Cleaner and Circular Bioeconomy 2 (2022) 100013

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Cleaner and Circular Bioeconomy

journal homepage: www.elsevier.com/locate/clcb

A review on treatment processes of chicken manure

M. Devendran Manogaran a, Rashid Shamsuddin a,∗, Mohd Hizami Mohd Yusoff a, Mark Lay b,
Ahmer Ali Siyal a
a
HICoE-Centre for Biofuel and Biochemical Research, Institute for Self-Sustainable Building, Chemical Engineering Department, Universiti Teknologi PETRONAS, Seri
Iskandar, Perak 32610, Malaysia
b
School of Engineering, Faculty of Science and Engineering, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand

a r t i c l e i n f o a b s t r a c t

Keywords: The poultry industry is a fast-growing industry fuelled by overwhelming customer demand. Of the different
Chicken manure poultry meat options, chicken is arguably the most popular as it is the second most staple food item in Malaysia
Circular economy after rice. Consequently, due to the overwhelming demand for chicken meat, chicken manure is produced in
Composting
abundance. In fact, a chicken produces 80 g to 100 g of manure daily, corresponding to 3-4% of its body weight.
Pyrolysis
Utilizing the raw manure as an organic fertilizer without any prior treatment results in adverse environmental
Gasification
Anaerobic digestion consequences as this common practice acts as a vector for propagation of pathogens, attracting flies and pests as
well as contributing to odour problems. Treatment methods using pesticides, effective microorganisms and daily
collection and disposal have been adopted by the farmers but these techniques are relatively costly and associated
with potential environmental threats. Other techniques such as composting, pyrolysis, gasification, anaerobic
digestion, hydrothermal liquefaction and torrefaction are drawing interest due to their ability to convert waste to
value-added products. Approximately, 77,209 tonnes of chicken manure produced per day in Malaysia in 2014
can potentially generate up to 3.86 million m3 of methane from anaerobic digestion, equivalent to potential
generation of 139.5 TJ of heat or 38.7 GWh of electricity theoretically. This paper reviews the technical and
practical aspects of the techniques mentioned above in terms of operation, performance and limitations. This
paper also examines the preferential treatment techniques in relation to the product outputs with good market
potential while being environmentally sustainable.

1. Introduction globally. The rise in chicken meat intake by Malaysians is attributed

to the versatility of the meat, the relative low cost in comparison to
The global population has increased significantly to 7 billion in 2020 other meat options, the boost in household income and the acceptance
and is foreseen to grow to approximately 10 billion by 2050. Con- of chicken meat by all religions (Ferlito, 2020). As Malaysia is also rich
sequently, the worldwide food demand has been on the rise and by in ethnical diversity with multiple religions and beliefs therefore, the
2050, it is predicted that the food requirement per capita will double pattern of food consumption differs from one religion to another partic-
(Manogaran et al., 2020). The past few years have observed a dietary ularly in terms of meat intake. For instance, pork is prohibited to Mus-
haul in developing nations, being on par with developed nations due to lims and beef is forbidden to Hindus; such boundaries with respect to
enhanced income, growing populations and rapid development which religious beliefs makes poultry meat of high demand amongst the meat
has caused an overwhelming demand for animal protein compared to commodities (Othman and Ruslan, 2020).
grains and other pantry staples (Godfray et al., 2018). Malaysia is one The high demand for chicken meat has resulted in a significantly
of the highest poultry consumers globally and the poultry industry is growing issue of manure treatment (Lee et al., 2017). It is estimated
observed as a vital reservoir for protein meat supply to Malaysians that a chicken produces 80 to 100 g of manure daily which is about
(Nematbakhsh et al., 2021). Of the plethora of protein meat options, 3-4% of its body weight (Abdeshahian et al., 2016). Similar to other
chicken meat has been met with an overwhelming demand. In fact, livestock droppings, chicken manure is a nutrient rich organic waste
chicken meat has been recognized as the second most staple food item that contains substantial amounts of nitrogen, phosphorus and potas-
after rice hence acting as the primary protein source in the Malaysian di- sium and is commonly used untreated as organic fertilizer in agricultural
etary regime (Chijioke et al., 2020). The yearly consumption of chicken fields. However, excessive use of such fertilizers leads to environmen-
per capita in Malaysia is close to 50 kg, which is the first in Asia and third tal pollutions such as deterioration of air quality, increased greenhouse

∗
Corresponding author.
E-mail address: mrashids@utp.edu.my (R. Shamsuddin).

https://doi.org/10.1016/j.clcb.2022.100013
Received 22 February 2022; Received in revised form 24 May 2022; Accepted 31 May 2022
2772-8013/© 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/)
M.D. Manogaran, R. Shamsuddin, M.H. Mohd Yusoff et al. Cleaner and Circular Bioeconomy 2 (2022) 100013

