Micro-constituents in Digester Gas: Sweating the Small Stuff

Increasingly wastewater treatment facilities that utilize anaerobic digestion are examining biogas beneficial use projects for energy recovery that transcend the current, and most common, practice of only capturing heat energy for process heating and flaring surplus biogas. Utilization of digester gas (DG) in energy recovery devices (e.g., hot water boilers, reciprocating internal combustion engines and microturbines) can be impacted by not only gas quantity but increasingly by gas quality. Digester gas quality considerations must include an evaluation of the major constituents (e.g., methane and carbon dioxide) to determine the overall energy content; however, the minor constituents (e.g., hydrogen sulfide and siloxanes) can most greatly influence the economic viability and mechanical reliability of a biogas beneficial use project.Specifically, results of single event on-site DG sampling for eight (8) different wastewater treatment facilities for both major and minor constituents are presented. These results are derived from plants ranging in design capacity from 15 to 75 million gallons per day (MGD) covering a wide range of other site specific operational criteria (e.g., digester residence time, primary/secondary sludge blend, etc.) all operating conventional mesophilic anaerobic digesters. Furthermore, results of multiple sampling events at a single wastewater treatment facility are presented. These results indicate significant variation exists in micro-constituent content between wastewater treatment facilities and that significant temporal variability exists between samples collected at a single wastewater treatment plant.Digester gas treatment options for removal of moisture, particulates, hydrogen sulfide and siloxanes to meet gas quality requirements for downstream use in the digester gas utilization equipment are also discussed herein.