Science of the Total Environment
Research progress in solid carbon source–based denitrification technologies for
different target water bodies
--Manuscript Draft-Manuscript Number:
STOTEN-D-20-19231R1
Article Type:
Review Article
Keywords:
solid carbon source, heterotrophic denitrification, nitrogen pollution, nitrogen removal,
water treatment
Corresponding Author:
Shiyang Li, Ph.D.
Shanghai University School of Environmental and Chemical Engineering
Shanghai, CHINA
First Author:
Feifan Zhang
Order of Authors:
Feifan Zhang
Chengjin Ma
Xiangfeng Huang, Ph.D.
Jia Liu, Ph.D.
Lijun Lu, Ph.D.
Kaiming Peng, Ph.D.
Shiyang Li, Ph.D.
Abstract:
Nitrogen pollution in water bodies is a serious environmental issue which is commonly
treated by various methods such as heterotrophic denitrification. In particular, solid
carbon source (SCS)–based denitrification has attracted widespread research interest
due to its gradual carbon release, ease of management, and long-term operation. This
paper reviews the types and properties of SCSs for different target water bodies. While
both natural (wheat straw, wood chips, and fruit shells) and synthetic (polybutylene
succinate, polycaprolactone, polylactic acid, and polyhydroxyalkanoates) SCSs are
commonly used, it is observed that the denitrification performance of the synthetic
sources is generally superior. SCSs has been used in the treatment of wastewater
(including aquaculture wastewater), agricultural subsurface drainage, surface water,
and groundwater; however, the key research aspects related to SCSs differ markedly
based on the target waterbody. These key research aspects include nitrogen pollutant
removal rate and byproduct accumulation (ordinary wastewater); water quality
parameters and aquatic product yield (recirculating aquaculture systems); temperature
and hydraulic retention time (agricultural subsurface drainage); the influence of
dissolved oxygen (surface waters); and nitrate-nitrogen load, HRT, and carbon source
dosage on denitrification rate (groundwater). It is concluded that SCS-based
denitrification is a promising technique for the effective elimination of nitrate-nitrogen
pollution in water bodies.
Response to Reviewers:
Reviewer #1：According to the authors, "Based on our meta-analysis we identify and
classify the denitrification principles of synthetic SCSs, commonly used SCSs,
influencing factors, and effluent parameters." However, I did not see any meta-analysis
of the existing results in the paper. More importantly, the authors mainly summarized
the findings of past studies and did not provide in-depth analyses of these results.
Especially in section 3, I expected the authors to give a critical analysis of the findings
of previous studies on the application of SCSs in different target water bodies, but in
my view that section of the paper merely presented a summary of the results of the
past studies. Therefore, I do not see it to be of much interest to the STOTEN
readership.
Answer:
Thanks a lot for the suggestion. Detailed descriptions of meta-analysis on synthetic
SCS-based denitrification and further discussion on the effect of different SCSs and
reaction conditions on the denitrification efficiency were supplemented in the revised
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manuscript as a section of ‘ Meta-analysis on synthetic SCS-based denitrification’.
During the meta-analysis, process of data compilation, determination of target factors
and application of software were detailedly introduced in the revised manuscript, and
the results of nitrate removal rate effect size and its standard deviation were expressed
visually in the form of five forest plots, which were also supplemented as Figure 2. The
actual data on mean removal rate, SD, number of studies and the classification of each
factors are listed in Table S1 in Supplementary Information.
Based on the results from meta-analysis, the effect of carbon source type, carbon
source species, influent N concentration, HRT and water temperature on nitrate
removal rates were discussed respectively, and finally provide scientific supports for
the future applications of synthetic SCS-based denitrification.
Due to the supplements of a novel section, some adjustments has been made in
Introduction and Conclusion parts.
In the Introduction: “In this paper, we provide an overview both lab and field studies of
the types and operating principles of commonly used SCSs based on a review in this
field. We identify and classify the denitrification principles of synthetic SCSs, commonly
used SCSs, influencing factors, and effluent parameters. Meta-analysis was further
applied for better understanding of the effect of different SCSs and reaction conditions
on the denitrification efficiency. Finally, we summarize the limitations governing the use
of SCSs to provide a scientific basis for the future development of SCS-based
denitrification techniques.”
In the Conclusion “Solid-phase carbon sources have good application prospects for
solid phase nitrification. Research with respect to wastewater treatment has mainly
been focused on the removal efficiency of nitrogen pollutants and DOC accumulation
in the effluent, while studies on recirculating aquaculture systems have focused on
product yield and water quality parameters. Agricultural subsurface drainage system
research was extensive, and focused on natural SCSs and the influence of
temperature and HRT on denitrification efficiency. The primary aspect of surface water
research was the influence of DO on denitrification efficiency, while studies on
groundwater were mainly focused on the influence of nitrate-nitrogen load, HRT, and
carbon source dosage on denitrification efficiency. The optimization of operation
parameters (especially HRT), the hybrid application of synthetic carbon sources and
the further design of low temperature-tolerated reactors are worthy of continued study.”
Reviewer #3：I especially notice one component that is missing from the review. That
is the scientific basis of all prominent technologies mentioned for the denitrification.
With the detailed underlying processes and pathways and then highlighting differences
in the process for which one method is better than the other will certainly enrich this
review.
Answer:
Thanks a lot for the suggestion. For better illustration of the scientific basis of SCSbased denitrification and comparing the differences of different SCSs during
denitrification process, a section of ‘Comparison of raw and synthetic SCSs’ was
supplemented in the revised manuscript.
Since raw and synthetic SCSs are the commonly used carbon sources for biological
denitrification, the comparison between the two types of SCSs were extensively
discussed in this section. We analyzed their differences in material sources, hydrolytic
ability and hydrolysate, as well as denitrification performance. Besides, their adaptive
applications in different target water bodies were also summarized and discussed.
Due to the supplement of new contents, the title for the second section has been
adapted as ‘Types, properties and utilization of SCSs’ for better summarization.
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Cover letter
[Feb 25, 2020]
Jay Gan
Editor in Chief
Science of the Total Environment
Dear Editor:
I wish to submit a revised version of our manuscript for publication in Science of the Total
Environment, titled “Research progress in solid carbon source–based denitrification technologies
for different target water bodies.” The paper was coauthored by Feifan Zhang, Chengjin Ma,
Xiangfeng Huang, Jia Liu, Lijun Lu, Kaiming Peng.
We would like to thank you for giving us the opportunity to submit a revised version of our
manuscript and the reviewers for their helpful suggestions and comments. We have replied to
each comment in detail and revised the manuscript accordingly. We specifically cleared some
unclarities about our collection efficiency, which was a concern of both reviewers. For a detailed
point-by-point response, please refer to the attached response letter.
This manuscript has not been published or presented elsewhere in part or in entirety and is not
under consideration by another journal. We have read and understood your journal’s policies, and
we believe that neither the manuscript nor the study violates any of these. There are no conflicts
of interest to declare.
Thank you for your consideration. I look forward to hearing from you.
Sincerely,
Shiyang Li
College of Environmental Science and Engineering, Tongji University, Shanghai 200092,
People’s Republic of China.
Tel: +86 021 65982399
Email: lishiyang@tongji.edu.cn
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Solid carbon source–based denitrification technologies for different
target water bodies: A review
Chengjin Ma a, Xiangfeng Huang a, Jia Liu a, Lijun Lu a, Kaiming Peng a, Shiyang Li a
a
College of Environmental Science and Engineering, State Key Laboratory of Pollution
Control and Resource Reuse, Ministry of Education Key Laboratory of Yangtze River Water
Environment, Tongji University, Shanghai 200092, People’s Republic of China
Correspondence information: Shiyang Li, College of Environmental Science and
Engineering, Tongji University, Shanghai 200092, People’s Republic of China. Tel: +86 021
65982399. Email: lishiyang@tongji.edu.cn
Reviewer #1：
According to the authors, "Based on our meta-analysis we identify and classify the
denitrification principles of synthetic SCSs, commonly used SCSs, influencing factors, and effluent
parameters." However, I did not see any meta-analysis of the existing results in the paper. More
importantly, the authors mainly summarized the findings of past studies and did not provide in-depth
analyses of these results. Especially in section 3, I expected the authors to give a critical analysis of
the findings of previous studies on the application of SCSs in different target water bodies, but in my
view that section of the paper merely presented a summary of the results of the past studies.
