Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Final Activity Report Summary - MEMBIOF (Interactions between water treatment membrane surfaces and biofilms)

The objectives of this project were to bring knowledge and techniques from the study of biofilms in environmental microbiology to bear on understanding how biofilms form on water treatment membranes and how such biofilms can affect mass transfer in the membrane processes through their structure and function.

Much work was done to show that bacterial attachment to synthetic membrane surfaces are strongly affected by roughness (roughest surfaces attract the most), electrostatic interactions (rate of deposition was affected by surface charge), hydrophilic bacteria were much more sensitive to ionic strength effects than hydrophobic bacteria. Hydrophilicity ultrafiltration membrane surfaces coated with hydrogels showed less attachment than the original membranes. There is no noticeable change in rate of attachment from hydrophobic membranes with different extents of hydrophobicity.

Bacteria with motility have higher deposition rates and are more resistant to surface chemical effects (electric double layer repulsion). A compound known to interfere with interbacterial communication was shown to inhibit attachment by physico-chemical effects alone. Antiscalants were shown to enhance bacterial attachment by making RO surfaces more hydrophobic. A theory was developed that allowed the mutual effects of physico-chemical surface forces that operate over 10's of nm to be combined with hydrodynamic forces via a single lumped mass transfer parameter that allows a relatively simple interpretation of experimental data in terms of the impact on this parameter. The basic idea is that the hydrodynamic transport of bacteria to the boundary layer occurs in series with the jumping of particles from the boundary layer to the surface and the tendency for this jump to occur is given by the single lumped parameter.

Biofilm structure and composition:
It was shown that biofilms formed on membranes from tertiary wastewater effluent form a classic mushroom structure, heterogeneous biofilm and that the mass transfer between the mushrooms is enhanced relative to the clean membrane. The presence of antiscalant encourages the formation of biofilm on RO membranes. Biofilm on membranes was more promoted by polycarboxylate antiscalants whereas biofilm on inert supports was more promoted by phosphonate antiscalants. The biological diversity of biofilms formed on hydrophobic membranes used for seawater desalination by membrane distillation, increases significantly as the biofilm develops from its initial to its full grown state. A particular genus - Alteromonas - is particularly prominent as a colonising bacteria.

Mass transfer and biofilms.
The formation and metabolic activity of biofilms on RO membrane surfaces can be monitored by drops in the oxygen content in the permeate relative to the feed solution. Biofilms enhance nucleation rate of mineral scale (gypsum) by 1-2 orders of magnitude. Biofilms have only a slight effect on crystal growth rates of gypsum on RO membranes. There are indications that heterogeneous biofilms on RO membranes actually improve the overall mixing next to the membrane (lower concentration polarization) even though there is local stagnation underneath the point of attachment of an actual macrocolony.

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