Electrochemical Generation of Hydrogen Peroxide in Wastewater Effluent for Destruction of Trace Organic Compounds

Abstract

As water scarcity increases, treatment of wastewater for reuse is seen as an alternative water source. Tertiary treatments, including advanced oxidation processes (AOP), are required to remove persistent trace organic compounds that remain after conventional treatments. Two types of AOPs, UV-H2O2 and a heterogeneous Fenton-like process are experimentally investigated to predict the fate of trace organic compounds (TOrCs) during the oxidation processes, specifically addressing the significant effect of water matrix on oxidation efficiencies. A flow-through process consisting of electrochemical generation of hydrogen peroxide followed by UV-H2O2 photolysis was utilized to treat wastewater effluent spiked with p-cresol. Superoxide radicals produced during the generation of hydrogen peroxide reacted with effluent organic matter (EfOM) to produce organic radicals that reacted with p-cresol and showed a decrease in concentration of p-cresol. This process does not occur in Milli-Q water. Semi-quantitative strategies were employed to establish the role of superoxide radicals during this process, including utilizing methyl-p-benzoquinone as a superoxide radical probe. A mechanism was proposed to predict the transformation of p-cresol with electrochemically produced organic radicals. The next step was to expose the hydrogen peroxide containing wastewater effluent to UV. A comprehensive UV-H2O2 model with the complete reaction mechanism including reactions of intermediates with hydroxyl radicals in Milli-Q water was used to predict the degradation of p-cresol in wastewater effluent. This model was used to fit a parameter to account for the shading effect of light absorbing compounds found in wastewater effluent. A model of an annular flow-through reactor with reflecting walls was then applied to demonstrate the predicted degradation of p-cresol in the electrochemical reactor effluent. The model had previously mathematically demonstrated that the wall reflectivity significantly enhanced the rate of conversion of the target, accounting for the UV light reflection from the reacting walls, as well as the hydrodynamics of the annular flow. This rate was decreased significantly in wastewater effluent due to the shading effect. The second AOP included the use of zeolite as an iron-support to be used as a catalyst in Fenton-like oxidation process of adsorbed p-cresol. Fenton’s reagent consists of a solution of hydrogen peroxide and ferrous iron reacting to form highly reactive and oxidative hydroxyl radicals that degrade contaminants at low pH. As iron precipitates above a pH of 3, this process is rendered impractical. In order to overcome this limitation, it was found that hydrophobic iron-amended zeolites could be used to destroy compounds in water in a broad pH range. The zeolite was successfully loaded with iron and then used in oxidation experiments. The zeolite was regenerated in Milli-Q water, showing stability in both batch and column experiments. In wastewater, zeolite regeneration was limited as the pore size may be too small for compounds found in wastewater to enter. In addition to investigating removal of contaminants in water through oxidation, additional methods were examined to attempt to recover valuable products found in wastewater, like nutrients such as phosphorous. Phosphorous is an essential fertilizer component that is in short supply, but is found in municipal wastewater. Struvite (MgNH4PO4·6H2O) is an insoluble mineral that forms in waters rich in phosphorous. The purpose of this study was to investigate the feasibility of using residual carbon dioxide for struvite control in both bench- and pilot-scale experiments. Models were developed to predict the pH upon carbon dioxide addition and validated using experimental results culminating in a design tool to be used at any wastewater treatment plant for the economic recovery of nutrients from wastewater to support expansion of sustainable farming practices.