Therefore, I do not see it to be of much interest to the STOTEN readership.
Answer:
Thanks a lot for the suggestion. Detailed descriptions of meta-analysis on synthetic SCS-based
denitrification and further discussion on the effect of different SCSs and reaction conditions on the
denitrification efficiency were supplemented in the revised manuscript as a section of ‘ Meta-analysis
on synthetic SCS-based denitrification’.
During the meta-analysis, process of data compilation, determination of target factors and
application of software were detailedly introduced in the revised manuscript, and the results of nitrate
removal rate effect size and its standard deviation were expressed visually in the form of five forest plots,
which were also supplemented as Figure 2. The actual data on mean removal rate, SD, number of studies
and the classification of each factors are listed in Table S1 in Supplementary Information.
Based on the results from meta-analysis, the effect of carbon source type, carbon source species,
influent N concentration, HRT and water temperature on nitrate removal rates were discussed
respectively, and finally provide scientific supports for the future applications of synthetic SCS-based
denitrification.
Due to the supplements of a novel section, some adjustments has been made in Introduction and
Conclusion parts.
In the Introduction: “In this paper, we provide an overview both lab and field studies of the types
and operating principles of commonly used SCSs based on a review in this field. We identify and classify
the denitrification principles of synthetic SCSs, commonly used SCSs, influencing factors, and effluent
parameters. Meta-analysis was further applied for better understanding of the effect of different SCSs
and reaction conditions on the denitrification efficiency. Finally, we summarize the limitations governing
the use of SCSs to provide a scientific basis for the future development of SCS-based denitrification
techniques.”
In the Conclusion “Solid-phase carbon sources have good application prospects for solid phase
nitrification. Research with respect to wastewater treatment has mainly been focused on the removal
efficiency of nitrogen pollutants and DOC accumulation in the effluent, while studies on recirculating
aquaculture systems have focused on product yield and water quality parameters. Agricultural
subsurface drainage system research was extensive, and focused on natural SCSs and the influence of
temperature and HRT on denitrification efficiency. The primary aspect of surface water research was the
influence of DO on denitrification efficiency, while studies on groundwater were mainly focused on the
influence of nitrate-nitrogen load, HRT, and carbon source dosage on denitrification efficiency. The
optimization of operation parameters (especially HRT), the hybrid application of synthetic carbon
sources and the further design of low temperature-tolerated reactors are worthy of continued study.”
Reviewer #3：
I especially notice one component that is missing from the review. That is the scientific basis of
all prominent technologies mentioned for the denitrification. With the detailed underlying processes
and pathways and then highlighting differences in the process for which one method is better than
the other will certainly enrich this review.
Answer:
Thanks a lot for the suggestion. For better illustration of the scientific basis of SCS-based
denitrification and comparing the differences of different SCSs during denitrification process, a section
of ‘Comparison of raw and synthetic SCSs’ was supplemented in the revised manuscript.
Since raw and synthetic SCSs are the commonly used carbon sources for biological denitrification,
the comparison between the two types of SCSs were extensively discussed in this section. We analyzed
their differences in material sources, hydrolytic ability and hydrolysate, as well as denitrification
performance. Besides, their adaptive applications in different target water bodies were also summarized
and discussed.
Due to the supplement of new contents, the title for the second section has been adapted as ‘Types,
properties and utilization of SCSs’ for better summarization.
Research progress in solid carbon source–based denitrification
technologies for different target water bodies
Feifan Zhang a, Chengjin Ma a, Xiangfeng Huang a, Jia Liu a, Lijun Lu a, Kaiming Peng a,
Shiyang Li a
a
College of Environmental Science and Engineering, State Key Laboratory of Pollution
Control and Resource Reuse, Ministry of Education Key Laboratory of Yangtze River Water
Environment, Tongji University, Shanghai 200092, People’s Republic of China
Correspondence information: Shiyang Li, College of Environmental Science and
Engineering, Tongji University, Shanghai 200092, People’s Republic of China. Tel: +86 021
65982399. Email: lishiyang@tongji.edu.cn
Abstract: Nitrogen pollution in water bodies is a serious environmental issue which is
commonly treated by various methods such as heterotrophic denitrification. In particular, solid
carbon source (SCS)–based denitrification has attracted widespread research interest due to its
gradual carbon release, ease of management, and long-term operation. This paper reviews the types
and properties of SCSs for different target water bodies. While both natural (wheat straw, wood
chips, and fruit shells) and synthetic (polybutylene succinate, polycaprolactone, polylactic acid, and
polyhydroxyalkanoates) SCSs are commonly used, it is observed that the denitrification
performance of the synthetic sources is generally superior. SCSs has been used in the treatment of
wastewater (including aquaculture wastewater), agricultural subsurface drainage, surface water, and
groundwater; however, the key research aspects related to SCSs differ markedly based on the target
waterbody. These key research aspects include nitrogen pollutant removal rate and byproduct
accumulation (ordinary wastewater); water quality parameters and aquatic product yield
(recirculating aquaculture systems); temperature and hydraulic retention time (agricultural
subsurface drainage); the influence of dissolved oxygen (surface waters); and nitrate-nitrogen load,
HRT, and carbon source dosage on denitrification rate (groundwater). It is concluded that SCSbased denitrification is a promising technique for the effective elimination of nitrate-nitrogen
pollution in water bodies.
Keywords: solid carbon source, heterotrophic denitrification, nitrogen pollution, nitrogen
removal, water treatment
1 Introduction
A fast increase in human population and rapid developments in the industrial and agricultural
sectors have resulted in nitrogen pollution–induced eutrophication of water bodies (Jafari et al.,
2015, Liu et al., 2019). In natural water bodies, nitrogen existing in forms other than nitrate-nitrogen
is gradually converted to nitrate-nitrogen by microorganisms, resulting in an increasingly severe
accumulation of nitrate-nitrogen in water bodies (Canfield et al., 2010, Zhang et al., 2014). Water
bodies containing excessive nitrate-nitrogen may cause damage to crop root systems and impact
crop yields when used as water sources for agricultural irrigation (Steidl et al., 2019). Furthermore,
drinking from polluted water bodies may cause toxic effects or even death in wildlife, and excessive
nitrate-nitrogen intake in humans can result in methemoglobinemia (blue baby syndrome) or may
cause toxic effects or even death in wildlife in extreme cases (Li et al., 2017). Studies have revealed
that nitrogen pollution in groundwater exists in approximately 110 countries and has caused safety
issues concerning drinking water globally (Chen et al., 2013, Feng et al., 2020b).
In response to existing nitrogen pollution issues, many physicochemical and bioecological
methods have been utilized for the removal of excess nitrate-nitrogen in water. Owing to the current
technological progress, the application of carbon sources under anoxic conditions to achieve
biological nitrogen removal through denitrification (Cheng et al., 2020, Zhang et al., 2017) has
emerged as the mainstream method employed in water treatment. Treatment processes in traditional
wastewater treatment plants usually involve the addition of external carbon sources to promote
microbial denitrification. Commonly used external carbon sources mainly consist of water-soluble
low-molecular-weight (LMW) organic compounds such as methanol, acetates, and LMW sugars
(Feng et al., 2013). However, the addition of dissolved carbon sources to natural water bodies
increases the organic pollution load and may even cause secondary pollution if the calculated dosage
amounts are unreasonable. Furthermore, the need for dosing facilities is also a limiting factor in the
widespread use of dissolved carbon sources. Conversely, solid carbon sources (SCSs) have attracted
a considerable research interest due to their appropriate carbon release rates, favorable conditions
for microbial biofilm growth, long carbon release duration, ease of management, and long-term
operation(Yang et al., 2020b, Zhang et al., 2020). Currently, the most commonly used SCSs can be
classified into two major categories, i.e, natural cellulosic materials and synthesized biodegradable
polymers (BDPs). Target water bodies for SCS-based denitrification include wastewater (including
aquaculture wastewater), agricultural subsurface drainage, surface water, and groundwater. To date,
studies regarding the application of SCSs to different target water bodies under specific conditions
and operating requirements have investigated the types and properties of commonly used carbon
sources, process characteristics, influencing factors, and effluent parameters.
