Competition for substrate and space in biofilms

Competition for substrate and space in biofilms was studied using a microelectrode technique and a micro-slicing technique. Three different kinds of biofilms were cultured using laboratory-scale, rotating drum biofilm reactors fed with synthetic wastewater. The measured concentration profiles provide direct experimental evidence of the competition in multispecies biofilms for substrates. Increases in organic loading or ammonium-nitrogen loading cause more consumption of oxygen, which results in competition for oxygen between heterotrophs and nitrifiers. Even in a pure nitrification system, heterotrophs, supported by soluble microbial products or metabolic products, could exist in the nitrification biofilm. Nitrifiers, however, have difficulty existing in the heterotrophic biofilms, and their populations were always 4 or 5 orders lower than those of heterotrophs. It was found that the value of criterion for transition between oxygen and ammonium in nitrifying biofilms (S-N/S-DO) was between 0.77 and 1.2, and the value decreased with an increase of glucose loading. The competition for substrate in biofilms resulted in a stratified structure with nonuniform spatial distributions of biofilm properties, such as density, porosity, and effective diffusivity. This stratified structure in turn affects the substrate transfer and substrate competition within the biofilm. It was found that a certain biofilm system may not have only one penetration depth, corresponding to the critical thickness, for the whole range of biofilm thicknesses.