01 Process Basics

Aerobic = with oxygen

Anaerobic = without oxygen

• Anaerobic fermented biogas consists of 45-70% CH4 and 25-55% CO2.

• Biological substrate used: Slurry, manure and renewable raw material (silage)

• Slurry is semi-liquid mixture of animal feaces and urine

• Manure is moist/dry mixture of animal feaces with straw, hay or saw dust

Complex, 4 stage process —> microbiologic, chemical and physical influence quantities

Hydrolytic bacteria + enzymes break down into complex compounds (protein, carbohydrates,fats) to simple organic compounds (amino acids, sugar, fatty acids)

—> From original material to simpel organic components

Used bacteria: Hydrolytic bacteria

Acid forming bacteria convert the simple organic compounds to lower fatty acids (propionic acid, butyric acids, CO2, H2) and small amounts of acetic and lactic acids

Simple organic components (amino acids, fatty acids, sugar)

—> Lower fatty acids (propionic acid, butyric acid)

—> Further products (lacticum acidum, alcohols)

Used bacteria: Fermentative bacteria

• Products: Acetic acids and H2 + CO2

• Partial pressure of hydrogen: If hydrogen content is too high it is bad for acetic acid bacteria —> must go into close relationship with methane-forming bacteria (4. stage)

Used bacteria: Acetogenetic bacteria

• Products are formed from products of acetogenesis (acetic acids, hydrogen, CO2)

• Products: Biogas (CH4 + CO2)

• All microorganisms have different requirements (temperature, humidity, air, pH and pressure) —> in all stages —> But 4th stage is the most sensitive

Used bacteria: Methanogentic archeas

• One-step: All steps of fermentation in one digester

• Wet fermentation (dry substrate content 5-12%)

• Advantage: High gas quality, heat value until 20 MJ/m3

• Humidity - Certain levels of humidity is always needed

• Oxygen - Insertion of air cannot be fully avoided for anaerobic fermentation

• Temperature

  • For stable fermentation optimum temperature for mesophil bacteria is 30-45C

  • For fast fermentation optimum temperature for thermophil bacteria is 45-55C

• pH

  • pH level for hydrolyzed and acid-forming is 5-6.3 —> Can tolerate higher pressure

  • pH level for acetic acid-forming and methanogens is 6.5-8 —> more sensitive

  • optimum pH-value for plant is 7

—> Factors for optimization / construction

1. Digester Loading- rate at which substrate is fed into the anaerobic digester

2. Degradation degree- rate to which substrate in digester is broken down by microorganisms

3. Hydraulic Retention time - average time the liquid portion of the substrate remains in the digester

4. Methane production

Formular for calculating dry matter content.

BR is amount of organic dry matter per m³ which is added to the digester daily.

—> The higher the digester loading the lower the retention time

—> Gas yield depends on the retention time

• Minimum time: low time —> bacteria become less —> stop of the process

• Maximum time: Regarding to economic aspects

Floating layers

—> Too dry or too light material rises and forms coherent layers at the top making fermentation process to fall down. Can be prevented by stirring.

Inhibitors

—> Negatively affect digestion process.

Intermediates

—> Compounds that form as byproducts during the multi-step process of anaerobic digestion.

Retardants

—>substances that inhibit the anaerobic digestion process, reducing efficiency and biogas yield.