Organic Waste Recycling

Biofuels production 151

Anaerobic

Organic matter CH 4 + CO 2 + H 2 + NH 3 + H 2 S (4.1)

digestion

In general, anaerobic digestion is considered to occur in the following stages:

1) Liquefaction or polymer breakdown

2) Acid formation, and

3) Methane formation

Figure 4.2 shows the main intermediate compounds formed during anaerobic
decomposition of protein, carbohydrate, and fat. Descriptions of reactions
occurring in each of the three stages are as follows:

Figure 4.2 Anaerobic digestion of organic compound

Stage 1: Liquefaction

Many organic wastes consist of complex organic polymers such as proteins,
fats, carbohydrates, cellulose, lignin, etc., some of which are in the form of
insoluble solids. In this stage, these organic polymers are broken down by
extracellular enzymes produced by the hydrolytic bacteria, and dissolved in the
water. The simple, soluble, organic components (or monomers) which are
formed are easily available to the acid producing bacteria. It is difficult to
distinguish this stage from what is known as stage 2 (acid-formation stage), for

Organic matter

Carbohydrate Simplesugars Volatile fatty acids Acetate

Hydrogen

Carbon dioxide

Long chain

fatty acids

Fat and oil

Liquefaction Acid formation Methane formation

Carbon dioxide

Methane

Amino acids

Ammonia

Bacterial cells

(Acid-formers and

methane formers)

Protein