and Frąc 2012). The concentration of VFAs may differ
according to the conditions and design of the digester. The
produced VFAs during this phase will reduce slurry pH. The
bacteria involved in this stage are less sensitive, which
require a pH of 5 and above compared to methanogens
(Kumaran et al. 2016).
3.1.3 Acetogenesis
During the acetogenesis stage, VFAs and alcohols are converted into hydrogen, carbon dioxide and acetate. Long
chain VFAs of more than four carbon chains could not be
used directly by methanogens. The long chains must be first
converted into acetate prior to being converted into methane.
In this stage, acetate will be accumulated into a higher
concentration and will affect the pH value (Wang et al.
1999).
3.1.4 Methanogenesis
Methanogenesis is the most crucial phase in the entire AD
process where methane is produced by the methanogenic
bacteria. Up to 70% of methane formed originated from
acetate while the remaining 30% is produced from the
conversion of hydrogen and carbon dioxide. This stage has
the slowest biochemical reaction among the four stages of
anaerobic digestion. Limitation of methane production may
occur if there is a large entry of oxygen, temperature change,
pH change (optimum pH is between 6.5 and 7.2) or overloading of digesters as the methanogens are very sensitive
towards the environmental conditions (Kumaran et al. 2016).
3.2 Parameters Affecting Anaerobic Digestion
Factors which affect the anaerobic digestion of palm oil
by-products are listed in Table 3, while Table 4 summarizes
selected operational conditions on various anaerobic digestion processes of palm oil by-products. The tables summarize key information of the process such as the types of
feedstock materials, reactor used, application of inoculum,
operating OLR, HRT and their findings. To conclude, the
anaerobic digestion process depends on various feedstock
parameters such as the pH and C/N ratio, as well as on the
processing conditions of temperature, OLR, HRT and mixing. Wide range of research done as shown in Table 4 prove
that anaerobic digestion is a suitable process for palm oil
by-products to treat the wastes prior to disposal while
simultaneously generating value-added products.
3.3 Anaerobic Co-digestion of Palm Oil Mill
Waste Derivative
Lately, the co-digestion method is preferred for treating a
mixture of solid and liquid waste simultaneously which can
increase the efficiency of the overall process by enhancing
the stability of the process and stabilizing the macro and
micronutrient content to sustain microbial growth.
Co-digestion of substrates can give synergistic or antagonistic effects (Labatut et al. 2011). A synergistic effect can be
defined as having cumulative advantage from various
organic matters in the feedstock formulation, for example,
co-digestion of POME with DC can increase the biogas and
methane production compared to mono digestion of POME
alone (Tepsour et al. 2019). In contrast, an antagonistic effect
is a counter-productive effect that can come from pH inhibition, toxicity, highly acidic environment and others when
two or more incompatible materials are mixed together
(Labatut et al. 2011).
Table 4 highlights various studies of palm oil by-products
co-digestion for biogas production. A synergistic effect of
co-digestion can be seen in many feedstock combinations,
for example; EFB and DC (Tepsour et al. 2019), EFB and
POME (Kim et al. 2013) and EFB and POME with chemical
and biological sewage sludge (Suksong et al. 2017). However, the author reported an antagonistic effect at sewage
sludge contents between 6 and 42% in EFB and POME
co-digestion system (Suksong et al. 2017). The positive
effect may be due to the balance of chemical composition or
an increase in microbial activity, while too much of one
component will result in methane reduction. Another successful co-digestion application was POME and cattle
manure digestion. It was revealed that the addition of
ammonium bicarbonate improved biogas production significantly (Zaied et al. 2020). This could suggest that the
addition of alkali compound may enhance the overall stability of the process by balancing the pH from acid accumulation due to microbial action.
Co-digestion is one of the ways to improve the overall
production of biogas. Other than co-digestion, POME pretreatment which consist of POME de-oiling, POME sedimentation, POME pre-hydrolysis, inorganic additive
supplementation, biological additive supplementation and
bioreactor modification may also be used to improve the
overall quality of the biogas and methane production
(Choong et al. 2018). Besides enhancing methane yield,
co-digestion provides an avenue for by-products utilization
132
R. Shamsuddin et al.
according to the conditions and design of the digester. The
produced VFAs during this phase will reduce slurry pH. The
bacteria involved in this stage are less sensitive, which
require a pH of 5 and above compared to methanogens
(Kumaran et al. 2016).