In this paper, we provide an overview both lab and field studies of the types and operating
principles of commonly used SCSs based on a review in this field. We identify and classify the
denitrification principles of synthetic SCSs, commonly used SCSs, influencing factors, and effluent
parameters. Meta-analysis was further applied for better understanding of the effect of different
SCSs and reaction conditions on the denitrification efficiency. Finally, we summarize the limitations
governing the use of SCSs to provide a scientific basis for the future development of SCS-based
denitrification techniques.
2 Types, properties and utilization of SCSs
2.1 Commonly used SCSs
Commonly used SCSs can be classified into three major categories: natural (cellulosic),
synthetic (polymeric), and other SCSs (e.g., acidogenic liquids from food waste, hydrolyzed sludge
and other reprocessed organic materials) (Guo et al., 2017, Zhang et al., 2016a). Due to the scarcity
of relevant existing literature and applications, other SCSs have not been discussed in this paper.
Most early applications of SCSs involved the use of natural carbon sources, including
cellulosic agricultural and forestry wastes, such as corn cobs, corn stover, wheat straw, cardboard
fibers, leaf litter, tree bark, wood chips, and fruit kernels, which were reported in previous studies
(Chang et al., 2016, Christianson et al., 2012, Chun et al., 2009, Gomez et al., 2000). Natural SCSs
have several advantages such as low cost and ease of acquisition. Because they consist primarily of
agricultural and forestry wastes, they are commonly used as filling materials in denitrification
located near the outflow of agricultural subsurface drainage systems.
However, the wider application of natural SCSs in denitrification is limited by unstable carbon
release rates, slow denitrification rates, excessive release of dissolved organic carbon (DOC), and
increased color intensity in the effluent. Therefore, while the denitrification efficiencies of natural
SCSs were reported more frequently in earlier studies, more recent literature has focused on the
simultaneous removal of total nitrogen (TN) and nitrate-nitrogen (Si et al., 2018), aerobic
denitrification (Cheng et al., 2020), characteristics of leached pollutants (Abusallout and Hua, 2017,
Jia et al., 2019), and functional microbial communities (Hu et al., 2019).
In recent years, the use of BDPs as synthetic SCSs has attracted considerable research attention.
BDPs only decompose under the action of extracellular enzymes secreted by specific
microorganisms; hence, they can avoid or mitigate many of the aforementioned problems associated
with natural SCSs. BDPs refer to a class of high-molecular-weight (HMW) materials that are
degraded or enzymatically hydrolyzed under biological action to generate LMW compounds which
can be utilized by organisms. In addition to serving as carbon sources, BDPs as synthetic SCSs also
act as carriers for the growth of denitrifying microorganisms (Zhang et al., 2016c; d) and have been
used in the denitrification of water bodies since the 1990s. In 1992, Müller et al. reported the use of
polyhydroxybutyrate (PHB) granules as an SCS for denitrification through the construction of a
laboratory-scale up flow fixed-bed reactor (Muller et al., 1992). They found that the denitrification
rate at 10 ºC was 11 mg·(L·h)-1, and cells co-immobilized with the PHB granules exhibited a higher
denitrification rate compared to suspended cells. Currently, only a few types of synthetic SCSs are
commonly applied to the denitrification of wastewater, including polylactic acid (PLA) (Fan et al.,
2012), polycaprolactone (PCL) (Chu and Wang, 2011a; b, Li et al., 2016, Wu et al., 2013),
polybutylene succinate (PBS) (Wang and Chu, 2016), and polyhydroxyalkanoates (PHAs) (Lopardo
and Urakawa, 2019). Table 1 illustrates a summary of the denitrification efficiencies reported in a
number of studies on SCSs.
Table 1. Summary of denitrification performance of different solid carbon source (SCSs) reported in
previous literature
2.2 Basic principles of synthetic SCS-based denitrification
2.2.1 Synthetic SCSs Utilization
Anoxic biological denitrification usually takes place under the action of microorganisms and
fungus. It involves biological redox reactions in which organic carbon sources and nitrates serve as
electron donors and acceptors, respectively. This results in the reduction of nitrate to nitrogen, which
is subsequently removed from the water body through the denitrification process. The overall
process consists of the following steps:
𝑁𝑂3− → 𝑁𝑂2− → 𝑁𝑂 → 𝑁2 𝑂 → 𝑁2
However, only soluble biodegradable carbon source such as acetic acid, formic acid, and
methanol can be directly utilized by denitrifying microorganisms during biological denitrification,
whereas SCSs must be converted into LMW compounds prior to utilization by microorganisms.
The utilization of BDPs as carbon sources and bacterial carriers for SCS based denitrification
occurs under microbial action, with SCS biodegradation enabling the growth and metabolism of
microorganisms attached to the surfaces of SCSs and the . Polymer degradation can be characterized
as a process that results in the breakage of a large and complex molecule into smaller molecules
(dos Santos et al., 2018). First, biofilms are formed through the attachment and growth of
microorganisms on polymer surfaces; then, polymer chains are cleaved by extracellular enzymes,
leading to the hydrolysis of the polymers into soluble LMW compounds (Hocking et al., 1996, Shah
et al., 2008). Subsequently, LMW compounds enter functional microorganisms through
semipermeable membranes and are utilized as carbon sources and electron donors. In short, the
utilization of BDPs involves hydrolysis and denitrification, with the former being the rate-
determining process (Takahashi et al., 2011).
Figure 1. Schematic of the utilization of biodegradable polymers (BDPs) as carbon sources by denitrifying
microorganisms
Microbial degradation has various impacts on SCSs, including changes in chemical structure
(Lucas et al., 2008), significant reductions in the polymer molecular weight, increased surface
roughness, formation of perforations and pits, and mechanical deformation due to structural
dissolution and breakdown. In turn, these impacts change the carbon release rate of the material.
Microbial degradation also leads to a decrease in the crystalline phase content and a corresponding
increase in readily hydrolysable amorphous content, resulting in hydrophilicity changes.
2.2.2 Basic mechanisms of synthetic SCS-based denitrification
Müller et al. had reported the use of PHB (molecular formula: [C4H6O2]n) as a synthetic SCS
(Muller et al., 1992). And PHB based denitrification reaction with nitrate ions as electron acceptors
is as follows:
5[𝐶4𝐻6 𝑂2 ] + 18𝑁𝑂3 − → 9𝑁2 + 18𝐻𝐶𝑂3 − + 2𝐶𝑂2 + 6𝐻2𝑂
(1)
According to Boley et al., by assuming a yield coefficient (Yx/s) of 0.45 g biomass/g PHB, the
summarized denitrification equation including biomass formation when PHB is used as the carbon
source can be expressed as (Boley et al., 2000):
0.494[𝐶4 𝐻6𝑂2 ] + 𝑁𝑂3 −
→ 0.415𝑁2 + 𝐻𝐶𝑂3 − + 0.130𝐶𝑂2 + 0.169[𝐶5 𝐻7𝑂2 𝑁]
(2)
+ 0.390𝐻2 𝑂
where C5H7O2N is the molecular formula of the microbial cells.
Because the molecular formulae of commonly used synthetic SCSs (PCL, PBS, PLA, and
PHAs) can be represented as CxHyOz, the basic denitrification reaction for synthetic SCSs is as
follows:
5[𝐶𝑥 𝐻𝑦 𝑂𝑧 ] + (4𝑥 + 𝑦 − 2𝑧)𝑁𝑂3 −
→ (2𝑥 +
𝑦
−
− 𝑧)𝑁2 + (4𝑥 + 𝑦 − 2𝑧)𝐻𝐶𝑂3 + (𝑥 − 𝑦 + 2𝑧)𝐶𝑂2
2
(3)
+ (2𝑦 − 2𝑥 + 𝑧)𝐻2𝑂
Using Equation (3), the calculated mass of PHB required to reduce the unit mass of nitratenitrogen is 2.92 g PHB/g NO3--N when biomass formation is excluded and 3.03 g PHB/g NO3--N
when biomass formation is considered. Based on theoretical calculations, the masses of glucose,
methanol, and ethanol required to reduce the unit mass of nitrate-nitrogen when biomass formation
is excluded are 2.68, 1.90, and 1.37 g/g NO3--N, respectively. When biomass formation is considered,
the required masses of methanol and ethanol are 2.47 and 2.01 g/g NO3--N, respectively (Mateju et
al., 1992).
