Mechanisms Involved in Osmotic Backwashing of Fouled FO Membranes

Organic matter leads to one of the biggest problems in membranes: fouling. Developing efficient cleaning processes is therefore crucial. This study systematically examines how alginic acid fouling formed under different physical and chemical conditions affect osmotic backwashing cleaning efficiency in forward osmosis (FO). The fouling layer thickness before and after osmotic backwashing was measured by confocal laser scanning microscopy in order to assess cleaning efficiency, along with pure water flux (PWF) measurements. Osmotic backwashing was found to be very efficient. In the absence of Ca2+ in the feed solution, the alginate fouling thickness was <33 μm. The presence of 2.5 mM Ca2+ in the feed solution promoted the formation of a compact fouling layer, with a thickness of 173 μm. One minute of backwashing using 0.7 M NaCl, fully restored the PWF and reduced the fouling layer thickness down to <6 μm. However, backwashing with less than 0.7 M NaCl was less effective, with 26 μm of fouling remaining, despite a complete PWF recovery. Backwashing also became less effective when the initial membrane fouling flux increased using a draw solution (DS) of 4 M NaCl, with 91 μm of fouling remaining, despite a full PWF restoration. The use of Ca2+ in the osmotic backwashing DS caused the fouling layer to expand and not be removed due to flux reversal and the interaction between the alginic acid layer and Ca2+. A reduction in the PWF recovery was obtained, showing the type of salt used for backwashing has a severe influence on cleaning efficiency.