ABSTRACT
Most microorganisms live and grow in aggregates such as biofilms, flocs (“planktonic biofilms”), and sludge. This form of microbial life is described by the somewhat inexact but generally accepted term “biofilm”. The feature which is common to all these phenomena is that the microorganisms are embedded in a matrix of extracellular
polymeric substances which are responsible for the morphology, structure, coherence and physico-chemical properties of these aggregates. Biofilms are ubiquitously distributed in natural soil and aquatic environments, on tissues of plants, animals and man as well as in technical systems such as filters and other porous materials, reservoirs, pipelines, ship hulls, heat exchangers, and separation membranes (Costerton et al., 1987; Flemming and Schaule, 1996). Biofilms develop adherent to a solid surface (substratum) at solid-water interfaces, but can also be found at water-air and at solid-air interfaces. They are composed of accumulations of microorganisms (prokaryotic and eukaryotic unicellular organisms), extracellular polymeric substances (EPS), multivalent cations, biogenic and inorganic particles as well as colloidal and dissolved compounds. EPS are mainly responsible for the structural and functional integrity of biofilms and are considered as the key components that determine the physicochemical properties of biofilm. EPS form a three-dimensional, gel-like, highly hydrated and locally charged biofilm matrix, in which the microorganisms are embedded and more or less immobilized. In technical systems, not only the biological but also the physico-chemical properties of biofilms are of practical importance. For example, if a biofilm has to be removed from a surface, the forces which keep the matrix of the EPS together (cohesion) and attached to the surface (adhesion), have to be overcome. In other words, the mechanical stability of the microbial aggregate plays a key role in cleaning processes, regardless of the viability of the biomass. Compared to the biological and biochemical properties of biofilms, the consideration of the physicochemical properties have been much less in the focus of biofilm research. However, they include such important aspects (examples in parentheses) as frictional resistance (e.g. on ships’ hulls and in water pipes), mechanical stability (in cleaning and for prediction of biofilm sloughing), mass transfer resistance and diffusion (in biofilm reactors), heat transfer resistance (on heat exchangers), hydraulic resistance (on separation membranes), sorption properties, mechanisms and capacity (as a sink and source of pollutants), binding of water (in sludge dewatering and change of surface properties), optical properties (in colouring of surfaces), and tolerance against biocides (in surface disinfection).
