General Fate Model for Microconstituents in an Activated Sludge System

Abstract

Mathematical models elaborated for removal of microconstituents (MCs) in activated sludge (AS) system have not incorporated developments in International Water Association (IWA) models. In this thesis, sorption and biodegradation kinetic models that can be applied to describe transformation of MCs in an AS process were evaluated (volatilization and photodegradation were considered negligible). Bisphenol-A (BPA), 17α-ethinylestradiol (EE2), and triclosan (TCS) have been selected as target compounds in this thesis. Sorption batch tests were conducted to retrieve kinetic and equilibrium data. Nine lab-scale continuous flow porous-pot bioreactors operating at various solids retention times (SRTs) and hydraulic retention times (HRTs) were studied for biodegradation of MCs. The effects of SRT, HRT and the biomass concentration on sorption and removal of MCs were also investigated and the results of each phase were incorporated into MCs fate models. Freundlich and linear sorption isotherms and pseudo-second-order kinetic models with different kinetic rates were found to best fit the sorption and desorption results. The result of biodegradation study in the presence of 20 µg/L of MCs demonstrated 90-98, 63-91 and 97-98% mean removal rates of BPA, EE2 and TCS, respectively, in systems operating at SRT of 5–15 d and HRT of 4-10 h. Calculation of mass fluxes of selected MCs in the dissolved and particulate phases showed that biotransformation was the principal removal mechanism of targeted MCs. The fate models for the degradation mechanism of selected MCs were evaluated by applying various mathematical models. The pseudo-second-order model was found to best fit the results when active MCs degraders (XC) were used in the model. It was found that biodegradation studies should incorporate XC and not mixed liquor suspended solids concentration in their kinetic formulations. Therefore, the result of this study could be seen in the context, where the active MCs degraders are proposed to reduce the variability of biodegradation kinetic rates in AS systems operating at different operational conditions. Finally nitrification inhibitors, allylthiourea (ATU) were added to reactors and it was found that although nitrification process affects the fate of MCs in AS system, heterotrophs were most likely responsible for the biotransformation of the targeted MCs.