70
5.1.3.2 pH and Alkalinity
McCarty (1964) determined that the biological process of AD works optimally in a
pH environment of 6.6–7.6. However, Yadvika et al. (2004) and Gerardi (2003)
reported that the optimum pH range for AD is at 6.8–7.2. At a pH lower than 5.5,
acidogenic bacteria are still active but methanogenic bacteria are inhibited. Inhibition
of methanogenic bacteria lowers the methane content of the biogas; therefore, low
pH conditions are avoided. During the AD process, pH will usually drop lower than
6.6 if there is excessive accumulation of fatty acids in the acidogenesis stage.
Decrease in pH is caused by either overloading of substrates or because the toxins
in the feedstock inhibit the activity of methanogens. In this case, substrate feed must
be stopped so that acid production will stop or decrease and acetogens and methanogens will be able to degrade excess acid that was produced. Another solution is to
use lime for neutralizing the acid and increase the pH to an optimum range. An
increase in pH (greater than 8.0) also has an inhibitory effect on AD. At pH 9.0, the
methanogenesis process completely stops (Clark and Speece 1971).
Alkalinity is a measure of the capacity of a solution to neutralize acid. Bicarbonate
(HCO 3
−
), carbonate (CO 3
2−
), and hydroxide (OH
−
) are the ions that are used to
increase alkaline conditions in the digester. Alkalinity is also considered as the buffering capacity of a solution and is essential for controlling and maintaining a stable
pH for the anaerobic digestion process. High alkalinity conditions (1500–3000 mg
CaCO 3 L
−1
) enhance the pH stability of the anaerobic digestion process
(Gerardi 2003).
5.1.3.3 Redox Potential
The redox potential is a measure of oxidation capacity or reducing capacity. Biogas
is produced effectively in an anaerobic environment where the redox potential must
be less than −150 mV. Redox always reaches a negative value (less than −100 mV)
under anaerobic conditions (Wiese and König 2009).
In general, the use of substrates including oxygen, nitrate and sulfate promotes
oxidation that can significantly change the redox potential and cause changes in
pH. Redox potential can be used to predict impending changes in pH of the digester
(Wiese and König 2009).
Methane begins to form and CO 2 and H 2 are converted into CH 4 when the redox
value is less than −250 mV; H 2 O and H 2 S are produced when the redox value is less
than −150 mV (Laanbroek 1990).
5.1.3.4 Salinity
High salinity and ammonium concentrations have detrimental effects on biological
processes such as anaerobic digestion (Fang et al. 2011; Chen et al. 2008; Reinhart
and Townsend 1998; Kargi and Dincer 1996). High salt concentrations dehydrate
N. V. C. Ngan et al.
5.1.3.2 pH and Alkalinity
McCarty (1964) determined that the biological process of AD works optimally in a
pH environment of 6.6–7.6. However, Yadvika et al. (2004) and Gerardi (2003)
reported that the optimum pH range for AD is at 6.8–7.2. At a pH lower than 5.5,
acidogenic bacteria are still active but methanogenic bacteria are inhibited. Inhibition
of methanogenic bacteria lowers the methane content of the biogas; therefore, low
pH conditions are avoided. During the AD process, pH will usually drop lower than
6.6 if there is excessive accumulation of fatty acids in the acidogenesis stage.
Decrease in pH is caused by either overloading of substrates or because the toxins
in the feedstock inhibit the activity of methanogens. In this case, substrate feed must
be stopped so that acid production will stop or decrease and acetogens and methanogens will be able to degrade excess acid that was produced. Another solution is to
use lime for neutralizing the acid and increase the pH to an optimum range. An
increase in pH (greater than 8.0) also has an inhibitory effect on AD. At pH 9.0, the
methanogenesis process completely stops (Clark and Speece 1971).
Alkalinity is a measure of the capacity of a solution to neutralize acid. Bicarbonate
(HCO 3
−
), carbonate (CO 3
2−
), and hydroxide (OH
−
) are the ions that are used to
increase alkaline conditions in the digester. Alkalinity is also considered as the buffering capacity of a solution and is essential for controlling and maintaining a stable
pH for the anaerobic digestion process. High alkalinity conditions (1500–3000 mg
CaCO 3 L
−1
) enhance the pH stability of the anaerobic digestion process
(Gerardi 2003).
5.1.3.3 Redox Potential
The redox potential is a measure of oxidation capacity or reducing capacity. Biogas
is produced effectively in an anaerobic environment where the redox potential must
be less than −150 mV. Redox always reaches a negative value (less than −100 mV)
under anaerobic conditions (Wiese and König 2009).
In general, the use of substrates including oxygen, nitrate and sulfate promotes
oxidation that can significantly change the redox potential and cause changes in
pH. Redox potential can be used to predict impending changes in pH of the digester
(Wiese and König 2009).
Methane begins to form and CO 2 and H 2 are converted into CH 4 when the redox
value is less than −250 mV; H 2 O and H 2 S are produced when the redox value is less
than −150 mV (Laanbroek 1990).
5.1.3.4 Salinity
High salinity and ammonium concentrations have detrimental effects on biological
processes such as anaerobic digestion (Fang et al. 2011; Chen et al. 2008; Reinhart
and Townsend 1998; Kargi and Dincer 1996). High salt concentrations dehydrate
N. V. C. Ngan et al.