Researchers uncover new membrane fouling mechanism
Researchers have uncovered "an additional reverse osmosis (RO) membrane fouling mechanism" arising from organic compounds present in ultrafiltration (UF) permeate during wastewater desalination.The study by scientists in Israel and Germany found that organic films that form on RO membranes in a membrane bioreactor from organic material in UF permeate irreversibly reduce flux in the RO membrane. But flux reduction by large molecules secreted by microorganisms - extracellular polymeric substances (EPS) - can be reversed.
They proposed that the permanent damage inflicted by UF permeate arose from its arrest of the swelling in the membrane polyamide which the researchers had measured. The swelling effect is crucial to water transport through the membrane, the researchers said.
The scientists ran parallel, fouling experiments with UF permeate and EPS in an RO plate-and-frame unit. They saw reversible changes in the RO permeate flux as pH values were altered from 6 to 8 and back to 6 using pristine and fouled membranes with EPS.
On putting UF permeate-fouled membranes throughout he same pH variation the scientists found "irreversible flux decline" in the RO. The pH change, they said, promoted interaction between the UF permeate organic matter and the membrane to reduce further membrane permeability. They concluded that acids in the permeate, originating from decayed organic matter, were binding irreversibly with the membrane surface: "More studies need to be carried out to analyse the effects of humic acids, abundant in the UF permeate fouling layer, that irreversibly interact with the polyamide surface.
They found that the film deposited from UF permeate enhanced the attachment of bacteria, but had no effect on EPS adsorption.
The swelling changes and film build-up were measured using quartz crystal microbalance with dissipation - a technique that uses shifts in resonant frequency caused by changes in the mass of the membrane.
The work was published in the Journal of Membrane Science.