An Evaluation Study of Santa Rosa's Existing Co-digestion Program: Why Truck Along when You can Optimize?

INTRODUCTION Trucked waste programs allow WRRFs to turn organic 'waste' into a valuable resource and reduce greenhouse gas emissions through energy production. Implementing a trucked waste program maximizes the economic and societal value of feedstocks once considered environmental risks but requires oversight and maintenance to be successful. As WRRFs consider codigestion trucked waste programs, it is important for utilities to evaluate costs versus benefits, and to understand the associated operational, maintenance, administration, and monitoring requirements. The City of Santa Rosa installed a high strength waste (HSW) receiving station in 2017. The program accepts HSW such as fats, oils, and grease (FOG), brewery waste, and dairy waste while collecting revenue from tipping fees, generating energy, and recovering heat for plant processes. In addition to HSW, the City receives low strength wastes (LSW) including leachate, septic and chemical toilet waste, groundwater, gray water, and wine rinseate. While the trucked waste program operates at a net positive revenue, the City sought to further refine the program by taking a closer look at the impact from individual trucked wastes to their treatment processes to identify optimization opportunities. Brown and Caldwell (BC) and the City collaborated on a detailed study to complete the following: 1. Summarize baseline conditions 2. Identify optimization enhancements 3. Recommend enhancements while rebalancing tipping fees to maintain net positive program revenue BASLINE CONDITIONS Historical data was reviewed to identify the impacts of LSW and HSW on biogas and biosolid production. A solids-water-energy balance was completed to identify volatile solids reduction (VSR) and specific biogas production values for the WRRF. Results from the solids-water-energy balance are presented in Figure 1. Results demonstrated a 10 percent HSW flow and loading contribution in digester feed, which resulted in a 2.6 percent biosolids contribution and 15 percent biogas contribution. The relatively high VSR associated with HSW and validated through the solids-water-energy balance impacted relatively the high biogas contribution and low biosolids contribution. The LSW loadings were identified to contribute only about 1 percent of biogas, energy, and biosolids production; however, LSW sources were not sampled regularly, and assumptions were made based on values from textbooks and other local facilities for the solids-water-energy balance. Differences in total solids (TS), volatile solids (VS), and biochemical oxygen demand (BOD) concentrations impact the estimated LSW influence on plant performance. Interviews with operations identified ultraviolet transmissivity (UVT) impacts from leachate waste, which had been observed during wet weather events. Leachate impacts to UVT have been reported at flows above 1 percent by volume (gallon of leachate per gallon of wastewater), but the threshold for UVT impacts may vary depending on wastewater characteristics. The City accepts both pipeline and trucked leachate wastes. Figure 2 was created to provide a method for determining the limit of leachate that could be accepted depending on the leachate flow contribution (e.g., contributions of 1 to 4 percent by volume were plotted). Review of baseline operating conditions identified that program administration and trucked waste monitoring could be improved. Improvements to these areas required an increase in plant staffing, and the baseline assessment documented that trucked waste program administration responsibilities were shared among three environmental compliance roles. Consolidation of these responsibilities into a single program manager was recommended. Review of historical cost and revenue showed a net positive operating revenue from the trucked waste program. Tipping and permit fees were the main source of income. Energy generation, though significant (HSW was estimated to increase energy production by at least 15 percent), accounted for only 12 percent of total revenue for the City's program. This finding demonstrates the importance of setting appropriate tipping fees, although tipping fee revenues for the City include LSW as well. The three highest operating costs were evaluated as treatment costs from HSW loading, administration and compliance staffing costs, and biosolids hauling costs. OPTIMIZATION AND ENHANCEMENT OPPORTUNITIES The enhancement opportunities were divided into three categories: administration, process, and physical. A universe of potential enhancement projects was recommended based on the baseline condition assessment. The City screened enhancements and the remaining were advanced to an economic life-cycle analysis, as shown in Figure 3. LIFE-CYCLE ANALYSIS AND TIPPING FEE RECOMMENDATIONS A life-cycle analysis was completed in tandem with tipping fee rebalancing. Tipping fees were rebalanced to match the City's current net positive revenue, on average, for a 10-year lifecycle. This methodology allowed the City to determine which optimizations and enhancements to prioritize. Figure 4 demonstrates the baseline tipping fees versus recommended increases over the next 10 years. Increasing rates were necessary to keep up with escalating plant operating costs, and addition of enhancement projects (e.g., hiring additional staff, constructing a new waste hauler check-in station, etc.) was shown to have a significant impact on certain rates. BC recommended the following approach when increasing tipping fees: 1. Increase tipping fee rates for no greater than 25 percent of waste categories per year; Do not change tipping fees for most of the waste types to minimize risk of losing customers and revenue. 2. Increase rates by no more than 20 to 25 percent relative to the previous year. SUMMARY Diversion of organics from landfills provides a pathway for WRRFs to receive food waste and generate additional biogas for beneficial use. As these opportunities are considered, utilities should be aware of the requirements associated with implementing and operating a codigestion program. The lessons learned and recommendations from this study may help other utilities as they consider ways to improve plant performance and minimize risks and costs associated with the implementation, operation, and management of a trucked waste program. The lifecycle cost analysis highlights the importance of tipping fees to operate a program at net positive revenue.