Gas-Liquid Mass Transfer along Small Sewer Reaches

Municipal and industrial sewers may be localized sources of volatile organic compound (VOC) emissions to the ambient atmosphere. Previous studies of VOC emissions from sewers have focused on sewers with large diameters that are often characterized as having mild channel slopes and as conveying relatively large wastewater flow rates. The study described in this paper was completed to better understand VOC emissions from sewer reaches with small diameters, steep channel slopes, and relatively low wastewater flow rates (e.g., as might be typical for building laterals, street sewers, and on-site industrial sewers). Mathematical models were developed to investigate the nature of mass transfer kinetics and equilibrium conditions in such sewers. A series of 20 experiments were then completed to determine liquid-phase and gas-phase mass transfer coefficients for a range of sewer operating conditions and chemical properties. Experiments were completed in an experimental sewer reach (60 m length, 0.2 m diameter) using five volatile chemicals (acetone, ethyl acetate, toluene, ethylbenzene, and cyclohexane, listed in order of increasing Henry's law constants). Experimental stripping efficiencies were as high as 47% for cyclohexane and as low as 0.3% for acetone. Experimental and mathematical results indicate that VOCs with low Henry's law constants (e.g., acetone) can reach equilibrium conditions rapidly in sewers. However, emissions of VOCs with high Henry's law constants (e.g., cyclohexane) are kinetically limited, allowing for the sewer to be treated as an “open” system. The findings described herein suggest that a large fraction of VOCs with high Henry's law constants may be emitted to the ambient atmosphere in the near vicinity to the point of discharge.