Anaerobic Digester Operating Conditions Affects Acetate Utilization Capacity

Anaerobic digestion of wastewater sludges relies on complex microbial communities for the hydrolysis and degradation of organic matter. Methanogenesis, the production of methane gas, is a rate limiting step for stable anaerobic digestion, and about two-thirds of the methane is generated from the conversion of acetate. The acetate-utilizing methanogens have a fundamental role in digester capacity and stability. They are highly sensitive to digester operating conditions in which the digester loading and volatile fatty acid (VFA) production rate exceeds their capacity, resulting in inhibition of their activity with subsequent decreased gas production, increased VFA concentrations, and decreased pH. The VFA degradation rate in anaerobic digestion is a key component to digester stability and is closely tied to acetate production. It is hypothesized that the acetate degradation rate is related to the digester operating condition; thus for a particular operating condition a unique acetate-utilization rate, or capacity, exists. In addition, the acetate-utilization capacity is related to the methanogenic population and perhaps to the type of methanogen. The acetate-utilizing methanogens are comprised of two genera, Methanosaeta and Methanosarcina. Typically, Methanosaeta dominates mesophilic anaerobic digestion, however Methanosarcina is capable of higher growth and acetate-utilization rates and confers greater stability to anaerobic digestion through its ability to better accommodate transient loadings.The objectives of this research were to determine how the acetate-utilization capacity of the methanogens varies for different full-scale anaerobic digesters and to investigate if the variation could be related to different digester operating conditions. In this study a laboratory method was used to measure the maximum acetate-utilization capacity, termed Vmax, for several full-scale anaerobic digesters. The Vmax values were compared to estimated acetate production rates, based on the digesters loading and volatile solids reduction, and to determine the digester acetate utilization capacity relative to the actual plant loading and operating condition. A measure of the excess substrate utilization capacity of the digesters was determined from this comparison. Monod based kinetic equations were adapted to predict the substrate utilization capacity of anaerobic digesters at different operating conditions. In addition, molecular methods, specifically quantitative polymerase chain reaction (qPCR), were applied to the digester samples to enumerate the acetate-utilizing Methanosaeta and Methanosarcina populations.The results illustrated a useful tool for digester operation and control through prediction and monitoring of digester acetate utilization capacity. Previous work and these results demonstrate that monitoring Vmax of an anaerobic digester is a sensitive tool for measuring digester stability, predicting digester upsets, and monitoring recovery. Analysis of kinetic equations and acetateutilization capacities of full-scale digesters identified several operating conditions, particularly solids retention time and solids loading rates, which exhibit higher acetate-utilization capacities, and thus greater ability to process transient loadings. Monod-based kinetic equations help predict acetate-utilization rates of anaerobic digesters and may assist operational control of digesters.The plant evaluations showed that thermophilic digesters have greater ability to process acetate than mesophilic digesters, and that the first stage digesters of two digesters in series have greater ability to process acetate than the second stage digesters. Molecular methods indicate thermophilic digesters have a greater population of Methanosarcina than mesophilic digesters. While the Methanosaeta population numbers varied greatly between different digesters, this genera clearly predominated in all mesophilic digesters. Digester acetate-utilization capacity could be predicted from operating conditions such as solids retention time and volatile solids loading rate using microbial substrate utilization kinetics. Good predictions were found for primary/first stage digesters, and results could be used to estimate the maximum loading a digester can receive without overloading the methanogenic population.