Cost comparison of advanced oxidation processes for wastewater treatment using accumulated oxygen-equivalent criteria

Advanced oxidation processes (AOPs) have received a lot of attention over the years as advanced physico-chemical polishing wastewater treatments to remove biorefractory pollutants. Additionally, many studies report their excellent degradation and mineralization performance as stand-alone technologies too, demonstrating the versatility of these processes; however, there is a lack of suitable methods to compare the performance (in terms of removal efficiency and operating costs) of different AOPs in the same conditions. In this context, the goal of this paper is to propose a systematic investigation by introducing a novel criterion, namely the accumulated oxygen-equivalent chemical-oxidation dose (AOCD), to systematically compare the diverse AOPs available: ozonation, H₂O₂ photolysis, Fenton, photo-Fenton, electro-Fenton and photoelectro-Fenton (paired with anodic oxidation, for the latter two). For each of these, the cost efficiency was determined by optimizing the operating conditions for the removal of phenol, selected as a model pollutant (1.4 mM, equivalent to 100 mg-C L^-1). The operating costs considered sludge management, chemical use and electricity consumption. Among all AOPs, electro-Fenton was the most cost-effective (108 - 125 € m^-3), notwithstanding the mineralization target (50%, 75% and 99%), owing to its electrocatalytic behavior. Chemical Fenton proved competitive too up to 50% of mineralization, meaning that it could also be considered as a cost-effective pre-treatment solution. AOCD was the lowest for electro-Fenton, which could be attributed to its excellent faradaic yield, while UV-based processes generally required the highest dose. The AOCD criterion could serve as a baseline for AOP comparison and prove useful for the legislator to determine the "best available techniques" as defined by the Industrial Emissions European Union Directive 2010/75/EU.