gas (GHG) emissions, accumulation of harmful trace metals, eutroph- ity and the end-products generated based on the waste-to-wealth and
ication in water bodies, soil acidification and enhanced nutrient loss circular economy concepts. Apart from recognized methods like com-
primarily nitrogen and phosphorus from soil due to leaching, erosion posting, pyrolysis, gasification and anaerobic digestion, hydrothermal
and runoff prompted by lack of consideration for the nutrient require- liquefaction and torrefaction are evaluated as well. Although not com-
ments of crops (Zhang et al., 2018). The unpleasant odour triggered by mon for treating animal manure, these methods are utilized for biomass
untreated chicken manure draws the attention of flies, pest and rodents treatment hence, they are incorporated in this review which adds to its
which in addition to propagation of pathogens and antibiotic resistance novelty. This should assist decision makers in selecting the most ade-
results in an apparent threat to human health (Duan et al., 2019). These quate chicken manure treatment method.
factors drive the need to examine alternatives for treatment and use of
chicken manure so it can be used sustainably and economically.
2. Treatment methods of chicken manure
When evaluating waste management strategies, the concept of waste-
to-wealth is one that has been drawing much attention. The objective of
Common treatment methods practiced by farmers for chicken ma-
this concept is to encourage a sustainable way of life where waste recov-
nure are using pesticides as well as daily collection and disposal. The
ery and valorisation is viewed for its inherent benefits to the environ-
practice of direct utilization of chicken manure as an organic fertil-
ment as well as to improve livelihoods, increase job opportunities and
izer for soil conditioning puts the environment at jeopardy due to over-
promote new technologies (Xu et al., 2019). Waste-to-wealth efforts are
fertilization (Tańczuk et al., 2019a). Effective microorganism (EM) sup-
parallel with the circular economy model. The circular economy model
plementation is the addition of mixed cultures of beneficial and natu-
is a progressive shift from the linear economy model which operates on
rally occurring microorganisms as inoculants to chicken manure to im-
the cradle-to-grave flow which capitalizes on single use materials, con-
prove the microbial diversity of soil to which it is applied. One study
tributing to adverse environmental effects. The circular economy model
revealed positive findings on application of EM-treated manure as an
on the other hand, minimizes waste by means of recycling and regener-
organic fertilizer however, the environmental sustainability of this ap-
ating resources which leads to cleaner production. Adopting the circular
proach has yet to be clearly established (Gunawan et al., 2020). Other
economy model will ideally lead to zero-waste and will subsequently
alternatives of treating chicken manure are land application and incin-
generate value chains using renewable energy and natural resources in
eration, which have contributed towards leaching of nutrients into wa-
connected loops instead of resources being consumed and disposed of
terways, malodours, and particulate emissions, which can adversely af-
in linear flows (Lacy and Rutqvist, 2016). Many treatment methods for
fect human health and wildlife (Arena et al., 2015; Maya et al., 2016).
chicken manure yield circular economy outputs; energy being one of
The treatment methods reviewed in this paper are composting, pyrol-
them hence, this calls for a revamp in terms of dealing with chicken
ysis, gasification, anaerobic digestion as well as other relatively new
manure.
approaches. Apart from composting, the other methods are not as com-
In recent years, energy security has been recognized as one of the
mon amongst farmers due to lack of expertise and high capital cost even
main concerns globally due to limited energy supply and its increasing
though they can result in useful outputs rather than only serving as treat-
production cost, challenges in energy generation and growing human
ment method. This review paper can increase the awareness on potential
population that demands for more energy (Zhu et al., 2020). Malaysia’s
treatment strategies for animal manure, specifically for chicken manure
energy source breakdown in the year 2019 observes that up to 94%
that is projected to increase rapidly in tandem with human population.
of energy is derived from fossil fuels. In fact, the nation has devel-
oped a high dependence on fossil fuels for energy generation since 2010
(Jones, 2021). Our continued dependency on fossil fuels gives rise to 2.1. Composting
environmental concerns such as carbon dioxide (CO2 ) and other GHG
emissions. Consequently, such circumstances result in global warming Composting is where the manure is mixed with other organic mat-
and climate change with adverse effects to humankind and the environ- ter and bulking agents to facilitate aerobic microbial breakdown and
ment (Zhang et al., 2019). Energy security relies on dependable access stabilization of organic matter under conditions that expedite the de-
to multiple types of energy sources in sufficient quantities and afford- velopment of thermophilic temperatures to generate an end product
able prices which does not reflect their potential effects on the environ- that is stable, free of pathogens and suitable for land application
ment and economy (Lee et al., 2022). Generating renewable energy from (Akdeniz, 2019). The composting process can be segregated into four
waste sources such as chicken manure contribute economically towards stages. The first stage is hydrolysis of organic matter such as proteins and
energy sustainability. sugars by mesophilic microorganisms which grow in the temperature
This paper reviews the available treatment processes of chicken ma- range of 20-45 °C. This microbial activity heats the compost to approx-
nure, including the important considerations and limitations with re- imately 65-68 °C, during which, the mesophilic microorganisms are re-
spect to the physical and chemical properties of the manure. This is placed by thermophilic microorganisms. Pathogens are also eliminated
as the physical and chemical traits of the manure significantly affect at this stage of the process due to the high temperatures (Tuomela et al.,
the ease of operation and efficiency of the treatment method. For in- 2000). The easily digestible matter is then consumed resulting in the
stance, it has been reported that the moisture content of chicken ma- compost temperature decreasing and fungi proliferate degrading the cel-
nure is high (Singh et al., 2018) hence, the treatment method should lulose, hemicellulose and lignin over a period of 3 to 6 months which
be able to adapt to this. Accordingly, gasification might not be suitable produces a stable humic material (Sánchez et al., 2017). The compost
as feedstock with high moisture content causes feeding and fluidization at the end of the process is often utilized as safe and nutrient-enhanced
problems within the process system (You et al., 2016) whereas, anaer- fertilizer (Li et al., 2020). The degree of composting is dependent on
obic digestion on the other hand adapts well with feedstock of varying the change in temperature of a composting sample when placed in a
moisture content (Ward et al., 2008). Furthermore, to the best of the au- dewar or from measuring the CO2 evolution. Raw compost experiences
thors’ knowledge, there has been no comprehensive review comparing a temperature change of greater than 40 °C and an oxygen demand of
the different treatment strategies for chicken manure specifically in the 900 mg per gram volatile matter per hour, while finished compost ex-
last five years. Review papers on livestock manure or animal manure in perience a temperature change of 0-10 °C and an oxygen demand of 1
general is common while some papers are dedicated to a particular treat- mg per gram per hour (Godlewska et al., 2017). Composting techniques
ment approach. The current manuscript however, delves into different applied include in-vessel systems, aerated or static bins, dynamic con-
treatment methods for chicken manure while evaluating the ease of the tinuous composting and windrows (Sánchez et al., 2017).
process as well as its drawbacks. Additionally, this review also evalu- Factors such as temperature, moisture, porosity, aeration rate, pH
ates the impact of the treatment methods on environmental sustainabil- and C/N ratio affect the microbial community and metabolism which

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M.D. Manogaran, R. Shamsuddin, M.H. Mohd Yusoff et al. Cleaner and Circular Bioeconomy 2 (2022) 100013

Table 1
Conventional operating conditions and output yield relative to pyrolysis process.

