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Utilization of waste tea leaves to suppress the odors in compost processes

March 2011
ACS National Meeting Book of Abstracts

March 2011

Conference: 241st National Meeting and Exposition of the American-Chemical-Society
Volume: 241

Authors:

Chitsan Lin

National Kaohsiung University of Science and Technology (NKUST)

Meng-Ru Wu

National Yunlin University of Science and Technology

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Chemosphere 299 (2022) 134488

Available online 3 April 2022

Evaluate the role of biochar during the organic waste composting process: A

critical review

Minh Ky Nguyen

, Chitsan Lin

, Hong Giang Hoang

, Peter Sanderson

, Bao Trong Dang

Xuan Thanh Bui

, Ngoc Son Hai Nguyen

, Dai-Viet N. Vo

, Huu Tuan Tran

Ph.D. Program in Maritime Science and Technology, National Kaohsiung University of Science and Technology, Kaohsiung 81157, Taiwan

Department of Marine Environmental Engineering, National Kaohsiung University of Science and Technology, Kaohsiung, 81157, Taiwan

Faculty of Health Sciences and Finance - Accounting, Dong Nai Technology University, Bien Hoa, Dong Nai, 76100, Viet Nam

Global Centre for Environmental Remediation (GCER), Faculty of Science, The University of Newcastle, Callaghan, NSW, Australia

HUTECH University, 475A, Dien Bien Phu, Ward 25, Binh Thanh District, Ho Chi Minh City, Viet Nam

Key Laboratory of Advanced Waste Treatment Technology, Vietnam National University Ho Chi Minh (VNU-HCM), Linh Trung Ward, Thu Duc District, Ho Chi Minh

City, 700000, Viet Nam

Faculty of Environment and Natural Resources, Ho Chi Minh City University of Technology (HCMUT), Ho Chi Minh City, 700000, Viet Nam

Faculty of Environment, Thai Nguyen University of Agriculture and Forestry (TUAF), Thai Nguyen, 23000, Viet Nam

Center of Excellence for Green Energy and Environmental Nanomaterials (CE@GrEEN), Nguyen Tat Thanh University, Ho Chi Minh City, 700000, Viet Nam

School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300, Nibong Tebal, Penang, Malaysia

Laboratory of Ecology and Environmental Management, Science and Technology Advanced Institute, Van Lang University, Ho Chi Minh City, Viet Nam

Faculty of Technology, Van Lang University, Ho Chi Minh City, Viet Nam

HIGHLIGHTS GRAPHICAL ABSTRACT

•Using biochar as an additive improved

the performance and quality of

composting.

•Biochar affects the dynamic and struc-

ture of the microbial community during

composting.

•Biochar reduced the availability of

heavy metals and odorous gases

emissions.

•Biochar improved the compost maturity

by promoting enzymatic activity and

germination index.

ARTICLE INFO

Handling Editor: Derek Muir

Keywords:

Additives

Microorganism

Nitrogen losses

ABSTRACT

Composting is very robust and efcient for the biodegradation of organic waste; however secondary pollutants,

namely greenhouse gases (GHGs) and odorous emissions, are environmental concerns during this process. Bio-

char addition to compost has attracted the interest of scientists with a lot of publication in recent years because it

has addressed this matter and enhanced the quality of compost mixture. This review aims to evaluate the role of

biochar during organic waste composting and identify the gaps of knowledge in this eld. Moreover, the research

* Corresponding author. Laboratory of Ecology and Environmental Management, Science and Technology Advanced Institute, Van Lang University, Ho Chi Minh

City, Viet Nam.

** Corresponding author. Ph.D. Program in Maritime Science and Technology, National Kaohsiung University of Science and Technology, Kaohsiung 81157,

Taiwan.

E-mail addresses: ctlin@nkust.edu.tw (C. Lin), tranhuutuan@vlu.edu.vn (H.T. Tran).

Contents lists available at ScienceDirect

Chemosphere

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

https://doi.org/10.1016/j.chemosphere.2022.134488

Received 16 January 2022; Received in revised form 18 March 2022; Accepted 30 March 2022

Chemosphere 299 (2022) 134488

Humication

Enzyme activity

Greenhouse gases emission

direction to ll knowledge gaps was proposed and highlighted. Results demonstrated the commonly referenced

conditions during composting mixed biochar should be reached such as pH (6.5–7.5), moisture (50–60%), initial

C/N ratio (20–25:1), biochar doses (1–20% w/w), improved oxygen content availability, enhanced the perfor-

mance and humication, accelerating organic matter decomposition through faster microbial growth. Biochar

signicantly decreased GHGs and odorous emissions by adding a 5–10% dosage range due to its larger surface

area and porosity. On the other hand, with high exchange capacity and interaction with organic matters, biochar

enhanced the composting performance humication (e.g., formation humic and fulvic acid). Biochar could

extend the thermophilic phase of composting, reduce the pH value, NH

emission, and prevent nitrogen losses

through positive effects to nitrifying bacteria. The surfaces of the biochar particles are partly attributed to the

presence of functional groups such as Si–O–Si, OH, COOH, C

–

O, C–O, N for high cation exchange capacity and

adsorption. Adding biochars could decrease NH

emissions in the highest range up to 98%, the removal efciency

of CH

emissions has been reported with a wide range greater than 80%. Biochar could absorb volatile organic

compounds (VOCs) more than 50% in the experiment based on distribution mechanisms and surface adsorption

and efcient reduction in metal bioaccessibilities for Pb, Ni, Cu, Zn, As, Cr and Cd. By applicating biochar

improved the compost maturity by promoting enzymatic activity and germination index (>80%). However,

physico-chemical properties of biochar such as particle size, pore size, pore volume should be claried and its

inuence on the composting process evaluated in further studies.

1. Introduction

Composting is an effective technique to convert organic wastes into

the nal product called “mature compost” through various metabolisms

(Awasthi et al., 2018; Chowdhury et al., 2014; S´

anchez-Monedero et al.,

2019). During the composting process, volatile organic compounds

(VOCs), greenhouse gases (GHGs: N

O, CO

, CH

), and other odor

emission (H

S, NH

) could lead to serious secondary pollution, which

may pose adverse environmental and health effects (Chung et al., 2021;

Guo et al., 2020; S´

anchez-Monedero et al., 2019). The previous studies

illustrated that default N

O and CH

emissions factors were 0.6 g kg

−1

and 10 g kg

−1

waste during the composting process, respectively (Pipatti

et al., 2006). In certain conditions, for instance poor aeration, will lead

to anaerobic conditions that favors H

S and NH

related odorous emis-

sions (Lin et al., 2021; Tran et al., 2021a). The rapid increase in VOCs

and GHGs, leading to the emission of chemical precursors to ozone (O

has led to the enhancement of ground-level ozone and raised concerns

about the impact of its pollution (Wang et al., 2017; Xu, 2020), which

may be related to changing ambient environment characteristics (Phan

et al., 2020; Pusede et al., 2015; Wu et al., 2017a). The effects of severe

secondary and primary pollution caused by VOCs and GHGs from

composting process impact natural ecosystems and health issues, which

is a concern and needs to be solved. Therefore, it is necessary to manage

those gaseous emissions, which has become one of the most signicant

concerns for sustainable development goals.

Adding biochar as an additive into the compost mixture is a helpful

way to decrease GHGs and VOCs emissions during the composting

(Chowdhury et al., 2014; Dias et al., 2010; He et al., 2017; Wang et al.,

2018). The porous structure of biochar could enhance aeration rate and

reduce bulk density of the compost pile, also provide main shelters for

microorganisms, thus reducing the anaerobic zone generated, and as a

result, minimize GHGs and odors emissions (Jindo et al., 2012a; Steiner

et al., 2011). Owing to physical properties (e.g., high nano-porosity and

large SSA), biochar can enhance aeration capacity and water retention.

The Brunauer-Emmett-Teller (BET) method revealed the specic surface

area (SSA) of nonactivated and rice straw–derived biochars varied from

0.2 to 35.4 m

−1

which have been demonstrated by its helpful appli-

cations (Hwang et al., 2018; Lap et al., 2021; Nguyen et al., 2018; Tran

et al., 2022). Additionally, the BET SSA of nonactivated biochars lightly

changed consistent with the increase related to pyrolysis temperature.

Biochar formed from various feedstocks has a SSA of more than 520 m

−1

, and the SSA of biochar derived from diverse feedstocks is as follows:

shell, straw, wood, manure, and sludge (Yuan et al., 2016). The benets

of the application of compost combined with the presence of biochar

have reduced the bulk density inside the composting piles. The exist a

strong correlation between bulk density and the microbial community

(bacterial and fungi), indicated depending on the initial organic wastes,

and change in bulk density affecting microbial colonization/structure

and its biomass (Bapat et al., 2022; Jindo et al., 2012a). Biochar mi-

crospores may be responsible for adsorbing moisture, and biochar par-

ticles may also contribute some secondary porosity structure, which is

essential for improving aerobic conditions during composting (Steiner

et al., 2011). The bulk density is inuenced signicantly by the com-

posting time and experimental process. Reducing bulk density obtained

by using biochar in the composts and reaching potential benets for

their application. Furthermore, biochar agent has been investigated as a

promising and efcient technology to increase the organic matter

degradation and adsorbs GHGs (Wang et al., 2018). The previous illus-

trated that poultry manure mixed with wood biochar showed that the

organic matter was approximately degraded by 73–75%, and also GHG

emissions were reduced around 42.8% (Chowdhury et al., 2014; Dias

et al., 2010). Similarly, coffee husk and sawdust biochar were reported

to decrease GHG emissions by around 46% and 55%, respectively (Chen

et al., 2010; Jiang et al., 2016).

