Relations Between Biofilm Viscoelasticity and Biofouling Extent of Reverse Osmosis Membranes: Case Study of Cellulose In the EPS Matrix

Biological fouling, mainly bacterial originated, can be an extent acute problem in the operation of seawater desalination facilities even when the water inlet contains low levels of nutrients. Microbial biofilm in general and bacterial biofilm in particular, has destructive influences on the desalination process, including decrease in the permeate water flux, its quality and an increase in frequency of the membrane cleaning. The content of microbial biofilm is living cells protected by extra-cellular polymeric substances (EPS) composed mainly of polysaccharides, proteins and DNA secreted by the microorganisms. The EPS constitutes a scaffold that provides protection on the microenvironment of the organisms: moist, toxins, pH, osmotic pressure and nutrients together with mechanical stability and elasticity. The fouling/biofouling layer that reduces the performance of the reverse osmosis (RO) membrane, the mean of the desalination treatment, is composed mainly by living cells, extracellular matrix, organic matter, colloids and inorganic sediments.

This project aims to investigate in a controlled manner, if changes in the EPS compartments will influence the biofilm-membrane relationship that are in charge for changes in permeate flux and salt rejection. This reduced performance is induced directly by enhanced concentration polarization together with the overall increase in hydraulic resistance caused by the biofilm. A model component that was chosen to be investigated in the EPS matrix of the marine bacterium, Vibrio fischeri, is cellulose. We examine the effects of cellulose, contained in the EPS, on its coherence and viscoelastic properties. Also, we show how the extent of cellulose in the EPS influences its adherence to RO membranes and the consequent reduction in permeate flux and salt rejection during the desalination process in a laboratory scale. We claim that there is a relation between the followings: (i) mechanical strength and elasticity that cellulose provides to the extracellular matrix; (ii) increased adsorption to the membrane surface, and (iii) decrease in membrane performance.

Three different strains of the marine bacterium V. fischeri were used: a wild type, and two isogenic strains, up- and down-regulating cellulose production and secretion.  Biofilm formation on RO membranes was analyzed with confocal microscopy and EPS extraction from a biofilm that was grown on polyamide surface mimicking RO membrane was carried out. Distinguishable differences in the architecture and amount of biofilm on the RO membranes were observed both with rich medium and seawater. In addition, the organic and polysaccharide concentrations have shown a greater amount in the over-expression strain compared to the wild type and to the strain down regulating cellulose production and secretion. EPS adherence and viscoelastic properties were evaluated in a quartz crystal microbalance with dissipation (QCM-D) monitoring technology. While no effect of cellulose concentration on EPS adherence properties was observed, higher cellulose concentration caused a rising trend of the matrix elasticity.  These bacterial strains, altered in cellulose expression also showed interesting changes in biofouling of RO membranes during desalination process. Higher amount of cellulose has shown to reduce permeate flux attributed to increase in the biofilm hydraulic resistance, while in the absence of cellulose, enhanced concentration polarization governs. To our knowledge, this is the first time, both elastic properties and structure of the biofouling layers are shown to directly affect membrane performance. Force interaction between the EPS and the membrane and within the EPS is currently being characterized with atomic force microscope in order to acquire better physical understanding of this fouling system. In all, it seems that there is a high significance of the presence of cellulose in the EPS matrix influencing biofilm mechanical properties, EPS adsorption to the membrane, and biofouling extent.