3.1.3 Acetogenesis
During the acetogenesis stage, VFAs and alcohols are converted into hydrogen, carbon dioxide and acetate. Long
chain VFAs of more than four carbon chains could not be
used directly by methanogens. The long chains must be first
converted into acetate prior to being converted into methane.
In this stage, acetate will be accumulated into a higher
concentration and will affect the pH value (Wang et al.
1999).
3.1.4 Methanogenesis
Methanogenesis is the most crucial phase in the entire AD
process where methane is produced by the methanogenic
bacteria. Up to 70% of methane formed originated from
acetate while the remaining 30% is produced from the
conversion of hydrogen and carbon dioxide. This stage has
the slowest biochemical reaction among the four stages of
anaerobic digestion. Limitation of methane production may
occur if there is a large entry of oxygen, temperature change,
pH change (optimum pH is between 6.5 and 7.2) or overloading of digesters as the methanogens are very sensitive
towards the environmental conditions (Kumaran et al. 2016).
3.2 Parameters Affecting Anaerobic Digestion
Factors which affect the anaerobic digestion of palm oil
by-products are listed in Table 3, while Table 4 summarizes
selected operational conditions on various anaerobic digestion processes of palm oil by-products. The tables summarize key information of the process such as the types of
feedstock materials, reactor used, application of inoculum,
operating OLR, HRT and their findings. To conclude, the
anaerobic digestion process depends on various feedstock
parameters such as the pH and C/N ratio, as well as on the
processing conditions of temperature, OLR, HRT and mixing. Wide range of research done as shown in Table 4 prove
that anaerobic digestion is a suitable process for palm oil
by-products to treat the wastes prior to disposal while
simultaneously generating value-added products.
3.3 Anaerobic Co-digestion of Palm Oil Mill
Waste Derivative
Lately, the co-digestion method is preferred for treating a
mixture of solid and liquid waste simultaneously which can
increase the efficiency of the overall process by enhancing
the stability of the process and stabilizing the macro and
micronutrient content to sustain microbial growth.
Co-digestion of substrates can give synergistic or antagonistic effects (Labatut et al. 2011). A synergistic effect can be
defined as having cumulative advantage from various
organic matters in the feedstock formulation, for example,
co-digestion of POME with DC can increase the biogas and
methane production compared to mono digestion of POME
alone (Tepsour et al. 2019). In contrast, an antagonistic effect
is a counter-productive effect that can come from pH inhibition, toxicity, highly acidic environment and others when
two or more incompatible materials are mixed together
(Labatut et al. 2011).
Table 4 highlights various studies of palm oil by-products
co-digestion for biogas production. A synergistic effect of
co-digestion can be seen in many feedstock combinations,
for example; EFB and DC (Tepsour et al. 2019), EFB and
POME (Kim et al. 2013) and EFB and POME with chemical
and biological sewage sludge (Suksong et al. 2017). However, the author reported an antagonistic effect at sewage
sludge contents between 6 and 42% in EFB and POME
co-digestion system (Suksong et al. 2017). The positive
effect may be due to the balance of chemical composition or
an increase in microbial activity, while too much of one
component will result in methane reduction. Another successful co-digestion application was POME and cattle
manure digestion. It was revealed that the addition of
ammonium bicarbonate improved biogas production significantly (Zaied et al. 2020). This could suggest that the
addition of alkali compound may enhance the overall stability of the process by balancing the pH from acid accumulation due to microbial action.
Co-digestion is one of the ways to improve the overall
production of biogas. Other than co-digestion, POME pretreatment which consist of POME de-oiling, POME sedimentation, POME pre-hydrolysis, inorganic additive
supplementation, biological additive supplementation and
bioreactor modification may also be used to improve the
overall quality of the biogas and methane production
(Choong et al. 2018). Besides enhancing methane yield,
co-digestion provides an avenue for by-products utilization
132
R. Shamsuddin et al.