Biochar amendment for composting has also recently been regarded

as a cost-effective and environmentally friendly solution to improve

composting humication and performance, increasing microbial activ-

ities and decreasing the available of heavy metal contents and organic

contaminants (Agegnehu et al., 2017; Guo et al., 2020; Wu et al., 2018;

Xiao et al., 2017). Biochar has favorable physicochemical properties,

such as large SSA, high porosity, carbon-residue derived from the

thermal conversion related to organic waste and cation exchange ca-

pacity, which allows it to interact with essential nutrient cycles and

promotes microbial development during the composting (Awasthi et al.,

2017; Gudimella et al., 2022; Qayyum et al., 2017). The results showed

that compost mixed with 8–12% biochar became more humied after

composting (35 days), and the compost maturity not only revealed that

this could be a much more feasible approach to increased important

nutrients such as NO

3−

, PO

43−

, Na

, K

, DOC and DON, but also

bioavailability of heavy metal (i.e., Zn, Cu, Ni, and Pb) was reduced

when compared to control. Regarding the effect of biochar on the dy-

namic of a microbial community, the increased temperature of com-

posting piles is related to changes in the richness and diversity of

compost mixture (Jindo et al., 2012a; Le et al., 2021; Steiner et al., 2011;

Thakur et al., 2022; Yen et al., 2020). Biochar impacts the microbial

community structure and changes in the phospholipid fatty acid analysis

(PLFAs) patterns are related to major composting proles (i.e., tem-

perature, C/N ratio, bulk density) as the critical drivers. During this

stage, the raw materials were mineralized by the biochemical and bio-

logical processes in which enzyme (i.e., dehydrogenase) reected the

microbial dynamic of the composting process. Also, biochar is an

effective tool for producing high-quality compost-based on reducing

nutrient losses during the composting process and increasing compost

maturity (Awasthi et al., 2016; Jiang et al., 2016). On the other hand,

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

composting sewage sludge and animal waste with biochar has been

shown to reduce mobility and bioavailability of toxically heavy metals

(Antonangelo et al., 2021). Biochar⸻as an amendment to reduce

the bioavailability of heavy metals and enhance the composting effec-

tiveness during composting by adding 1, 3, 5 and 7% biochar into a

mixture (Liu et al., 2017b). During co-composted biochar investigated

that these heavy metals are usually formed strongly complex with the

organic substances in composting materials. The biochar pyrolysed at

temperature conditions between 450 and 500 ◦C demonstrated excellent

ability in immobilization of heavy metals (particularly Zn, Cu) during

biochar blended composting (Li et al., 2015). Biochar was observed to

reduce the bioavailability and toxicity of Cd and Zn to E. fetida earth-

worms in the vermicomposting experiments as well as enhancing the

number of juveniles, cocoons in the compost mixture.

In recent years, numerous studies have investigated the biochar ef-

fect on the composting process (Guo et al., 2020; Sanchez-Monedero

et al., 2018). Sanchez-Monedero et al. (2018) indicated that as an ad-

ditive, biochar enhanced the composting performance and humication

process, resulting in enhanced quality and maturity of the compost

mixture. Similarly in Guo et al. (2020) reported that biochar is a valu-

able technique to enhance microbial activities, reducing GHGs and

odors emissions, and the availability of heavy metals. However, the

interaction between microbial community and biochar during the

composting process has not been addressed yet. Also, a comprehensive

review of the role of biochar on compost quality is still ambiguous.

Therefore, the aims of the study are to evaluate the role and effect of

biochar combined with composting as an amendment on VOCs, GHGs

emissions, enhancement of compost quality/maturity and their rela-

tionship with the microorganism activities during the organic waste

composting process. The key benets that can be reached by the biochar

mixed composting will also be investigated, especially related to the

organic matter degradation, microbial community, humication,

reduction of nitrogen losses, VOCs, and GHGs emissions. Finally, the

future perspectives will be pointed out in this review.

2. Effect of biochar on the physio-chemical properties during

composting process

As an additive agent, biochar helps to improve the performance of

composting process (Fig. 1). Also, biochar addition could enhance the

efciency and decrease available nutrients due to their characteristic

such as larger porosity, functional groups, water holding capacity

(WHC), cation exchange capacity (CEC) (Guo et al., 2020; Schmidt et al.,

2014). The critical effects of added biochar on composting physico-

chemical proles and their performance are shown in Table 1.

2.1. Moisture

Biochar was added into the compost mixture as a bulking agent, that

could decrease the initial moisture content by absorbing excess moisture

of the mixture (Zhang et al., 2016). Also, biochar properties reduced

bulk density and enhanced the aeration of compost, leading to a

decrease the initial moisture content (Chowdhury et al., 2014; Wang

et al., 2013a). On the other hand, biochar addition has been illustrated

to increase the water retention capacity of the mixture as an absorbent,

implying it prolonged the optimal moisture content (50–60%) for

composting, and enhanced the performance and humication of the

composting process (L´

opez-Cano et al., 2016; Prost et al., 2013). The

positive effect of biochar addition on the humication related to organic

matters during the composting process and has been illustrated in pre-

vious research (Dias et al., 2010; Jindo et al., 2012b; Zhang et al.,

2014a). For instance, the combined addition of compost (35%) mixed

biochar (20%) contributed efciently to moisture content during the

composting (Zhang et al., 2014a). Also, Awasthi et al. (2017) showed

that 8–12% biochar was blended into biosolids co-composting to

improve humication within 35 days of the experiment.

2.2. Oxygen content

Oxygen content plays a crucial role during the composting process,

their presence in the biochar contributes to the adsorption process,

which is also the key factor for aerobic microbial to degrade the organic

substrate of the mixture (Tran et al., 2021b). Biochar addition into the

mixture was observed enhancing the effectiveness of oxygen supply

during the composting process due to higher porosity and their large

SSA (Mujtaba et al., 2021; Tran et al., 2022). For instance, the presence

of biochar has increased the range from 21% to 37% in oxygen (O

)

uptake rates on the rst day of the sludge composting process (Zhang

Fig. 1. Effects of biochar addition on physicochemical properties during composting.

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

et al., 2014a). Similarly, Steiner et al. (2011) indicated that a com-

posting pile with biochar doses (20% v/v) improved oxygen content

availability, accelerating organic matter decomposition through faster

microbial growth. Biochar with large surface area provided a home for

microorganisms, that signicantly enhanced the microbial structure

(richness and diversity) of mixture (Laird et al., 2010). Furthermore,

biochar’s porous properties can help enhance the physical structures of

compost by increasing pile porosity, which prevents anaerobic fermen-

tation by promoting oxygen supply (Xiao et al., 2017).

2.3. pH

pH is a crucial factor that indicates the microbial activities and

population community during the composting process (Tran et al.,

2020). Increased pH at a later stage (i.e., in composting development)

was thought to impair biochar’s potential for phenolic component

retention, thus causing biochar materials to degrade even more (Bernal

et al., 2009; Tran et al., 2021a). The previous studies illustrated that

critical condition for composting is affected by optimal pH during the

compost mixture (6.5–7.5) (Godlewska et al., 2017; Hoang et al., 2022;

Tran et al., 2021a; Yunus et al., 2020; Zainudin et al., 2020). Also, the

mobility of ions (e.g., heavy metals) is usually determined by pH. The

solubility of their toxic metals is reduced in compost with a higher pH,

lowering its toxicity when used as fertilizer. Because some soluble

alkaline components in biochar leak away, biochar can elevate compost

pH shortly after being added (Li et al., 2015; Tran et al., 2021b). The pH

prole during biochar mixed composting varies depending on biochar

characteristics, composting methods/techniques, their compositions (e.

g., C/N ratio, nutrients, etc.) and composting materials such as food

waste, animal waste, biosolids, yard waste, etc. (Godlewska et al., 2017).

Furthermore, He et al. (2017) indicated that negative charge surfaces on

biochar could absorb the generated ammonia/ammonium, resulting in a

pH reduction.

Table 1

Main effects of added biochar on compost physico-chemical properties and their performance.

Composting Biochar Amended rate Scale Periods Effects on composting performance and their

quality

References

Poultry manure Wood 50% (w/w) Conical piles: 1.5 m high

The turned-pile system

210

days

A high polymerization degree of humic compounds

leads to reduce TN losses

(Dias et al., 2010)

Cow dung +hydrilla

+sawdust

Wood 2.5, 5, 10%

(w/w)

A rotary drum composter:

550 L

20 days Increased TN around 45% (Jain et al., 2018)

Pig manure +sawdust Bamboo 0.03% (w/w) A tractor-pull windrow

turner

74 days A shorter time for thermophilic phase and a higher

temperature during thermophilic

phase

(Wang et al., 2014)

Poultry litter +

sugarcane straw

Poultry litter 10% (w/w) 220 L compost bin 60 days Decreasing N2O and CH4 emissions

Increased TN by 40%

(Agyarko-Mintah

et al., 2017a)

Chicken litter +

sawdust

Hardwood

shaving

5, 10% (w/w) Spherical plastic bins

(153 L)

133

days

The increase in CEC was 6.5 times (Khan et al., 2016)

Sewage sludge +

wheat straw

Wheat straw 2, 4, 8, 12%

(w/w)

130-L PVC composter

reactors

56 days Decreasing and degrading volatile fatty acids

(VFAs) and odor emission index

(Awasthi et al.,

2018)

Poultry litter +

sugarcane straw

Green waste 10% (w/w) 220 L plastic compost

bins

60 days Decreased TN losses by 51%

Improved N retention

(Agyarko-Mintah

et al., 2017b)

Municipal solid waste Wood chips 1.5, 3, 5% (w/

Real conditions

Full-scale

72 days A positive impact on the compost quality and

reduction of nitrogen losses

Indicated higher moisture level and lower density

(Malinowski et al.,

2019)

Dewatered sewage

sludge +wheat

straw

Wheat straw

biomas

2, 4, 6, 8, 12,

18% (w/w)

130-L reactor in-vessel 56 days Increased water-soluble nutrients (i.e., NO3–, DOC,