2.3 Comparison of raw and synthetic SCSs
Compared with liquid carbon sources, SCSs can be used as biofilm carriers for microorganisms,
which are more convenient for transportation, operation and storage (Boley et al., 2000). In previous
studies, most solid carbon sources can release organic matter needed in denitrification systems,
especially in low C/N wastewater treatment.
In terms of material sources, raw and synthetic SCSs come from multiple sources. Cellulosic
agricultural and forestry wastes are the main components of raw SCSs. Synthetic SCSs are mainly
produced by petrochemical engineering, while some studies focus on the utilization of endogenous
accumulation of PHAs by microorganisms (He et al., 2018, Wu et al., 2021). With the development
of material science, more and more new materials are used in sewage treatment. BDPs with excellent
biocompatibility, biodegradability and non-toxicity, such as PCL and PBS, are ideal materials for
environmental protection (Wu et al., 2012).
The hydrolytic ability of carbon sources is regarded as a key factor of denitrification (Hang et
al., 2016). Denitrification rate is inhibited when available carbon from carbon source is insufficient,
while an over-release of carbon may on the contrary, cause carbon loss and organic pollution (Healy
et al., 2012). Regarding the characteristics of carbon release, first-order kinetics can basically
describe the carbon release process of all SCSs. Raw SCSs can release more organic matter, and
synthetic SCSs have an advantage in the sustainability of carbon release. In order to take advantage
of the advantages of both kinds of SCSs in terms of carbon release, some carbon sources mixed with
raw and synthetic SCSs have been prepared (Jiang et al., 2020, Liu et al., 2018b, Xiong et al., 2019) .
The hydrolysate of raw and synthetic SCSs are quite different. Raw SCSs often contain
multiple components. Taking lignocellulosic materials as an example, they are mainly comprised of
lignin, cellulose and hemicellulose. Among them, cellulose can be easily used by microorganisms
and hydrolyzed into glucose, hemicellulose can also be used after enzymatic hydrolysis into small
molecular organic matter, and lignin is difficult to be degraded (Forrest et al., 2010, Zhong et al.,
2020). It has been reported that the denitrification efficiency of woody biomass is lower than that
of herbaceous biomass due to its lignin content and natural structure in woody biomass (Kim et al.,
2016).
The hydrolysate of the synthetic SCSs is related to its own structure, and its degradation
involves the joint effect of several processes. Taking PHB as an example, PHB can directly undergo
abiotic hydrolysis in water because it contains -COO- group as a polyester. When PHB is used as
SCS in a biological denitrification system, biodegradation of PHB also plays a significant role.
Enzymes in the intracellular and extracellular matrix can disrupt long-chain polymer chains and
hydrolyze oligomers, and these oligomers will be further hydrolyzed into polymer monomers after
a short period of time (dos Santos et al., 2018, Kessler et al., 2014). Water temperature, pH value,
microbial community and the supplement of nutrients will all affect the degradation process of SCSs.
In terms of denitrification, both SCSs can promote denitrification in low C/N wastewater
treatment. Due to the difference in carbon release patterns between the two types of carbon sources,
and considering the sustainability of carbon release, synthetic SCSs are ideal slow-release carbon
sources for low TN water treatment (such as groundwater and drinking water), raw SCSs may be a
more ideal carbon source to enhance the denitrification of secondary wastewater (Chu and Wang,
2013, Xiong et al., 2020).
3 Application of SCSs in different target water bodies
Currently, the target water bodies for SCS-based denitrification technologies include
wastewater (Duan et al., 2016, Xu et al., 2018a, Yang et al., 2020b) (including aquaculture
wastewater (Gutierrez-Wing et al., 2012, Zhang et al., 2016b)), agricultural subsurface drainage,
(Christianson et al., 2016, Chun et al., 2010), surface water(Feng et al., 2019), and groundwater
(Zhang et al., 2018).
3.1 Wastewater
Studies on the application of SCSs to wastewater denitrification can be classified into two main
categories; (1) advanced treatment of ordinary wastewater or wastewater treatment plant effluent
and (2) purification of nitrogen pollutants in recirculating aquaculture systems.
3.1.1 Treatment of ordinary wastewater
For wastewater with characteristically low C/N ratios, nitrogen removal is often achieved via
dosing with external carbon sources. Generally, SCSs are used as filling materials and microbial
carriers in fixed-bed reactors for the purification of wastewater by denitrification. In a study by Rout
et al. (Rout et al., 2017), organic solid waste substances were used as carbon sources to investigate
the influence of experimental parameters, such as influent nitrate concentration, hydraulic retention
time (HRT), and bed depth, on denitrification efficiency. The found that a low HRT reduced nitrate
removal efficiency, increased nitrite accumulation, and decreased effluent chemical oxygen demand
(COD). When the influent nitrate concentration was 70, 50, and 30 mg/L, the effluent nitrate
concentration could be maintained at < 10 mg/L for 31, 39, and 49 days, respectively, with the
denitrification process following first-order reaction kinetics. Sun et al. (Sun et al., 2019a) utilized
alkali-pretreated corn cobs as solid carbon sources and biofilm carriers for the removal of nitrates
and refractory organic pollutants from coking wastewater. They found that the treatment process
could concurrently achieve the stable removal of over 90 % of residual nitrate and the degradation
of typical refractory organic matter. In another study, Duan et al. compared the performances of
PBS, PHBV, and PCL as carbon sources for the treatment of nitrified swine wastewater (Duan et al.,
2016). They found that the denitrification reaction time was shortest when PCL was used, with the
nitrate removal rate exceeding 95 % after 20 days of cultivation, and total organic carbon (TOC)
and NH4+-N were absent in the effluent. Xu et al. constructed a packed-bed bioreactor using a
PHBV/PLA blend as a carbon source and biofilm carrier for the removal of ammonia-nitrogen,
nitrite-nitrogen, and nitrate-nitrogen from the effluent of a secondary settling tank in an activated
sludge wastewater plant (Xu et al., 2019b). They found that the nitrogen removal system effectively
removed all three nitrogen pollutants. Furthermore, the nitrogen removal efficiency was influenced
by temperature (the denitrification rate at 30 ºC was five times greater than at 10 ºC); however,
higher temperatures also promoted TOC accumulation. Sun et al. (Sun et al., 2020) also used a
PHBV/PLA blend for the purification of sewage treatment plant effluent, and achieved removal
efficiency of 98.1 ± 2.9, 87.2 ± 6.8, and 89.3 ± 6.3 % for NH4+-N, NO3—N, and TN, respectively.
These results indicate that the reactor system was capable of simultaneous nitrification and
denitrification under appropriate aeration conditions.
Previous studies on SCS-based wastewater treatment utilized both natural and synthetic SCSs
and mainly focused on the impact of the type of carbon source, temperature, HRT, and pH on
treatment efficiency. In light of the objectives of wastewater treatment, a significant number of
relevant studies have also examined nitrite and ammonia-nitrogen accumulation and effluent TOC,
whereas other studies have investigated changes in microbial communities and the abundance of
functional genes.
