Are Your Digesters Up to the Task? Aligning Resource Recovery Planning with Reality

Abstract Anaerobic digestion serves as the linchpin for much of the wastewater solids treatment in the United States. In many major metropolitan areas, digesters were constructed between the 1950s and 1970s, and are reaching the end of their useful life. As utilities seek to increase resource recovery and intensify digestion, these older vintage digesters may not be able to accommodate simple upgrades that allow for higher temperature or higher total solids operation. Introduction Anerobic digestion sits at the nexus between traditional wastewater treatment and modern utility climate goals, including greenhouse gas (GHG) reduction, landfill diversion, and green energy generation. Utilities performing solids and energy planning around these goals are typically relying heavily upon their anaerobic digesters to be efficient and high performing, often seeking to intensify digestion so as to accommodate imported feedstocks (e.g. fats, oil and grease, food waste, outside sludge, etc.) or simply to provide capacity for future flows and loads within the same tankage. In many major metropolitan areas, digesters were constructed between the 1950s and 1970s, and are reaching the end of their useful life. As utilities seek to increase resource recovery and intensify digestion, these older vintage digesters may not be able to accommodate simple upgrades that allow for higher temperature or higher total solids operation. Results and Discussion When seeking to achieve higher loading rates (either due to increased population in the service area or a desire to import high strength wastes), it is typical for utilities to start the planning process by assessing the potential of existing anaerobic digestion tankage. Intensifying digestion via established processes such as thermophilic digestion or thermal hydrolysis can be an appealing way to expand capacity (see Figure) with a seemingly incremental investment (e.g. heat exchangers, thermal hydrolysis skid). The ability to intensify digestion can be especially important in older cities where sites may be constrained and expansion of a digester complex is not possible. Despite these advantages, however, digestion infrastructure may not be capable of realizing the full potential of intensification due to either structural inadequacy or suboptimal configuration. As a general industry practice, digester tanks are expected to remain in service for 50 to 75 years, with the associated mechanical equipment having a shorter service life of approximately 20 years. Factors that can affect this service life include freeze/thaw cycles, which can weaken concrete, and seismic activity. In addition, many older digesters were constructed with floating covers, and cover integrity, as well as interior and exterior coating integrity, can expose the digester structure to wear inducing conditions. While not a hard trigger, replacement of the digesters prior to reaching 75 years of life serves as a sound rule of thumb. Very few comparable facilities have digester structures of this age operational, although a small number remain in service with plans to replace prior to 2030 (e.g., San Francisco's Southeast Water Pollution Control Plant). In addition, considerations about capacity, given that older digesters were simply not designed to perform at the higher loading rates needed for codigestion applications, amplify the need for eventual replacement of these structures. Other unknowns, including structural vulnerabilities, could accelerate this replacement timeline. Operating conditions associated with higher rate digestion that can be problematic for older digestion infrastructure include higher gas production rates, biogas entrainment, and changes to viscosity. Older gas mixing systems, for example, are likely inadequate for realizing the full active volume of a digester generally, a condition that is exacerbated by sludges of higher total solids content. Changes to viscosity associated with higher total solids content (thicker sludge) and/or inclusion of food waste can, in addition to impacting mixing performance, can entrain biogas, contributing to acute operational issues such as digester foaming. Changes to sludge rheology will also impact ancillary systems, such as pumps and heat exchangers, which should be assessed for upgrades. Higher digester loading limits could potentially be realized if existing digesters are upgraded with features such as improved mixing and surface withdrawal, both of which will help realize the full active volume of the digester and minimize gas and foam entrainment (see Figure). Such upgrades can be included in existing digesters, assuming the structural integrity of the tank is intact. A detailed condition and structural assessment is recommended prior to implementing digester enhancements intended to accommodate higher rate digestion. Conclusions When assessing the capability of existing digesters to process higher organic loads, a thorough assessment must be undertaken. This includes a robust condition assessment, as well as an evaluation of retrofits that can maximize the benefits of intensification. Full costs of such upgrades should be compared against the cost and feasibility of construction of new, modern digesters to help utilities realize their full plans for resource recovery.