DON, PO43–, K+and Na+)

Reduced bioavailability of heavy metals

(Awasthi et al.,

2017)

Poultry manure +

barley straw

Holm oak 3% (w/w) Pilot-scale

Trapezoidal piles (1.5 m

high)

Turned pile (windrow)

system

weeks

Biochar accelerated organic matter (OM)

degradation

Reduce the composting time by around 20%

(S´

anchez-García

et al., 2015)

Poultry manure +

wheat straw

Woodchips 5, 10% (w/w) Reactors: 165

42 days Addition of biochar caused increasing temperature

and shortened the thermophilic phase

Biochar increased CO2 emission

(Czekała et al., 2016)

Corn wastes Corn wastes 1, 2% (w/w) Plastic pots (35 cm height

and 25 cm diameter)

150

days

Improves the physicochemical properties of soil

Exhibited high CEC and soil organic carbon (SOC)

(Liu et al., 2021a)

Hen manure +wheat

straw

Bamboo 5, 10, 20% (w/

Small laboratory reactor

Cylindrical (inner

diameter: 0.25 m, total

height: 0.40 m)

28 days Reduced CO2, CH4, N2O and NH3 emissions (Liu et al., 2017a)

Swine manure +

maize straw

N/A 5, 10% (w/w) Medium-scale

PVC reactors (100 L)

52 day Decomposition of dissolved organic carbon (DOC)

Biochar promoted the composting humication

and increased the P-bioavailability

(Cui et al., 2022)

Green waste Yellow pine N/A Plots

Lab-scale

N/A Biochar and compost divergently impacted

functional groups of soil

(Hale et al., 2021)

Farm yard manure +

vermicompost

Rice husk due N/A Plot size (3.6 m ×2.6 m) N/A Improving soil hydro-physical properties, crop

yield

(Sharma et al., 2021)

Distilled grain waste Coconut

shells

5, 10, 15, 20%

(w/w)

A computer-controlled

28 L reactor

65 days Adding 10% biochar reduced nitrogen loss up to

25.69% and accelerated OM degradation, thereby

shortening the composting cycle

(Wang et al., 2021)

Food waste digestate

+sawdust +mature

Tobacco stalk 2.5, 5, 10%

(w/w)

20 L composters 42 days 10% biochar distribute to reduce 58% of NH3

emission and 50% of nitrogen loss

(Manu et al., 2021)

Fresh chicken manure Rice husk 3, 5, 10% (w/

100-L plastic, cylindrical

vessels

Pilot-scale

50 days Signicant reduction in gaseous emissions (GHGs,

NH3 and CO2), microbial pathogens

(Chung et al., 2021)

Remarks: CEC: Cation exchange capacity, TN: Total nitrogen, OM: Organic matter, DOC: Dissolved organic carbon, DON: Dissolved organic nitrogen, PVC: Polyvinyl

chloride, N/A: Not available.

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

2.4. Temperature

Temperature is known as a key vital indicator of the composting

process and reects the activities of microorganisms and also the organic

matter degradation in the compost piles (Huang et al., 2019). The added

biochar to the compost has been observed to activate the process, evi-

denced by a temperature increase and extending the thermophilic phase

(Chen et al., 2010; Steiner et al., 2010). The presence of biochar in the

composting process leads to temperature rising quickly, and so does the

duration of the thermophilic phase. The temperature during composting

process increased faster in case of biochar addition than compared to the

control without biochar (Wei et al., 2014). Biochar added at the start of

the composting process, leads to increased water holding capacity

(WHC), thus ensuring the desired moisture level in the range from 50%

to 60% w/w (Prost et al., 2013). The biochar addition resulted in

obtaining higher temperature, ascribed fewer heat losses, and increased

microbial activity (Li et al., 2015). The higher temperature from the

composting may accelerate the abiotic oxidation process of the biochar

surface, resulting in more hydrophilic functional groups (e.g., carbonyl

groups, hydroxyl) that are available for microbial breakdown (Cheng

et al., 2006). The inclusion of biochar enhances aeration and hence the

number of microorganisms, speeding up the transformations and

increasing the heat produced (Godlewska et al., 2017).

2.5. C/N ratio

Biochar is used popularly to amend the elemental composition dur-

ing the composting, and the C/N ratio is one of the most primary factors

that have inuenced this process (Lin et al., 2021; Nguyen et al., 2020).

Because of the refractory carbon produced from the biochar addition,

most studies indicated that adding biochar enhanced the C/N ratio

(Chowdhury et al., 2014; Jindo et al., 2012a; Zhang et al., 2014b). A

suitable initial C/N ratio should be reached around 25:1 for aerobic

organic wastes composting (Wu et al., 2017c). The carbon to nitrogen

ratios (C/N) of various feedstock-derived biochars and composts varies,

which has a direct effect on the rate of organic matter decomposition

(Godlewska et al., 2017). The rates of labile carbon mineralization

remained high due to these biochar features, and the biochar in the

compost provided more excellent durability when utilized as the soil

amendments, which has crucial implications for C sequestration (Dianey

et al., 2021; Doan et al., 2022; Godlewska et al., 2017; Kamaruzaman

et al., 2022; Steiner et al., 2010). A high mineralization intensity leads to

decompose/oxidize to easily available forms and conservation of N

levels that are favorable during the N-rich composting process, e.g.,

manures and organic wastes. Adding raw materials that contain a high

C/N ratio (e.g., biochar) could improve immobilizing N compounds, and

the nitrogen retained might be plant available for their growth. Owing to

the presence of not only acidic functional groups but also in the condi-

tion of low pH, biochar has been reported as an absorber of

water-soluble NH

or NH

, thus reducing N losses during composting

process (Kastner et al., 2009; Steiner et al., 2010). Biochar may impact

the C/N ratio, which is an essential factor in compost microbiology

(Wang et al., 2015). Biochar produces a favorable micro-environment

for nitrifying bacteria, which convert ammonia (NH

) to nitrate

(NO

3−

), resulting in higher nitrogen content in biochar-treated compost

(Zhang et al., 2014a).

In short, biochar was added to the compost mixture as a bulking

agent that might reduce the initial moisture content of the mixture by

absorbing excess moisture. The presence of oxygen in biochar contrib-

utes to the adsorption process, which is a signicant component for

aerobic microorganisms to decompose the organic substrate of the

mixture during the composting process. The optimal conditions of ox-

ygen content (15–20%) and moisture content range of 50–60% could

boost enzyme production and accelerate microbial activity. When bio-

char is mixed into the composting piles, it increases the water-holding

capacity (WHC), ensuring the optimum moisture content. The pH

prole of biochar mixed compost varies based on biochar properties,

composting techniques, and their compositions (e.g., C/N ratio, nutri-

ents, etc.), and the ideal pH during the composting mixture should be

reached 6.5–7.5. For aerobic organic waste composting, a suitable initial

C/N ratio should be about 20–25:1, which is essential for C

sequestration.

3. Effect of biochar on the dynamic of microbial community

during the composting process

3.1. Effect of biochar on the dynamic of microbial community

The biochar itself possesses a highly porous structure that contains

valuable substances such as inorganic nutrients, labile aliphatic com-

pounds, and minerals (Quilliam et al., 2013; Xiao et al., 2017). Conse-

quently, the biochar amended compost essentially contributes to an

increase in natural ventilation, temperature, moisture content, a favor-

able niche, and nutrition for native microorganisms (Atkinson et al.,

2010). In this regard, added biochar could positively improve the per-

formance by changing the microbial community in the compost pile

(Agegnehu et al., 2017). However, how much biochar dosage to ensure

maximum activity and diversity of the microbial community is still

being evaluated.

Table 2 presents some studies on the microbial community changes

driven by different raw materials and biochar dosage. Most studies

suggested that biochar is hugely compatible with various feedstock

(Bello et al., 2020; Du et al., 2019b; Li et al., 2021). The biochar dosages

are commonly used from 1% to 20% (w/w). Besides, adding a high

dosage of biochar (10%) could reduce NH

, hydrogen sulde (H

S), and

total VOCs, while N-cycling microorganisms such as genus Pusillimonas

and Pseudomonas became more active (Li et al., 2022; Liu et al., 2017a).

Moreover, the enzyme’s activity of the bacterial community was

strengthened by the biochar addition (10% and 20%) into sewage sludge

and sawdust mixtures (Li et al., 2022). It indicated that the high dose of

biochar might accelerate the metabolic activity of the adapted strain.

However, the observed richness (Chao1) and diversity (Shannon-Wi-

ener) varied with biochar dosage. Some studies have suggested that

increasing biochar dosage to 20% recorded lower alpha diversity indices

than control compost (Zainudin et al., 2020). Possibly, a high dose of

biochar might reduce the biodiversity index. It is well known that

increasing the decomposition rate can help facilitate treatment times,

but high microbial diversity could promote the complete breakdown of a

broad type of persistent pollutants (Bird et al., 2011; Novak et al., 2016).

Thus, this creates a trade-off to balance the crucial benets between

metabolic rate and microbial community diversity driven by biochar

dosage. Excessive biochar (>10%) might cause severe heat dissipation

and water loss, adversely affecting on the composting process (Liu et al.,

2017a). In addition, the relative abundance of heavy metals resistant

bacteria (HMRB) in composting process was decreased with elevated

biochar dosages (0–10%). Heavy metals and biochar content signi-

cantly reduced Firmicutes (52.88–14.32%), Actinobacteria

(35.20–4.99%), while increased phylum of Bacteroidetes (0.05–15.07%)

and Proteobacteria (0.01–20.28%) (Zainudin et al., 2020). A moderate

addition of biochar (6%) was considered to have the most abundant

HMRB among all treatments (poultry manure, wheat straw, and added

chicken manure biochar (0–10%) (Li et al., 2021). Furthermore, the

added 7.5% biochar enhances the removal of recalcitrant keratinized

waste during pig manure composting (Duan et al., 2020). Taken

together, these results point to the use of an appropriate amount of

biochar that can help balance the diversity and metabolic activity of the

community.

