A Comprehensive Biochemical Characterization of Methane Generation Within 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: 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.The model architecture consisted of eight calculation modules solved simultaneously for each pipe section included in the 21 miles of model domain. These modules included the following:1. Pipe and wastewater characteristics—pipe geometry, wastewater flow temperature, pH, alkalinity, sulfate, sulfide, nitrate, VSS, COD, dissolved N2, dissolved O2, dissolved carbon dioxide, and dissolved methane were entered for each tributary.2. Hydraulics—pressure and energy grade lines were determined at each node3. Stream mixing—wastewater constituents and dissolved gas concentrations were combined between contributing side sewers and upstream flow for each pipe junction.4. Organic carbon oxidation reactions—consumption of soluble COD and generation of products was estimated based on several possible electron acceptors, including oxygen, nitrate, and sulfate; including denitrification with organic carbon or sulfide.5. Sulfur transformations—conversion between sulfide and sulfate was estimated based on several oxidation and reduction reactions, including hypochlorite dosing.6. Anaerobic digestion—anaerobic digestion of volatile settled solids and generation of carbon dioxide and methane was estimated.7. Gas bubble formation—rate of gas bubble formation was estimated for each of five gases based on the solution of Dalton's and Henry's laws and mass conservation.8. Gas emissions—gas bubbles were allocated to points of high elevation (low pressure).The model components were then calibrated to field data. The hydraulic grade line was calibrated to field pressure monitoring stations. The reaction rate of sulfate reduction was calibrated to measured sulfate and sulfide concentrations along the length of the force main. The rate of anaerobic digestion was calibrated to gas venting records. The calibrated model was able to characterize the range of physical, chemical, and biochemical processes occurring in the force mains.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 describes a pH shock dosing pilot study which resulted in an order of magnitude decrease in gas emissions.