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	Home Plant construction   > Digestion of organic waste   > Industrial standard biogas plants Engineering and licensing Project development and operation The company Biogas plants   > Working principle of a biogas plant   > Biological processes in biogas plants Links Contact Sitemap		Biological processes in biogas plants Short introduction to biological basics – these are very important for the AD-process! 4-step degradation: Fig. 1: Click picture to enlarge Anaerobic digestion occurs in 4 steps of degradation that are closely linked: Hydrolysis of polymers (carbohydrates, proteins, fats) to monomers (sugars, fatty acids, amino acids) Acidogenesis to volatile (short chain) fatty acids (e.g. acetate, propionate) and alcohols Acetogenesis to acetate, CO2 and H2 and Methanogenesis to Biogas at the same time (i.e. syntrophic)		Methanogenesis is the last step of degradation and it is the most sensitive one! Accumulation of intermediate acids will reduce the pH-value and inhibit methanogenesis. Therefore overloading of the process has to be prevented. Conditions necessary for the growth of methanogenic bacteria are: a temperature of approx. 38°C or 55°C (only slow changes of temperature can be tolerated) absence of oxygen (O2 is toxic) neutral ph (pH < 6.5 will inhibit biogas production completely) sufficient retention time (due to very slow growth of bacteria) unsuitable input material: Wood (lignin cannot be degraded at anaerobic conditions) Mineral oil (note: vegetable oil is suited) Toxic material (e.g. antibiotics) Quantity and quality of biogas Typical composition of biogas: CH4 50-75 % CO2 25-50 % H2S 200-5000 ppm Heating value 5.5-7 kWh/m³ Ignition limit in ambient air 6-12% In theory according to the "Buswell formula" In practise, mainly dry matter content and organic content are considered.