Influence of permeation on air/water cleaning of spiral wound membrane NF/RO elements

To improve hydraulic conditions, periodic air/water cleaning (AWC) is applied to a nanofiltration (NF) spiral wound membrane (SWM) element under permeation conditions to control biofouling and particulate fouling. A pilot study was carried out for 212 days with a vertically positioned SWM element fed by tap water enriched with a biodegradable compound (60 μg acetate-C l⁻¹). Operational parameters were daily recorded, rinse water was collected and analysed and a membrane autopsy was performed at the end of the experimental run. Normalized pressure drop (NPD) increased as a result of biofouling and particulate fouling, and could be controlled by periodic AWC, while the membrane transport coefficient (MTC) and retention based on the conductivity remained constant. Rinsing water from AWC contained biomass and particulate matter (iron), predominantly during the first few minutes of AWC. Membrane autopsy revealed active biomass and inorganic deposits (mainly iron and copper) at the inlet and outlet of the membrane element. The use of AWC in (vertically positioned) NF/RO SWM elements under permeation conditions improved the control of membrane fouling (especially biofouling and particulate fouling) and did not compromise the integrity of the membrane element (until day 60).