Biosolids Planning for an Uncertain Future - How Regulatory Drivers and Technology Developments are Influencing Long-Term Infrastructure Decisions

Introduction The current era is challenging to plan for long-term biosolids infrastructure upgrades. Over the past decade, many of the traditional solids management outlets have dwindled, with incinerators shutting down due to increasingly strict air regulations, and many landfills starting to limit biosolids loads. Land application is one of the strongest remaining pillars of traditional biosolids management, with its relatively low costs and effective nutrient recycling making it a favorable option. As of 2021, 43% of all biosolids in the USA were land-applied. Given the uncertainty around future regulations restricting land application due to PFAS and other emerging contaminants, this outlet is also facing its existential crisis. While confidence in traditional biosolids outlets waver, at the same time many utilities across the country are grappling with their fast-approaching need for renewal of solids infrastructure. Many of these solid systems built in the 1970s have needed capital upgrades repeatedly deferred to the point that major investment is now needed as these systems reach the end of their service life. Additionally, there has been attrition in O&M staffing across the country due to Covid and other global forces. With limited experienced O&M staff, it becomes difficult to embrace advanced technology, especially purported to destroy PFAS which are very early in their development phase and whose efficacy is as unclear as the future regulations they intend to address. Waiting for these regulatory and technology questions to be answered would be ideal but not feasible for many utilities whose time to act is now. The case study for the City of Toledo, OH forms the basis of this paper. Toledo is planning and procuring funding for one of the largest upgrades in its Bay View WWTPs history. This impending solids system upgrade is viewed as an opportunity to enhance sustainability and reduce GHG emissions by recovering more renewable energy from biosolids. There is also trepidation, however, over pouring major investments into digesters with the looming uncertainty around land application due to future federal PFAS regulations. Existing Conditions This paper presents an overview of the recent solids planning effort for the Bay View WWTP (ADF - 68 MGD). In 2022 the City developed a facility plan that included demolishing plant digesters (built in the 1930s) to clear space for new liquid stream process. The plan included constructing a new solids facility on new plant land created by filling part of the bay on which the WWTP sits. This WWTP currently produces ~ 40,000 wet tons of Class B biosolids annually, which consists of combined thickened primary and WAS that is anaerobically digested and dewatered to Class B cake. These biosolids are land-applied using a 3rd party cake management partner, while occasional loads are sent to landfills during seasonal outlet restrictions. The biogas generated is utilized on-site for digester heating with boilers. Toledo had previously shown advanced thinking and ambition in energy recovery as it constructed a combined cycle turbine CHP plant at the WWTP designed to run of digester gas and landfill gas piped in via a trans-city pipeline. This facility was sitting currently unused due to lack of sufficient fuel, as landfill gas volumes were less than half of the anticipated design values. The City engaged the engineering team to explore various solids processing alternatives and recommend a path forward for their new solids processing facility. The team focused on maximizing energy recovery and enhancing sustainability while recognizing that the City was staff-limited without capacity to take on new advanced technology adding considerable O&M effort. Another driver was to reduce final solids volume and develop alternatives centered around Pyrolysis and Gasification addressing PFAS concerns. Since the City was capital and staff limited, these alternatives were developed with the leading technology manufacturers as 3rd party turnkey projects to be owned and operated by a 3rd party commercial entity. Methods A comprehensive and holistic solids and energy modeling effort was developed to understand complex and interacting impacts of implementing various solids and energy processes in one framework (Figure 1). Each potential combination of processes was quantitatively analyzed for annualized cost, greenhouse gas (GHG) reductions, net power generation, and initial capital cost. Along with determining an operationally sound and cost-effective treatment facility, processes were also examined to meet the goals of generating Class A biosolids, maximizing energy recovery, and utilizing technology that shows promise for reducing PFAS contained in residuals. The technologies investigated under this framework include: Pre-digestion - Gravity Thickening of Primary Sludge - Mechanical Thickening of WAS - Thermal Hydrolysis - Thermal-Alkaline Hydrolysis Digestion - Single Stage Mesophilic Digestion Post Digestion - Dewatering - Thermal Drying Biogas Utilization - Digester heating via boilers - Existing Combined Cycle Turbine CHP - New Combine Heat and Power Engine - Renewable Natural Gas -On-site and 3rd Party - Thermal Drying Biochar Processes - Pyrolysis - Gasification Leading technology manufacturers were consulted to develop 3rd party proposals for Biochar Processes. The proposals included the technology provider and a solids management contractor that would serve as O&M staff and manage all residuals. Two different project delivery methods were considered for biochar technology - turnkey contracting and city finance. Under turnkey contracting, the 3rd party would finance and construct the facility, operate, maintain, and charge Toledo a tipping fee for cake delivered. For the city finance option, the City would pay to build the facility and engage in a long-term O&M and residuals management contract with the 3rd party entity. Results The quantitative results (Table 1) from the energy flow modeling were used as a guide to provide recommendations to the stakeholders, some of which are described below. - Reusing digester biogas for generating renewable natural gas results (RNG) in the increased monetary value of the gas. Current solids loading would generate about $1,700,000 gross RIN revenue annually. Adding the RNG system alone to the existing facility did not meet the project goals since there was no final solids volume reduction. - Addition of a pre-digestion hydrolysis process for WAS changed the overall outlook from a solids volume and energy endeavor. Hydrolysis allowed additional solids to be digested, greater biogas generation and increased the dewaterability of sludge. This alternative reduces GHG emissions more than others while reducing dewatered solids by 33% compared to the baseline. This alternative retain the same solids management program, optimizing the cost and sustainability of the existing solids facility; however, does not create a Class A product or destroy PFAS. - Adding a thermal dryer to the digestion system with enhanced WAS lysis system is recommended to create Class A granules eliminating the export of Class B biosolids. It was still financially advantageous to send biogas to RNG and fuel the dryer with NG. This scenario reduces the amount of solids transported out of the facility by 78% but increases the annualized cost of solids management while increasing GHG emissions. - For mitigation against future PFAS regulations as an end goal, a 3rd Party turnkey solution with either pyrolysis or gasification would need to be considered as the only viable option. Pyrolysis systems appeared to have better financials but did not include as much redundancy as the Gasification system proposed. The syngas produced helped the energy balance of these processes, but both were net energy consumers, driving increased GHG emissions. The turnkey project delivery would reduce the capital spent over the baseline scenario by about $100 million by digesters and eliminating cake storage. The case study findings provided a need to stay flexible while investing capital in upgrading the biosolids facilities. While federal regulatory action is not imminent, using a phased project delivery approach for the solid's upgrade project will be prudent while re-investing in digestion. The initial phase would include the installation of a single WAS-lysis treatment train as a large-scale pilot treating ~50% of the WAS load. Biogas will be utilized to produce RNG, thus increasing the value of the energy resources. Adding a thermal dryer or biochar processes will increase the solids handling systems' cost, complexity, and GHG emissions. These options need to be planned for both budgeting and space allocation, but should be phased for implementation when there is evidence that the market for land-applied cake is reduced or eliminated due to regulatory concerns.