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Abstract
Electrical Impedance Spectroscopy (EIS) is a non-destructive, real time monitoring technique that has the ability to detect inorganic and organic foulants on a RO membrane surface.
For the inorganic fouling study, calcium sulphate (CaSO4) and calcium carbonate (CaCO3), which are two typical, scalants observed in RO plants, were used to assess feasibility of EIS to detect inorganic fouling. EIS signals were obtained across a frequency range of 10-1 to 106 Hz and it can be concluded that conductance values obtained at low frequencies (10-100 Hz), co-relates to the environment on the membrane surface. Scale formation was detected before any significant permeate flux decline was observed. Hence, conductance at low frequencies is the most relevant EIS parameter for scale detection in RO systems.
Given the instability of EIS signals at high frequencies, the same approach of measuring conductance at low frequencies was also applied to the organic fouling experiments. Different concentrations of Tryptophan, alginate and humic acid, in the presence of divalent ions (Ca2+) were used to mimic typical organic foulants and were filtered in batch mode till 85% recovery was achieved. EIS was operated throughout the filtration and the conductance was measured across a range of low frequencies.
Based on conductance data at 38Hz, EIS is able to successfully characterize the initial deposition of organic fouling on the membrane. However, results obtained from experiments using higher feed concentrations did not detect significant changes highlighting the limitation in using EIS for monitoring organic fouling formation for feed solutions above a fouling threshold limit. Trends obtained during this study further ascertain the ability of EIS to detect the formation of fouling and to characterize the organic fouling layer during the initial adsorption phase on the membrane surface.
In conclusion, EIS is capable of monitoring inorganic scaling and organic fouling in situ and in real time through the analysis of the electrical properties of the membrane surface. In particular, in systems that experience organic fouling, EIS can identify and determine the composition during the initial stages of organic fouling on the RO membrane. Therefore, proving that EIS is a novel and effective technique that can be applied to RO systems for wastewater treatment.