Development of monitoring tools based on electrical impedance spectroscopy for water treatment and integrated process for wastewater recovery
Ho, Jia Shin
Date of Issue2017
School of Physical and Mathematical Sciences
Singapore Membrane Technology Centre
In this thesis, a real time and non-invasive electrical impedance spectroscopy (EIS) fouling monitor was employed for determination of threshold flux, to monitor biofouling and fouling behavior in a real water reclamation plant. EIS is able to provide reliable information on fouling events in reverse osmosis (RO) and to give insights into the phenomenological events happening in the diffusion polarization (DP) layer during colloidal as well as biofouling. When RO was operated near the threshold flux for silica fouling, the nature of the flowing colloidal suspension of silica on the membrane surface changed. This transition was well-defined and was reflected in the changes of the slope of transmembrane pressure (TMP) with flux and the conductance of the diffusion polarization (DP) layer determined by EIS. The threshold flux increased with increasing crossflow velocity. The effect of the presence of a spacer in the feed channel was also investigated and resulted in a higher threshold flux. The conductance of the diffusion polarization layer (GDP) derived from the low frequency region in the EIS was identified as the most importance parameter for signaling the onset of fouling in this thesis. EIS was also used to monitor the biofilm formation on the membrane surface. In this case, the GDP showed two stages of biofilm formation, first increasing then decreasing. The first stage was related to the accumulation of bacterial cells and the formation of the respiration products from the bacteria. The second stage referred to the accumulation of extracellular polymeric substances (EPS) which was the main component for the formation of the biofilm matrix. The effect of sodium azide which acts as the biostat was also evaluated and its presence slowed down the growth of bacterial on the membrane surface. Sodium azide’s effect was reflected in the normalized GDP plot. The sustainable flux, at which the fouling was minimal, could be estimated from the rate of change of the initial increase of normalized GDP with respect to flux. In order to investigate the applicability of EIS fouling monitor in real plant, it was used to monitor the performance of a RO treatment train in one of the wastewater reclamation plants in Singapore. The EIS measurement system coupled with the canary cell was installed in a side stream of the RO train. The canary cell was operated at the same average hydrodynamic and flux conditions of the spiral wound modules of the RO train. The response of the EIS fouling monitor was found to be correlated well with the plant performance. A maxima was observed from the normalized real impedance of the diffusion polarization layer (Zreal-DP) versus time plot well before several cycles of chemical cleaning. The occurrence of Zreal-DP can be used as an indicator to initiate mitigation measures to control fouling.