3.2. Effect of biochar on the structure of microbial community

Biochar creates a distinct microbial population as a result of its

intervention in the composting process. Proteobacteria, Bacteroidetes,

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

Table 2

Different raw materials and biochar dosages drive microbial communities to change over time.

No. Materials Composting

ratios

Biochar

(%)

Max.

Temp.

(◦C)

Max.

Shannon

Max.

Chao1

Phylum Major Genus (Early-Middle

periods)

Major Genus (Middle -Later

periods)

Refs.

1 Poultry manure +rice straw 2:1 20 58.9 9.9 5.2 887 Firmicutes, Proteobacteria,

Bacteroidetes

Sinibacillus,

Ammonibacillus,

Pseudofulvimonas,

Pusillimonas,

Petrimonas

Pusillimonas,

Pseudomonas,

Pseudofulvimona,

Petrimonas,

Sinibacillu

(Zainudin

et al., 2020)

Poultry manure +rice straw

+biochar

2:1 20 71.5 11 4.9 653 Firmicutes,

Proteobacteria, Bacteroidetes

Ammonibacillus,

Sinibacillus, Halomonas,

Pusillimonas,

Pseudofulvimonas

Halomonas,

Pusillimonas, Pseudofulvimona,

Nitriliruptor,

Truepera

2 Chicken manure +peanut

straw (3–5 cm)

2.5:1 0 60.2 8.94 4.1 312 Firmicutes, Bacteroidetes,

Proteobacteria,

Halanaerobiaeota,

Actinobacteria

Gallicola, Proteiniphilum,

Bacillus, Ammoniibacillus

Pseudomonas,

Pusillimonas,

Ignatzschineria,

Thiopseudomonas,

Flavobacterium.

(Li et al.,

2022)

Chicken manure +peanut

straw +biochar

2.5:1 10 64.6 8.71 3.9 318 Firmicutes, Bacteroidetes,

Proteobacteria,

Halanaerobiaeota,

Actinobacteria

Gallicola, Proteiniphilum,

Bacillus,

Ammoniibacillus

Pseudomonas,

Pusillimonas,

Ignatzschineria,

Thiopseudomonas,

Flavobacterium

3 Sewage sludge +corn cob (7

mm)

5:3 (v/v) 0 54 8.08 4.6 – Proteobacteria, Bacteroidetes,

Firmicutes, Actinobacteria

Bacillus, Ochrobactrum,

Proteiniphilum

Alicycliphilus, Ochrobactrum,

Proteiniphilum

(Liu et al.,

2021c)

Sewage sludge +corn cob +

biochar

5:3 (v/v) 8.2 55 8.07 4.0 – Proteobacteria, Bacteroidetes,

Firmicutes, Actinobacteria

Sphingobacterium,

Glutamicibacter,

Ochrobactrum,

Rhodanobacter,

Enterobacter

Bacillus, Ochrobactrum,

Rhodanobacter

Sewage sludge +corn cob +

biochar

5:3 (v/v) 15.2 55 8.13 3.9 – Proteobacteria, Bacteroidetes,

Firmicutes, Actinobacteria

Glutamicibacter,

Microbacterium,

Ochrobactrum,

Enterobacter,

Rhodanobacter

Bacillus, Ochrobactrum,

Rhodanobacter

4 Cow manure +sugarcane

straw (1 cm)

5:1 (v/v) 0 60 – – – Firmicutes, Actinobacteria,

Proteobacteria, Chloroexi,

Bacteroidetes

Corynebacterium,

Romboutsia,

Pseudoxanthomonas,

Thermomonospora,

Clostridium

Thermopolyspora,

Thermomonospora, Ureibacillus

(Yan et al.,

2021)

Cow manure +sugarcane

straw (1 cm) +biochar

5:1 (v/v) 5 68.5 – – – Firmicutes, Actinobacteriota,

Proteobacteria, Chloroexi,

Bacteroidetes

Corynebacterium,

Romboutsia,

Pseudoxanthomonas,

Thermomonospora,

Clostridium

Thermopolyspora,

Thermomonospora, Ureibacillus

5 Sewage sludge +straw (1 cm) 4:1 0 59.9 8.4 4.7 582 Firmicutes, Proteobacteria,

Chloroexi,

Actinobacteria, Bacteroidetes

Pseudomonas,

Chloroexi,

Pedobacter,

Planomicrobium,

Microtrichales

Vulgatibacter, Anaerolineaceae,

Thermobida

(Xue et al.,

2021)

Sewage sludge +straw (1 cm)

+biochar

4:1 3.85 61.6 8.2 4.6 556 Proteobacteria, Chloroexi,

Firmicutes, Actinobacteria,

Bacteroidetes

Pseudomonas, Pedobacter,

Planomicrobium

Psychrobacillus,

Paenisporosarcina, Ureibacillus

4:1 0 61.4 8.2 5.0 619

(continued on next page)

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

Table 2 (continued )

No. Materials Composting

ratios

Biochar

(%)

Max.

Temp.

(◦C)

Max.

Shannon

Max.

Chao1

Phylum Major Genus (Early-Middle

periods)

Major Genus (Middle -Later

periods)

Refs.

Sewage sludge +straw (1 cm)

+aerobic bacteria

Proteobacteria, Chloroexi,

Firmicutes, Actinobacteria,

Bacteroidetes,

Pseudomonas,

Chloroexi,

Pedobacter,

Planomicrobium,

Microtrichales

Vulgatibacter, Thermobida,

Ureibacillus, Chelativorans

Sewage sludge +straw (1 cm)

+aerobic microorganism

agent +biochar

4:1 3.82 62.4 8.25 5.0 576 Proteobacteria Chloroexi,

Firmicutes, Actinobacteria,

Bacteroidetes

Pseudomonas,

Microtrichales,

Planomicrobium,

Chloroexi

Thermopolyspora, Anaerolineae,

Chloroexi, Limnochordaceae,

Ureibacillus

Sewage sludge +straw (1 cm)

+facultative anaerobic agent

4:1 0 61.1 8 4.9 583 Proteobacteria, Chloroexi,

Firmicutes,

Actinobacteria,

Bacteroidetes

Pseudomonas,

Microtrichales,

Chloroexi,

Pedobacter,

Planomicrobium

Bacillus, Chelativorans,

Thermobida,

Pseudoxanthomonas

Sewage sludge +straw (1 cm)

+facultative anaerobic agent

+biochar (3–5 mm)

4:2 3.82 62.2 8.2 5.0 594 Proteobacteria and Chloroexi,

Firmicutes,

Actinobacteria,

Bacteroidetes

Microtrichales, Chloroexi,

Pedobacter, Planomicrobium

Pseudoxanthomonas,

Anaerolineaceae, Thermobida,

Anaerolineae, Chloroexi

6 Pig manure +wheat straw 2:1 0 64 7.2 5.5 820 Actinobacteria, Firmicutes,

Bacteroidetes, Chloroexi,

Proteobacteria

Bacilli,

Clostridia,

Tenericutes, Actinobacteria

Firmicutes, Bacteroidetes,

Proteobacteria, Chloroexi

(Yang et al.,

2020)

Pig manure +wheat straw +

bean dregs (15%)

2:1 0 68 8.1 4.8 840 Actinobacteria, Firmicutes,

Bacteroidetes, Chloroexi,

Proteobacteria

Bacilli, Clostridia,

Genus of Tenericutes and

Actinobacteria

Firmicutes, Bacteroidetes,

Proteobacteria, Chloroexi

Pig manure +wheat straw +

biochar (10%)

2:1 10 68 8.3 5.5 900 Actinobacteria, Firmicutes,

Bacteroidetes, Chloroexi,

Proteobacteria

Bacilli, Clostridia,

Tenericutes, Actinobacteria

Firmicutes, Bacteroidetes,

Proteobacteria Chloroexi

Pig manure +wheat straw +

bean dregs (15%) +biochar

(10%)

2:1 10 69 8.15 6.0 820 Actinobacteria, Firmicutes,

Bacteroidetes, Chloroexi,

Proteobacteria

Bacilli, Clostridia,

Tenericutes, Actinobacteria

Firmicutes, Bacteroidetes,

Proteobacteria, Chloroexi

7 Tomato stalk +chicken

manure +biochar

6:5 1 56 7.2 2.58 – Proteobacteria, Bacteroidetes,

Firmicutes, Actinobacteria

Flavobacterium,

Actinobacterium,

Chitinophaga sp.,

Pseudomonas sp.

Acinetobacter,

Chitinophaga sp.,

Flavobacterium,

Actinobacterium

(Wei et al.,

2014)

Tomato stalk +chicken

manure +peat bog

6:5 1 50 7.3 2.03 – Proteobacteria, Firmicutes,

Actinobacteria

Pusillimonas sp.,

Microbacterium sp.,

Geobiacillus sp.,

Pseudomonas sp.,

Rhizobiales sp.

Rhizobiales sp.,

Acinetobacter sp.,

Chitinophaga sp.

Pusillimonas sp.,

Tomato stalk +chicken

manure +zeolite

6:5 1 50 8 1.46 – Proteobacteria Rhizobiales sp.,

Chitinophaga sp.,

Acinetobacter sp.,

Actinobacterium

Pseudomonas sp.,

Chitinophaga sp.,

Rhizobiales sp.,

Pusillimonas sp.,

8 Cattle manure +maize straw 5:1 0 75 8.1 4.57 871 Firmicutes,

Actinobacteria, Proteobacteria,

Chloroexi,

Bacteroidetes

Corynebacterium,

Bacillus,

Atopostipes,

Marinilabiaceae,

Turicibacter

Thermopolyspora, Thermobida,

Anaerolineaceae

(Bello et al.,

2020)

Cattle manure +maize straw

+biochar

5:1 10 75 8.2 4.65 913 Actinobacteria

Firmicutes,

Proteobacteria,

Chloroexi,

Bacteroidetes

Corynebacterium,

Bacillus,

Atopostipes,

Marinilabiaceae,

Turicibacter

Actinomadura, Longispora,

Streptomyces

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

Firmicutes, and Actinobacteria were the most predominant phyla in bio-

char amended compost due to their excellent compatibility with biochar

and feedstock (Duan et al., 2019a; Li et al., 2022; Yang et al., 2020).