Solid Retention Output Composition (%)

Pyrolysis Process Time (s) Heating Rate (K/s) Particle Size (mm) Temperature (°C)
Oil Char Gas

Slow 450–550 0.1–1 5–50 277–677 30 35 35

Fast 0.5–10 10–200 <1 577–977 50 20 30
Flash <0.5 >1000 <0.2 777–1027 75 12 13

significantly affect the composting process and quality of compost pro- this period, components in the vapour phase continuously react with
duced (Wang et al., 2018). Moisture needs to be maintained at around each other, favouring solid char formation in addition to other liquids
50-60% wet basis, C/N ratio between 25 and 30, pH between 5.5 and (Al Arni, 2018). The drawback of this approach is its inadequacy to syn-
9, temperature between 55 and 63 °C and oxygen content greater than thesis high quality bio-oil. High retention time results in cracking of the
5%. The pile needs to be bulky to allow ready access of air and have suf- primary product in the process and which often jeopardises the produc-
ficient water holding capacity without blocking the air space and pores tion of bio-oil. Moreover, a significantly lower hear transfer and longer
(Yu et al., 2015). Chicken manure has low porosity, high moisture con- retention time calls for additional energy input (Selvarajoo, 2021).
tent, low C/N ratio and high pH which is overcome by adding high C/N Fast pyrolysis on the other hand occurs by rapid heating of the
ratio bulking agents such as rice husk, wood chips and sawdust which biomass to a high temperature in the absence of oxygen. The critical
increases the C/N ratio, pile porosity and aeration channels while simul- traits of this process are its closely monitored reaction temperature,
taneously reducing the water content (Zhang and Sun, 2016). rapid vapour retention time and swift cooling of vapours and aerosol
Addition of compost to soil enhances the organic matter content in particularly favourable for rich bio-oil production (Al Arni, 2018). Fast
soil as well as the soil structure and agricultural productivity due to pyrolysis is garnering much attention due to its potential in producing
the additional nutrients in the compost and presence of plant growth- liquid fuels and a variety of commodity and speciality chemicals. The
promoting organisms (Luo et al., 2017). Consequently, addition of liquid product is economically transported with ease and stored along
compost aids food security. More notably, replacing synthetic fertil- with effective handling of the solid biomass from utilization. Fast pyrol-
izer with compost improves soil biodiversity and reduces environmen- ysis also requires comparatively low investment cost and high energy
tal risk of nutrient leaching (Pose-Juan et al., 2017). Composting con- efficiency compared to other processes, particularly on a small scale
tributes favourably to pollution control (Uyizeye, 2019), bioremediation (Venderbosch and Prins, 2010).
(Ventorino et al., 2019), weed control (Zubair et al., 2020) and plant dis- Flash pyrolysis on the contrary, is a favourable process to produce
ease control (Pane et al., 2019). Composting organic waste compared to solid, liquid and gaseous fuel from organic matter enriched biomass
landfilling prevents ground water from being polluted due to leachate which can yield up to 75% bio-oil. Basically, the process is described
from landfilling organic matter (Ayilara et al., 2020). Stable compost as rapid devolatilization in an inert atmosphere indicating a high heat-
is also much easier to handle in terms of storage and transport com- ing rate of the organic matter and a very narrow gas retention time
pared to raw organic waste which in this case is raw chicken manure which is less than 1 s to be exact in a high temperature ambience in
(Akdeniz, 2019). the range of 450–1000 °C (Patel et al., 2020). Despite the many bene-
However, composting is a time consuming process, requiring 3 to 6 fits of flash pyrolysis, there are notable technological constraints which
months to produce a mature compost, hence the footprint of the site include unsatisfactory thermal stability, presence of solids in the bio-oil
is quite large. Odours can be generated from composting piles if the synthesized, corrosiveness of the oil, enhanced viscosity of the oil over
C/N ratio, aeration and water is not managed well, resulting in the piles time due to catalytic action of char, alkali concentrated in the char dis-
generating ammonia at low C/N ratios as excess nitrogen is volatilised solves in the oil and production of pyrolytic water (Gupta et al., 2021).
(Pardo et al., 2015) or the piles become anoxic at low oxygen content There are multiple pyrolysis reactor designs such as fixed bed
resulting in fermentation products such as alcohol and leachate. Addi- reactors, fluidized-bed reactors and ablative reactors (Ore and Ade-
tionally, pathogens that are able to tolerate high temperature conditions biyi, 2021). All these different reactors have a defined feedstock size
can sporulate and present a potential health hazard if the compost is de- limitation to ensure functional heat transfer and trouble-free operational
rived from food crops (Ayilara et al., 2020). performance. Accordingly, it is essential to prepare the feedstock which
in this case is chicken manure in the suitable size. This is often done by
2.2. Pyrolysis mechanical operations such as grinding and cutting. The chicken ma-
nure needs to be dried as well such that the moisture content is unmis-
Pyrolysis is described as the thermal decomposition of biomass in the takably below 10 wt%. This stage is of upmost importance as is prevents
absence of oxygen resulting in generation of bio-oil, solid biochar and adverse implications of moisture on the pH, viscosity, stability, corro-
non-condensable gas products. The process is particularly intricate and siveness and other liquid properties of the final product. Although in-
comprises of both simultaneous and successive reactions when organic troducing mechanical operations and drying enhances the liquid prod-
matter is heated in a non-reactive atmosphere (Kan et al., 2016). Pyrol- uct attained from the process, it increases the production cost as well
ysis can be segregated into three primary categories which are slow, fast (Hong et al., 2020). After subjected to mechanical operations and dry-
and flash pyrolysis as depicted in Fig. 1. ing, the feedstock is fed into the pyrolysis reactor of choice. The char
The different types of pyrolysis are distinguished with respect to the produced in the reactor emulates the duty of a vapour cracking cat-
operating parameters which are inclusive of the heating rate, process alyst. Cyclones are often incorporated into the process flow to sepa-
temperature, biomass particle size and solid retention time. The rela- rate the char from the reactor after the pyrolysis process has been com-
tive distribution of products generated is also essentially correlative to pleted to achieve complete char removal. However, often times, some
the type of pyrolysis and its operating variables as depicted in Table 1 small char particles pass through the cyclones and ends up mixed in
(Hu and Gholizadeh, 2019). the liquid product. After the solids have been separated, the vapours
Slow pyrolysis is often used to enhance char yield given a low heating and gases are quenched quickly to prevent continual cracking of the or-
rate and low temperature setting. This is because its vapour residence ganic molecules. Pyrolysis liquid condensers are normally utilized for
time is significantly higher in comparison to the other two variations quenching of the vapours in which the vapours are cooled directly with
of pyrolysis, anywhere between five minutes to half an hour. During the bio-oil or a hydrocarbon liquid (Xin et al., 2021).

