS and PEPS are configured in a lead-lag setup. Reduction in peak flow provided by FEQ reduced the runtime on the lag unit, overnight baseline flows are augmented but did not transfer the run hours to a different time of day. Primary effluent is split between east and west PEPS. Hourly pump speed data were compared between FEQ and non-FEQ periods for hours above 100% pump speed, indicating that a lag unit was running. Lag unit runtimes were reduced in all three pump stations, as shown in Table 2. Controls Stabilization While peak flow attenuation provides for energy and equipment efficiency, augmentation of overnight low flows can keep control systems operating in a more optimal range. One example of this can be found in the bioreactor air flow control valves. The dissolved oxygen (DO) setpoint is maintained through a cascading control loop where the span between the measured and setpoint DO is converted to a change in air flow rate, achieved by modulating an actuated air flow control valve. A control valve can modulate between 15 and 100% through many cycles during the low flow period of the day when FEQ is not used. The modulation is reduced to between 20-30% with FEQ (Figure 2). Current Work CWS is again engaged with ETO on a project to refine FEQ operation to maximize energy savings by creating new FEQ control programs. Power data from the IPS is being leveraged for asset management and initial analysis has found that one of the 450 hp pumps operates at significantly lower efficiency (kW/MGD) than the other. PEPS data has been examined more closely for operational changes that would have the east and west PEPS work more as a singular pump station. Initial analysis estimates are that an additional 500,000 kWh could be saved at the IPS, 100,000kWh at the PEPS, and 700,000 kWh by improving the efficiency of the underperforming PEPS pump. This project is expected to be completed and ready for inclusion at WETFEC 2024.