3. Biology of anaerobic digestion (after Peter Weiland, Institute of Technology and Biosystems Engineering Federal Agricultural Research Centre, Braunschweig, Germany) Microbiology and biochemistry of anaerobic digestion Physical and chemical parameters Substrates Setting up and managing the process Synthesis and outlook
Microbiology and biochemistry of AD 1 st stage Hydrolysis 2 nd stage Acidogenesis 3 rd stage Acetogenesis 4 th stage Methanogenesis Methanogens Biomass Polysaccharides Proteins Fats Hydrolytic bacteria Sugars Amino acids Fatty acids Fermentative bacteria H 2 /CO 2 Carbonic acids: Acetate Propionate Butyrate Succinate Alcohols Acetogens H 2, CO 2 Acetate Biogas CH /CO 4 2
Microbiology and biochemistry of AD ca. 70% ca. 30%
Microbiology and biochemistry of AD In anaerobic digestion, the microorganisms can only make use of a small part of the chemical energy bound in the substrates they digest. Most of the energy is lost to them as useless biogas. A direct consequence of this low energy yield is the small amount of bacterial biomass which is formed. Another consequence is the small amount of waste heat released: AD is an isothermic process.
Physical and chemical parameters Comparative formation of bacterial biomass during aerobic degradation (with oxygen) and anaerobic degradation (digestion) aerobic degradation anaerobic degradation C fraction (100%) C fraction (100%) biomass biomass
Physical and chemical parameters Generation times of different microorganisms Anaerobic microorganisms Acidogenic bacteria Bacterioides < 24 hours Clostridium 24-36 hours Acetogens 80-90 hours Methanogens Methanosarcina barkeri 5 15 days Methanococcus ca. 10 days Aerobic microorganisms Escherichia coli 20 minutes Belebtschlamm 2 hours
Physical and chemical parameters Medium requirements for the anaerobic digestion of input materials and waste Parameter Hydrolysis / Acidogenesis Methanogenesis Temperature 25-35 C mesophilic: 32-42 C thermophilic: 50-58 C ph 5.2 6.3 6.7 7.5 C:N ratio 10-45 20-30 Total solids < 40 %TS < 30 % TS Redox potential +400-300 mv < -250 mv Nutrients: C:N:P:S ratio 500 : 15 : 5 : 3 600 : 15 : 5 : 3 Trace elements no special requirements essential: Ni, Co, Mo, Se
Physical and chemical parameters Comparison between mesophilic and thermophilic digestion Range of application Mesophilic methanogenesis Range of application Thermophilic methanogenesis
Physical and chemical parameters Inhibiting and toxic factors Inhibitor Inhibiting concentration Comment Oxygen > 0,1 mg/l O 2 Inhition of the methanogens (obligatory anaerobes). Hydrogen sulphide > 50 mg/l H 2 S Inhibitory effect increases as ph decreases. Volatile fatty acids > 2000 mg/l HAc (ph = 7,0) Inhibitory effect increases as ph decreases. The bacteria exhibit high adaptability to this factor. Ammonium-N > 3500 mg/l NH (ph = 7,0) Inhibitory effect increases as ph decreases and temperature rises. The bacteria exhibit high adaptability to this factor. Heavy metals Desinfectants Antibiotics Cu > 50 mg/l Zn > 150 mg/l Cr > 100 mg/l n.d. Only dissolved metals are inhibitory. Deztoxification by precipitation as sulphides. Inhibitory effect is product spezific.
Physical and chemical parameters Inhibition by ammonia Inhibition of methanogenesis by NH 3 (methane formation from acetic acid) Inhibition [%]
Physical and chemical parameters Inhibition by sulphides Inhibition of methanogenesis (methane formation from acetic acid) Inhibition [%]
Physical and chemical parameters Inhibition by carbonic acids Inhibition of methanogenesis Inhibition [%] Acetic acid Propionic acid mg/l acids (undiss.)
Substrates Theoretical gas yield of various classes of substances Gas yield Nl/kg org. TS Fats Proteins Starch
Substrates Composition of the biogas according to the digestion substrate Class of substances Biogas yield [l/kg odm] Methane content [Vol.-%] Calorific value [kwh/m³] Carbohydrates 700-830 50-55 5.0 5.5 Proteins 700-900 70-75 7.0 7.5 Fats 1.000-1.400 68-73 6.8 7.3 Biowaste 350-500 55-68 5.5 6.8 Renewable raw materials 500-700 50-62 5.0 6.2
Substrates Composition of the biogas according to the average oxidation level of the carbon Mean oxidation level of the C oxalic acid formic acid citric acid proteins butyric acid carbohydrates, acetic acid propionic acid fats methanol
Cow manure 25 Pig manure 36 Fodder beet 95 Silage maize 190 Ray grass 110 Biowaste 120 Grease separator residue 400 Waste fat 800 Food waste 240 Substrates Biogas yields of various digestion substrates Biogas yield [m 3 gas/t substrate] Agricultural waste Agricultural raw materials Non-agricultural waste
Synthesis and outlook Knowledge of the microbial metabolic processes is indispensable to operate a biogas plant. Operational stability is only possible if account is taken of the microbial requirements and chemical parameters. Sustainable biogas production is only economically feasible in the long term if the full value can be extrated from the biomass by optimising the digestion process and using all the energy.