The undesired deposition of microorganisms and the formation of biofilms is called “biofouling”. The consequences of biofouling for water purification, transport and storage are considerable both economically and ecologically. Countermeasures against biofouling include three steps: i) detection, ii) sanitization and iii) prevention of biofouling. The detection has to refer to surfaces. Cell counts in water water samples do not reflect the location or the extent of biofilms. Biocides display only limited value in terms of removal of biofouling layers. First, biofilm organisms are protected against biocides and tolerate 10 to 1000-fold higher concentrations. Second, water systems usually cannot be kept sterile. Thus, dead biofilms provide nutrients and suitable surfaces for further growth of cells imported with the raw water. Cleaning of a system is an integral part of sanitization and even more important than disinfection. It has to be based on a designed strategy. Efficiency control is mandatory and has to occur on representative surfaces. The prevention of biofouling is frequently achieved by continuously dosage of biocides. However, this is only possible with suitable raw waters and many failures are reported. Chlorine is still the biocide most frequently used. Concerns about effectivity and environmental protection give rise to other strategies. “Good housekeeping” is recommended as a general countermeasure. It includes frequent cleaning, efficiency control, biofilm monitoring, limitation of nutrients, maintenance of high shear forces, and a cleaning-friendly design. “Biofouling” is operationally defined and refers to biofilm development which exceeds a given “threshold of interference”. Keeping biofilm development below that threshold offers a new strategy, which considers biofouling as a biofilm reactor in the wrong place. Nutrient limitation is an option to curb biofilm development in sensitive areas. If large colonization areas are offered elsewhere than in the system to be protected, biofilms will develop there, sequestering dissolved nutrients and turning them into immobilized biomass in a place where it can be handled more easily than, e.g., inside a heat exchanger or a membrane module. Combined with effective monitoring techniques, a biocide-free antifouling-strategy can be realized.

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