Early stages of RO membrane fouling during tertiary wastewater desalination

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In tertiary wastewater desalination, the effective minimisation and cleaning of membrane fouling requires an understanding of the physicochemical interactions between conditioning film–membrane surface, conditioning film–bacteria, bacteria–EPS, and within the extracellular polymeric substances (EPS).

This Israeli–German study analyses the effect of conditioning film on the attachment of bacteria and EPS (isolated from RO membrane biofouling layers) to the surface, using a quartz crystal microbalance with dissipation (QCM-D), equipped with polyamide-coated sensors.

The conditioning film originates from the membrane bioreactor (MBR) ultrafiltration permeate (UFP), and mainly consists of effluent organic matter (EfOM). This conditioning film is shown to enhance bacterial attachment, but has no effect on the EPS adsorption.

The UFP and EPS were then characterised, and their effects on the swelling behaviour of the model polyamide surface were delineated using QCM-D, by altering pH values from 6 to 8 and back to 6.

In parallel, fouling experiments with UFP and EPS were conducted in a parallel RO plate-and-frame unit. Reversible changes in the RO permeate flux were observed as pH values were altered (from 6 to 8 and back to 6) with pristine and fouled membranes with EPS.

In contrast, irreversible flux decline was observed for the UFP fouled membrane: pH variation, back to 6, promoted further interaction between the UFP organic matter and the membrane, leading to further reduced membrane permeability.

These results are probably related to the swelling phenomenon and its inhibition by the UFP layers observed in the QCM-D. More studies need to be carried out to analyse the effects of humic substances, abundant in the UFP fouling layer, that irreversibly interact with the polyamide surface.

Reduced membrane swelling capability, which is important for water transport, by different foulants is therefore proposed as an additional RO membrane fouling mechanism.

Journal of Membrane Science, Volume 466, 15 September 2014, Pages 26–35.