Understanding the Stability and Capacity of Full-scale Anaerobic Digesters Using a Bench-scale Test

Introduction Despite being essential to treat the solids stream at Water Resources Recovery Facilities (WRRF) around the world, anaerobic digesters are typically operated with imprecise and limited process information. Instead, industry standard values with generous safety factors are used to buffer any perturbations. Understanding the risk of upset is particularly important when the digesters are operated close to their design capacity, under variable loading conditions, or co-digesting extrinsic feedstocks. Clean Water Services (CWS) operates digesters at two facilities. The Durham WRRF runs two digesters that receive imported Fats, Oils, and Greases (FOG) for co-digestion which increases the variability of the loading conditions. The Rock Creek WRRF digesters are not fed co-digestion materials routinely but have limited online measurements. Total feed flow and overall gas generation of the five-digester system are the only values tracked online, each by a single meter. Discrete measurements of physicochemical parameters such as volatile acids (VA), alkalinity, pH and volatile solids (VS) content are performed routinely and individually to satisfy typical operational requirements. However, the status of the digesters can often be misrepresented when using only grab sample values. Furthermore, the diagnosis of upset events can be slowed by miscalculated loading rates, restricted visual inspections and the use of laboratory analyses that take time to be completed. CWS has refined and is using a bioassay to monitor the Durham and Rock Creek digesters. This bioassay was developed based on the understanding that a method that measures the capability of the microbial community to use a key intermediate, such as acetate, can help determine how close a digester is to failure (Conklin et al., 2008). Monitoring the ability to complete this metabolic step allows staff to troubleshoot and take corrective action prior to upset events, and better understand operational conditions that can affect the health of the digesters and limit their capacity. The bioassay has shown reproducible results during development, and validation against operational conditions (Sosa-Hernandez & Schauer, 2023). This presentation will 1) include updated validation of bioassay results against full-scale operation, and 2) provide example insights into the digestion process. Weekly bioassays have been conducted at the Durham WRRF starting in September of 2021. Testing was extended to Rock Creek in June of 2022 with the intention of identifying responses of the indicators to usual operational conditions. The goal is to capture routine changes that are hypothesized to burden the digesters microbial metabolism, some of which include: FOG loading changes, digester restarts, and mechanical difficulties. Bioassay Implementation The bioassay consists of measuring the ability that fresh digestate samples from active digesters have to utilize additional acetate. Figure 1 shows real experimental results and the bioassay indicators, which are: - Rate of biogas production in an acetate fed bottle (rac) - Rate of biogas production in a bottle containing only digestate (rb) - Ratio of the two measured biogas production rates (rac/rb) - Biogas generation per mass of added acetate or biogas recovery - Acetate depletion time Changes to volatile solids loading rate (VSLR) and hydraulic detention time (HDT) of a full-scale digester can impact the rates of biogas production (rac and rb) and the ratio rac/rb. Hypothetically then, the ratio rac/rb represents the excess capability of the digestate to use acetate which could decrease when volatile acids accumulate due to organic overloading or due to low detention times that could cause biomass wash-out. Responses to Volatile Solids Loading Rate and Hydraulic Detention Time Figure 2 shows the relationships between rac/rb and the VSLR and HDT of the Durham digesters which are fed FOG. A correlation, although weak, is observed, which suggests an impact of operational conditions on the capacity to use excess acetate. The variability of the results could be influenced by our inability to properly measure characteristics (e.g. volatile solids) of the digester feed and flows, which worsens with co-digestion feedstocks like FOG. Physicochemical parameters that are tracked routinely (VA, Alkalinity, pH, etc.) show no signs of instability during the monitored period, despite the loading fluctuations and operating close to the operational benchmarks shown in Figure 2. A validation of the risk of failure when operating at industry standards might be required, that accounts for operational history and other microbial health indicators. Additionally, the organic loading capacity of FOG fed digesters might not be understood using solely biogas production from additional acetate, and other anaerobic digestion intermediaries could be used to evaluate this hypothesis. Responses During Digester Re-start and Mechanical Limitations The results obtained during the following events will be included in the presentation and manuscript: a)The Rock Creek digesters were put offline for cleaning and restarted in late 2022.The bioassay allowed us to determine the time at which a newly restarted digester and the system reached steady state. The changes in the ratio rac/rb can be used to determine how quickly fresh feed loading can be increased to the digesters, and to observe impacts to the other digesters. b)Overflowing of the Rock Creek digester contents has been observed on different occasions. Bioassay results indicated that the ability of the biomass to use the additional acetate was not perturbated during these events, which helped understand that the causes of overflowing were not related to organic overloading. This focused troubleshooting on mechanical issues that caused temperature to decrease, and provided confidence that the digester could quickly return to steady state if proper operating conditions could be resumed. Comparison of Digesters and Type of Feed Figure 3 presents a comparison of the results obtained from both the Durham and Rock Creek digesters, and the daily VSLR at which these are operated. The Durham digesters are operated under significant loading variability on a daily basis, and the ratio does not seem to be sensitive to these changes. This could also indicate a more robust biomass that is able to take shock loads or that the changes, despite their magnitude, are consistent on the long term which help maintain stability. Co-digestion with FOG could cause a microbial population shift that may relieve some of the burden over aceticlastic methanogens, but can affect earlier degradation steps (Ram Mohan et al., 2022). Due to their consistent loading, the Rock Creek digesters could be more susceptible to overloading if changes occur on a short timeframe. Conclusions and Next Steps We were able to identify responses of the bioassay indicators to different operational conditions. The insight gained from these results include: - The ratio rac/rb, which represents the capacity to use additional acetate, could be an indicator of the ability to take in higher loads. - Daily and highly variable loadings from FOG do not seem to impact the ability of the biomass to use additional acetate, which also seems to be increased. This could indicate greater capacity to take in higher organic loads, robustness of the microbial community or that the aceticlastic methanogens are more available. - The indicators could be used to distinguish process failures that are caused by mechanical problems from those that are caused by organic/hydraulic overload. These can also help identify long term impact of mechanical failures on the microbial metabolism. - A higher loading rate at which the capacity to utilize acetate is inhibited has not been identified using the bioassay. This suggests that conventional loading rate criteria may be overly conservative. The digesters have been generally stable during the monitored period, as denoted by other process parameters and observations. Monitoring of our digesters will continue during suspected risk of failure. Additionally, this study offers the opportunity to collaborate with other utilities that experience process difficulties which can help test our hypotheses elsewhere.