Influence of organic fouling layer characteristics and osmotic backwashing conditions on cleaning efficiency of RO membranes

Abstract

Fouling remains a prevalent and serious problem in industries using membrane processes.Efforts to mitigate fouling are improving, however, membrane fouling cannot be completely eliminated. Therefore fouling control via development of sustainable cleaning methods are crucial. Despite osmotic backwashing showing promise, little is understood about this cleaning method for removal of fouling from reverse osmosis (RO) membranes. This paper systematically examines how organic fouling characteristics and osmotic backwashing parameters influence cleaning efficiency. Alginic acid was used as a model foulant and numerous microscopy techniques, including confocal microscopy, scanning electron microscopy and atomic force microscopy were used to examine the membrane fouling before and after cleaning to gain a clearer understanding of the mechanisms involved. Increasing CaCl2 concentration in the fouling solution resulted in an increase in fouling layer thickness from 37 to 179 µm, due to the complexation of Ca 2+ and the carboxyl groups in the alginate.Osmotic backwashing efficiency with 0.7 M NaCl decreased as the fouling layer became thicker and the pure water flux (PWF) recovery decreased from 92% to 81%. Osmotic backwashing efficiency also decreased with increasing initial permeate flux, as less fouling was removed: the fouling generated at higher initial fluxes is largely irreversible, resulting in a denser and more compact fouling layer. In an effort to increase osmotic backwashing flux, a CaCl2 draw solution was used, however, the Ca 2+ ions were found to interact with the alginate in the fouling layer, rendering this method inefficient, when compared to NaCl draw solutions which originated similar osmotic backwashing fluxes. Interestingly, the fouling layer was found to swell from 16 µm to 141 µm, when osmotic backwashing was carried out with a NaCl draw solution, followed by contact with a low ionic strength solution used for PWF testing. This phenomenon does not occur to the same extent after backwashing with CaCl2. The same trends were obtained for bovine serum albumin (BSA) fouling, whilst humic acid (HA) did not display any swelling phenomena. However, it showed the same cleaning inefficiency when using CaCl2 as a draw solution.

Osmotic backwashing of forward osmosis membranes to detach adhered bacteria and mitigate biofouling

“…fact that the osmotic backwashing flux is increasing, indicating an interaction between the adhered bacterial cells and the DSobw, even if just for 1 minute. This immediate influence of Ca 2+ was also demonstrated in previous studies with alginic acid fouling on FO and RO membranes [86,87].…”

Progress in Research and Application of Nanofiltration (NF) Technology for Brackish Water Treatment

“…At the same time, the deterioration of water quality accompanied by climate change and population growth further exacerbates this problem [ 2 ]. According to one estimation, 60% of the world’s population will live in water-deficient areas by the year 2025 [ 3 ].…”

Overview of Fouling – An Industrial Jeopardy

“…A negative interaction of the Ca 2+ in the draw solution with the AA fouling layer shows a smaller swelling potential relative to that disclosed to NaCl solutions. The monovalent salts found in the draw solution disperse into the fouling layer, enhancing its ionic strength and thereby the corresponding osmotic pressure [70].…”