3.1.2 Recirculating aquaculture systems
Recirculating aquaculture systems have emerged as a novel aquaculture technology that
involve the treatment of aquaculture wastewater and subsequent recycling and reuse of the treated
water. The removal of nitrate-nitrogen represents a key step in the wastewater purification process
(Bao et al., 2019, Podduturi et al., 2020). In a study by Luo et al. (Luo et al., 2019), the denitrification
performance and bacterial properties of recirculating aquaculture systems using PCL and PHBV as
SCSs were compared over a 102 day period. They found that the denitrification rates achieved with
PCL and PHBV under influent nitrate-nitrogen concentrations of 81.1–132.75 mg/L and an influent
flow rate of 1 L/h were 0.27 and 0.19 g·(L·d)-1, respectively. For the removal of the same mass of
nitrate-nitrogen, the mass of PCL consumed was significantly lower than the mass of PHBV, and
the effluent nitrate-nitrogen and ammonia-nitrogen concentrations achieved using PCL were also
lower. Deng et al. investigated the influence of operating conditions such as dissolved oxygen (DO)
concentration and salinity on nitrogen removal performance and microbial communities in a
recirculating aquaculture system that utilized PBS as the carbon source (Deng et al., 2017). They
found that salinity decreased the number and diversity of operational taxonomic units, while DO
had no significant influence on the microbial community. Zhu et al. constructed a denitrification
bioreactor using PBS as the carbon source and compared the denitrification performance using real
and synthetic recirculating aquaculture system wastewater to determine the influence of salinity and
nitrate concentration on heterotrophic denitrification (Zhu et al., 2015). They found that the nitrate
volumetric removal rate increased with influent nitrate loading. Conversely, salinity had little
influence on nitrate removal (an increase of salinity from 0‰ to 25‰ led to an increase of
denitrification rate from 0.53 to 0.66 kg NO3--N·(m3·d)-1); however, it did increase the likelihood of
excessive DOC and ammonia-nitrogen accumulation in the effluent. Li et al. prepared a novel beadshaped SCS using semen litchi (SL), poly (vinyl alcohol) (PVA), and sodium alginate (SA) as raw
materials (Li et al., 2019). They found that the denitrification rate was up to 243.5 ± 7.08 mg
N·(L·d)-1 when the beads were used in the SCS based denitrification of mariculture wastewater.
Zhang et al. (Zhang et al., 2016b) introduced PHB as a denitrification carbon source into an
aquaculture system in which 120 tilapias were reared. After 120 days of culture without water
exchange, it was found that the nitrate-nitrogen concentration of the system was maintained at a
certain level, thereby effectively avoiding the toxic effects of nitrate-nitrogen on aquatic animals.
Existing studies have indicated that the SCSs used in most recirculating aquaculture systems
mainly consist of synthetic SCSs and other novel SCSs. This is because aquaculture systems are
sensitive to effluent quality, requiring the use of synthetic carbon sources which can provide stable
carbon release rates, less DOC accumulation, and lower effluent color intensity. The main aspects
of interest in relevant studies were the conventional performance indicators of denitrification,
including nitrate concentration, nitrite, and ammonia accumulation; TOC; and microbial community
composition. In certain studies, quality changes in aquatic products (e.g., fishes) reared in systems
with and without denitrification were compared; however, significant differences were not observed
between the experimental and control groups (Boley and Müller, 2005).
3.2 Agricultural subsurface drainage systems
Agricultural subsurface drainage systems are a commonly used agricultural drainage method
in which excess groundwater and surface water is removed through underground (subsurface)
drainage pipes (Schipper et al., 2010). The controlled agricultural drainage enables the elimination
of waterlogging and control of groundwater levels, which are beneficial to the prevention of soil
swamping and salinization, creating favorable conditions for agricultural production. In most
existing subsurface drainage systems in China, water is directly discharged into nearby water bodies,
and has a deleterious impact on the ecological environment (Chun et al., 2009). Consequently, the
installation of denitrification bioreactors prior to drainage discharge has become widely accepted.
The use of corn cobs, corn stover, wheat straw, cardboard fibers, leaf litter composts, tree bark,
wood chips, and almond shells as carbon sources have been reported in previous studies
(Christianson et al., 2016, Chun et al., 2009). Camilo et al. (Krause Camilo, 2016) constructed a
horizontal flow reactor filled with wheat straw and pine bark mulch for the removal of nitratenitrogen and the herbicide agent atrazine (ATR) from subsurface drainage water. At 21 ºC and a
HRT of 0.43 d, the removal rates of nitrate-nitrogen and ATR were 30 g N·(m·d)−1 and 22 mg
ATR·(m·d)−1, respectively. David et al. (David et al., 2016) performed a three-year evaluation of
two wood chip bioreactors and found that nitrate-nitrogen removal requirements could be satisfied
during year one and the early part of year two due to the adequate release of soluble carbon. However,
as operating time increased, temperature became the primary limiting factor of the nitrate-nitrogen
removal rate. In another study, Li et al. (He et al., 2018) utilized wood chips and fly ash in tandem
for the simultaneous removal of nitrate-nitrogen and phosphate pollution from subsurface drainage
water. They found that the nitrate-nitrogen removal efficiency changed significantly with HRT.
However, changes in phosphate removal efficiency with HRT were not significant, and
orthophosphate adsorption by fly ash was far less than the saturated capacity determined from a
previous study.
The use of biomass denitrification beds for the purification of agricultural subsurface drainage
water represents the main practical application of SCS-based denitrification technologies. Research
in this field is also relatively well-established and highly relevant to practical applications. SCSbased denitrification beds located at the end of agricultural subsurface drainage systems mainly
utilize natural SCSs as fillers, with wood chips being one of the most commonly used materials.
Given the heterogeneity in application locations and substantial variations in water quality and
volume, relevant studies have focused on the influence of temperature and HRT on denitrification
and the measures required to overcome denitrification inhibition. Other key research directions
include the age of filling materials (Ghane et al., 2018), decomposition and degradation of fillers
(Seres et al., 2018), and leaching characteristics of DOC (Abusallout and Hua, 2017).
3.3 Surface water
The input of pollutants beyond the carrying capacity has led to the aggravation of surface water
eutrophication. For large water bodies with low pollutant concentrations, SCS-based denitrification
technologies are a promising method to remove excessive nitrate-nitrogen and bring less side effect
to water body (Chang et al., 2016).
By adopting pretreated corn cobs, rice straw, and rice hulls as SCSs, Feng et al. (Xie et al.,
2017) compared the impact of different carbon sources and pretreatment methods on SCS-based
denitrification and analyzed the differences in effluent nitrogen pollutant concentration and
microbial communities. They found that SCSs pretreated with acid or alkali achieved higher
denitrification rates and lower effluent concentrations of ammonia-nitrogen and nitrites. In another
study, Feng et al. (Feng et al., 2019) developed solid-phase denitrification systems using alkalipretreated rice husks, pomelo peels, and durian peels as biodegradable carriers for the simultaneous
nitrification and denitrification of ammonia-nitrogen-polluted wastewater. High nitrogen removal
rates (0.56–0.68 mg NH4+-N·(L·h)-1) and the identification of multiple new aerobic denitrifiers were
realized.
Another typical application of SCS-based denitrification technologies is in the removal of
nitrogen pollutants in constructed wetlands. Jia et al. (Jia et al., 2019) utilized agricultural wastes
(wheat straw) as carbon sources for the removal of nitrogen pollutants in a constructed wetland. The
average dissolved organic carbon release rate was 5.24 mg·(g·d)−1, and three months assessment
revealed TN removal efficiencies of 66.75–93.67 %. The DOM generated from the various
agricultural wastes mainly consisted of humic and fulvic acid-like compounds. In another study,
Shen et al. prepared cornstarch/PCL blends for use as SCSs in constructed wetlands. They found
that the average denitrification rate and nitrate removal efficiency were 0.069 kg·(m3·d)-1 and 98.23 %
(25℃, 72 h HRT), respectively, and the major component of DOM was polysaccharides which
mainly consisted of reducing sugar (Shen et al., 2015). Si et al. selected wheat straw, cotton, PBS,
and newspaper as external carbon sources for the comparison of NO3--N and TN removal rates under
low and high temperatures, and found that newspaper achieved the highest removal rates under all
temperature conditions (Si et al., 2018). Subsequently, 16S rRNA metagenomic sequencing was
employed to investigate the influence of different SCSs on the structure and function of bacterial
communities. Liu et al. constructed a wetland using PBS as the SCS for the treatment of ammonianitrogen-polluted wastewater under aerated conditions (Liu et al., 2018c). They found that TN
removal rates of up to 99 % could be achieved, and simultaneous nitrification and denitrification
was the main microbial nitrogen removal pathway.
In addition to investigating nitrate-nitrogen removal, studies with respect to surface water
bodies, including constructed wetlands, have mainly focused on ammonia-nitrogen and TN removal
efficiencies and the simultaneous nitrification and denitrification process in the presence of SCSs.
Furthermore, the impact of different carbon sources, temperatures, and DO levels on removal
efficiencies and the DOM characteristics and functional microorganisms in effluents have also been
frequently studied.
3.4 Groundwater
The pollution of groundwater by nitrate-nitrogen is extremely severe in China (Ma et al., 2012).
