Scaling-up of microbial electrochemical advanced oxidation processes (MEAOPs) for efficient degradation of emerging pollutants

Abstract

Microbial electrochemistry technology (MET), which converts organic matter in wastewater into electric energy through electrochemical active microorganisms, and further produces valuable chemicals (e.g. H2 and H2O2), is becoming a promising and sustainable technology. Recently, microbial electrochemical advanced oxidation processes (MEAOPs), mainly represented by the bio-electro-Fenton (BEF) process, have proven to be an efficient and energy-saving emerging water treatment technology for the removal of refractory emerging pollutants from wastewater, by combining MET with traditional electrochemical advanced oxidation. However, so far, there is no research available on the scaling up of the MEAOPs, especially for the BEF process, for treating such wastewaters. Therefore, the aim of this PhD thesis is devoted to scaling up the MEAOPs in two ways: ⅰ) designing and building a scaled-up BEF process and further investigating its feasibility on H2O2 production and continuous pollutant (e.g. dyes and pharmaceuticals) removal; ⅱ) designing and building novel scaled-up UV-induced MEAOPs, including MEUC and MEUPS processes, for the efficient and cost-effective removal of pollutants without pH adjustment and iron sludge issues that exist in the BEF process. First, a dual-chamber 20-L scaled-up BEF reactor with multiple electrodes is developed herein for the first time, to explore the feasibility of cathodic in-situ H2O2 production. A maximum H2O2 yield of 10.82 mg L-1 h-1 and a cumulative H2O2 concentration of 454.44 mg L-1 within 42 h were obtained under optimal operating conditions. Electrical energy consumption in terms of direct input voltage was only 0.239 kWh kg-1 H2O2, which is much lower than the lab-scale BEF reactor and even electrochemical systems. Next, under the BEF operating mode that requires the addition of iron, the feasibility of treating dye-containing wastewater and the continuous treatment of pharmaceutical-containing wastewater as a tertiary treatment process in wastewater treatment plants (WWTPs) was tested. Regarding treating dye-containing wastewater, 20 mg L-1 of methylene blue was efficiently removed following apparent first-order kinetics, with corresponding rate constants for decolorisation and mineralisation at 0.68 and 0.20 h-1 under optimal conditions, respectively. Moreover, when the hydraulic retention time (HRT) was 28 h in the continuous operating mode, methylene blue decolorisation and mineralisation efficiencies were observed as high as 99% and 77%, respectively. Regarding treating pharmaceutical-containing wastewater, the selected six model pharmaceuticals (500 µg L-1 for each) were completely removed by implementing 0.1 V voltage, 0.3 mM Fe2+ and HRT for 26 h. Moreover, taking clofibric acid as an example, its transformation products in the scaled-up BEF process, such as 4-chlororesorcinol, were tested, and the possible transformation pathway was inferred accordingly. Additionally, the application of real wastewater matrix also exhibited good performance. Second, to overcome further the two major bottlenecks in the BEF process, namely pH adjustment before and after treatment and the iron sludge problem, an innovative microbial electrochemical ultraviolet photolysis cell (MEUC), using UV irradiation instead of adding iron, was developed for the efficient and cost-effective treatment of pharmaceutical-containing wastewater, only with electrical energy input. The effect of crucial operating parameters on system performance, including applied voltage, the cathodic aeration rate, UV intensity and HRT, were systematically investigated. Furthermore, five major transformation products were detected while degrading the model compound carbamazepine, and the probable transformation pathways were proposed. Moreover, the luminescence inhibition test on Vibrio fischeri shows that the treated effluent had no obvious ecological toxicity. Furthermore, the MEUC system was further assessed with real WWTPs secondary effluent, which again demonstrates the effective removal of carbamazepine. Third, since the MEUC system may be inefficient in treating wastewater with high chroma, due to low UV transmittance, a novel microbial electrochemically assisted UV/persulfate (MEUPS) process is proposed, in order to expand further the applicable wastewater type for this system. It was observed that the decolorisation and mineralisation of 40 mg L-1 of methylene blue during the MEUPS process can reach 100% within 140 min and 97% within 5 h under optimal operating conditions. This hybrid process exhibits a better treatment effect than that of the individual process, and the synergy factor is quantified as 6.42. Regarding the reactive radicals involved in this process, it was proven that •SO4−, •OH, and •O2− are the three major examples in this regard. In addition, the treated effluent exhibited non-toxic properties when using Lemna minor as an indicator. Overall, the above results highlight the feasibility of scaling-up MEAOPs, including BEF, MEUC and MEUPS processes, as efficient and cost-effective technologies that can be applied to treat wastewater containing refractory emerging pollutants for future commercial application.