(Bio)fouling in Cross-flow Membrane System: Investigating the Role of Transparent Exopolymer Particles (TEP)

There is no doubt that global water consumption in near future will increase exponentially. To meet this upcoming water demand, membrane technologies seem to be one of the most promising ones. Membranes have numerous advantages over other technologies; however, membrane fouling, i. e. the clogging of the pores by organic and inorganic materials and/or the growth of microorganisms, makes these membranes difficult to operate and to be energy inefficient. Fouling slows down the production, requires more operational pressure, cleaning reagents and equipment, and in many cases damages membranes irreversibly. From all fouling types, biological fouling is probably the most difficult to overcome because it involves self developing process - the microbial growth.
Recent discovery revealed the abundance of transparent microscopic sticky substances abundant in natural waters, formed from components excreted by microorganisms (mostly algae). This material has been named transparent exopolymer particles (TEP). It has been observed that these invisible organic substances are usually generated during algal bloom seasons and algae are the main excretor of the TEP. In membrane filtration processes, TEP is suspected to be able to form a sticky layer and facilitate initial attachment of particles and microorganisms on otherwise clean membrane surfaces. The transparent exopolymer particles are therefore recently getting more attention in fouling prevention studies.
This research illustrates how the TEP enhances particular and biological fouling in cross-flow membrane systems (RO and UF). For RO, the studies have been accomplished employing a recently developed tool – membrane fouling simulator (MFS). For UF - a small-scale hollow fiber filtration setup was used. To test the TEP effect on particulate fouling, 1µm polysterene microspheres have been applied. To test the effect on biological fouling, a pure culture of Escherichia coli (ATCC 25922) as well as a mixed natural microbial culture from the water of North Sea were utilised. In order to attain objective results, the experimental plan has been set up to exclude as many external factors that may interfere with the results as possible, leaving the only variance – the presence and the abscence of the TEP. The cultured TEP used in the experiments was extracted from laboratory-grown common strain of seawater diatom Chaetoceros affinis.
To illustrate the effect of TEP on biological fouling, the decline in membrane performance (membrane permeability) over time was monitored and membrane autopsies (TOC, ATP) have been done. The short term tests with pure bacterial culture and the long term tests with natural microbial community have illustrated that the TEP enhances bacterial deposition and growth.
To illustrate the effect of TEP on particulate fouling, the particle concentration that goes in and comes out of the membrane was measured, and deposition of particles on the membranes was evaluated. Surprisingly, it has been shown that the presence of the TEP reduces the deposition of solid non-sticky particles. The backtransport velocity theory was employed to explain this phenomenon.
It has been verified that during the algal bloom periods, TEP should be removed from the feed water of reverse osmosis membranes. Ultrafiltration is an appropriate pre-treatment approach for substantial removal of TEP and minimisation of biofouling in downstream RO system. Moreover, TEP demonstrated flocculant properties on particulate/colloidal materials in the water. Further studies should be carried out to investigate its potential application as a natural coagulant.