With the application of nitrogen fertilizers and the haphazard discharge of domestic sewage,
nitrogen pollutants migrate to groundwater through the infiltration of surface runoff. Consequently,
the adoption of SCS-based denitrification technologies for groundwater purification has received
considerable attention.
Chu et al. used a PHBV and bamboo powder blend as a carbon source and biofilm carrier in a
packed-bed reactor for nitrate removal in groundwater (pump to surface). They found that the
reactors achieved a rapid start-up without external inocula, nitrate removal efficiencies of up to
87.4 %, and less adverse effects in terms of nitrite accumulation (0.5 mg/L) and DOC release (10.5
mg/L) (Chu and Wang, 2016). When Xie et al. utilized a PHA/cellulose blend as a slow-release
carbon material for the removal of nitrates from groundwater, they found that the blend exhibited
excellent nitrate removal efficiency and less adverse effects in terms of nitrite accumulation during
stable operations (Xie et al., 2017). In another study, Ye et al. developed a PHBV and ceramsite
based permeable reactive barrier system, which was used in packed-bed reactors for the treatment
of groundwater polluted with nitrate-nitrogen (Ye et al., 2017). Results of a continuous experiment
conducted over a 35 day period indicated that more than 95 % of nitrate-nitrogen was removed and
a maximum denitrification rate of 241 mg N·(L·d)-1 was achieved. Furthermore, they found that
shortening the HRT significantly reduced the release of DOC. Jin et al. constructed a sawdust/pyrite
mixotrophic denitrification reactor and analyzed the influence of sawdust dosage and HRT on
reactor performance for in situ groundwater remediation (Jin et al., 2019). They found that an overdosage of sawdust increased nitrite-nitrogen and ammonia-nitrogen accumulation, and increasing
the HRT from 12 to 24 h did not significantly enhance removal efficiency.
The main aspects of interest in studies related to SCS-based groundwater treatment are similar
to those of ordinary wastewater treatment and include the influence of the nitrate-nitrogen load,
HRT, temperature, and carbon source dose on denitrification efficiency. Effluent parameters of
interest mainly include nitrogen pollutant concentration, DOC concentration, and microbial
community structure.
3.5 Summary of SCSs applications
Table 2 provides a summary of the applications of SCSs in different target water bodies.
Table 2. Characteristics of studies on the application of SCSs in different target water bodies
Table 2 illustrates that differences in the characteristics and treatment requirements of the various
target water bodies result in different carbon sources, process characteristics, factors of interest, and
effluent parameters. For ordinary wastewater treatment, which is primarily aimed towards pollutant
removal, effluent quality indicators are the primary concern. Furthermore, operating temperature
and HRT and the accumulation of byproducts (e.g., DOC, nitrite-nitrogen, and ammonia nitrogen)
in the effluent should also be examined. Consequently, studies on the application of SCS-based
denitrification to ordinary wastewater treatment have mainly focused on achieving optimal effluent
quality through parameter control. For recirculating aquaculture systems, effluent quality
requirements are more stringent and treatment costs are usually higher compared to ordinary
wastewater treatment. Consequently, synthetic SCSs with a low likelihood of secondary pollution,
rapid carbon release, and high denitrification efficiency are commonly used. The influence of
temperature on denitrification efficiency was rarely investigated with respect to recirculating
aquaculture systems because the temperature is typically maintained within a certain range for
aquatic product survival; however, the aquatic product yield was a key indicator. For agricultural
subsurface drainage systems, natural agricultural and forestry wastes are the most common primary
carbon sources due to the adaptability to local conditions and cost requirements. Research in this
area is relatively well-established, with a key issue being the selection of the appropriate HRT to
address large fluctuations in the quality and volume of drainage water. Because these drainage
systems are mainly located in the open, reducing the constraints imposed by low temperatures on
denitrification efficiency is also a key concern. Other areas of interest include denitrification
efficiency at low HRT, byproduct concentrations in the effluent, carbon source operating life, and
the simultaneous removal of nitrogen and phosphorus. Surface water is characterized by low
pollutant concentrations and high flow rates, limiting microorganism enrichment, the formation of
an adequate supply of localized carbon sources, and the denitrification ability of the microorganisms.
This problem can be effectively resolved through the adoption of SCS-based denitrification
technologies. Most existing studies have focused on conventional influencing factors (e.g. type and
quantity of the carbon source and nitrogen pollutant load) and effluent quality parameters. Given
the geographical and environmental characteristics of surface water, many researchers have also
explored the influence of DO on the performance of carbon sources in surface water denitrification.
In groundwater, which is characterized by low concentrations of easily oxidizable organic carbon
and low temperatures, denitrification rates under natural conditions are usually lower (Wu, 2002).
The microbial growth induced by the dosing of nutrient solutions in groundwater leads to a reduction
in the pores of the water-containing medium, which consequently results in blockages; hence, the
adoption of SCS-based denitrification technologies (especially denitrification barriers) has provided
a novel means of nutrient supply. Existing studies on groundwater have mainly focused on the
influences of nitrate-nitrogen load, HRT, and carbon source dosage on denitrification efficiency.
Furthermore, the functional genes and microbial community structures related to the denitrification
process have also been explored in research on the various target water bodies.
4 Meta-analysis on synthetic SCS-based denitrification
In order to better understand the effect of different SCSs and reaction conditions on the
denitrification efficiency, we applied meta-analysis on the published literature in the area of solidphase denitrification in recent years. Since the denitrification rates of synthetic carbon sources are
much larger than that of raw carbon sources, in this study, we focus on the application of synthetic
carbon source on nitrate removal.
The data came from published journal articles that presented nitrate removal rates from flowthrough, synthesized SCS-based denitrification bed or lab-scale column reactors with nitrate as the
main target containment. We searched for literature in the database of Web of Science with
synthesized carbon sources (i.e. PBS, PCL, PHA, PHB, PHBV and PLA) and denitrification.
Literature without long-term stable operation and key experimental data were excluded. 23 papers
published focusing on synthesized carbon source based solid phase denitrification in recent three
years were finally analyzed in our study.
The core measure of solid-phase denitrification in our study was denitrification rates (DR) in
the units of nitrate removal per volume of bioreactor per time (gNL-1d-1), and necessary calculation
were applied using other information from papers when DR was not given directly. The data
required for meta-analysis in our study were the mean value of DR and the corresponding standard
deviation (SD).
Based on our former discussion and available data, we chose carbon source type (CS type),
carbon source species (CS species), influent N concentration, HRT and water temperature as target
factors, which were further categorized into two or three levels, adapted from the meta analysis
study on woodchip denitrification reactors (Addy et al., 2016).
CS type is categorized into ‘mix CS’ (mixed synthetic carbon source or the mixture of synthetic
and raw carbon sources) and ‘single CS’; three kinds of synthetic carbon source (i.e. PHBV,
PHBV/PLA, PCL) with abundant experimental data are analyzed; Influent N concentration is
divided into low, intermediate and high categories split by 20 and 50 mgN/L; HRT and water
temperature are categorized in similar manner, split by 2 h and 5 h, 22℃ and 25℃, respectively. The
actual data on mean removal rate, SD, number of studies and the classification of each factors are
listed in Table S1.
The response ratio (lnR) and the response variance (VlnR) were calculated (Addy et al., 2016)
and MetaWin 2.0 was used for the calculation of nitrate removal rate effect size and its standard
deviation. Forest plots of the meta-analysis results were shown in Figure 2.
Figure 2. Mean nitrate removal effect size and 95% bias-corrected confidence interval by different
categories of (a) CS type, (b) CS species, (c) Influent N concentration, (d) HRT and (e) Temperature.
Numbers labeled in the figures are the mean value of effect size, and n represent for the number of studies in
meta-analysis.
There was no significant difference in nitrate removal rates between mix carbon source and
single carbon source (Fig 2a), and while species of CS (PHBV, PHBV/PLA and PCL) were decisive
as shown in Fig 2b. The mixing of carbon sources is generally for two purposes, one is to minimize
the cost while ensuring the denitrification efficiency, and the other is to treat multiple pollutants
simultaneously or in stages (Jiang et al., 2020, Yang et al., 2020a, Yang et al., 2020c). The small
difference in denitrification performance showed that mixing of SCSs under specific water bodies
and specific environments is worth further exploration.
