Modeling air stripping of volatile organic compounds (VOCs) from biological treatment processes

Abstract

The removal (biodegradation and air-stripping) of some common VOCs, including benzene, toluene, TriChloroEthylene (TCE), methylene chloride, and Methyl IsoButyl Ketone (MIBK) was evaluated for aerobic biological treatment processes. The methodology used laboratory-scale data (aqueous input and output) for oxygen demand and VOCs to estimate mass transfer parameters and eliminate the need to simultaneously calculate removal by biodegradation and air stripping. First, a dimensionless parameter (ratio of VOC removed to estimated VOC air-stripping rate), ã, was developed, where ã < 1.0 implied that air stripping could be significant. This compared well to laboratory-scale data in each case (for example: toluene: ã = 0.615, air stripped = 8.18%; MIBK: ã = 42.1, air stripped = 0.18%). Finally, a stochastic model was developed (based on steady-state mass balances) to predict the amount of VOC air stripped. Conventional process parameters (normal distributions) were used to estimate the overall mass transfer coefficient for oxygen, (KLa)O, and this was used to estimate the overall mass transfer coefficient for each VOC, (KLa)VOC (based on a two-film model). The calculated results compared well with laboratory scale measurements, i.e., the calculated mean air stripping values were all within ±1.5 standard deviations of the mean observed experimental values. This prediction method is generally useful because the calculation does not contain the additional uncertainty from the bio-kinetic parameters.