1.3 Energy Flow
Oxidation of organic substrates yields large amounts of energy. The electron transport systems provide for a smooth release of this energy. Some energy can be readily
utilized by the microorganisms, and the remaining can be converted into the
phosphate bond energy of ATP for storage. The amount of energy available from
oxidation of substrates depends on the nature of the substrate and on the metabolic
pathways used by an organism. The potential energy available from the complete
oxidation of glucose with molecular oxygen, according to McCarty [6], is 28.7 kcal/
electron equivalent or 688,000) cal/mol of glucose. Conversion of glucose to alcohol
and CO 2 has a potential energy of only 58,000 calories. The alcoholic fermentation
of a molecule of glucose results in the net generation of two molecules of ATP.
Assuming the bond energy of ATP is 7000 cal, it can be calculated that approximately 25% of the potential energy can be tapped for use by the microorganisms. In
the case of complete oxidation, one molecule of glucose yields 38 molecules of ATP
or approximately 266,000 cal. The efficiency, at 40% of the potential energy, is high
compared to mechanical systems.
The potential energy (Gibbs free energy change, ΔG) for a substrate oxidation
reaction can be calculated from published data on free energy for half reactions. The
following is a list of substrates commonly found in wastewater and their potential
energy with complete oxidation in the presence of molecular oxygen:
Substrate
ΔG
Substrate
ΔG
kcal/electron mole
kcal/electron mole
Glucose
À28.7
Glutamate
À26.3
Fructose
À28.7
Butanol
À25.8
Lactose
À28.7
Benzoate
À25.6
Sucrose
À28.7
Butyrate
À25.5
Glycine
À27.l
Propionate
À25.3
Alanine
À26.3
Acetate
À25.3
In an activated sludge process, the substrates in the wastewater are assimilated by
heterotrophic bacteria. Part of the energy is spent for supporting various metabolic
activities. The bacterial population supports the growth of protozoa with a further
energy loss as a result of the predator activities. A similar loss in energy takes place
at the higher level (metazoa as predators). The utilizable energy in the process is
therefore gradually diminishing. Given a long aeration time in the process, more
energy is consumed and a higher effluent quality is obtained as a result.
The chemical transformation brought about by biological treatment processes
involves oxidation-reduction reactions. The general equation for oxidation-reduction
potential is given as:
84
L. K. Wang et al.
Oxidation of organic substrates yields large amounts of energy. The electron transport systems provide for a smooth release of this energy. Some energy can be readily
utilized by the microorganisms, and the remaining can be converted into the
phosphate bond energy of ATP for storage. The amount of energy available from
oxidation of substrates depends on the nature of the substrate and on the metabolic
pathways used by an organism. The potential energy available from the complete
oxidation of glucose with molecular oxygen, according to McCarty [6], is 28.7 kcal/
electron equivalent or 688,000) cal/mol of glucose. Conversion of glucose to alcohol
and CO 2 has a potential energy of only 58,000 calories. The alcoholic fermentation
of a molecule of glucose results in the net generation of two molecules of ATP.
Assuming the bond energy of ATP is 7000 cal, it can be calculated that approximately 25% of the potential energy can be tapped for use by the microorganisms. In
the case of complete oxidation, one molecule of glucose yields 38 molecules of ATP
or approximately 266,000 cal. The efficiency, at 40% of the potential energy, is high
compared to mechanical systems.
The potential energy (Gibbs free energy change, ΔG) for a substrate oxidation
reaction can be calculated from published data on free energy for half reactions. The
following is a list of substrates commonly found in wastewater and their potential
energy with complete oxidation in the presence of molecular oxygen:
Substrate
ΔG
Substrate
ΔG
kcal/electron mole
kcal/electron mole
Glucose
À28.7
Glutamate
À26.3
Fructose
À28.7
Butanol
À25.8
Lactose
À28.7
Benzoate
À25.6
Sucrose
À28.7
Butyrate
À25.5
Glycine
À27.l
Propionate
À25.3
Alanine
À26.3
Acetate
À25.3
In an activated sludge process, the substrates in the wastewater are assimilated by
heterotrophic bacteria. Part of the energy is spent for supporting various metabolic
activities. The bacterial population supports the growth of protozoa with a further
energy loss as a result of the predator activities. A similar loss in energy takes place
at the higher level (metazoa as predators). The utilizable energy in the process is
therefore gradually diminishing. Given a long aeration time in the process, more
energy is consumed and a higher effluent quality is obtained as a result.
The chemical transformation brought about by biological treatment processes
involves oxidation-reduction reactions. The general equation for oxidation-reduction
potential is given as:
84
L. K. Wang et al.