“Biofouling” is a biofilm problem which is operationally defined. It applies to biofilms which exceed a given threshold of interference. If this is the case, countermeasures are taken. However, conventional antifouling strategies are hampered by some intrinsic problems which can identified as follows: (1) there are no early warning systems, (2) detection of biofouling is not performed on surfaces but by analysis of water samples which will not indicate site or extent of biofilms, (3) disinfection is misunderstood as cleaning, (4) nutrients are not limited although they must be considered as potential fouling biomass, and (5) the efficacy of countermeasures is performed by process performance or product quality. Avoiding mistakes 1–3 and 5 requires monitoring systems which provide relevant information on line, in situ, in real time, non-destructively and suitable for computer-aided automatization. Sample removal, staining or other secondary procedures should not be necessary. A suitable monitoring device is the fibre optical sensor. It consists of a measuring head integrated into the surface to be monitored and contains a sending and a reading optical fibre. If material is deposited on the tip of the fibres, light emitted from the sending fibre will be scattered back by particles and collected by the reading fibre. Increasing deposit accumulation results in an increasing signal of backscattered light. This sensor has been successfully integrated into a water piping system and responded quantitatively to build up and removal of deposits. Under conditions in which the particles were represented by bacteria, a calibration was performed. A stable signal could be acquired above a cell density of 105 cells cm-2. The system is, in principle, suitable to be integrated into membrane modules at representative locations.

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