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M.D. Manogaran, R. Shamsuddin, M.H. Mohd Yusoff et al. Cleaner and Circular Bioeconomy 2 (2022) 100013

Fig. 1. Different types of pyrolysis processes

and the products synthesized (Lee et al., 2019).

The primary products acquired from biomass pyrolysis are bio-char, pose an apparent hazard to environmental sustainability as well as hu-
pyrolytic gaseous species and vapours which condense to liquid prod- man health (Mustafi et al., 2006).
ucts at ambient temperature (Hu and Gholizadeh, 2019). Compositions
of pyrolysis product can be enhanced as follows (Uddin et al., 2018): 2.3. Gasification
bio-char – lower temperature and heating rate procedure liquid prod-
ucts – lower temperature setting but higher heating rate procedure py- Gasification is the thermochemical conversion of carbon rich feed-
rolytic gaseous species – higher temperature setting and lower heating stock into combustible product gas utilizing gasifying agents such CO2
rate procedure (Widjaya et al., 2018). Gasification consists of four stages as shown in
Bio-oil, a dark brown, viscous organic liquid is the primary yield Fig. 2 which are drying, devolatilisation, also known as pyrolysis, com-
of interest from pyrolysis. It is mainly constituted of oxygenated com- bustion and reduction.
ponents which result in high thermal insecurity and low heating point The drying stage necessitates the evaporation of free and bound wa-
(Hu and Gholizadeh, 2019). Another undesirable trait of bio-oil is its ter in the feedstock by heat often supplied by exothermic reactions in
low pH value in the range of 2–3.7 because of the presence of carboxylic the subsequent stages. The temperature is normally between 100 °C and
acids. Consequently, bio-oils are more likely corrosive to common struc- 200 °C which satisfies the fundamental function of this stage in the over-
tures and highly instable during storage attributed to its ongoing chem- all process without thermally decomposing the feedstock. This is be-
ical reactions for instance, etherification, esterification and polymeri- cause the temperature condition does not meet the mark to execute such
sation to form larger molecules (Kan et al., 2016). Much attention has heavy duties (Patra and Sheth, 2015). The drawback of this stage is the
been drawn to means of enhancing the yield of bio-oil qualitatively and emission of certain air pollutants such as volatile organic compounds.
quantitatively with researches emphasizing on reactor designs and con- Nonetheless, the inclusion of this step is significant in the case of a high
figurations taking precedence over other operational parameters. Up- moisture content feedstock. The reason behind this would be that the
grading of bio-oil is imperative prior to practical utilization in engines drying stage prevents feeding or fluidization problems such as agglom-
(Uddin et al., 2018). For instance, transportation liquid fuel can be syn- erate formation and jamming which are often associated to feedstock
thesized from bio-oil through upgrading by means of high-pressure hy- with high moisture content such as chicken manure. In the absence of
dro processing (Elliott et al., 2012) and catalytic cracking technologies the drying step, the reduced heating value of the product gas adversely
(Ibarra et al., 2019). Bio-char on the other hand, is extensively recog- effects the overall energy efficiency of the gasification reaction. Such
nized for its efficacy as soil amendment attributed to its enriched plant circumstances yield a significantly enhanced tar content in the product
nutrient content, aiding carbon sequestration which effectively allevi- gas due to the declining reaction temperature (You et al., 2018). Essen-
ates atmospheric carbon (Vilas-Boas et al., 2021). tially, the drying rate is controlled by the heat and mass transfer between
The pyrolytic gaseous species, often recognized as syngas constitutes feedstock particles and their ambient atmosphere corresponding to the
of primarily hydrogen (H2 ) and carbon monoxide (CO) with minute temperature difference, particle surface area, moisture and convection
quantities of water (H2 O), nitrogen (N2 ), (CO2 ) as well as hydrocarbons velocity of surrounding flows as well as diffusivity of moisture within
such as methane (CH4 ), ethylene (C2 H4 ), ethane (C2 H6 ), tar and ash feedstock particles and moisture (Arima et al., 2018).
with respect to the feedstock material and pyrolysis operating param- The consecutive stage is devolatilisation, otherwise regarded as py-
eters. The potential of syngas for utilization as an alternative fuel for rolysis which has been discussed in Section 2.2. The essence of this stage
industrial combustion processes and internal combustion engines has is further degradation of the feedstock particles into volatile matter and
been evaluated for application such as transportation and power gener- carbonaceous solid residue, also known as biochar under elevated tem-
ation (Honus et al., 2018). However, a study on a single cylinder spark perature conditions in the absence of oxygen (Wani et al., 2020).
ignition engine utilizing syngas as fuel observed a drawback in terms of The next stage is combustion which encompasses of complete or par-
the emission of nitrogen oxides (NOx ) emission and such circumstances tial oxidation of carbonaceous output and certain gas species yield from

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M.D. Manogaran, R. Shamsuddin, M.H. Mohd Yusoff et al. Cleaner and Circular Bioeconomy 2 (2022) 100013

Fig. 2. Consecutive stages of gasification (Safarian et al., 2019).

