BIOFOULING - THE ACHILLES-HEEL OF MEMBRANE PROCESSES

Microorganisms in membrane systems tend to adhere to surfaces and to f orm a gel layer called biofilm, which participates in the separation p rocess as a secondary membrane. On the raw water side, it causes an in crease of fluid friction resistance which increases Delta p(feed/brine ). Also, overall hydraulic resistance of the membrane Delta p(membrane ) Can increase due to the biofilm. If these effects exceed a certain t hreshold of interference, they are addressed as biofouling. Countermea sures require a three step protocol: (1) detection, (2) sanitation, an d (3) prevention. Detection has to be performed on the surface as plan ctonic cell numbers released randomly from the biofilm do reflect neit her site nor extent of biofilm growth. The analysis includes microbiol ogical and biochemical parameters; the differentiation between other k inds of fouling such as scaling or organic fouling can be performed by FTIR-ATR spectroscopical analysis. Sanitation should be focused on re moval of the biomass rather than on killing the microorganisms attache d to the surface. First, the slime matrix, consisting mainly of polysa ccharides and proteins, must be weakened. This requires interference w ith the binding forces, which are weak physico-chemical interactions s uch as hydrogen bonds, van der Waals and electrostatical interactions. Then, increased shear forces can remove the biomass. A preventive con cept should acknowledge the fact that biodegradable substances in the water represent the biofouling potential. Biofouling can be regarded a s a ''biofilm reactor in the wrong place''. Reduction of the nutrient content of the raw water can be achieved by a ''biofilm reactor in the right place'', i.e., a biofilter on which microorganisms form biofilm s and sequester the nutrients from the water phase. Mandatory for any optimized antifouling strategy is monitoring of biofilm development; a fiber optical device which provides real-time, on-line, in situ infor mation non-destructively is proposed which can be adjusted to membrane modules.