Predicting Oxygen Uptake and VOC Emissions at Enclosed Drop Structures

Abstract

Drop structures used during wastewater collection and treatment are sources for volatile organic compound (VOC) emissions. To assist in the reduction of such emissions, pilot-scale experiments were completed using municipal wastewater to study the effects of drop height, liquid flow rate, and tailwater depth on oxygen transfer, and to evaluate the effects of the same parameters on the stripping of 10 VOC tracers. Results were used to develop predictive models for oxygen and VOC transfer. Oxygen uptake at the pilot drop structure suggests that the drop height is the most important parameter influencing oxygen uptake at enclosed drop structures. Tailwater depth had little effect on oxygen transfer at the drop structure. Stripping of VOCs at drop structures was seen to be a strong function of Henry's law coefficient. This sensitivity was related to gas-phase resistance in mass-transfer and/or VOC accumulation in the air bubbles. Incorporating gas-phase resistance and an appropriate a factor for wastewater into the model allowed the prediction of VOC deficit ratios and estimation of VOC stripping at drop structures for both clean water and wastewater.