Performance Evaluation of Pilot Digestion Optimization Facility (PDOF) for Digestion Intensification of Metro Vancouver Anaerobic Digestion Facilities

Introduction Anaerobic digestion has proven effective and reliable for stabilizing sludge for decades1. As municipalities grow, digestion intensification can offset or defer major capital expenditures. Testing operational limits for existing systems and trialing new technologies at full-scale carries considerable risk as potential process failures can lead to long recovery times or not meeting regulatory requirements. One approach to address these challenges is conducting bench-scale tests; however, a direct transition to full-scale can impose risks that were not fully considered at bench-scale. Metro Vancouver has built a Pilot Digestion Optimization Facility (PDOF) to conduct tests at a representative scale without risk to full-scale operations. The PDOF consists of three 8 m3 digesters that can be fed with mixed sludge or any combination of primary and secondary sludge. This paper presents results of the first three tests of a multi-year test plan: baseline digestion, thermophilic digestion and temperature-phased anaerobic digestion (TPAD). The test plan for 2023—2024 is outlined in Figure 1. Baseline digestion aimed to replicate the full-scale mesophilic digesters and achieve stable PDOF operation at various hydraulic retention times (HRTs) to identify the most comparable HRT to the full-scale digesters and to develop a volatile solids removal (VSr) performance curve. Thermophilic digestion will run two pilot digesters at 55 degrees C and 60 degrees C while the third runs at 38 degrees C as a mesophilic control. Stress tests will determine HRT operating limits. TPAD involves separating the digestion process into a fermentation phase in a separate tank from methane production. Higher temperatures (55 degrees C) in the first stage increase the rate of hydrolysis, while lower temperatures (38 degrees C) for the methane forming stage reduce energy input and maximize process stability2. The results of baseline digestion test are complete and discussed here. Results for the other tests will be available at the time of paper submission. Methods The PDOF is located at Lulu Island Wastewater Treatment Plant (LIWWTP) in British Columbia, Canada where two 5,400 m3 mesophilic anaerobic digesters operate with 25-to-30-day HRT. The PDOF feeds the same mixed sludge as the full-scale digesters to three 8 m3 pilot digesters. The PDOF was commissioned in June 2023 and achieved steady state mesophilic digestion after 8 weeks. Baseline digestion testing was conducted from July to September 2023. Performance data includes organic loading rate (OLR), VSr and destruction (VSD), biogas and methane yields, volatile fatty acids (VFA), alkalinity, ammonia, and microbial consortia. Genomic samples are collected, extracted and sequenced using shallow shotgun sequencing. Results Baseline Digestion Key process parameters for the pilot digesters were within 5 to 15 % of the full-scale digesters, suggesting direct scalability between the two. Figures 2—4 highlight the similarity between TS, VFA, and ammonia concentrations. The start-up period used diluted digested sludge and can be seen from June 2023 to mid-August 2023 when the digesters achieved steady-state operation. PDOF metrics including VSr and biogas yield were comparable to the full-scale digesters (Table 1). As biogas data from the test was not completely reliable due to condensate issues, the data reported here relies on measured chemical oxygen demand (COD) and a theoretical conversion of 0.35 standard litres of methane per gram of COD destroyed3. Following baseline digestion, modifications to the gas meters improved the accuracy for further tests. There was good agreement between the microbial consortia population in the pilot and full-scale digesters (Figure 5). Thermophilic digestion with LIWWTP sludge in the PDOF is expected to develop a completely new population as it is not seeded from thermophilic digesters. Thermophilic Digestion A single digester was transitioned to thermophilic temperatures (55 degrees C) without a thermophilic digestate seed by increasing the temperature instantly, with no added feed. To ensure that the thermophilic transition was progressing as expected, VFA and alkalinity were monitored daily (Figure 6). A second digester will be converted to 60 degrees C using different methods where mixed mesophilic sludge will be continuously fed to the digester. The two transition methods will be compared once complete. TPAD The test for TPAD is anticipated to be complete by the time of paper submission. Conclusions The three PDOF digesters demonstrated distinct individual performances according to their HRT during the steady-state period. All three digesters were stable throughout the tests based on digested sludge characterization. The VSr results for Digester B with 20-day HRT were closest to the full-scale digester with 28-day HRT. This may indicate that a scalability factor or inefficiencies at full-scale need to be considered in future tests. With the shorter HRT of Digester C (15 days), the product still meets the Organic Matter Recycling Regulation (OMRR)4 requirements for Class B Biosolids. In addition, this study provides valuable insights into the performance and scalability of the PDOF, operational limits for various digestion processes at reduced HRTs, and potential redefines the minimum requirements to achieve biosolids reguirements.