Novel permeable catalytic anodic system for enhancing biodegradability of RO concentrate

Abstract

The water scarcity has been increased over time and the wastewater reuse has been considered as one of alternative water sources. The membrane-based process, especially reverse osmosis (RO) process, has been frequently applied as a core technology in wastewater reuse processes, however, a RO concentrate, which contains a high concentration of non-biodegradable organic matters, is produced inevitably. Currently, RO concentrate is being returned to the biological processes and induced the inhibition of microbial activity by the accumulation of non-biodegradable organic matter. Advanced oxidation processes (AOPs) including electrochemical oxidation (EO) has been used to mineralize or to increase the biodegradability of non-biodegradable organic matters in RO concentrate using reactive oxygen species (ROS) such as hydroxyl radicals ($\cdot$OH) and singlet oxygens ($\cdot$O). However, the limited ROS generation on the electrode surface, corrosion of electrode, and the slow mass transfer from bulk to electrode surface have been suggested as tackles to hinder the application of electrochemical oxidation (EO) processes. In order to solve these problems, EO system with permeable catalytic membrane anode was suggested to mineralize and to increase the biodegradability of organic matters in RO concentrate in this research. The conductive hollow-fiber membrane (CHM) electrode was firstly fabricated from multi-walled carbon nanotubes, and TiO$_2$ (TiO$_2$-CHM) and V$_2$O$_5$-doped TiO$_2$ (V$_2$O$_5$/TiO$_2$-CHM) were incorporated into CHM to maximize the ROS generation. The measurement of radical generation via electron spin resonance spectroscopy showed that the methyl radical ($\cdot$CH$_3$), atomic oxygen ($\cdot$O) and hydroxyl radical ($\cdot$OH) were generated from all CHMs and the linear sweep voltammetry (LSV) exhibited that the TiO$_2$-CHM showed the highest radical evolution potentials. The changes in composition, molecular weight distribution and biodegradability of organic matters in RO concentrate were investigated after the electrochemical oxidation. After the application of EO, dissolved organic carbon (DOC) was not decreased significantly, while fluorescence excitation emission matrix (FEEM) exhibited that the characteristics of organic matter was totally changed. The result of molecular weight distribution for each permeate showed that the portion of low molecular weight organic matter was increased in the permeate from CHM with the 2 times elevated oxygen uptake rate (OUR), which suggested the increase in biodegradability of organic matters in RO concentrate. However, the decomposition of carbon nanotube and partial oxidation of organic matter were suggested as the limitations of proposed EO system.