Hollow fiber membrane contactors—A means to study the reaction kinetics of humic substance ozonation

This paper describes the use of membrane contactors to investigate the reaction kinetics of the ozonation of humic substances (HS). Humic substances are a collection of large, complex organic molecules in soil and surface water originating from microbial decay of dead organisms. They give and undesired yellow color to (drinking) water. A methodology based on theoretical mass transfer considerations was used to evaluate and interpret results from ozonation experiments. Mass transfer experiments were conducted in a hollow fiber membrane contactor in order to allow stationary ozonation conditions. Because mass transfer conditions for hollow fiber membranes can be controlled well and in and outgoing streams can be analyzed precisely, this way of performing ozonation enables a thorough and precise investigation of the reaction kinetics. For this purpose, an ozone resistant PVDF hollow fiber membrane module was developed. Theoretical considerations on mass transfer with and without reaction and experimental mass transfer data indicate that the initial ozonation reactions occur instantaneously compared to the diffusive ozone mass transfer. During the oxidation process the reaction rate shifts from instantaneous to fast compared to the diffusive ozone mass transfer. Even when the ozone consumption is 0.25 mol O3/mol DOC, the enhancement factor is >2 and no ozone is found in the liquid bulk. This indicates that the reaction is at least fast. The average HS diffusion coefficients during the initial ozonation reaction can be estimated from the assumption that the reactions are instantaneous.