Biochar is likely to promote the proliferation of certain individual bac-

terial phyla rather than entire communities due to biochar affecting

physicochemical properties during composting (Dang et al., 2021).

Proteobacteria was related to the nitrogen and carbon cycle, while Acti-

nobacteria played an essential part in degrading lignin and refractory

cellulose. Actinobacteria can utilize biochar as a source of C and miner-

alize it to CO

; these biochars may have guided the metabolism of

Actinobacteria and enriched it through a process known as Copiotrophs

(Bello et al., 2020).

The relative abundance of this bacterium can be altered by C/N

value, biochar dosage, composting phase, and temperature (Czepiel

et al., 1996). Biochar simultaneously increases the temperature and

prolongs the thermophilic phase compared to non-biochar compost

piles, thereby promoting the metabolism of adapted strains as well as

changing the bacterial community over time (Zainudin et al., 2020). The

Actinobacteria and Firmicutes play an important role in decomposing

complex organic materials during the thermophilic phase and are

thermos-tolerant (Bello et al., 2020). With increased temperature, the

relative abundance of genus belonging to Bacteroidetes and Proteobac-

teria decreased and later increased as the compost temperature declined

(Bello et al., 2020). Genera of Proteobacteria and Bacteroidetes are known

to be resistant to antibiotics, and this fact may explain why increased

peak temperature by biochar amendment compost would be benecial

to enhance the elimination of antibiotic resistance genes (ARG) (Fu

et al., 2021; Li et al., 2017; Siedt et al., 2021).

3.3. Effect of biochar on the microbial community – nitrogen during the

composting

Absorption of biochar increases the activity of nitrifying bacteria,

reduces methanogens, and increases heavy metal xation. The surfaces

of the biochar particles are partly attributed to the presence of functional

groups such as Si–O–Si, OH, COOH, C

–

O, C–O, N for high cation ex-

change capacity and adsorption (Agegnehu et al., 2017). As a result,

biochar aids in absorbing both NH

and greenhouse gas released from

the compost pile, increasing oxygen diffusion to reduce the anaerobic

zone (Xiao et al., 2017). After the NH

is adsorbed by biochar, this is

reported to benet the growth of nitrifying bacteria, which convert the

ammonium to nitrate and thus retain nitrogen in the compost products

(Godlewska et al., 2017; Li et al., 2022). Consequently, it tends to in-

crease N–NO

3−

concentration while decreasing the volatilization of NH

(Chen et al., 2017a). The concentration of NO

3−

was increased twice

compared to conventional compost (L´

opez-Cano et al., 2016). In addi-

tion, biochar also affects the genus denitrication population, such as

O−producing bacteria, by improving oxygen diffusion into the

compost pile, so less N

O is produced (Wang et al., 2013b). The pre-

dominant aerobic zone is responsible for a decrease in the population of

anaerobic species such as denitrifying bacteria and methanogens. By

contrast, biochar likely increased the activity of methane-oxidizing

bacteria that convert CH

to CO

(Liu et al., 2017a). In addition, when

the compost pile has few anaerobic zones, it results in a limited number

of Methanogens and Methanotrophs present to directly reduce CH

emission (He et al., 2019a; Mao et al., 2018).

In summary, a proper biochars dosage could reduce organic and

inorganic pollutants by altering the prole and activity of the microbial

community. A higher rate of biochar addition will increase pollutant

uptake and carbon sequestration but be more expensive and might hurt

biodiversity. Biochar not only helps to create a distinct microbial pop-

ulation in the compost pile, but its high adsorption capacity is benecial

for heavy metal xation, improved nitrication, and reduced GHGs

emissions. To put such results into practice, it is necessary to carry out

large-scale experiments.

4. Effect of biochar on VOCs and GHGs emission during

composting process

During composting, VOCs (e.g., hydrocarbons, alcohols, aldehydes,

esters, etc.), GHGs (N

O, CO

, CH

), and odorous gases (H

S, NH

) are

emitted into the ambient air, which may pose an environmental risk and

health concerns. Biochar amendment is considered as an efcient solu-

tion to adsorb VOCs, GHGs, odorous gases through various previous

studies (Awasthi et al., 2016; Chowdhury et al., 2014; Dias et al., 2010;

He et al., 2017; Janczak et al., 2017; Jiang et al., 2016; Wang et al.,

2018). Table 3 and Fig. 2 illustrate the effects of biochar on the VOCs

and GHGs emissions during composting.

4.1. Volatile organic compounds (VOCs)

The biochar addition signicantly reduced the emissions of oxygen-

and nitrogen-containing VOCs during the composting process

(S´

anchez-Monedero et al., 2019; Tran et al., 2018). During the ther-

mophilic phase belongs to composting process, VOC were classied

based on its abundance into three main groups, including nitrogenous,

oxygenated and other compounds. By using 90% poultry manure plus

10% straw (within 3% biochar addition), improving aerated conditions

reduced up to 50% these concentrations during the thermophilic phase

in composting process. The greatest efciency was illustrated in the

OVOCs compounds, with most ketones, phenols and volatile fatty acids

(VFAs) concentrations reduced signicantly in a pile mixing biochar.

The addition of biochar promoted the aeration rate in the composting

matrix due to its higher porosity, leading to increasing gas exchange and

preventing the anaerobic zones formation, which could be a source of

VOCs (Sanchez-Monedero et al., 2018). Pore structure and surface acid

functional groups in biochar could trap toxic emissions, thus preventing

toxic volatilization and reducing their pollution. Biochar’s strong sorp-

tion capacity may represent a mechanism for VOCs elimination in the

composting piles, that is aided by their large SSA (S´

anchez-Monedero

et al., 2019). For example, wooden biochars showed high removal ef-

ciencies for acetone, toluene and cyclohexane in the value from 50 to

100 mg VOC g

−1

(Zhang et al., 2017). Janczak et al. (2017) investigated

that 10% biochar can decrease VOCs emissions during biosolid com-

posting. Biochar could absorb TVOCs (about 17.55%) in the experiment

based on distribution mechanisms and surface adsorption (Li et al.,

2021). Furthermore, the most efcient VOCs reduction was investigated

in OVOCs compounds (e.g., phenols, ketones, and organic acids) and

dramatically reduced the amounts of volatile nitrogen compounds,

which are produced by microbial modication of N-compounds. These

results indicate the importance of biochar application not only impacts

the composting progress but also their sorption capacity as key drivers

for VOCs reduction.

In addition, the environmental conditions are characterized such as

temperature, moisture, etc. during composting that may affect the

sorption of VOCs on biochar surface (Hwang et al., 2018;

S´

anchez-Monedero et al., 2019; Tran et al., 2018; Zhang et al., 2017).

The impact of biochar could modify these conditions inside the com-

posting matrix, and affecting on vital parameters including aeration,

temperature, moisture, microbial activity that is related to the for-

mation/degradation of VOCs compounds during composting (Mauli-

ni-Duran et al., 2014). More clearly, composting biochar increases pH,

aeration, enhances water holding capacity, and thus improving oxygen

content and redox conditions (Wu et al., 2017b). Interestingly, the

composting piles mixed biochar enhanced favorable environmental

conditions for microbial growth, leading to the reduction of VOCs. This

work may explain that biochar is a promising alternative sorbent and

favorable effective in reducing gaseous VOCs.

4.2. Greenhouse gases (N

O, CO

, CH

)

During composting several major greenhouse gas compounds such as

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

nitrous oxide (N

O), carbon dioxide (CO

) and methane (CH

) can be

emitted due to organic matter degradation (Chowdhury et al., 2014).

Adding biochar to composts increases carbon stabilization, which raises

the potential benets of employing co-composting, such as minimizing

nutrient losses through leaching and lowering GHG emissions. Biochar

addition to the co-composting is benecial approach in the reduction of

emissions, that is due to the better aeration, reduced bulk density,

gas diffusion, and creating suitable conditions for methanotrophs can

consume CH

(Vandecasteele et al., 2016). Illustration of the GHG

emissions rates measured during the composting process by amending

biochar resulted in a clear reduction in CH

. The average emission rates

for the whole process of the control and the biochar mixed compost were

8.1 and 1.5 g CH

−1

, respectively. Biochar addition in the feedstock

mixture of green and organic wastes enhance the composting process

and shows the different effects before and after adding biochar 10% into

the composting piles. The removal efciency of CH

emissions has been

reported with a wide range from 10.8% up to greater than 80%

(Agyarko-Mintah et al., 2017b; Chen et al., 2017a; Vandecasteele et al.,

2016). Also, Steiner et al. (2010) found improved aeration when biochar

addition at doses of 5% and 20% reduced emissions by 58% and 71%,

respectively.

He et al. (2019b) showed that granular bamboo biochar has a high

potential for reducing CO

emissions. The positive effect of additives

during co-composting may have signicant potential to reduce CO

emissions. In another study, biochar addition led to a reduction of CO

emissions up to 44% compared to the control (Barthod et al., 2016). This

can be explained by the adsorption of organic matters on the biochar

surface, and biochar has been used as a co-composting agent to reduce

carbon emissions. The major mechanism can trap of CO

during com-

posting related to biochars addition, leading to decreased CO

emis-

sions, with biochar as the alkaline agent. In addition, it is seen that the

organic matter decomposition during the composting process (bio--

oxidative phase) were partially limited by the biochar adsorption for

reduction. The average CO

emission rates for the whole com-

posting process of the control and the biochar mixed experiments were

401 and 195 g m

−1

, respectively (Vandecasteele et al., 2016).

