analysis of a pilot-scale anaerobic baffled reactor treating domestic ...

• Disintegration of composite particulate material into smaller carbohydrate, protein and lipid
fractions. This is usually achieved by mechanical means including agitation or as a result of
dissolution of binding agents in a complex particulate structure.
• Extracellular enzymatic hydrolysis of large molecular weight compounds to long chain fatty
acids, simple sugars and amino acids. Hydrolysis is accompanied by some release of energy
(Batstone et al., 2002).
2.1.1.2 Anaerobic oxidation
Anaerobic oxidation is the process by which long chain fatty acids are oxidised to simple organic acids
(otherwise called volatile fatty acids or VFA). The long carbon chain is sequentially shortened by the
removal of two carbon atoms (an acetate molecule) at each step. The final product of anaerobic
oxidation of fatty acids with an even number of carbon atoms is acetate only; when the fatty acid has
an odd number of carbon atoms, one mole of propionate is produced per mole of substrate.
CH
CH
3
3
( CH )
2
( CH ) COOH + ( n −1)
⋅ H O → CH CH COOH + CH COOH + ( n −1)
⋅ H (n is odd)
2
n
n
⎛ n + 2 ⎞
COOH + n ⋅ H 2O
→ ⎜ ⎟CH
3COOH
+ n ⋅ H 2
⎝ 2 ⎠
(n is even)
2
3 2
⎛ n −1
⎞
⎜ ⎟
⎝ 2 ⎠
3
11
Eq. 2-1
Anaerobic oxidation requires the use of hydrogen ions (H + ) as electron carriers. These may be
assumed to be derived from the dissociation of water as shown in Eq. 2-1. Consequently relatively
large amounts of dissolved elemental hydrogen (H2) are released in this process (Batstone et al., 2002).
2.1.1.3 Acidogenesis
Amino acids and simple sugars are fermented by acidogens or acid formers that produce VFA
including acetic, propionic, butyric and lactic acid. Acidogenesis can occur without an additional
electron donor or acceptor (Gujer and Zehnder, 1983). The VFA end-product of acidogenesis is
determined by the environmental conditions (Mosey, 1983). Different amounts of H2 are produced
during acidogenesis depending on the acidogenesis end-product. Eq. 2-2 shows the production of
acetic acid from glucose by acidogenesis with H + ions acting as electron acceptor.
C H O +
2H O → 2CH
COOH + 2CO
+ 4H
6
12
6
2
3
2
2
Eq. 2-2
This is the thermodynamically preferred reaction since it provides acid-forming bacteria with the
biggest energy yield. Other VFA including propionic, butyric and lactic acids or a combination thereof
may also be produced by acidogenesis, according to the type of micro-organisms present and solution
thermodynamics, largely governed by dissolved H2 concentration (Mosey, 1983). Production of
butyric and propionic acid occurs in response to accumulation of dissolved H2 following high organic
loading incidents, since the H2 yield of these reactions is less (or in the case of propionic acid,
reversed) than the reaction that produces acetic acid; through this self-regulated microbial switching of
biochemical pathways, micro-organisms are able to play an active role in controlling redox potential
(Mosey, 1983).
Acidogenesis of amino acids usually occurs via Stickland oxidation where different amino acids are
fermented pairwise, with one amino acid in a pair acting as electron donor, while the other acts as
electron acceptor (Batstone et al., 2002). There is typically a 10% shortfall of electron acceptor and
2