Nitrate removal rates were significantly effected by influent N concentration, as shown in Fig
2c. Reactors with high influent N concentration (>50 mgN/L) obtained higher denitrification rates
than those with intermediate (20-50 mgN/L) and low (<20 mgN/L) influent N concentration. Higher
nitrate concentration always resulted in larger reaction rates, and higher nitrate removal rates could
also be obtained unless the exceeding of maximum denitrification capability of the system (Jiang et
al., 2020, Xu et al., 2018a).
HRT with different levels also significantly influenced nitrate removal rates (Fig 2d). However,
the results showed that higher HRT (>5h) achieved the inferior performance on nitrate removal,
which is contradictory to the results in a certain literature with the consideration of HRT levels (Ding
et al., 2020, Yi et al., 2020, Zhang et al., 2021). This is mainly because, in some studies, researchers
set a relatively long HRT in order to achieve a stable low nitrate effluent concentration (Feng et al.,
2020a, Han et al., 2018, Lan et al., 2020). In fact, synthetic carbon sources often have short lag time
and brilliant carbon release efficiency, and in many cases, HRT of 2h is sufficient for the thorough
removal of nitrate (Fang et al., 2020, Shen et al., 2020).
The nitrate removal rate effect sizes under different temperature were shown in Fig 2e. Low
temperature (<20℃) significantly affect nitrate removal, where less COD release, nitrite
accumulation and shift of denitrifying genus were detected at low-temperature (Shen et al., 2020,
Xu et al., 2019b). Nevertheless, the high and intermediate categories are not significantly different
in nitrate removal. There are two main reasons for this. First, the research literature analyzed mainly
conducted lab-scale column reactors around room temperature (around 25℃), which was reasonable
for the treatment of RAS, municipal waste water and surface water; second, the effect of low
temperature on enzyme activities and microbial community related to carbon hydrolysis and
denitrification are more obvious when it is below 15℃ (Jiang et al., 2020, Shen et al., 2020).
Therefore, realizing high-efficiency denitrification under low temperature conditions (generally
groundwater) is still a topic worthy of continued research.
Synthetic solid-phase carbon sources have good application prospects for solid phase
nitrification. The optimization of operation parameters (especially HRT), the mixing of synthetic
carbon sources for actual complex water bodies and the further design of low temperature-tolerated
reactors are important considerations for future study.
5 Advantage and disadvantage summary
The eutrophication of water bodies remains a serious problem in many parts of the world;
hence, research on the removal of nitrate-nitrogen pollution is extensive. In particular, research has
focused on SCSs due to their ease of transport, low tendency for secondary pollution, long service
life, and ease of management. In existing research, target water bodies for SCS-based denitrification
include wastewater (including aquaculture wastewater), agricultural subsurface drainage, surface
water, and groundwater. Relevant research on practical applications in agricultural subsurface
drainage systems is extensive and well-established, while studies on practical applications in other
water bodies are relatively scarce.
Although SCS-based denitrification technologies have received widespread attention, they also
possess certain shortcomings. Natural SCSs are inexpensive, easily acquirable, and have a long
operating life; however, their applications are often limited due to unstable carbon release rates,
excessive DOC release, and increased color intensity during the early stages of denitrification.
Synthetic SCSs are readily utilized by microorganisms due to their strong bioaffinities, have a low
tendency to cause secondary pollution due to their simple composition, and exhibit rapid carbon
release rates and high denitrification efficiencies. Synthetic SCSs are superior to natural SCSs in
most applications; however, they are expensive, have limited carbon release rates, and their
denitrification performance is strongly influenced by temperature. While these factors currently
limit the broader application of synthetic SCSs, it is anticipated that ongoing research will
successfully address these issues. In conclusion, despite the presence of certain limitations, SCSbased denitrification technologies show promise for applications in many fields due to their superior
advantages.
6 Conclusion
Solid-phase carbon sources have good application prospects for solid phase nitrification.
Research with respect to wastewater treatment has mainly been focused on the removal efficiency
of nitrogen pollutants and DOC accumulation in the effluent, while studies on recirculating
aquaculture systems have focused on product yield and water quality parameters. Agricultural
subsurface drainage system research was extensive, and focused on natural SCSs and the influence
of temperature and HRT on denitrification efficiency. The primary aspect of surface water research
was the influence of DO on denitrification efficiency, while studies on groundwater were mainly
focused on the influence of nitrate-nitrogen load, HRT, and carbon source dosage on denitrification
efficiency. The optimization of operation parameters (especially HRT), the hybrid application of
synthetic carbon sources and the further design of low temperature-tolerated reactors are worthy of
continued study.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Grant No.
51809195), Postdoctoral Science Foundation of China (No. 2018M642083) and National Water
Pollution Control and Treatment Science and Technology Major Project of China (Nos.
2017ZX07204004 and 2017ZX07204002).
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*Graphical Abstract
*Highlights (for review : 3 to 5 bullet points (maximum 85
characters including spaces per bullet point)
Highlights:
-
Nitrogen pollution in water bodies is a serious environmental concern
Solid carbon sources are an effective denitrification treatment method
Synthetic carbon sources were more effective compared to natural carbon sources
Different water bodies required varied approaches to optimize denitrification
Research progress in solid carbon source–based denitrification
technologies for different target water bodies
Feifan Zhang a, Chengjin Ma a, Xiangfeng Huang a, Jia Liu a, Lijun Lu a, Kaiming Peng a,
Shiyang Li a
a
College of Environmental Science and Engineering, State Key Laboratory of Pollution
Control and Resource Reuse, Ministry of Education Key Laboratory of Yangtze River Water
Environment, Tongji University, Shanghai 200092, People’s Republic of China
Correspondence information: Shiyang Li, College of Environmental Science and
Engineering, Tongji University, Shanghai 200092, People’s Republic of China. Tel: +86 021
65982399. Email: lishiyang@tongji.edu.cn
Abstract: Nitrogen pollution in water bodies is a serious environmental issue which is
commonly treated by various methods such as heterotrophic denitrification. In particular, solid
carbon source (SCS)–based denitrification has attracted widespread research interest due to its
gradual carbon release, ease of management, and long-term operation. This paper reviews the types
and properties of SCSs for different target water bodies. While both natural (wheat straw, wood
chips, and fruit shells) and synthetic (polybutylene succinate, polycaprolactone, polylactic acid, and
polyhydroxyalkanoates) SCSs are commonly used, it is observed that the denitrification
performance of the synthetic sources is generally superior. SCSs has been used in the treatment of
wastewater (including aquaculture wastewater), agricultural subsurface drainage, surface water, and
groundwater; however, the key research aspects related to SCSs differ markedly based on the target
waterbody. These key research aspects include nitrogen pollutant removal rate and byproduct
accumulation (ordinary wastewater); water quality parameters and aquatic product yield
(recirculating aquaculture systems); temperature and hydraulic retention time (agricultural
subsurface drainage); the influence of dissolved oxygen (surface waters); and nitrate-nitrogen load,
HRT, and carbon source dosage on denitrification rate (groundwater). It is concluded that SCSbased denitrification is a promising technique for the effective elimination of nitrate-nitrogen
pollution in water bodies.
Keywords: solid carbon source, heterotrophic denitrification, nitrogen pollution, nitrogen
removal, water treatment
1 Introduction
A fast increase in human population and rapid developments in the industrial and agricultural
sectors have resulted in nitrogen pollution–induced eutrophication of water bodies (Jafari et al.,
2015, Liu et al., 2019). In natural water bodies, nitrogen existing in forms other than nitrate-nitrogen
is gradually converted to nitrate-nitrogen by microorganisms, resulting in an increasingly severe
accumulation of nitrate-nitrogen in water bodies (Canfield et al., 2010, Zhang et al., 2014). Water
bodies containing excessive nitrate-nitrogen may cause damage to crop root systems and impact
crop yields when used as water sources for agricultural irrigation (Steidl et al., 2019). Furthermore,
drinking from polluted water bodies may cause toxic effects or even death in wildlife, and excessive
nitrate-nitrogen intake in humans can result in methemoglobinemia (blue baby syndrome) or may
cause toxic effects or even death in wildlife in extreme cases (Li et al., 2017). Studies have revealed
that nitrogen pollution in groundwater exists in approximately 110 countries and has caused safety
issues concerning drinking water globally (Chen et al., 2013, Feng et al., 2020b).