Table 3

Effects of biochar on VOCs and GHGs emissions during composting.

Composting Biochar Characteristics Effects and benets Mechanism and limitation References

Green waste +

bagasse +

chicken manure

Waste willow

wood (Salix spp.)

Pyrolysis: 550 ◦C

Compost

windrows

Field-scale

Lowered N2O emissions

Improved soil quality

Yield increase

Biochars are more stable in soil, it is resulting in

lower CO2 loss

(Agegnehu et al.,

2016)

Green waste +

bagasse +

chicken manure

Waste willow

wood (Salix spp.)

Pyrolysis: 550 ◦C

9% w/w

Field-scale

Period: 98 days

Amendment

windrows

Reduced N2O emissions

Increased Na, K, Mg, P, NO3–,

NH4+and soil carbon

Biochar may be less suited for reducing N2O

ux in some agricultural soils, at least on

shorter temporal scales

Biochar could act as an electron shuttle, which

enhances the last step from N2O to N2

(Agegnehu et al.,

2016)

Farm manure Garden peat

Pyrolyzer: 450 ◦C

Rate: 2% w/w

Laboratory scale

Periods: 3 months

Moisture: 60%

Reduced CO2 emission, especially

with a higher proportion of biochar

in the compost

Increased SOC, yield, N and K

contents in plant

Effect of biochar mixed composts illustrated

stabilization of native and labile organic carbon

present in farm manure (FM), resulting in

carbon stabilization in the soil

(Qayyum et al., 2017)

Pig manure Rice straw

Pyrolysis: 450 ◦C

Reactors: 120 L

cylindrical plastic

Period: 84 days

Passive aeration

composting

Signicantly reduced N2O

emissions

The low degradability of biochar could be led to

the core reason for these ndings

(Vu et al., 2015)

Poultry manure +

wheat straw

Wood woodchips

Pyrolysis: 350 ◦C

5% and 10%

Laboratory scale

Reactors: 165 L

Period: 42 days.

5% and 10% of biochar can reduce

NH3 emission by 30% and 44%,

respectively

Biochar addition to poultry manure contributes

to nitrogen retention in the solid

Biochar’s benecial effect on nitrogen loss is

due mainly to its adsorption properties and the

presence of surface acid groups

(Janczak et al., 2017)

Chicken mortality Woodchips

Gasication:

520 ◦C

1, 5, 10, and 15%

Period: 11 weeks

Pilot-scale

Composting test:

32-gallon bins

Aerated rate: 1.5 L

min–1

Biochar amendment at 10 and 15%

could reduce the cumulative NH3

emissions up to 40% and 57%

The potential to decrease NH3 release is due to

the adsorption of NH3/NH4+by biochar pores

Acid functional groups on biochar surfaces can

trap NH4+and prevent their volatilization

(Wang et al., 2018)

Sewage sludge +

zeolite +lime

Wheat straw Bench-scale PVC:

130 L

Period: 56 days

12% biochar

12% biochar +zeolite could

signicantly reduce the CH4

58.03–65.17% and N2O

92.85–95.34%

Biochar enhanced the gaseous NH3 adsorption

and as a potential additive

Rapid mineralization of total organic matter

(TOM)

(Awasthi et al., 2016)

Chicken manure

+straw

Holm oak

Pyrolysis: 650 ◦C

3% biochar

addition

Trapezoidal piles:

1.5 m ×2 ×3

Pilot scale

Period: 20 weeks

Biochar efciently reduced the

levels of VOC during the

thermophilic phase

The most efcient VOC reduction

was observed in OVOCs compounds

(e.g., ketones, phenols and organic

acids)

Biochar dramatically reduced the amounts of

volatile nitrogen compounds, which are

produced by microbial modication of N-

compounds

Biochar’s strong sorption capacity may serve as

a mechanism for VOCs removal, which is aided

by the original biochar’s surface area

(S´

anchez-Monedero

et al., 2019)

Swine manure Non-activated

biochar

Pyrolysis:

495–505 ◦C

Pilot-scale

Non-activated

Biochar can be a promising and

comparably-priced option for

reducing NH3 emissions from swine

manure

The biochar’s NH3 mitigation is likely related

to creating a semi-porous crust layer over the

surface of the manure

(Maurer et al., 2017)

Fresh chicken

manure +

peanut straw

Biochar made

from charcoal at

400 ◦C

Surface area:

35.48 m2 g–1

10% biochar

Reactors: 60 L.

Aeration rate: 2 L

min–1

Period: 40 days

NH3, H2S, and TVOCs emission

decreased by 20.04%, 16.18%, and

17.55% in the experiment

Biochar could absorb TVOCs based on

distribution mechanisms and surface

adsorption

(Li et al., 2021)

Remarks: PVC: Polyvinyl chloride, VOCs: Volatile organic compounds, TVOCs: Total volatile organic compounds, OVOCs: Oxygenated volatile organic compounds,

SOC: and soil organic carbon.

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

Chowdhury et al. (2014) also reported that the addition of biochar

during barley straw with hen manure co-composting reduced by

27–32% total GHGs emissions (CO

equivalents) compared to barley

straw addition alone. However, some cases illustrated that biochar

might facilitate CO

emissions due to increasing the temperature factor

during composting (Guo et al., 2020). As mentioned above, declined

emission was a result of absorption by biochar properties, while

increase in their emission was related to strongly enhanced composting

substrate degradation and improved aeration environment during

composting. Thus, the effect of biochar on CO

emission should be

claried in further studies (Fan et al., 2008; Steiner et al., 2010, 2011).

Meanwhile, nitrous oxide (N

O) is also currently examined as the

third most important long-lived GHG (i.e., after CO

and CH

S´

anchez-García et al. (2015) found N

O emissions in both thermophilic

and mesophilic phases, which they attributed to the intensive nitrica-

tion occurring in these composting piles, where hydroxylamine was

Fig. 2. Effect of biochar on VOCs and GHGs emissions during composting (Org-N: Organic nitrogen).

Fig. 3. Effects of biochar addition on nitrogen transformation during the composting process.

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

likely formed as a nitrication intermediary (Maeda et al., 2011).

Reduced emission of N

O was commonly conrmed during the com-

posting, which is enhanced with biochar addition (L´

opez-Cano et al.,

2016; Vandecasteele et al., 2016). In the case of the average N

emission levels for the biochar blended compost and the control were

182 and 213 mg N

O m

−1

, respectively. According to Janczak et al.

(2017), adding biochar (10%) to poultry manure composting reduced

nitrogen loss by 38.3% when compared to the control. Furthermore, the

effect of added biochar on nitrogen transformation during the com-

posting process is shown in Fig. 3. Biochar’s potential to reduce N

emissions during composting is dependent on how it affects some pro-

cesses. Wang et al. (2013a) illustrated that biochar increased the gene

expression of N

O-reductase (nosZ), and the last enzyme in the deni-

trier reduction chain from N

O to N

. The nitrogen transformation and

O reduction mechanism include (i) rst, biochar absorbs NO

3−

reducing the amount of substrate accessible to denitrifying bacteria; (ii)

second, metals bonded to the biochar surface, such as iron and manga-

nese, aid in the chemical reduction from NO

3−

to N

; (iii) third,

improved aeration aids nitrication while suppressing denitrication;

and (iv) nally, denitrifying bacteria of N

O-reductase (nosZ) with high

expression through converting N

O to N

are enriched during the

composting process (Antonangelo et al., 2021; Guo et al., 2020).

4.3. Other gaseous emissions

The properties of biochar have facilitated the adsorption of odorous

gaseous emissions (e.g., NH

, H

S) during composting (Antonangelo

et al., 2021; Vandecasteele et al., 2016). Biochar can absorb large

quantities of NH

/NH

on the surfaces areas and biochar pores could be

led to signicantly decreased NH

emission (Dias et al., 2010; Mali´

nska

et al., 2014). Most of the experiments demonstrated that the biochars

could decrease NH

emissions in a range from 7% to 98%, with biochar

used rates of range from 2% to 28% (Sanchez-Monedero et al., 2018).

This capacity can be explained by relation to the internal pore volume,

micropores structure, large SSA, the occurrence of several surface acidic

function groups (i.e., OH•, C

–

O, amino group, etc.), and also the cation

exchange process that can reduce the NH

emissions during composting

(Chowdhury et al., 2014; Khan et al., 2014; Mali´

nska et al., 2014; Steiner

et al., 2010).

Compared to composting without biochar, composting with biochar

produced lower NH

emissions and more signicant NO

3−

concentra-

tions (Chen et al., 2010; Mali´

nska et al., 2014). NH

/NH

is converted

to NO

3−

by nitrifying bacteria from nitrication process that was

improved due to biochar addition, whereas the reverse pathway (i.e., the

ammonication process) was reduced (L´

opez-Cano et al., 2016). Biochar

addition to composting has been frequently investigated to reduce NH

emissions and prevent nitrogen losses (Mali´

nska et al., 2014). Similarly,

Maurer et al. (2017) used inactive biochar on the surface of pig manure

to minimize odor and found that NH

emissions were reduced by

13–23%.

Most biochars, on the other hand, are alkaline and have a highly

catalytic center dispersion throughout the pore system, making them as

excellent ideal for successful H

S oxidation (Agyarko-Mintah et al.,

2017b). During chicken litter composting, biochar (5% and 20% w/w)

reduced H

S emissions by 58–71%, respectively (Steiner et al., 2010).

Also, Vandecasteele et al. (2016) has demonstrated that the biochar’s

ability to absorb hazardous chemicals (e.g., H

S) aims to reduce their

negative effects on organic matter (OM) degradation during composting.

Furthermore, biochar used during composting can enhance aeration,

thus reducing toxic gas (i.e., H

S) emission from the decomposition of

OM (e.g., sulfur-containing organic compounds, protein, etc.) (Steiner

et al., 2010).

