Use of Force Main Modeling to Evaluate Gas Emissions from Long Force Main Systems: Case Studies of Methane and Nitrogen Gas Control

Hampton Roads Sanitation District (HRSD) owns two wastewater collection force mains each approximately 21 miles long with detention times longer than 24 hours. In one of the force mains, this has resulted in a tendency for anaerobic digestion of settled solids and formation of methane, hydrogen sulfide, nitrogen, and carbon dioxide gases within the force main. These gases strip out and collect at high points and must be manually vented to the atmosphere. Gas composition was often as high as 80 to 90% methane and also contained greater than 1,000 ppmv of hydrogen sulfide. In the second force main, the gas emissions were heavily influenced by nitrate discharge from a pretreatment system at the upstream end of the force main. While this second force main was also influenced to some degree by methane formation, nitrogen gas proved to be a major contributor to off gassing. Both situations represented an odor nuisance risk and safety concern and cost many man-hours for manually venting the force mains.A mathematical model was developed to reproduce the relevant physical, chemical and biological processes present in the force mains; calibrate the model to available field data; and then run scenarios to evaluate the effect of potential mitigation options. This paper presents the modeling in the form of two case studies, one for each of the two force mains.The force main modeling included consideration of: solid and liquid and gas phases, flowing and stationary fluids, varying pressures, and biochemical reactions within the force main. The model consisted of eight calculation modules solved simultaneously for each pipe section included in the 21 miles of model domain. These modules included consideration of the following:1. Piping configuration and wastewater characteristics2. Hydraulics to determine pressure and energy grade lines3. Side sewer mixing of wastewater constituents and dissolved gas concentrations from the side sewers4. Organic carbon oxidation reactions based on consumption of soluble COD and generation of products based on possible electron acceptors including oxygen, nitrate, and sulfate5. Sulfur transformations including conversion between sulfide and sulfate6. Anaerobic digestion of volatile settled solids and generation of carbon dioxide and methane was estimated.7. Gas bubble formation for each of five gases based on the solution of Dalton’s and Henry’s laws and mass conservation.8. Gas emissions allocated to points of high elevation (low pressure).The model components were 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 first case study shows results of a pH shocking program that was successfully used to reduce methane gas formation. This program has resulted in over 90% reduction in off gassing and demand of HRSD staff time for venting. The pH shocking program has proven successful over a two year period thus far.The second case study shows off gas projections related largely to nitrogen gas production caused by relatively high doses of nitrate discharged from an upstream pretreatment process from a local food manufacturing facility. This case study presents evaluations used towards establishing updated pretreatment criteria that would minimize off gassing issues in the force main.