pyrolysis. The products of the combustion reaction are often H2 O, CO2 , biochar that functions as the reactant for the Boudouard reaction and
CO and H2 . This strongly exothermic reaction often occurs between water gas reaction during the subsequent stage as well (You et al., 2018).
700 °C and 1500 °C (Patra and Sheth, 2015) and is responsible for sup- The next stage is the reduction stage whereby the temperature
plying gasifier heat required in the subsequent reduction reaction as well range is approximately 800-1000°C (Janajreh et al., 2021; Patra and
as the drying and pyrolysis stages of the process which are endothermic Sheth, 2015). The reduction stage is a platform for the biochar to react
in nature (Janajreh et al., 2021). The primary reactions taking place at with H2 O, CO2 and H2 from the previous stage to generate a mixture
this stage are expressed as follows; of combustible gases constituting of CO, CO2 , H2 , CH4 as well as light
1 hydrocarbons for instance, acetylene and ethylene. Generally, the reac-
Carbon partial oxidation ∶ C(s) + O (g) → CO(g)(ΔH = −111kJ∕mol) (1) tion of biochar with H2 O occurs faster in comparison to the reaction of
2 2
biochar with CO2 (Cao et al., 2020). The primary reactions taking place
Carbon complete oxidation ∶ C(s) + O2 (g) → CO2 (g)(ΔH = −394kJ∕mol)
are as depicted in Eqs. (4)–(7);
(2)
Boudouard reaction ∶ C(s) + CO2 (g) ↔ 2CO(g)(ΔH = +172kJ∕mol) (4)
1
Gas oxidation ∶ H2 (g) + O2 (g) → H2 O(g)(ΔH = −242kJ∕mol) (3)
2
Water gas reaction ∶ C(s) + H2 O(g) ↔ CO(g) + H2 (g)(ΔH = +131kJ∕mol)
1
CO(g) + O (g) → CO2 (g)(ΔH = −283kJ∕mol) (5)
2 2
Water gas shif t reaction ∶ CO(g) + H2 O(g) ↔ CO2 (g)
1
CH4 (g) + O (g) → CO(g) + 2H2 (g)(ΔH = −35.7kJ∕mol)
2 2 + H2 (g)(ΔH = −41kJ∕mol) (6)
The oxidation of volatile gas species as illustrated in Eq. (3) occurs
Methane reaction ∶ C(s) + 2H2 (g) ↔ CH4 (g)(ΔH = −75kJ∕mol) (7)
rapidly resulting in a large portion of the oxygen being utilized be-
fore diffusing to the surface of the biochar, often preventing biochar From an environmental conservation point of view, there has been
oxidation as expressed in Eqs. (1) and (2). In the circumstance of a fruitful evidence in favour of gasification in comparison to other con-
sub-stoichiometric oxygen condition, partial oxidation of carbon occurs ventional methods of disposing chicken manure particularly in terms of
dominantly, yielding CO as described in Eq. (1). The steam generated net GHG emissions. A life cycle assessment (LCA) drawing comparison
during this stage of the process acts as the reactant for the water gas re- between the net GHG emissions when disposing of manure using the
action and water gas shift reaction which will occur during the consecu- common approach, land application and gasification observed signifi-
tive stage. Furthermore, oxygen deficiency will also result in remaining cant GHG emissions reduction using the latter approach as compared to

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M.D. Manogaran, R. Shamsuddin, M.H. Mohd Yusoff et al. Cleaner and Circular Bioeconomy 2 (2022) 100013

Table 2 chemicals can be synthesized by either catalytic conversion or anaerobic

GHG emission with respect to the LCA framework for land application scenario. fermentation; one of which is H2 . H2 generation can also be significantly
kg CO2 -eq per tonne enhanced by syngas reforming through catalysed reaction for instance,
Life cycle stage dry feedlot manure Mass (%) steam reforming of CH4 and higher hydrocarbons as well as the wa-
ter gas shift reaction utilizing Fe and Ni catalysts at the temperature
Manure collection 0.99 0.34
Transportation 0.64 0.22 range of 200-1100 °C and pressure range between 1 bar and 30 bar
Spreading 1.90 0.64 (Chianese et al., 2016).
Manure emissions 292 98.80
Displacing fertilizer utilization −177 100
2.4. Anaerobic digestion
Net emissions 119 NA

