Gas- and liquid-phase mass transfer resistances of organic compounds during mechanical surface aeration

Abstract

The mass transfer behavior of a suite of six volatile halogenated organic compounds and oxygen was investigated in a laboratory scale reactor representative of mechanical surface aeration during activated sludge treatment. The rate of mass transfer of the organics was related to that of oxygen by a proportionality factor Ψ. Significant deviations were observed for the least volatile solute from the expected constant value of Ψ under the assumption of liquid phase controlled mass transfer. Data analysis and interpretation according to two-resistance mass transfer models with appropriate correction for the slight variation in molecular diffusivities indicates that the gas phase resistance to mass transfer is more important than previously assumed. Based on this work and on the data analysis of two other studies it is recommended that a value of k G/ k L ≈ 40 be used to estimate the relative importance of gas and liquid phase mass transfer resistances in mechanical surface aeration under conditions of high specific power input. Consequently, only compounds with a dimensionless Henry's constant ⩾0.55 can be considered to exhibit complete (⩾95%) liquid phase controlled mass transfer behavior. It appears that the liquid phase mass transfer rate constant, k L a is proportional to the square root of the diffusivity in accordance with the penetration and surface renewal theories. The dissolved constituents present in treated wastewater reduce the rate of mass transfer of organic solutes and oxygen in a proportional fashion. Since the conditions of operation and the observed rates of oxygen mass transfer in this work are similar to those encountered in the field, there is reason to believe that these findings are also valid under conditions typical of water and wastewater treatment.