Electrochemical treatment of reverse osmosis concentrates generated in water recycling

Bagastyo, Arseto Yekti (2013). Electrochemical treatment of reverse osmosis concentrates generated in water recycling PhD Thesis, School of Chemical Engineering, The University of Queensland.

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Author Bagastyo, Arseto Yekti
Thesis Title Electrochemical treatment of reverse osmosis concentrates generated in water recycling
Formatted title
Electrochemical Treatment of Reverse Osmosis Concentrates Generated in Water Recycling
School, Centre or Institute School of Chemical Engineering
Institution The University of Queensland
Publication date 2013
Thesis type PhD Thesis
Supervisor Damien J. Batstone
Jurg Keller
Korneel Rabaey
Jelena Radjenovic
Total pages 172
Total colour pages 25
Total black and white pages 147
Language eng
Subjects 090409 Wastewater Treatment Processes
030604 Electrochemistry
090703 Environmental Technologies
090404 Membrane and Separation Technologies
Formatted abstract
Reverse osmosis membrane processes are widely used for sustainable water manufacturing, including reclamation from municipal wastewater effluents. Virtually all contaminants are concentrated in 20% of the hydraulic flow in a stream termed reverse osmosis concentrates (ROC). The contaminants may consist of dissolved nutrients, salts, and in particular, recalcitrant organic pollutants (i.e., trace organics). Existing strategies for ROC treatment are energy intensive and at high capital cost due the large amount of chemicals employed and possibly cost of waste sludge disposal. Electrochemical treatment is a promising alternative since the high concentration of chloride ions in the ROC lowers the energy demand of the process and promotes indirect oxidation of organics via electrogenerated active chlorine. However, it may also lead to the formation of more toxic and persistent chlorinated by-products. This thesis focuses on the evaluation of electrode materials, operational parameters and cell configurations for electrochemical oxidation of ROC. Additionally, strategies were investigated in order to minimise the formation of toxic by-products, such as combining electrooxidation and electroreduction processes, controlling the operational pH in electrooxidation and applying electrodialysis pre-treatment of ROC. The results demonstrate that the main mechanism of electrooxidation at mixed metal oxides electrodes was indirect oxidation via active chlorine species. At a current density of 10 mA cm-2, Ti/Pt-IrO2 and Ti/SnO2-Sb were the most effective electrodes in removing chemical oxygen demand (COD) and dissolved organic carbon (DOC). However, high concentrations of formed hazardous trihalomethanes (THMs) and haloacetic acids (HAAs) were also observed for Ti/Pt-IrO2 and Ti/SnO2-Sb due to intense electrochlorination of organics in the ROC. An improved removal of COD and DOC was obtained using a boron-doped diamond (BDD) electrode with 12.5 mA cm-2 of current density at circumneutral pH, mainly due to the increased contribution of hydroxyl radicals (OHâ–ª) to the oxidation of organic matter. Nevertheless, halogenated by-products were continuously formed, even at an extended electrolysis time, (i.e., 10.9 Ah L-1). Electroreduction post-treatment using a carbon cathode at high supplied charge was able to decrease the toxicity of the oxidised ROC. However, a minor release of chloride ions was observed during reduction, suggesting that this decrease was likely achieved by the adsorption of halogenated by-products onto the cathode rather than their dechlorination. In contrast, when the concentration of chloride ions in the ROC was lowered using electrodialysis prior to oxidation, the formation of chlorinated by-products was minimised, i.e., less than 1 μM of THMs and HAAs were detected in electrooxidation of the electrodialysed ROC. From the tested Ti/Pt-IrO2, Ti/SnO2-Sb and BDD electrodes, the latter one was found to be the most efficient in the presence of a low concentration of chloride ions, i.e., 142 mg L-1. In addition, BDD performance was further improved with the addition of sulfate ions due to the generation of persulfate ion and, more importantly, sulfate radicals. Overall, the obtained results highlight the environmental risk of electrooxidation of ROC in both undivided and divided cell configuration due to halogenated by-products even when mitigation strategies are applied. Further research is required on the development of electrode materials and performance of different reactor configurations, optimisation of operational parameters as well as implementation of downstream and/or upstream treatments to ensure a safe application of electrochemical treatment of brine.
Keyword Reverse osmosis concentrates
Electrochemical oxidation
Electrochemical reduction
Dimensionally stable anode
Boron-doped diamond electrode
Dissolved organic pollutants
Halogenated by-products

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Created: Thu, 25 Apr 2013, 14:54:04 EST by Mr Arseto Yekti Bagastyo on behalf of Scholarly Communication and Digitisation Service