Anaerobic digestion (AD) exploits several groups of bacteria and

Table 3 substrates, exhausted under strict anaerobic conditions specifically,
GHG emission with respect to the LCA framework for gasification scenario. oxidation–reduction potential under 200 mV to convert organic material
kg CO2 -eq per tonne into biogas, mostly constituting of CH4 and CO2 . The process consists
Life cycle stage dry feedlot manure Mass (%) of four pivotal stages which are fermentation (hydrolysis), acidogenesis,
Manure collection 0.99 1.35
acetogenesis and methanogenesis as portrayed in Fig. 3. Each consecu-
Manure transportation 1.02 1.38 tive degradation stage is facilitated by a different class of microorgan-
Manure emissions 46.90 63.70 ism, which partially act in syntrophic interrelation under distinct ambi-
Gasification plant emissions 23.40 31.80 ent conditions (Bharathiraja et al., 2018).
Biochar transportation 1.03 1.40
Hydrolysis is the stage during which complex, insoluble organic ma-
Biochar spreading 0.22 0.29
Avoided electricity generation −545 76.10 terials such as lipids, carbohydrates, proteins and nucleic acid are hy-
Carbon sequestration −143 20.00 drolysed into basic water-soluble monomers by exoenzymes excreted
Biochar effects on agronomy −28.40 3.96 by hydrolytic bacteria such as cellulase, cellobiase, xylanase and li-
Net emissions −643 NA pase. The monomers which are the end product of hydrolysis experi-
∗
Negative net emission value address GHG emission mitigation and vice versa. ence acidogenesis by another facultative and obligatory anaerobic bac-
teria after which these monomers are degraded even further into short-
chain organic acids, alcohols, H2 , CO2 and volatile fatty acids (VFAs)
the former. Land application decreased GHG emissions by taking over (Bharathiraja et al., 2018). The concentration of the intermediately
the application of mineral fertilizers for crop cultivation. However, it is formed hydrogen ion has a significant effect on the variety of prod-
vital to note that the surplus in GHG emissions abatement is exclusively uct formed. The products from the acidogenic stage play an imperative
attributed to this merit only in the case of land application. Gasification role as substrate in the acetogenic stage whereby homoacetogenic mi-
on the other hand, notably reduced GHG emissions through three major croorganisms continuously reduce exergonic H2 and CO2 to acetic acid
aspects which are avoided electricity generation, carbon sequestration (Kremp et al., 2018). During this stage, organic acids and alcohols are
and the consideration of biochar effects on agronomy. Overall, the net converted into acetate as well and this is crucial to note as acetate acts as
GHG emissions for one ton of dry feedlot manure using land application a substrate for methane-forming bacteria. Moreover, past studies have
and gasification is 119 and −643 kg CO2 -eq, respectively observing the also observed that acetogenic bacteria nourish in a symbiotic relation-
apparent edge of the latter as shown in Tables 2 and 3 (Wu et al., 2013). ship with methane-forming bacteria (Mutungwazi et al., 2021). This is
Other conventional means of disposing chicken manure include in- relevant for the last stage of the AD process which is the methanogenesis
cineration and combustion which also have significant environmental which involves two groups of bacteria which are the acetotrophic bac-
drawbacks as compared to gasification. Incineration for instance, re- teria that reduces acetate into CH4 and CO2 (Szuhaj et al., 2016) and
sults in 4 times more emissions of sulfur dioxide, SO2 than gasifica- the hydrogenotrophic methanogens that consume H2 to produce CH4
tion which lowers the pH of air consequently resulting in air acidifica- (Bharathiraja et al., 2018).
tion. Furthermore, opting for gasification to dispose of chicken manure There are a few variables that exhibit apparent effects on the effi-
results in 33% less NOx emissions, observing notably less smog pro- ciency of the AD process, directly influencing the biogas yield as well.
duction (Maya et al., 2016). Another study comparing gasification and Many studies in the past have stressed on the crucial impacts of tem-
combustion discovered that PM2.5 emissions of the gasification process perature on the microbial community, process kinetics as well as sta-
was 3 times lower than that for the combustion process because of the bility and methane yield. Psychrophilic temperature conditions for AD
lower emission of NOx compounds. Gasification also observed better have observed deterioration in microbial growth, substrate utilization
performance in the non-carcinogens impact category as the vinyl chlo- rates and biogas production in past studies (Xu et al., 2022). Ther-
ride equivalent production was 54 times lower than combustion pro- mophilic temperature setting on the other hand reduce biogas yield
cess (Arena et al., 2015). On the other hand, there are arguable en- contributed by the production of volatile gases such as ammonia which
vironmental concerns with respect to the gasification process. Gasifica- strains methanogenic activities. Accordingly, past studies suggested that
tion can produce many inorganic and organic contaminants for instance in the case of chicken manure as the sole feedstock for AD, mesophilic
NH3 , hydrogen sulfide (H2 S) and tar (Islam, 2020). These pollutants can temperature is more favourable in terms of stability of biogas generation
significantly affect the post-stream applications of the product gas and (Bi et al., 2019; Yin et al., 2018).
cause consequential concerns such as pipeline clogging, catalyst deacti- Another factor affecting the AD process is pH. The pH adequate
vation, conversion efficiency reduction and adverse emissions to the en- for hydrolysis is pH 4.0-5.0 whereas for acidogenesis and methano-
vironment (Tańczuk et al., 2019b). With respect to these circumstances, genesis are pH 5.0-6.5 and pH 6.8-7.5 respectively (Chatterjee and
adopting the gasification technology also requires cost allocation for gas Mazumder, 2019). Chuenchart et al. (2020) has observed that optimum
cleaning and conditioning. pH range for AD of chicken manure is pH 6.5–7.5 however, the pH of
The mixture of combustible gases, predominantly rich in H2 , CO, chicken manure falls short as it has been reported to be 6.1 (Singh et al.,
CO2 and CH4 with minute amounts of low molecular weight hy- 2018) hence, there is a need to adjust it prior for optimum biogas gen-
drocarbons such as C2 H6 and C3 H8 is often recognized as syngas eration.
(Janajreh et al., 2021). The potential of generating heat and electric- Furthermore, another parameter which affects the biogas yield dur-
ity from syngas has been well established (Cao et al., 2020). The quality ing the AD process is the organic loading rate (OLR). OLR is described
of syngas is also often time enhanced such that value-added biofuels and as the amount of volatile solids (VS) or chemical oxygen demand (COD)

6
M.D. Manogaran, R. Shamsuddin, M.H. Mohd Yusoff et al. Cleaner and Circular Bioeconomy 2 (2022) 100013

Fig. 3. Different stages of AD for biogas generation (Shamsuddin et al., 2021).

compounds fed daily per unit digester volume (Chandra et al., 2012). rated as a co-substrate in the AD process to enhance the C/N ratio. An ad-
Higher OLRs can lessen the digester’s size and as the result of this, the equate C/N ratio results in prolonged protein solubilization rate which
capital cost can be reduced as well however, the retention time should induces low total ammonium nitrogen (TAN) and free ammonia nitro-
be sufficient for AD to take place for biogas production (Jahn et al., gen (FAN) concentrations within the system. Hence, ammonia inhibition
2020). The activity of methanogens can fluctuate considerably with re- can be prevented by feeding the anaerobic digester with raw material
spect to carbon requirements and growth feedback to organic additions. which exhibits an optimal C/N ratio. However, an excessively high C/N
However, the addition of large volumes of new material on a daily basis ratio deprives the system of elemental nitrogen to sustain cell biomass
may cause alterations in the digester’s environment and briefly inhibit and causes accelerated nitrogen decomposition by microbes which leads
the bacterial activity during the premature stages of hydrolysis. This to significantly reduced biogas production. A system with an extremely
bacterial inhibition which may result in disruption to the AD process low C/N ratio on the other hand faces the risk of ammonia inhibition
may occur due to the significantly high OLR leading to higher hydrol- whereby this circumstance has adverse effects on methanogens leading
ysis and acidogenesis bacterial activity than methanogenesis bacterial to inadequate application of carbon sources (Hakimi et al., 2021).
activity consequently enhancing VFA production which in due course In addition to the factors discussed earlier, retention time also plays
results in an irreversible acidification. Accordingly, the pH of the di- an imperative role to ensure the sound performance of AD process. Re-
gester plummets causing the hydrolysis process to be inhibited such that tention time is the time taken to complete degradation of organic mat-
the defective methanogenesis bacteria are not able to transform as much ter. The retention time can be linked with the microbial growth rate and
VFA to CH4 which is why the maximum threshold OLR limit has been is also heavily affected by the OLR, substrate composition and process
extensively studied and observed in previous studies. In a recent study, temperature (Khan et al., 2016).
Bi et al. (2019) observed in an AD system with chicken manure as the Biogas produced through AD is an energy-efficient approach with
sole feedstock under thermophilic temperature condition, the setup with environmental friendly benefits that has an upper edge over other forms
higher OLR (2.5 kg volatile solids m−3 d−1 ) experienced a significant de- of bioenergy (Hanafiah et al., 2022). Fig. 4 depicts data from 2012 on
terioration in CH4 generation due to NH4 inhibition in comparison to the potential of biogas generation from different types of farm animals
lower OLR (1.6 kg volatile solids m−3 d−1 ). in Malaysia.
Another factor affecting the AD process is the carbon-to-nitrogen Based on the data in Fig. 4, it is apparent that manure produced by
(C/N) ratio. Previous studies suggested a C/N ratio in the range of 20–30 the poultry farming industry has a significantly enhanced potential for
or 20–35, with a ratio of 25 being optimum for AD process (Igbum et al., biogas generation over the manure produced by other types of farm ani-
2019). Cahyono et al. (2021) reported that the C/N ratio of fresh chicken mals. Furthermore, as indicated before, chicken meat is arguably one of
manure is 11.348 indicating that carbon adjusters need to be incorpo- the more popular options of poultry meat hence it can be deduced that

