The adhesion of bacteria to inert surfaces and the subsequent biofilm development have received considerable attention. In aquatic environments, adhesion to a surface is thought to provide the colonizing organisms with a supply of nutrients, since inorganic and organic nutrients are
concentrated at this inter face. Furthermore, once they adhere to a surface which is contacted by flowing water, the bacteria are exposed to a high concentration of nutrients
because of a filtration effect. Adhesion also functions to maintain microorganisms in a nutritionally favorable environment in upflow anaerobic sludge blanket and upflow anaerobic sludge bed and filter reactors in which there are no inert substrata available (excluding the walls of the reactor). In this situation, the upflow velocity will select for organisms which can adhere to each other to form well-settling granular sludge. The granular sludge will remain in the reactor and will then be exposed to the continuous supply of nutrients which is injected into the bottom of the reactor.