The R2E2 Experience -- Energy Performance Two Years On at the Green Bay Facility

NEW Water, the brand of the Green Bay Metropolitan Sewerage District, has completed a program, with initial planning starting in 2008, to replace its biosolids management system at the Green Bay Facility (GBF) in Green Bay, Wisconsin. The facility is permitted to treat 49 MGD of wastewater and process biosolids from the GBF and waste activated solids from their other wastewater facility, the DePere Facility (DPF). The GBF was constructed in 1975 and included a thermal conditioning system (TCS), belt filter press dewatering and two multiple hearth furnaces (MHFs) to process wastewater solids. This process train was chosen because agricultural land application in Wisconsin was deemed not to be practical. Subsequently, the TCS was shut down and in 2008, NEW Water took over operations of the DPF. The almost 40-year-old GBF did not have sufficient treatment capacity into the future and the equipment required replacement due to its age and increasing maintenance costs. In addition, the existing MHFs would not meet the pending federal Clean Air Act Maximum Achievable Control Technology (MACT) air pollution regulations for sewage sludge incinerators (SSIs) that became effective in 2016. Between 2008 and 2011, NEW Water and CH2M (now Jacobs) developed a Solids Management Facility Plan that evaluated numerous solids processing technologies and process trains to respond to these issues. Seventy-three solids unit processes were considered, some were eliminated and the remaining 52 unit processes were used to develop 17 process configurations. Of these 17 configurations, six alternative configurations were selected and evaluated in detail. The Digestion with Thermal Processing and Electrical Generation alternative was selected and later named the Resource Recovery and Electrical Energy Project, or R2E2. The R2E2 process flow configuration, shown in Figure 1, is an integrated solids processing system comprising mesophilic anaerobic digestion (MAD), dewatering and fluidized bed incineration with waste heat recovery (WHR). MAD was chosen as a first processing step to generate biogas for combined heat and power (CHP). High strength liquid organic wastes are co-digested in the MAD process to enhance biogas production and maximize power production and waste heat utilization. Dewatering using centrifuges, followed by fluidized bed incineration was selected as the most efficient method of managing the biosolids, given there is not land within economical distance of Green Bay for agricultural utilization. The fluidized bed reactor (FBR) is equipped with a partial scalping dryer, utilizing waste heat recovered from the FBR and using thermal oil to ensure autothermal combustion. Phosphorus is recovered from the dewatering centrate to produce a fertilizer product. In addition, beneficial uses of the residual ash are being pursued. The CHP, dewatering and FBR equipment trains were purchased and the balance of plant design completed in 2014. Construction commenced in August 2015 and commissioning and performance testing were completed by the end of 2018. Figure 2 shows operating data from the startup of the two digesters through August 2019. The system has been operating since November 2018, as an integrated system. Figure 3 shows a summary of the system energy distribution and usage at the design year of 2035 at annual average wastewater flow conditions. The R2E2 project is expected to generate more than 65% of the GBF projected electrical power requirements in 2035 using biogas only and up to 85% using supplemental natural gas to the full CHP loads. With the system thermal efficiency of about 60% based on heat recovered, the R2E2 project will meet NEW Water's goal of producing electricity in the most efficient way while maintaining autothermal combustion. The paper will discuss the energy recovery and production performance of the facility during the two years of full operation, including the Covid-19 period. Since operation of both digesters commenced, NEW Water has initiated its program of importing high strength organic waste (HSW) and has been increasing quantities to achieve design quantities. Biogas production has increased, with increased electricity production of green energy. The monthly biogas and natural gas electricity production, together with the monthly HSW imported quantities and generator run times in the first year are shown in Table 1. As part of the energy management system, SCADA options allow determining when is the optimum time to operate the CHP and the number of units to operate to reduce demand and other electrical charges. NEW Water continues to optimize energy production with waste heat recovery and matching plant heating requirements. The presentation will be of interest to municipal plant operators, wastewater utility managers, planners and design engineers who are considering integrated biosolids processing systems to maximize energy recovery and provide resource recovery opportunities.