Photocatalysis in Membrane Bioreactor Effluent: Assessment of Inhibition by Dissolved Organics

Abstract

Photocatalysis has been proposed as a synergistic addition to ultraviolet (UV) dosing systems to provide a form of advanced oxidation. The photocatalytic process, however, is significantly inhibited by dissolved organic matter (DOM). Membrane bioreactors (MBRs) offer an opportunity to simultaneously control DOM and provide a structure to support photocatalysts. Here, effluent organic matter (EfOM) samples from two MBR facilities employing membranes with different nominal pore sizes were assessed for their inhibition of TiO2 photocatalysis. Photocatalytic activity was quantified by measuring the photodegradation of a probe compound, para-chlorobenzoic acid. Analysis of inhibition profiles of both EfOM and a well-known ·OH quencher, tert-butyl alcohol (t-BuOH) showed that MBR EfOM inhibited photocatalysis via competitive adsorption and surface reactions as well as reactions in the bulk phase. EfOM concentrations below 0.34 mgC/L did not inhibit photocatalysis, whereas EfOM concentrations of above 1.8 mgC/L completely negated photocatalytic action. The results suggest that avoiding unwanted quenching reactions may be possible with proper operational control of the upstream MBR process. Insights into the relevance of radical quenching mechanisms are offered with regard to applying TiO2 to wastewater treatment.