5. Effect of biochar on compost quality and maturity

Biochar is known to improve the quality of compost through

enhancing the structure, reducing contaminant bioavailability,

improving the nutrient status and increasing efciency of composting

(humication) and these effects vary with the nature of the biochar

(feedstock, pyrolysis, etc.) (Godlewska et al., 2017; Guo et al., 2020;

Xiao et al., 2017). Thus, the beneciation of organic wastes by adding

biochar has been investigated in recent years to gain insight into the

process. Of interest is the effect of different biochar on various organic

waste composts (e.g., food waste, green waste, non-hazardous wood

waste, biowastes, biomass, manures, etc.) (Al-Gheethi et al., 2021;

Godlewska et al., 2017; Steiner et al., 2011; Tran et al., 2021b) and the

optimal biochar addition ratio with respect to the composting process

(the effect on organic matter), heavy metals availability, and compost

nutrient status. The ratio of biochar addition examined has generally

been between 2% and 20% by weight, and the addition of biochar at

different times in the composting process has been investigated (Xiao

et al., 2017).

5.1. Heavy metals

Whilst biochar may contain small amounts of heavy metals

depending on the feedstock, it’s role in reducing the availability of

metals is well documented (Godlewska et al., 2017; Guo et al., 2020;

Xiao et al., 2017). This role of biochar is therefore highly applicable to

the use of organic waste composts due to the tendency of metals to be

sorbed and concentrated in organic matter. Recent research has afrmed

the role of biochar in immobilizing metals during composting. These

studies have investigated several sources of biochars and composts and

the effect on availability of a range of metals. Zhou et al. (2018) found

after composting pig manure with sawdust, wheat straw biochar and

humic acid exchangeable Cu, Pb and Zn was reduced between 60 and

95% with addition of 5–7.5% biochar and 2.5% humic acid. Owing to

biochar being generally alkaline, the pH value in the environment is

increased and leading to heavy metal ions could be converted to hy-

droxide (–OH) and absorbed on the biochar’s surface. The incorporation

of biochar increased the formation of surface complexes between func-

tional groups and Pb ions on their surface (Jiang et al., 2012). Liu et al.

(2017b) reported a range of reduction in metal bioaccessibilities for Pb,

Ni, Cu, Zn, As, Cr and Cd (1–59%), with a variable effect of the

amendment rate of biochar to compost. Analysis with principal

component analysis (PCA) identied three principal components inu-

encing the effect of biochar compost on the bioavailability of heavy

metals, with PC1 accounting for 47.1% of the variability, which was

mostly related to the passivation of Pb. The impact of biochar mixing

compost on heavy metal bioavailability illustrated that Cr accounted for

the most signicant contributions to PC2 (24.75%), and As mainly

contributed to the PC3 (20.57%). The three PCs results revealed an

understanding of the passivation effect of biochar amendments on the

bioavailability related to heavy metals (i.e., Pb, Ni, Cu, Zn, As, Cr and

Cd) during the composting process.

The surface Oxygen-functional groups on biochar could adsorb and

immobilize heavy metals based on the mechanisms such as cation and

anion metal attraction, ion exchange, reduction, precipitation, electron

shuttling, physisorption, and so on (Ahmad et al., 2014; Ding et al.,

2014; Zheng et al., 2021). The decreasing heavy metals concentration

has been linked to immobilization and accumulation of mobile metal

fractions in the biochar pores and organic substances from compost

piles. Biochar could change the physico-chemical proles, cation ex-

change capacity, and microbial community activities (Hern´

andez et al.,

2022; Wu et al., 2017a). Therefore, compost-added biochar can adsorb

and immobilize the various heavy metals from organic waste sources.

The interaction mechanism between biochar and compost stabilizes

heavy metals contaminants through adsorption, ion exchange, binding,

reduction, co-precipitation, electron shuttling, and physisorption are

shown in Fig. 4.

Studies have also linked the role of biochar in reducing heavy metal

availability to organic matter humication and the effect of biochar on

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

microbial community. Cui et al. (2020) reported the benecial effects of

biochar in promoting certain bacteria, microbial decomposition

contributed to reduction of heavy metal (Pb, Zn, Cd and Cu) bio-

accessibility by up to 44% using maize straw biochar with swine manure

during composting. The higher amendment rate (10%) was more

effective than a lower amendment rate (5%). In a study which compared

application of corn stalk biochar and montmorillonite for reduction of

bioavailability of heavy metals in chicken manure, the benet of biochar

to bacterial diversity was linked to the heavy metal fractions and bio-

accessibility reduction (Hao et al., 2019). Reduction of Cu by up to 90%

and 15% for Zn (Hao et al., 2019). Similarly, Song et al. (2021) reported

that the key factor in reducing the availability of heavy metals during

composting chicken manure and rice hull with biochar was the inter-

action of bacteria and organic components through the transformation

of organic matter by heavy metal resistant bacteria (63% and 73%

contribution to Cu and Zn bioavailability respectively).

Other studies by Awasthi et al. (2021) reported the addition of bio-

char affected the community distribution of metal resistant bacterial.

Chen et al. (2017b) reported reduced metal availability (DTPA extract-

able) by rice straw biochar addition to sediment and agricultural waste

compost and a signicant inuence on the bacterial community di-

versity. However, the main drivers of change in bacterial community

composition were related to pile temperature, C/N ratio, organic matter

content, and water soluble carbon rather than DTPA extractable heavy

metals.

5.2. Nutrients

Due to the properties of biochar (including surface area, pore vol-

umes, functional groups, CEC) it has been found to play a benecial role

on the dynamics and retention of nutrients during composting (Joseph

et al., 2018; Lee et al., 2018; Wang et al., 2021). Lee et al. (2018) re-

ported increased soil pH, organic carbon and exchangeable K, reduced

loss of ammonium and nitrate from soils (by ~40% relative to control

and ~20% relative to compost only) and avoided loss of phosphorus

relative to compost only (similar to control) with the application of

wood biochar at 4% to green dreg waste compost.

The addition of biochar to compost plays an important role in ni-

trogen dynamics. Fig. 5 demonstrates a comparison between biochar

application aims to improve N cycling and reduce N losses during the

composting process. Biochar addition signicantly impacted mineral N

dynamics, improved N cycling by increasing NO

3−

concentrations due to

biochar creating a suitable microenvironment for nitrifying bacteria

activity (Godlewska et al., 2017; L´

opez-Cano et al., 2016). The benecial

effects during the composting has enhanced nitrication with the pres-

ence of high concentrations of NO

3−

, leading to decreasing the amount

of N

O emissions released compared to without biochar. L´

opez-Cano

et al. (2016) reported an addition of 4% oak wood biochar to composting

of olive mill waste with sheep manure aided nitrication by inhibition of

ammonication and increased total nitrogen content. Liu et al. (2017b)

reported addition of 0–7% wheat straw biochar to sewage sludge

resulted in ammonia loss of 22.4–35.6% of the control and proliferation

of nitrifying bacteria increased nitrate substantially (62–310%). In

contrast, Plaimart et al. (2021) reported 10% coconut husk biochar

amended to anaerobic dairy/pig slurry digestate increased nutrient

adsorption (N and P) and reduced abundance of nitrifying bacteria and

as a result reducing nitrate leaching.

Ammonia volatilization is also reduced with increasing biochar

application rates (Janczak et al., 2017; Mandal et al., 2019). Though a

meta-analysis by Sha et al. (2019) found that biochar and soil pH were

Fig. 4. Interaction between biochar and composting in stabilizing heavy metals (M: Heavy metals).

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

important considerations as biochar applied to acidic soil could stimu-

late ammonia volatilization. In general, the acidic functional groups of

biochar were a factor in reducing NH

and CH

volatilization from

compost. Other studies have also reported reduced nitrogen loss by

biochar addition to compost (Hestrin et al., 2020; Qu et al., 2020; Wang

et al., 2021). Biochar oxidation was reported as an important factor for

reducing loss of NH

(Hestrin et al., 2020). Qu et al. (2020) reported

biochar in combination with gypsum reduced both NH

and CO

loss. A

10% rate of biochar addition was reported as optimal for reducing ni-

trogen loss with a reduction relative ton control of 15% (Wang et al.,

2021). Biochar also plays a role in phosphorus availability with com-

posting. Biochar derived from wheat straw added to pig manure was

reported to increase the concentrations of water-soluble nutrients,

including PO

43−

(5.6–7.4%), K

(14.2–58.6%), and Ca

(0–12.5%) and

was correlated with biochar amendment rate (at a rate of 10–15%)

(Zhang et al., 2016). Conversely, Xu et al. (2019) reported biochar

decreased the risk of eutrophication by enhancing P retention.

5.3. Humic/Fulvic acid

The ratio of humic acid (HA) to fulvic acid (FA) relates to compost

maturity, with a ratio of 1.6 HA:FA reecting mature compost (Jindo

et al., 2016). Bamboo biochar amendment of chicken manure increased

humication during composting with HA/FA ratio ranging from 0.21 to

4.92, with the highest amendment rate of biochar (10%) (Duan et al.,

2019b). Li et al. (2021) reported the synthesis of humic substances was

substantially higher than control (<5% in control to >6% HA in

amended) when compost was amended with biochar, tea residue and

bean dregs between 0 and 20%. The degradation of organic matter was

also around 28–50% higher with additives and the bacterial diversity

was increased (Li et al., 2021).

