Empirical modeling of air-to-water oxygen transfer in gravity sewers

To evaluate existing empirical models that have been used for prediction of air-to-water oxygen transfer in gravity sewers, held and laboratory investigations were conducted. A series of 16 oxygen-transfer coefficients were obtained in noncleaned operating sewers using a gaseous tracer method. In addition, experiments on the effect of contaminants on air-to-water gas transfer, expressed through the alpha factor, were conducted in a laboratory vessel. Based on the obtained oxygen-transfer coefficients, five empirical models, all previously used within the sphere of sewer problems, were evaluated. Furthermore, previously reported transfer coefficients for gravity pipes were included in the evaluation. The model of Parkhurst and Pomeroy was found to be the best for simulation of oxygen transfer. Estimates from the worst alternative tend to overestimate the oxygen-transfer coefficient by-approximately a factor of 25 on the average. Except for the case when foam was present, the investigations on the alpha factor showed no significant deviation from unity. Finally, the coefficients in the equation of Parkhurst and Pomeroy were evaluated by a nonlinear regression on the available observations from operating sewers. A new set of coefficients based on a statistical evaluation were obtained.