Evaluation of advanced oxidation processes for water and wastewater treatment - A critical review.

This study provides an overview of established processes as well as recent progress in emerging technologies for advanced oxidation processes (AOPs). In addition to a discussion of major reaction mechanisms and formation of by-products, data on energy efficiency were collected in an extensive analysis of studies reported in the peer-reviewed literature enabling a critical comparison of various established and emerging AOPs based on electrical energy per order (E) values. Despite strong variations within reviewed E values, significant differences could be observed between three groups of AOPs: (1) O (often considered as AOP-like process), O/HO, O/UV, UV/HO, UV/persulfate, UV/chlorine, and electron beam represent median E values of <1 kWh/m, while median energy consumption by (2) photo-Fenton, plasma, and electrolytic AOPs were significantly higher (E values in the range of 1-100 kWh/m). (3) UV-based photocatalysis, ultrasound, and microwave-based AOPs are characterized by median values of >100 kWh/m and were therefore considered as not (yet) energy efficient AOPs. Specific evaluation of 147 data points for the UV/HO process revealed strong effects of operational conditions on reported E values. Besides water type and quality, a major influence was observed for process capacity (lab-vs. pilot-vs. full-scale applications) and, in case of UV-based processes, of the lamp type. However, due to the contribution of other factors, correlation of E values with specific water quality parameters such as UV absorbance and dissolved organic carbon were not substantial. Also, correlations between E and compound reactivity with OH-radicals were not significant (photolytically active compounds were not considered). Based on these findings, recommendations regarding the use of the E concept, including the upscaling of laboratory results, were derived.