The addition of biochar accelerates and enhances the humication

through sorption of humus substances on the biochar surface which

increases the formation of aromatic polymers (Awasthi et al., 2018;

Jindo et al., 2019). This was supported by Liu et al. (2021b) with rice

straw biochar addition to swine manure promoting humic acid aroma-

tization and carboxylation, which mainly occurred during the cooling

stage of composting. In addition, biochar was found by near edge X-ray

adsorption spectroscopy to result in diversication of C, N and O coor-

dination in humic acid (Liu et al., 2021b). Guo et al. (2021) investigated

the structural changes in compost related to organic matter degradation

and humication. The amendment of compost with biochar plays an

important role with a large increase in total porosity and decrease in

bulk density mainly related to the decomposition of organic matter. The

degree of humication was enhanced by up to 31% with biochar

addition.

5.4. Phytotoxicity

The application of immature compost to the soil can be toxic and

inhibit plant growth. This is due to N–NH

and hazardous wastes,

which inhibit germination and root elongation. Commonly, N–NH

will increase rapidly in the early stages of composting due to ammoni-

cation. In addition, some hazardous wastes contain active compounds

harmful to germination, such as heavy metals, phenols, ethylene com-

pounds, salts, and organic acids. Therefore, to assess the quality of

whether the compost is ready for use, several parameters could be

considered to track potential phytotoxicity such as temperature, NH

3−

ratio, C/N ratio, germination capacity, the ratio of humic acids to

fulvic acids (HA/FA), SUV

254

/DOC trend, oxygen-uptake rate (OUR)

and CO

and biochemical composition (Godlewska et al., 2017; Xiao

et al., 2017). However, some parameters such as C/N ratios show un-

certain results corresponding to different types of feedstocks and addi-

tives (Godlewska et al., 2017; Xiao et al., 2017).

To date, the germination index (GI) is by far the most practical

parameter to check the phytotoxicity in compost. This is because the GI-

related method is reliable when directly related to the seed germination

event (Chen et al., 2017b; Liu et al., 2017a; Wei et al., 2014). Moreover,

this method can be used to test the degree of phytotoxicity on germi-

nation caused by salinity, soil pathogens, toxic substances, some other

physical and chemical properties of the compost (Selim et al., 2012).

According to Zucconi (1981), compost samples at different phase

treatments were extracted by shaking fresh samples with distilled water

at a ratio of 1:10 w/v for 1 h. Then, 10–20 mL of the water sample was

applied to lter paper in a Petri dish containing 20 seeds, such as radish

or cucumbers (Liu et al., 2017a; Wei et al., 2014). Each Petri dish was

incubated at room temperature in the dark for 48–72 h. Seed germina-

tion rate and root length were then estimated and compared with the

control condition (distilled water only). The presence of components

such as NH

, organic acids, and phenols can reduce GI value (<35%),

which is considered as a potential for plant toxicity (Chen et al., 2017b).

However, as the compost matures, these toxic substances are gradually

metabolized or returned to the ground state, resulting in an improved GI

value. For example, reducing ammonium and heavy metals during

Fig. 5. Biochar improved N cycling and reduced N losses during composting (Org-N: Organic nitrogen, AOB: Ammonium oxidizing bacteria, NOB: Nitrite

oxidizing bacteria).

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

composting can increase the GI (>80%), which indicates that the

compost is virtually free of phytotoxins (Chen et al., 2017b; Du et al.,

2019b).

Recent results indicate that biochar in compost can further reduce

phytotoxicity. By adding biochar, the GI value at the early stage can start

at less than 20% and reach 128% at the mature stage (Awasthi et al.,

2020; Chen et al., 2017b; Wei et al., 2014). The addition of biochar

(10%) to compost (i.e., chicken manure and peanut straw) helped to

achieve a GI of 97% vs. 92% as control (without biochar) (Li et al.,

2022). Similarly, the addition of bamboo biochar (3.8%) to the compost

(i,e, sewage sludge and rice straw) achieved a GI of 116% vs. 101% as

control (Xue et al., 2021). The proposed mechanism is that the added

biochar can reduce N–NH

through high nitrication activity, while

the reduction of heavy metals such as Cu, Zn may be due to the

complexation of metals of this type with chelating organic compounds

(Selim et al., 2012). Overall, the difference in feedstocks and biochar

dosage is considered the main factor affecting the nal GI value.

5.5. Enzymatic activity

There are many enzymes involved in catalyzing the reactions of the

composting process, such as Amylase, Aryl-sulfatase, Beta-glucosidase,

Cellulase, Dehydrogenases, Protease, Phosphatase, Peroxidase, Xylanase,

Urease, etc. (Awasthi et al., 2020; Gong et al., 2021; Sun et al., 2016).

Cellulase, Protease, and Phosphatase are essential enzymes that act as

catalysts to disintegrate cellulose, proteins, and phosphorous, respec-

tively. Besides, Dehydrogenases are intracellular enzymes that catalyze

the respiration of organic compounds. It was reported that the Dehy-

drogenase and Xylanase activities were enhanced with a higher added

biochar dosage (10%) during the composting of poultry manure

(Awasthi et al., 2020). These results were attributed to the proliferation

of enzyme-producing microorganisms thanks to biochar increased

porosity, surface area, nutrients, and temperature. However, mixed

poultry manure, straw, and biochar can reduce Beta-glucosidase and

Phosphatase activity at mesophilic and thermophilic phases (Sun et al.,

2016). According to this study, the use of rice straw biochar (2%) did not

promote enzyme activity because the small particle size (<0.5 mm) of

biochar was thought to increase the adsorption capacity of enzymes (Sun

et al., 2016). Similarly, ne biochar (0.2–0.4 mm) at higher dosage

(10%) causes the decrease in Protease and Urease activity (Du et al.,

2019a, 2019b). By contrast, the application of large size biochar (20–40

mm) reduces the peak temperature and increases Aryl-sulfatase,

Beta-glucosidase, and Dehydrogenase activities (Du et al., 2019a). By

adding biochar into vermicomposting, the enzymatic activities (Prote-

ase, Phosphatase, and Cellulase) were improved, which is attributed to the

use of big biochar (20 and 50 mm) (Gong et al., 2021). These results

implied that the particle size of biochar might be a key to improving

enzyme activity. In addition, high doses of biochar are thought to reduce

alpha diversity, thereby selecting for a specic microbial population,

leading to limited production of some specic enzymes. Therefore, some

in-depth studies on the dosage and particle size of biochar are needed to

clarify the above arguments and evaluate its inuence on the com-

posting process.

6. Conclusions and future perspectives

The addition of biochar as an additive has improved the quality of

composting process due to their specic physicochemical properties (e.

g., larger surface area and porosity, and high ion exchange capacity).

Biochar application signicantly affects the moisture content, oxygen

available, temperature, pH, and C/N during organic wastes composting.

Moreover, biochar promoted aeration rate, increased gas exchange, and

prevented the formation of anaerobic zones, leading to signicantly

reduced GHGs and odor emissions during composting. Biochar is a

crucial factor to promote the interaction of heavy metal resistant bac-

teria and organic matter; results reduce the availability of heavy metals

during composting. With biochar, the richness and diversity of the mi-

crobial community were observed to improve, leading to enhanced

microbial activities, thereby promoting nutrient contents and humi-

cation during composting. To achieve the maximum efciency for bio-

char application during composting, some of the commonly referenced

ranges should be considered such as pH (6.5–7.5), moisture content

(50–60%), initial C/N ratio (20–25:1), biochar doses (1–20% w/w) aim

to improve oxygen content availability, enhancing the performance and

humication, accelerating organic matter decomposition and growth of

the microbial community. Also, need to select the biochar materials that

contain signicantly functional groups, e.g., Si–O–Si, OH, COOH, C

–

C–O, N for enhancing remediation mechanisms such as adsorption,

distribution, ion exchange,

−

interaction, etc. Our review has pro-

vided current knowledge on the role of biochar during the organic waste

composting process; however, many aspects are still ambiguous in this

area. Therefore, we highly recommended these knowledge gaps

following for further research:

•Many studies have reported on the effectiveness of biochar addition

to the composting process. However, the effect of biochar properties

on composting is still not fully understood. Biochar types (feedstock,

pyrolysis conditions), dosage, and particle size need to be further

studied to promote efciency and improve the quality of the end

product.

•The effect of biochar on the nitrogen cycle has been evaluated

through biochar increasing the activity of nitrifying bacteria,

reducing methanogens. However, how it affects to carbon cycle

needs to be investigated to clarify the role and function of microbial

community and biomass balance during the composting process.

•The interaction of specic microorganisms (bacteria and fungi) with

biochar and its correlation with composting performance (e.g.,

biodegradation, VOCs and odors emission, maturity, and nutrients)

should be investigated to ll up the research gaps in this area.

•Biochar addition to the composting process has been successfully

applied at lab-scale with short period through a number of studies.

However, eld scale applications should be conducted to evaluate

the efciency and optimal conditions. Furthermore, long-term

studies need to be focused on providing an in-depth understanding

of its effects on biodegradation during the composting period.

•Porous biochar (mesopores, and macropores) will be dramatically

statured by organic matter of compost mixture, leading to reduced

performance and its efciency. Further studies should be conducted

to address this matter to extend the life cycle of biochar.

•Heavy metal contaminants have adverse effects on the microbial

community of compost mixture. Biochar addition to composting can

reduce the availability of heavy metals; however, heavy metal

inactivation mechanisms should be claried to elucidate this

concern.

Credit author statement

Minh Ky Nguyen: Methodology, Formal analysis, Writing – original

draft. Chitsan Lin: Data curation, Conceptualization, Supervision. Peter

Sanderson: Writing – review & editing. Bao Trong Dang: Writing –

original draft. Xuan-Thanh Bui: Data curation, Methodology. Nguyen

Ngoc Son Hai: Methodology. Dai-Viet N. Vo: Writing – original draft.

Hong Giang Hoang: Methodology. Huu Tuan Tran: Formal analysis,

Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing nancial

interests or personal relationships that could have appeared to inuence

the work reported in this paper.

M.K. Nguyen et al.

Chemosphere 299 (2022) 134488

Acknowledgements

The authors wish to express their gratitude to Van Lang University,

Viet Nam for nancial support for this research.

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