Improved Understanding and Use of Generated Oxidizing Species in Liquid Waste Disinfection

Abstract

The primary human exposure routes to microbial pathogens from the use of wastewater arise largely in an agricultural setting1. Some of the best results for pathogen inactivation have been obtained by applying thermal treatment, irradiation and pasteurization. However, such treatments require investments that may not be feasible for developing countries. Indeed, a key component in any strategy aimed at increasing the reuse of treated wastewater should be the application of appropriate technologies that are effective, simple to operate, and low cost (in investment and especially in operation and maintenance)2. Based on success in removing organics in wastewater treatment(3,4), it is anticipated that higher order oxidants can be used to inactivate difficult–to-treat pathogens such as helminth eggs. In addition, given the suitable conductivity of wastewater due to the presence of urine5, electrochemically-assisted disinfection is seen as a very interesting alternative. Recent research in our group showed that synthetic urine spiked with E. coli could be disinfected by means of electrochemically-generated chlorine at a boron-doped diamond (BDD) anode6. The current study provides greater understanding of the electrochemical generation of oxidizing species on diamond electrode. Determining their kinetic properties will help to better understand proximity effects and the rate limiting factors of electrochemical disinfection. This is expected to yield quantitative data on the reactivity of different oxidant species and ultimately result in greater energy efficiency for electrochemical disinfection systems. Given the short lifetime of reactive oxidative species (ROS) such as hydrogen peroxide or hydroxyl radicals(7,8), their electrogeneration on a BDD electrode was monitored indirectly using optical measurements: absorption and fluorescence spectroscopies, respectively(9-12). The experimental setup for H2O2monitoring is illustrated in Figure 1. Results show that hydrogen peroxide can be produced on BDD electrodes and monitored in-situ in an oxygen depleted solution. An initial step allows oxygen generation by electrolyzing water at an anodic voltage. Oxygen is then reduced at a negative voltage to generate hydrogen peroxide. Table 1 gives the concentration of H2O2generated and the current efficiency as a function of the applied cathodic voltage. This presentation will also discuss the energy efficiency of a narrow gap multi-plate reactor compared to other geometries. Another important consideration in the inactivation of pathogens is the brief periods of reverse polarity that are often necessary to descale diamond electrodes after anodic oxidation13. The initial oxidation step will generate ROS such as hydroxyl radicals and ozone, and a second reduction step will enable hydrogen peroxide evolution and eventually descale the electrode. This work will present recommendations for design of an electrochemical system optimized in terms of oxidant utilization efficiency.