Integrated spectral based monitoring, optimization and control of the combined ozonation and powdered activated carbon adsorption process to remove organic micropollutants from secondary effluent

Abstract

In this study, an innovative approach for the integrated monitoring, optimization and control of the combined ozonation (O3) and powdered activated carbon (PAC) adsorption process is introduced making use of spectral surrogates (UVA254 and EEM-PARAFAC components). The combined O3-PAC process is designed to remove organic micropollutants (µP) from secondary effluent. Therefore, the removal of 6 µP with varying ozone reactivity was systematically studied in both O3 and PAC as stand-alone systems and in the combined O3-PAC system. For the latter, adsorption experiments were performed with µP spiked into ozonated secondary effluent (sequential system) and with µP spiked into the initial secondary effluent before ozonation (integrated system). In accordance with Swiss standards, the goal was to achieve 80% atrazine removal (as a model O3 recalcitrant compound) at optimized O3 and PAC doses. An ozone dose ranging from 0.45 to 0.65 mg O3/mg DOC was more cost-effective in promoting subsequent PAC adsorption (less than 40%), particularly at low PAC doses (1-2 mg PAC/mg (initial (DOC))). The classical ideal adsorbed solution theory (IAST) model could not be used to predict the subsequent PAC dose after ozonation in the integrated system. Correlation models were established between (i) the reduction of spectral surrogates during both O3 and PAC dosing and µP removal, (ii) the reduction of spectral surrogates during ozonation and the related PAC dose of the subsequent adsorption process and (iii) between the reduction of spectral surrogates during ozonation and the integrated process. Based on these correlation models, an online spectral control strategy was developed and implemented. Finally, the optimal dosing strategy for 80% atrazine removal was determined as 0.5 mg O3/mg DOC and 1.4 mg PAC/mg (initial) DOC.