Navigating a Digester Upset Event Using a Bench-scale Digestion Stability Test

INTRODUCTION Anaerobic digester upsets are rare at Clean Water Services (CWS) Water Resources Recovery Facilities (WRRF), but when they occur, they can limit solids treatment and cause costly, plant-wide disruptions. Operations staff work hard to prevent them by regularly monitoring process data, ensuring reliable measurements, and applying conservative standards. Historically, identifying causes of instability has been difficult at the Rock Creek WRRF, which operates five digesters of varying sizes and mixing configurations. Discrete measurements of key parameters - such as Volatile Acids (VA), alkalinity, pH and Volatile Solids (VS)- meet routine needs but take time to complete and lag behind actual process changes. Online measurements are limited to a single feed flow meter and until recently, a single overall gas generation value. Gas monitoring has been increased in response to data collected from tests described in this paper. Limited process information makes preventing and recovering from upsets challenging. CWS operations and research teams work together to analyze available and new information and make data driven decisions that ensure digester stability. DIGESTION STABILITY TEST CWS' research group developed a bench-scale Digestion Stability Test (DST) (Sosa-Hernandez et al., 2022) that has been used to monitor the digesters since 2022 (Sosa-Hernandez & Schauer, 2023) and provide additional information to Operations about digestion health. DST consists of measuring the ability of the microbial community of an active digester to use acetate. The results of DST are: - Rate of biogas production in an acetate fed reactor (rac), is the acetate utilization rate which represents active methanogenic biomass and the digester current loading capacity. rac would change with biomass growth or reduction caused by loading or operational shifts. - Rate of biogas production in a reactor containing only digestate (rb) is comparable to full-scale gas production. rb indicates organic loading changes, and can be used to validate full-scale trends. - Acetate capacity (rac/rb), represents the availability of the microbial community to take in higher organic loads. The hypothesis is that rac/rb would decrease if the hydraulic or organic loading to the digesters increase or if an inhibitory event occurs. DIGESTER UPSET EVENT The digesters are fed sequentially using one pump and control valves. When a digester reaches its set volume, the pump stops, its valve closes and the next valve opens. Due to level differences between digesters, shorter units can experience overloading due to valve leakage. CWS considered installing individual feed flow meters to improve feed control, but maintenance needs and complex installation require further evaluation. This section describes an event caused by equipment failure that was not noted through typical means. DST was critical in providing information to identify potential causes of stress, validate full-scale observations and gas flow measurements, and monitor microbial health during recovery. Figure 1 shows DST parameters and gas productions for active digesters (RC-3, RC-4, RC-5, RC-6), and Figure 2 presents process data during the event, which consisted of the following periods: A)RC-4 gas increase. - Gas system connections and the reliability of individual gas flow meters were questioned due to recent installation and inconsistencies with total gas usage. - DST measurements indicated increased loading to RC-4. Increased rb matched full-scale observations, and increased rac, indicated higher activity or growth of biomass; however acetate capacity was maintained stable at a value of 2, indicating ability to adapt to loading changes and maintain capacity. - RC-4 had been activated within the previous four months, raising concerns about valve placement. Valves and seals were inspected, confirming integrity. - Other process parameters (VA, Alkalinity, VS reduction, pH) showed no abnormalities. Without additional data, operations may not have recognized the issue in time, risking delayed response and potential failure. - Flame arrestors were cleaned and gas meters were calibrated to ensure accuracy of gas values. - RC-4 gas production continued rising, while the gas production of taller digesters RC-5 and RC-6 gradually decreased. B)Valve replacement and actuator issue for RC-4. - The increased gas production prompted valve reinspection, confirming leakage and requiring repair work. The valve was replaced and RC-4 was returned to typical feeding regime. Afterward, RC-4 gas spiked again while other digesters dropped. Incomplete valve closure due to actuator placement issues directed all digester feed to RC-4. - In addition to increased gas flow, operators and laboratory staff noted increased odor before other process parameters showed a response to the overload. Temperature control became erratic with higher solids load. - DST acetate capacity dropped then to 1, which is an indication of overloading or suppressed microbial acetate utilization (Figure 1.B). This correlated to the observed VA accumulation and reduced alkalinity (Figure 2.B). - To prevent complete failure, a reactive response was required after valve actuator correction. Feeding was stopped for RC-4 and sludge was transferred to and from other digesters for re-seeding and to supply alkalinity. Operations monitored CO2, H2S and pH daily, and DST was conducted to verify microbial activity. - RC-4 acetate capacity increased to 2.8 within 8 days indicating successful recovery and resiliency. Feed was started at 20% of typical feed and then increased gradually to support biomass growth, which was confirmed by rac trending upward (Figure 1.C). C)RC-4 reactivated and gas discrepancies. - After stable operation was regained for RC-4, gas differences were observed with RC-5 and RC-6. - Laboratory measured gas, rb were equivalent for RC-5 and RC-6, at the beginning of period C(Figure 1), suggesting equivalent loading and pointing to inaccurate full-scale gas values. Other DST measurements indicated no shifts in loading. - As observed in Figure 3, rb values match full-scale daily average gas flows except when gas from RC-6 was captured in RC-5 due to shared gas piping. To verify, the gas connection upstream of flow meters was closed, which made RC-5 and RC-6 gas trends to realign as shown in Figure 1.C. VALIDATING OPERATIONAL BENCHMARKS Figure 4 shows a comparison of DST's acetate capacity in comparison to calculated volatile solids loading (VSL) and hydraulic detention time (HDT). Since individual feed flow meters are not available, the VSL and HDT were back-calculated using gas productions and an assumed gas yield of 6.3 CF/lbVS. Gas yields for the digester feed are routinely measured full and bench-scale, and were assumed to be consistent through the monitored period. As observed in Figure 4, an acetate capacity of 2 is a good indicator of higher than typical loading or that the digesters could be at risk of approaching hydraulic overload. This event helped confirm that a VSL benchmark of 0.2 ppd/ft3 is conservative enough to maintain microbial activity, however, the hydraulic capacity may be limiting. LESSONS LEARNED -Managing risk: Traditional operating parameters do not give early warnings of conditions that lead to digester stress. Using DST alongside helps monitor microbial health, especially when digesters are operated near capacity. Gas productions complemented with microbial activity can prompt earlier investigation. -Trusting instrumentation: This event helped gain confidence in full-scale gas flow meters. DST results are useful for operators to validate gas production trends and work to understand intricacies of gas system connections. -Enhancing process information: When used in conjunction with typical process metrics, DST indicators provide insight to the causes of instability. For example, to distinguish mechanical from biological causes. Regular measurements of microbial activity daylight the effects of operational changes on biomass growth and health. This is particularly important during start-up or reactivation of upset digesters, to establish procedures that ensure a smooth transition. This event highlighted the value of cross-functional dialogue between different groups within CWS, helping establish a framework for interpreting digestion data. This improves decision-making under uncertainty, strengthening organizational resilience. This troubleshooting example builds confidence and provides guidance to respond to future challenges and proposed operational changes.