7
M.D. Manogaran, R. Shamsuddin, M.H. Mohd Yusoff et al. Cleaner and Circular Bioeconomy 2 (2022) 100013

2% (Appels et al., 2008). The digestate produced is rich in nitrogen and

is often applied in plantations fields as an organic fertilizer (Angouria-
Tsorochidou and Thomsen, 2021) which effectively lessens the depen-
dency on energy intensive mineral fertilizers, further reducing GHG

12%
emissions.
AD is a microbial conversion approach that favours an aqueous
environment which indicates that chicken manure that constitutes of
significant water content can be processed without any pre-treatment
(Ward et al., 2008). This circumstance is not applicable in the case of
many other conversion methods. Combustion, for instance exclusively
provides a net positive energy balance if the feedstock consists of water
content below 60% and even under those circumstances, a large por-
tion of the energy stored in the feedstock is harnessed for evaporation
of the moisture. Additionally, in the case of pyrolysis and gasification,
the energetic efficiency of both processes reduces substantially parallel
86% to increase in water content and the presence of water in the synthesized
bio-oil is also an undesirable attribute (Van de Velden et al., 2010). In
a nutshell, utilization of these methods calls for an energy demanding
pre-drying step for wet feedstock which includes fresh chicken manure.
Furthermore, AD is not only practical in a large-scale industrial appli-
cation, it can be incorporated for small-scale utilization as well. This
beneficial aspect of AD provides an avenue for life style betterment es-

Buffalo Cattle Poultry

pecially in rural regions and developing countries which has limited
or even worse, no energy supply all together. For instance, rural com-
munities in India exploit AD using agricultural wastes and weed as the
Fig. 4. Biogas generation potential from different farm animal manure in feedstock to yield cooking gas for household use (Talevi et al., 2022).
Malaysia in 2012 (Abdeshahian et al., 2016).
2.5. Other treatment methods
Table 4
Estimation of CH4 generation and energy produced from chicken manure The treatment methods discussed in this section are hydrothermal
based on population of chickens in Malaysia in 2014.
liquefaction (HTL) and torrefaction, both not as commonly applied for
Parameter Unit Value chicken manure. Another thing to note is that both these methods do not
often yield circular economy outputs directly instead need to be paired
Chicken population a million birds 772
Chicken manure produced per bird daily a g/day 100 with methods scrutinized in the earlier sections.
Total mass of chicken manure produced daily tonnes/day 77,209 HTL is essentially a reaction utilizing biomass or organic material
CH4 generated per kg of chicken manure b m3 /kg 0.05 as the feedstock in the presence of water, otherwise regarded as the
CH4 generated daily m3 /day 3,860,458
solvent at hydrothermal conditions which is in the temperature range
Energy produced c TJ/day 139.5
GWh/day 38.75
of 250-450 °C and pressure range of 100–350 bar. Under these condi-
tions, water remains in liquid form or relatively dense supercritical state
a
Chicken population of 772 million in 2014 and assumption of (Castello et al., 2018). Before being treated under HTL, the feedstock un-
100 g/day of chicken manure produced per bird acquired from litera- dergoes a three-step process which are depolymerisation, decomposition
ture study (Kumaran et al., 2016).
b
and finally, recombination. Depolymerisation is subsequent dissolving
CH4 conversion factor is 0.05 m3 CH4 per kg of chicken manure
of macromolecules while taking advantage of their physical and chem-
(Ahmad, 2010).
c
Heat value of CH4 is 55 MJ/kg (Wongarmat et al., 2021) with gas ical traits to ease the process. The high temperature and pressure set-
engine conversion efficiency assumed at 100%. ting urges the hydrolysis of long chain polymers consisting of carbon,
H2 and O2 into shorter chain hydrocarbons (Amit et al., 2021). After
that, decomposition observes loss of water molecule (dehydration), loss
most of the poultry manure produced is chicken manure (Ahmad, 2015). of CO2 molecule (decarboxylation) and removal of amino acid content
This finding is crucial as biogas can be used directly for electricity gener- (deamination) (Jena et al., 2015). Dehydration and decarboxylation are
ation and heating as well as a replacement for fossil fuels for instance, as functional for the removal of O2 from the feedstock in the form of wa-
transportation fuel. The valorisation of synthesized biogas, constituting ter and CO2 respectively. The degradation yield primarily constitutes of
of 65% CH4 , 35% CO2 as well as small traces of H2 S, H2 and N2 is envi- polar organic molecules, furfurals, glycoaldehydes, phenols and organic
ronmentally friendly and energy efficient due to significantly low emis- acids that are highly soluble in water. Before HTL, recombination and
sion of hazardous substances which otherwise can have an adverse effect repolymerization of reactive fragments takes place. Should there be any
on human health. Most of the time biogas is valorised energetically in a free H2 molecules present in the organic matrix for later process, the free
combined heat and power (CHP) installation for production of heat and radicals will be capped resulting in the stable molecular weight species.
electricity at the same time. These installations normally provide a 33% However, if there are no H2 molecules or there is an abundance of free
electrical efficiency and a 45% thermal efficiency (Appels et al., 2011). radicals to the point of overwhelming, the fragments will recombine or
Table 4 highlights the potential of biogas generation from chicken ma- repolymerize to form high molecular weight char compounds otherwise
nure in Malaysia in 2014 given that all poultry farms managed chicken known as coke formation (Shah, 2015).
manure through AD. Considering that HTL demands for a state of wet reaction, chicken
Moreover, the emission of volatile organic compounds (VOCs) is manure is an adequate feedstock for this process due to its significant
also minimal since 99% of the VOCs are completely oxidized during moisture content (Singh et al., 2018). Additionally, no drying is required
combustion which is significantly better in comparison to incineration which means that operational cost can be effectively reduced. However,
which emits toxic materials such as dioxins that demand for extensive it is crucial to note that HTL is often applied for agricultural and forestry
flue gas purification, effectively increasing the capital investment cost waste comprising of plant and plant-based material, otherwise recog-

