Gas-liquid mass transfer along small sewer reaches

Municipal and industrial sewers may be localized sources of volatile organi c compound (VOC) emissions to the ambient atmosphere. Previous studies of V OC emissions from sewers have focused on sewers with large diameters that a re often characterized as having mild channel slopes and as conveying relat ively large wastewater flow rates. The study described in this paper was co mpleted to better understand VOC emissions from sewer reaches with small di ameters, steep channel slopes, and relatively low wastewater flow rates (e. g., as might be typical for building laterals, street sewers, and on-site i ndustrial sewers). Mathematical models were developed to investigate the na ture 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 condit ions and chemical properties. Experiments were completed in an experimental sewer reach (60 m length, 0.2 m diameter) using five volatile chemicals (a cetone, ethyl acetate, toluene, ethylbenzene, and cyclohexane, Listed in or der of increasing Henry's law constants). Experimental stripping efficienci es were as high as 47% for cyclohexane and as low as 0.3% for acetone. Expe rimental and mathematical results indicate that VOCs with low Henry's law c onstants (e.g., acetone) can reach equilibrium conditions rapidly in sewers . However, emissions of VOCs with high Henry's law constants (e.g., cyclohe xane) are kinetically limited, allowing for the sewer to be treated as an " open" system. The findings described herein suggest that a large fraction o f VOCs with high Henry's law constants may be emitted to the ambient atmosp here in the near vicinity to the point of discharge.