Characterization of Methane Generation and Emissions from a Wastewater Force Main

Hampton Roads Sanitation District (HRSD) owns two wastewater collection force mains each approximately 21 miles long with detention times longer than 24 hours. Both are over-sized relative to flow, which has resulted in anaerobic digestion of settled solids and formation of methane, hydrogen sulfide, nitrogen, and carbon dioxide gases within the force main. Gas strips out and collects at high points and must be manually vented to the atmosphere. In one of the force mains, the gas composition was often as high as 90% methane and also contained greater than 1,000 ppmv of hydrogen sulfide. This situation represents both an odor nuisance and safety concern and has cost HRSD thousands of man-hours for manually venting the force mains. HRSD engaged CH2M HILL to assess the force mains and provide guidance toward a solution to both the gas emissions and odor emission problem.CH2M HILL took the approach of building a mathematical model to reproduce the relevant physical, chemical and biological processes present in the force mains; calibrating the model to available field data; and then running scenarios to evaluate the effect of potential mitigation options. The force main modeling included consideration of the following: solid and liquid and gas phases, flowing and stationary fluids, varying pressures, and two potential biochemical regimes coexisting within the same pipe. Based on a literature review, no model currently exists capable of tracking the processes needed to grapple with the problem. Therefore, CH2M HILL sought to develop the needed model. This paper presents the model development and performance relative to field sampling completed by HRSD.A later companion paper will present the results of modeling scenarios used to evaluate several options for controlling gas emissions including oxygen injection, nitrate addition and pH shock dosing. The paper will describe a pH shock dosing pilot study which resulted in an order of magnitude decrease in gas emissions.