ANTIFOULING STRATEGIES IN TECHNICAL SYSTEMS - A SHORT REVIEW

The undesired deposition of microorganisms and the formation of biofil ms is called ''biofouling''. The consequences of biofouling for water purification, transport and storage are considerable both economically and ecologically. Countermeasures against biofouling include three st eps: i) detection, ii) sanitization and iii) prevention of biofouling. The detection has to refer to surfaces. Cell counts in water water sa mples do not reflect the location or the extent of biofilms. Biocides display only limited value in terms of removal of biofouling layers. F irst, biofilm organisms are protected against biocides and tolerate 10 to 1000-fold higher concentrations. Second, water systems usually can not be kept sterile. Thus, dead biofilms provide nutrients and suitabl e surfaces for further growth of cells imported with the raw water. Cl eaning of a system is an integral part of sanitization and even more i mportant than disinfection. It has to be based on a designed strategy. Efficiency control is mandatory and has to occur on representative su rfaces. The prevention of biofouling is frequently achieved by continu ously dosage of biocides. However, this is only possible with suitable raw waters and many failures are reported. Chlorine is still the bioc ide most frequently used. Concerns about effectivity and environmental protection give rise to other strategies. ''Good housekeeping'' is re commended as a general countermeasure. It includes frequent cleaning, efficiency control, biofilm monitoring, limitation of nutrients, maint enance of high shear forces, and a cleaning-friendly design. ''Biofoul ing'' is operationally defined and refers to biofilm development which exceeds a given ''threshold of interference''. Keeping biofilm develo pment below that threshold offers a new strategy, which considers biof ouling as a biofilm reactor in the wrong place. Nutrient limitation is an option to curt, biofilm development in sensitive areas. If large c olonization areas are offered elsewhere than in the system to be prote cted, biofilms will develop there, sequestering dissolved nutrients an d turning them into immobilized biomass in a place where it can be han dled more easily than, e.g., inside a heat exchanger or a membrane mod ule. Combined with effective monitoring techniques, a biocide-free ant ifouling-strategy can be realized. Copyright (C) 1996 IAWQ.