Biomethanization of mixed organic substances is the effect of coexistence of numerous groups of microorganisms. Methanogenic degradation of such substances involves at least three different trophic groups of anaerobes, namely fermentative heterotrophs, proton-reducing syntrophs and methanogenic archaea. The development of molecular techniques allowed to detect some new groups of bacteria and archea, which often stay unculturable. The cultivation of uncultured organisms is of great significance in recognizing the function of these organisms. In the past few years, newly discovered microorganisms have been successfully isolated from anaerobic sludges, and the information regarding their physiology in connection with phylogeny is updated regularly.

The paper describes the most important factors controlling the process of methanogenesis in the biomethanization technology. It discusses the operational regimes of temperature as well as pH, C/N ratio, the necessity for micronutrients and sensitivity to a number of toxic compounds. Components with an inhibitory effect are characterized as biostatic compounds (ammonia, VFAs, hydrogen sulfide and salinity-inducing substances) and biocidal substances (such as surfactants and pharmaceuticals). The threshold limits of the compounds in question securing the system against disturbances are introduced, as well as the measures counteracting inhibition. Some ways of overcoming the negative impact of environmental factors on the system are presented, including co-fermentation, supplementation of nutrients, removal of ammonia and hydrogen sulfide by different methods and acclimatization of methanogens to inhibitory substances.

The aim of this paper is to show the basic principles of the anaerobic digestion process. All the stages of degradation, such as hydrolysis, acidogenesis, acetogenesis and methanogenesis are characterized. Biodegradable organic matter consists of three main types of substances: carbohydrates, proteins and lipids; the metabolic pathways of their decomposition are described. The last part of the paper presents the co-digestion process, its benefits and technological parameters required to make that process attractive from an economical and environmental point of view.