Behavior of Volatile Sulfur Compounds in Mesophilic and Thermophilic Anaerobic Digestion

Sulfur containing-compounds are commonly present in sewage and undergo degradation through wastewater and sludge treatment system. In the anaerobic digestion processes, volatile sulfur compounds (VSCs) can be generated from the sulfur containing-compounds and released in the digester gases, which will ultimately cause problems in odor control and advanced biogas utilization. The ability to predict VSC concentrations in the biogas and assess the impact of digester configuration, temperature, and sludge source and retention time on VSC emissions would assist with design decisions for digesters and gas treatment systems. However, currently quantitative estimation is not possible because of the limited information on the extent and rates of the sulfur conversion processes in anaerobic digestion and the relationship between feed sludge sulfur content and VSC generation.In this study, the kinetics of volatile organic sulfur compound (VOSC) degradation and their generation from the amino acid, methionine, was estimated in batch tests that were conducted with and without methanogen inhibition at 35 and 55 °C.First order kinetics were found to best describe VOSC degradation and methylation processes. The degradation rate constants of methyl mercaptan (MM), dimethyl sulfide (DMS), and dimethyl disulfide (DMDS) were 0.027, 0.013, 0.034 l·mg−1·h−1 at 35 °C and 0.0083, 0.005, 0.22 l·mg−1·h−1 at 55 °C, respectively. The methylation rate constants for MM were 0.0047 and 0.0012 l·mg−1·h−1 at 35 and 55 °C, respectively. The degradation rate constant of DMDS at 55 °C was about 6.5 times higher than that at 35 °C while the degradation rate constants of MM and DMS were about 3.4 and 2.7 times higher at 35 °C as compared to 55 °C. Since MM and DMS are more dominant in the digester emissions, it suggests that, with the same retention time, thermophilic digesters may have more VOSC released in the biogas than mesophilic digesters.MM was the dominant sulfur containing product from methionine degradation and its generation could be described by zero order kinetics. The MM generation rate was independent of temperature with the zero order rates constant about 4×10−6 at both temperatures. The releases of ammonia and VSC-S were compared during methionine incubation and it was found that the release of ammonia could be used as a surrogate to predict VSC-S release.A model which simulates the generation and degradation of VOSC in anaerobic methionine digestion at mesophilic and thermophilic temperatures has been established. Model simulations showed good agreement with the monitored data.