Mechanisms Involved in Osmotic Backwashing of Fouled Forward Osmosis (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 fouling layer thickness down to 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 fullPWF 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.