Electrochemical Advanced Oxidation Processes for Distributed Water Treatment

Distributed water treatment has emerged as an alternative to centralized water systems as a means of providing drinking water in locations where population density is too low to support the distribution networks needed to move treated water to places where it will be used. By exploitation of non-traditional water sources, distributed water treatment also has the potential to address the problem of water scarcity. Electrochemical advanced oxidation processes are promising techniques for distributed water treatment because of their small footprint, flexible operation and ability to remove contaminants without any chemical input. However, their adaptation has been hampered by challenges including the high cost of electrodes, slow mass transport of contaminants, high energy consumption, and difficulties in scaling up treatment processes. This research attempts to develop new electrochemical advanced oxidation processes for distributed water treatment by providing both a mechanistic understanding of the chemical reactions taking place and a better understanding of practical engineering considerations needed to deploy the technologies.Urban stormwater is one of the non-traditional water sources that is attractive to water-stressed cities because it is available in large quantities and does not require large investments in conveyance systems. To provide a means of removing chemical contaminants in distributed water systems, an electrochemical advanced oxidation process that is compatible with high-capacity stormwater recharge systems (e.g., drywells) was developed. The treatment system consisted of an electrochemical module for H2O2 production and an ultraviolent (UV) reactor for converting hydrogen peroxide (H2O2) into hydroxyl radical (●OH). To minimize the energy consumption and system footprint, production of a concentrated H2O2 stock solution prior to the storm event was evaluated. The H2O2 generation was optimized in terms of its energy efficiency and oxidant production rate. The solution pH was identified as the main factor affecting the H2O2 stability, with basic conditions leading to greater loss through dismutation. Using this information, the stability was enhanced by mixing the basic H2O2-containing catholyte with the acidic anolyte. The proposed advanced oxidation process demonstrated effective removal of trace organic contaminants in full-scale stormwater treatment system. However, the energy efficiency was limited by the inefficient UV light utilization, due to light reflection and backscattering at the water-air interface in the UV reactor as well as competition for UV light absorption with H2O2. In response to the inefficient light utilization by the UV/H2O2 process, an alternative approach for converting H2O2 to ●OH was developed. In Chapter 3, the reaction mechanism and reactive oxidant yields produced by activation of H2O2 on an inexpensive stainless-steel electrode was investigated. The stainless-steel electrode was investigated at different pH values and applied potentials using probe compounds to differentiate the production of different reactive oxidants (●OH versus Fe[IV]) from H2O2 activation. The stainless-steel electrode demonstrated high yields for converting H2O2 into ●OH at circumneutral pH conditions. In the presence of H2O2 concentrations comparable to those that might be generated by electrochemical reduction of O2, the stainless-steel electrode removed trace organic contaminants from an authentic water sample. However, loss of ●OH on or near the electrode surface decreased the utilization efficiency of ●OH. Finally, a reagent-free dual-cathode treatment process was evaluated. The system coupled the air-diffusion electrode with the stainless-steel electrode. The performance of the system was optimized separately for H2O2 generation and H2O2 activation. The optimal potential for H2O2 production on the air-diffusion electrode was identified as -0.04 V and the optimal potential for trace organic contaminant transformation on the stainless-steel electrode was identified as +0.02 V. The different optimal potential for each process suggests that the treatment could overcome the imbalance between H2O2 generation and H2O2 activation encountered in previously described composite electrodes. In tests of the combined electrochemical treatment system, the dual-cathode treatment process oxidized uncharged compounds (e.g., atrazine) but was unable to oxidize negatively charged benzoate. The efficiency of the system was also much less susceptible to competition for oxidants from negatively charged macromolecules (e.g., humic substances). These findings suggest that electrostatic repulsion minimizes the scavenging effect of negatively charged oxidant scavengers. The dual-cathode treatment system consumed about 10 kWh/m3, which is comparable to that of anodic oxidation and point-of-use UV/H2O2 process. Collectively, results of the experiments described in this dissertation indicate that electrochemical advanced oxidation processes hold promise as distributed drinking water treatment systems. The air-diffusion electrode can produce concentrated H2O2 stock solutions in an off-line reactor or lower levels of H2O2 can be produced in situ. Two H2O2 activation techniques, UV light and a stainless-steel electrode, converted H2O2 into reactive oxidant species that are capable of removing trace organic contaminants. Additional research is needed to scale up the lab scale systems, understand long-term performance of the electrodes, and develop strategies for treatment automation.

Advisor: Sedlak, David