3.9.1 pH
The optimum pH for AD process is 6.5–7.5 (Mata 2003; Khalid et al. 2011). During
the process, the hydrolysis and acidogenesis take place at lower pH (~5.5–6.5) as
compared to methanogenesis (~6.5–8.2). Generally, in every digester there are two
buffering systems which ensure the optimum pH range: carbon dioxide–hydrogen
carbonate and the ammonia–ammonium. Alternatively, lime, sodium carbonate,
soda ash, and sodium hydroxide are also used for pH adjustments. In the case of
immediate response, the sodium salts are used (Igoni et al. 2007). Addition of alkali
making the enlargement of particulate organics leads to more susceptible enzymatic
attack on the cellular substances.
3.9.2 Temperature
Digester temperature is a vital factor which influences the speed and total amount of
biogas produced (Samah 2016). Normally, three widely used temperature ranges are
psychrophilic (<20
C), mesophilic (15–45
C), and thermophilic (40–65
C).
Microbes are somewhat susceptible to alteration in temperature. The mesophilic
are the most common bacteria used in AD process with varying temperature up to
3
C (Chen et al. 2014); thermophilic consume additional power due to its increased
temperature requirement (Chen et al. 2008). The psychrophilic are the slowest than
others. The biogas production is lower in mesophilic digestion when compared to the
thermophilic digestion.
3.9.3 Organic Loading Rate
The determination of biological transformation in the digestion process is known as
organic loading rate (OLR). In other words, volume of substrate is introduced per
digester volume in a given time. This is another controlling parameter of AD system,
as overloading leads to increased volatile fatty acid concentration can result in
bacterial degradation and system failure. This is the index parameter to declare the
stress enforced on the microbes which also affects the total biogas production, COD
stabilization, and alkalinity. The correct OLR is determined by the content of
biomass and its biodegradability, where higher TS leads to higher OLR. In
industrialized countries, OLRs are in the range of 4–8 kg VS/m
3 reactor per day
for biowaste treatment. OLR is important for continuously stirred digesters. In
non-stirred AD systems, the recommended OLR is below 2 kg VS/m
3 reactor
(Vandevivere et al. 2003).
3 Biogas: An Effective and Common Energy Tool – Part I
91
The optimum pH for AD process is 6.5–7.5 (Mata 2003; Khalid et al. 2011). During
the process, the hydrolysis and acidogenesis take place at lower pH (~5.5–6.5) as
compared to methanogenesis (~6.5–8.2). Generally, in every digester there are two
buffering systems which ensure the optimum pH range: carbon dioxide–hydrogen
carbonate and the ammonia–ammonium. Alternatively, lime, sodium carbonate,
soda ash, and sodium hydroxide are also used for pH adjustments. In the case of
immediate response, the sodium salts are used (Igoni et al. 2007). Addition of alkali
making the enlargement of particulate organics leads to more susceptible enzymatic
attack on the cellular substances.
3.9.2 Temperature
Digester temperature is a vital factor which influences the speed and total amount of
biogas produced (Samah 2016). Normally, three widely used temperature ranges are
psychrophilic (<20
C), mesophilic (15–45
C), and thermophilic (40–65
C).
Microbes are somewhat susceptible to alteration in temperature. The mesophilic
are the most common bacteria used in AD process with varying temperature up to
3
C (Chen et al. 2014); thermophilic consume additional power due to its increased
temperature requirement (Chen et al. 2008). The psychrophilic are the slowest than
others. The biogas production is lower in mesophilic digestion when compared to the
thermophilic digestion.
3.9.3 Organic Loading Rate
The determination of biological transformation in the digestion process is known as
organic loading rate (OLR). In other words, volume of substrate is introduced per
digester volume in a given time. This is another controlling parameter of AD system,
as overloading leads to increased volatile fatty acid concentration can result in
bacterial degradation and system failure. This is the index parameter to declare the
stress enforced on the microbes which also affects the total biogas production, COD
stabilization, and alkalinity. The correct OLR is determined by the content of
biomass and its biodegradability, where higher TS leads to higher OLR. In
industrialized countries, OLRs are in the range of 4–8 kg VS/m
3 reactor per day
for biowaste treatment. OLR is important for continuously stirred digesters. In
non-stirred AD systems, the recommended OLR is below 2 kg VS/m
3 reactor
(Vandevivere et al. 2003).
3 Biogas: An Effective and Common Energy Tool – Part I
91