Anaerobic Digestion in Four Stages: Hydrolysis, Acidogenesis, Acetogenesis and Methanogenesis

Anaerobic digestion is one of the most important technologies for putting biomass to work. It converts solid organic matter into dissolved organic matter, then converts the energy held in waste into biogas for burning or power generation, recovering both material and energy. The residue it leaves is small and stable, and because the reaction equipment is sealed, odour emission can be controlled. Those properties greatly improve the energy balance of organic waste treatment, which is why the process holds advantages on both the economic and environmental side.

A Long History, Briefly

Biogas production from organic waste has a history of more than a hundred years, though the underlying phenomenon was noticed longer ago. As early as 1630, Van Helmont observed that a combustible gas could be produced during the rotting of organic matter, and found the same gas present in animal intestines. In 1776, C.A. Voltal established that the amount of degradable organic matter and the amount of combustible gas produced were directly related. In 1808, H. Davy detected methane gas in the anaerobic digestion gas of cattle manure. The world's first digestion plant was built in Mumbai, India in 1859, and in 1896 an anaerobic digestion tank for treating domestic sewage sludge was built in the small British city of Exeter, where the biogas lit a street. Surveys suggest there are 6-8 million household or low-technology anaerobic digesters worldwide, with the biogas mainly used for cooking and lighting.

Substrates and the Four-Stage Theory

The organic matter in biomass falls into three categories: carbohydrates, proteins and fats. Carbohydrates are built from C, H and O and include starches, cellulose, polysaccharides and monosaccharides; large sugar molecules break down into small monosaccharides. Proteins are complex organic compounds built mainly from C, H, O and N, and generally containing P, S and other elements; amino acids are their basic units, joined by peptide bonds through dehydration condensation. Fats are built from C, H and O as triglycerides of glycerol and fatty acids, where glycerol is simple but the fatty acids vary in type and chain length.

Different organic matters take different degradation paths, and the four-stage theory divides the whole process into hydrolysis, acidification, acetogenesis and methanogenesis.

Hydrolysis

This stage converts complex solid organic matter into simple soluble monomers or dimers under the action of hydrolytic enzymes. Microorganisms cannot directly metabolize macromolecules such as starch, lignocellulose, proteins and fats; these must first be broken into soluble polymers or monomer compounds before acidifying bacteria can use them. Starch is hydrolyzed into maltose, glucose and dextrin by amylase. Cellulose - a fibre of sugar-glycosidic bonds forming cellobiose and then polymerizing - is hydrolyzed into sugars by the combined action of several cellulases. Because natural cellulose is generally bound with lignin into a highly polymerized state that resists microbial attack, cellulose degradation is one of the rate-limiting steps of biogas fermentation. Protein, under protease action, has its peptide bonds broken to yield dipeptides and polypeptides and then various amino acids. Fat is first hydrolyzed into long-chain fatty acids and glycerol; glycerol is converted into phosphoglycerol under glycerol kinase catalysis, then oxidized into phosphodihydroxyacetone, isomerized into phosphoglyceric acid, and finally enters the glycolysis pathway for complete oxidation.

Acid-producing fermentation

Here soluble monomers or dimers are converted into end products dominated by short-chain fatty acids or alcohols. The hydrolyzed monomers are further degraded into volatile fatty acids, lactic acid, alcohol, ammonia and hydrogen, and secreted outside the cell. Acid-producing bacteria grow fast and tend to produce acetic acid, which yields the most energy for their own growth. The mix of end products depends on the anaerobic conditions, the substrate type and which microorganisms take part. Amino-acid degradation proceeds first through a redox nitrogen reaction to achieve deamination, generating organic acids, hydrogen and carbon dioxide.

Acetogenesis

This stage converts organic acids or alcohols with more than two carbons, produced during hydrolysis and acid production, into small molecules that methanogenic bacteria can use directly: acetic acid, hydrogen and carbon dioxide. Under standard conditions, the hydrogen-producing and acetic-acid-producing steps of organic acid conversion cannot proceed spontaneously; hydrogen inhibits the step, and lowering the hydrogen partial pressure of the system favours product formation. If the hydrogen partial pressure exceeds atmospheric pressure, organic acid concentration rises and methane production is inhibited. Avoiding hydrogen accumulation at this stage is therefore especially important, and in an anaerobic process the reduction of hydrogen partial pressure must be performed by hydrogenotrophic bacteria.

Methanogenesis

In the final stage, strictly obligate anaerobic methanogenic bacteria convert acetic acid, one-carbon compounds, and H2 and CO2 into CH4 and CO2. About 72% of the methane comes from the decomposition of acetic acid, generated by aceticlastic methanogens metabolizing the methyl group of acetate, and the remaining 28% is synthesized from CO2 and H2. Methanogenic bacteria metabolize slowly, so for the anaerobic digestion of soluble organic matter the methanogenesis stage is the rate-limiting step of the whole process.

Parameters That Decide the Outcome

The carbon-nitrogen ratio is the ratio of total carbon to total nitrogen in the raw material, and it has an optimal range in anaerobic digestion, generally from 20:1 to 30:1. Too high or too low affects the fermentation. An inappropriate ratio causes either a large release of ammonia nitrogen or excessive accumulation of volatile fatty acids, and both are important intermediates whose concentration, if wrong, inhibits methane fermentation.

Temperature tolerance is wide, from cold to hot - methanogenic bacteria surviving at extreme cold have even been found in Arctic sewers. Three bands are usually distinguished by microbial activity: cryophilic, from 10 deg C to 20 deg C; mesophilic, from 20 deg C to 45 deg C, usually run at 37 deg C; and thermophilic, from 50 to 65 deg C, usually run at 55 deg C.

pH reflects the acid concentration in the aqueous phase, and anaerobic fermentation bacteria - methanogenic bacteria above all - are extremely sensitive to it. At low pH the growth of methanogenic bacteria is inhibited. Many researchers have studied the optimal pH of the different stages, and the optimum for methanogenic bacteria is about 7.20.

Organic load is the amount of volatile organic matter borne by unit volume of the digestion reactor per unit time, and it is a key design and operating parameter. Its appropriate level depends on the material, the digestion temperature and the process chosen. For easily degradable waste such as vegetables, fruit and kitchen waste, the organic load is generally 1-6.8 kg VS/(m3·d).