Living with biofilms - elements of an integrated anti - fouling strategy

Biofouling“ is referred to as the unwanted deposition and growth of biofilms. This phenomenon can occur in an extremely wide range of opportunities ranging from colonization of medical devices, production of ultrapure, drinking and process water, fouling of ship hulls, pipelines and reservoirs. It seriously damages the efficacy of membrane processes by forming a secondary membrane which, at a certain extent of growth, can interfere with the separation process. Countermeasures are usually based on a medical paradigm: if the causative microorganisms are killed, the problem is solved. However, biofilm organisms are much more resistant to biocides, and even if they are killed, they still can cause problems because of their physical presence. Low fouling membrane materials are of uttermost importance. Furthermore, dissolved or particulate substances in the water phase which serve as nutrients for microbial growth must be considered as potential biomass. Monitoring of biofilm accumulation provides early warning capacity and optimization of countermeasures. For a sustainable antifouling strategy, an integrated approach is suggested which includes the analysis the fouling situation, a selection of suitable components of the „anti-fouling menu“ and an effective and representative monitoring of biofilm development. Biofouling: “The biofilter in the wrong place” The term “fouling” has been adapted from heat exchanger technology where it is defined generally as the undesired deposition of material on surfaces (Epstein, 1981), including: Scaling, mineral fouling: deposition of inorganic material precipitating on a surface Organic fouling: deposition of organic substances (e.g. oil, proteins, humic substances) Particle fouling: deposition of, e.g., silica, clay, humic substances and other particles Biofouling: Adhesion of microorganisms to surfaces and biofilm development In the first three types of fouling, the increase of a fouling layer arises from the transport and abiotic accumulation of the material from the water phase on the surface. What is deposited on the surface originates quantitatively from the water. In these cases, fouling can be controlled by eliminating the foulants from the liquid phase. However, this is different in the case of biofouling: microorganisms are pseudo "particles" which can multiply. Even if 99.9 – 99.99 % of all bacteria are eliminated by pre-treatment, a few will enter the system to become protected, adhere to surfaces and multiply at the expense of biodegradable substances. Thus, microorganisms convert dissolved organic material into biomass locally. This is the same mechanism which supports biofilm technology biofouling can be considered as a “biofilm reactor in the wrong place and time”. Substances suitable as nutrients, which would not act as foulants per se, will support fouling indirectly. As most anti fouling measures target at the microorganisms, the role of nutrients as a potential source of biomass is overseen and biocides tend not to decrease the nutrient level. On the contrary, nutrients supplied by the oxidation of recalcitrant organics can support rapid aftergrowth (LeChevallier, 1991). As it is virtually impossible to keep a common industrial system completely sterile, microorganisms on