Scalability of Anaerobic Digestion: Evaluating pilot scale operations for full-scale implementation for Metro Vancouver's advanced digestion concepts

Anaerobic digestion is a process central to the stabilization of sewage sludges while also proven to be the effective first step in many resource recovery programs. Mesophilic anaerobic digestion has proven efficient, effective, and reliable process for decades. However, with increasing capital costs, increased value of recovered resources and a societal push toward decarbonization, in an attempt to limit the impacts of climate change; interest in intensification of the digestion process has grown. In the past decade the adoption of intensified or advanced digestion processes has increased. Processes like thermal hydrolysis, thermophilic digestion and co-digestion have demonstrated that conventional process limits, performance and stability indices need to be reassessed and defined for the next generation of water resource recovery facility. Metro Vancouver, has experience with advanced digestion technologies, like its Class A series thermophilic anaerobic digestion system at Annacis Island, pioneering its adoption in the late 1990's. However, since its adoption and nearly 20 years of effective operations the true capacity limits of this process have yet to be defined. To define the operational limits of a biological process, a range of process metrics must be considered these include: - Substrate loading capacity. -Minimum solids retention time - Acclimation potential to inhibitory compounds - Process reaction rates - Dynamic stress responses and recovery (load, temperature, feed composition, toxicity, etc.) To elucidate these metrics at scale is possible, but the impacts of potential process failure can be significant, including long recovery times, process restart, loss of treatment and odors. Any of which can represent significant costs when managing thousands of cubic meters of material (millions of gallons). These risks further increase when new processes are being implemented and/or augmenting an existing digestion process. Further, testing new unproven technologies at scale that on paper would appear to be effective often prove to be more complicated than first envisioned, resulting in increased costs. These costs could result in a premature cessation of process development. One approach researchers and early adopters have addressed these challenges is through the bench testing, on-site and at universities. While these often provide evidence for proof of concept, they are limiting in scale and scope; often not integrating the true variability in operation that a full-scale facility, leading to conservative designs or non-adoption due to risk avoidance. Ultimately this slows the progression of technology advancement. Metro Vancouver, has elected to accelerate the testing and adoption of new processes for its wastewater treatment plants through the construction of the Pilot Digestion Facility (PDOF). This facility consists of three 8 cubic meter digesters, and a feed skid. The digesters are capable of operating at range of temperatures, retention times, mixing strategy and process phasing strategies. The PDOF not only allows for new processes to be tested but also existing systems to be stress tested to defined the maximum capacity of their existing assets. Further, at this scale, it is possible to test processes under real-world conditions, to better assess implementation at scale and process viability at scale. In June of 2023, the PDOF completed construction and commissioning and was started-up at the Lulu Island Wastewater Treatment Plant, in Richmond, BC. One of the first test plans implemented was to evaluate the reproducibility of pilot results relieve to the full-scale operating digesters. Three PDOF digesters were operated at solids retention times of 15, 20 and 28 days respectively on a mix of thickened primary (TPS) and waste secondary sludge (TWSS). Core process metrics were monitored to determine, system performance and compared to the full-scale operating digesters. Figure 1, shows the relationship between digester operating SRT and volatile solids destruction, after each of the three test digesters had reached stead-state and compared it to reference data and LIWWTP's current digester operations. What is apparent from the PDOF and the reference data is that 15 days SRT appears to be an inflection point around which further reduction in operating residence time will result in notable reduction in solids destruction which ties directly to biogas production. The current industry primary practice for resource recovery is biogas generation and methane utilization. Understanding this inflection point and being able to accurately test around this point will provide significant understanding of the total benefit of future process optimizations. Further what is also apparent is that the sludge a LIWWTP is more degradable than a 'typical' sludge, based on the reference data information. This is likely due to a combination of very low industrial input, high volatile content and a low overall residence time in the secondary system, a trickling filter solid contact system. The fact that the PDOF system produced similar VSr to the full-scale, suggests the system is performing in a similar manner to the full-scale digester, making extrapolations to full-scale relatively direct. In terms of reproducing the performance of the anaerobic digesters at LIWWTP, there was generally good agreement in the data, between the pilot and the full-scale digesters in terms of solids destruction, volatile acids, alkalinity and ammonia, Table 1. Biogas data was not completely reliable during this time as condensate was impacting meter accuracy on the pilot units. Modifications to the meter orientation, appears to have improved the response. As part of understanding the overall potential of anaerobic digestion and to gather further in-sites as to the fundamental mechanisms driving the observed results, microbial ecology analysis was conducted to understand changes in the consortia present. As part of this baselining observation the result of these efforts will be discussed. Figure 2, presents the relative abundance of different phylum level populations with time for each of the PDOF digesters and a single steady-state point for the LIWWTP digesters. Figure 3, further refines that focusing Euryarchaeota, which contain the methanogens. With the successful demonstration of the mesophilic operation of the PDOF facility, completing commissioning, the first tests of intensified digestion have commenced. The process being converted over to thermophilic operations to evaluate the limitations of the process. As of authoring the digester temperature has been increased and the microbial population is transitioning to thermophilic, as denoted by the significant increase in volatile acids and deterioration of biogas production. A process condition understood to be typical. Once steady thermophilic operation has been reached a range of process conditions will be tested. These include, reduced SRT and increased organic loading rate. This not only will demonstrate the potential for LIWWTP to utilize thermophilic digestion to increase capacity but also further define the capacity at the larger Annacis Island WWTP. This paper will discuss the results of the completed baselining operations, including lessons learned, and observation due to different operating retention times for mesophilic digestion. It will further present relevant observations from the conversion to thermophilic, which are in progress now.