8
M.D. Manogaran, R. Shamsuddin, M.H. Mohd Yusoff et al. Cleaner and Circular Bioeconomy 2 (2022) 100013

Fig. 5. Circular economy outputs yield from HTL processing (Cao et al., 2017).

nized as lignocellulosic biomass (Cao et al., 2017). The primary purpose tainability attributes in comparison to conventional treatment methods.
of HTL is supposedly to produce bio crude from biomass waste, poten- However, the primary disadvantage of both the aforementioned meth-
tially a substitution for commercial fossil fuels although the economic ods are its energetic efficiencies decreases significantly should there be
feasibility is questionable due to the need for higher pressure setups excessive water content in the feedstock which in this case is chicken
which can be very costly (Gollakota et al., 2018). A proposed solution manure. Consequently, applying these treatment strategies requires an
for this is the addition of catalyst which can inhibit side reactions while energy consuming pre-drying stage. Anaerobic digestion on the other
lowering temperature and pressure settings (Cao et al., 2020). How- hand adapts well with feedstock of relatively high-water content such
ever, this requires additional cost which may upset the cost reduced as chicken manure to harness renewable energy in the form of biogas
on lowering the temperature and pressure conditions. Aside from bio- that is rich in CH4 . Future utilization of this approach is also relatively
crude, there are other output that can be acquired from HTL provided flexible as adapts well with most geographical locations where the tem-
that other technologies are integrated within the system as depicted in perature is above 15 °C. Hydrothermal liquification and torrefaction on
Fig. 5. the other hand, are relatively new treatment methods for chicken ma-
Another treatment that has scarce past research output is tor- nure with comparably scarce and limited past research hence, making it
refaction. Torrefaction, otherwise known as mild pyrolysis encom- a challenge to foresee if applying these methods are economically fea-
passes of heating the feedstock at temperatures of roughly 200-300 °C sible.
(Akdeniz, 2019) however, with respect to poultry manure, wet torrefac-
tion via the use of fluidized bed technology using superheated steam as a Declaration of Competing Interest
fluidizing medium is recommended as it accelerates manure torrefaction
by up to four times (Isemin et al., 2019). The product of interest from the The authors declare no competing interest. Author Rashid Sham-
process is biochar which can be further upgraded into activated carbon, suddin received research grant from the Ministry of Higher Education
an intriguing bioproduct with high specific surface area. Although still at Malaysia.
the early stages of investigation, Tabet (2021) observed the biochar ob-
tained from wet torrefaction of chicken manure which was subsequently Funding
processed into activated carbon. The activated carbon synthesized had
relatively exceptional traits as its specific pore surface area and specific Funding acquired from Fundamental Research Grant Scheme
volume of pores were 3392 m2 /g and 0.841 cm3 /g respectively with (FRGS/1/2019/TK10/UTP/02/8) awarded by the Ministry of Higher
a particle size of less than 2 nm. Often times, biochar rich in nitrogen Education Malaysia to provide allowance to postgraduate student.
acquired from wet torrefaction of poultry manure serves well as a pH
absorbent which indicated that it is a prime candidate for application CRediT authorship contribution statement
as sorption materials for water purification from heavy metals or from
organic pollutants due to the abundance of surface functional groups M. Devendran Manogaran: Conceptualization, Writing – original
(Straten et al., 2018). draft, Writing – review & editing. Rashid Shamsuddin: Conceptualiza-
tion, Writing – original draft, Writing – review & editing. Mohd Hizami
3. Conclusion Mohd Yusoff: Supervision, Visualization. Mark Lay: Writing – review
& editing. Ahmer Ali Siyal: Writing – review & editing.
The paper discussed treatment methods for chicken manure which
is produced in abundance compared to other poultry manure due to the Acknowledgement
high demand of chicken meat as a source of protein. One of the treat-
ment routes reviewed is composting which yields compost, favourable Fundamental Research Grant Scheme (FRGS/1/2019/TK10/
to be utilized as a form of organic fertilizer should it be stable and UTP/02/8) from the Ministry of Higher Education Malaysia is acknowl-
mature however, there are several drawbacks. For instance, compost- edged. The authors also recognize HICoE’s support to Centre for Biofuel
ing is a lengthy process which poses a significant biohazard threat if and Biochemical Research, Universiti Teknologi PETRONAS.
the compost is not stable and mature. Pyrolysis on the other hand has
three variations which are slow, fast and flash pyrolysis corresponding Supplementary materials
to different operating conditions yielding distinct products which are
either rich in bio-oil, bio-char or pyrolytic gaseous species. Gasification Supplementary material associated with this article can be found, in
is also a treatment route for chicken manure with environmental sus- the online version, at doi:10.1016/j.clcb.2022.100013.

9
M.D. Manogaran, R. Shamsuddin, M.H. Mohd Yusoff et al. Cleaner and Circular Bioeconomy 2 (2022) 100013

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