In response to existing nitrogen pollution issues, many physicochemical and bioecological
methods have been utilized for the removal of excess nitrate-nitrogen in water. Owing to the current
technological progress, the application of carbon sources under anoxic conditions to achieve
biological nitrogen removal through denitrification (Cheng et al., 2020, Zhang et al., 2017) has
emerged as the mainstream method employed in water treatment. Treatment processes in traditional
wastewater treatment plants usually involve the addition of external carbon sources to promote
microbial denitrification. Commonly used external carbon sources mainly consist of water-soluble
low-molecular-weight (LMW) organic compounds such as methanol, acetates, and LMW sugars
(Feng et al., 2013). However, the addition of dissolved carbon sources to natural water bodies
increases the organic pollution load and may even cause secondary pollution if the calculated dosage
amounts are unreasonable. Furthermore, the need for dosing facilities is also a limiting factor in the
widespread use of dissolved carbon sources. Conversely, solid carbon sources (SCSs) have attracted
a considerable research interest due to their appropriate carbon release rates, favorable conditions
for microbial biofilm growth, long carbon release duration, ease of management, and long-term
operation(Yang et al., 2020b, Zhang et al., 2020). Currently, the most commonly used SCSs can be
classified into two major categories, i.e, natural cellulosic materials and synthesized biodegradable
polymers (BDPs). Target water bodies for SCS-based denitrification include wastewater (including
aquaculture wastewater), agricultural subsurface drainage, surface water, and groundwater. To date,
studies regarding the application of SCSs to different target water bodies under specific conditions
and operating requirements have investigated the types and properties of commonly used carbon
sources, process characteristics, influencing factors, and effluent parameters.
In this paper, we provide an overview both lab and field studies of the types and operating
principles of commonly used SCSs based on a review in this field. We identify and classify the
denitrification principles of synthetic SCSs, commonly used SCSs, influencing factors, and effluent
parameters. Meta-analysis was further applied for better understanding of the effect of different
SCSs and reaction conditions on the denitrification efficiency. Finally, we summarize the limitations
governing the use of SCSs to provide a scientific basis for the future development of SCS-based
denitrification techniques.
2 Types, properties and utilization of SCSs
2.1 Commonly used SCSs
Commonly used SCSs can be classified into three major categories: natural (cellulosic),
synthetic (polymeric), and other SCSs (e.g., acidogenic liquids from food waste, hydrolyzed sludge
and other reprocessed organic materials) (Guo et al., 2017, Zhang et al., 2016a). Due to the scarcity
of relevant existing literature and applications, other SCSs have not been discussed in this paper.
Most early applications of SCSs involved the use of natural carbon sources, including
cellulosic agricultural and forestry wastes, such as corn cobs, corn stover, wheat straw, cardboard
fibers, leaf litter, tree bark, wood chips, and fruit kernels, which were reported in previous studies
(Chang et al., 2016, Christianson et al., 2012, Chun et al., 2009, Gomez et al., 2000). Natural SCSs
have several advantages such as low cost and ease of acquisition. Because they consist primarily of
agricultural and forestry wastes, they are commonly used as filling materials in denitrification
located near the outflow of agricultural subsurface drainage systems.
However, the wider application of natural SCSs in denitrification is limited by unstable carbon
release rates, slow denitrification rates, excessive release of dissolved organic carbon (DOC), and
increased color intensity in the effluent. Therefore, while the denitrification efficiencies of natural
SCSs were reported more frequently in earlier studies, more recent literature has focused on the
simultaneous removal of total nitrogen (TN) and nitrate-nitrogen (Si et al., 2018), aerobic
denitrification (Cheng et al., 2020), characteristics of leached pollutants (Abusallout and Hua, 2017,
Jia et al., 2019), and functional microbial communities (Hu et al., 2019).
In recent years, the use of BDPs as synthetic SCSs has attracted considerable research attention.
BDPs only decompose under the action of extracellular enzymes secreted by specific
microorganisms; hence, they can avoid or mitigate many of the aforementioned problems associated
with natural SCSs. BDPs refer to a class of high-molecular-weight (HMW) materials that are
degraded or enzymatically hydrolyzed under biological action to generate LMW compounds which
can be utilized by organisms. In addition to serving as carbon sources, BDPs as synthetic SCSs also
act as carriers for the growth of denitrifying microorganisms (Zhang et al., 2016c; d) and have been
used in the denitrification of water bodies since the 1990s. In 1992, Müller et al. reported the use of
polyhydroxybutyrate (PHB) granules as an SCS for denitrification through the construction of a
laboratory-scale up flow fixed-bed reactor (Muller et al., 1992). They found that the denitrification
rate at 10 ºC was 11 mg·(L·h)-1, and cells co-immobilized with the PHB granules exhibited a higher
denitrification rate compared to suspended cells. Currently, only a few types of synthetic SCSs are
commonly applied to the denitrification of wastewater, including polylactic acid (PLA) (Fan et al.,
2012), polycaprolactone (PCL) (Chu and Wang, 2011a; b, Li et al., 2016, Wu et al., 2013),
polybutylene succinate (PBS) (Wang and Chu, 2016), and polyhydroxyalkanoates (PHAs) (Lopardo
and Urakawa, 2019). Table 1 illustrates a summary of the denitrification efficiencies reported in a
number of studies on SCSs.
Table 1. Summary of denitrification performance of different solid carbon source (SCSs) reported in
previous literature
2.2 Basic principles of synthetic SCS-based denitrification
2.2.1 Synthetic SCSs Utilization
Anoxic biological denitrification usually takes place under the action of microorganisms and
fungus. It involves biological redox reactions in which organic carbon sources and nitrates serve as
electron donors and acceptors, respectively. This results in the reduction of nitrate to nitrogen, which
is subsequently removed from the water body through the denitrification process. The overall
process consists of the following steps:
𝑁𝑂3− → 𝑁𝑂2− → 𝑁𝑂 → 𝑁2 𝑂 → 𝑁2
However, only soluble biodegradable carbon source such as acetic acid, formic acid, and
methanol can be directly utilized by denitrifying microorganisms during biological denitrification,
whereas SCSs must be converted into LMW compounds prior to utilization by microorganisms.
The utilization of BDPs as carbon sources and bacterial carriers for SCS based denitrification
occurs under microbial action, with SCS biodegradation enabling the growth and metabolism of
microorganisms attached to the surfaces of SCSs and the . Polymer degradation can be characterized
as a process that results in the breakage of a large and complex molecule into smaller molecules
(dos Santos et al., 2018). First, biofilms are formed through the attachment and growth of
microorganisms on polymer surfaces; then, polymer chains are cleaved by extracellular enzymes,
leading to the hydrolysis of the polymers into soluble LMW compounds (Hocking et al., 1996, Shah
et al., 2008). Subsequently, LMW compounds enter functional microorganisms through
semipermeable membranes and are utilized as carbon sources and electron donors. In short, the
utilization of BDPs involves hydrolysis and denitrification, with the former being the rate-
determining process (Takahashi et al., 2011).
Figure 1. Schematic of the utilization of biodegradable polymers (BDPs) as carbon sources by denitrifying
microorganisms
Microbial degradation has various impacts on SCSs, including changes in chemical structure
(Lucas et al., 2008), significant reductions in the polymer molecular weight, increased surface
roughness, formation of perforations and pits, and mechanical deformation due to structural
dissolution and breakdown. In turn, these impacts change the carbon release rate of the material.
Microbial degradation also leads to a decrease in the crystalline phase content and a corresponding
increase in readily hydrolysable amorphous content, resulting in hydrophilicity changes.
2.2.2 Basic mechanisms of synthetic SCS-based denitrification
Müller et al. had reported the use of PHB (molecular formula: [C4H6O2]n) as a synthetic SCS
(Muller et al., 1992). And PHB based denitrification reaction with nitrate ions as electron acceptors
is as follows:
5[𝐶4𝐻6 𝑂2 ] + 18𝑁𝑂3 − → 9𝑁2 + 18𝐻𝐶𝑂3 − + 2𝐶𝑂2 + 6𝐻2𝑂
(1)
According to Boley et al., by assuming a yield coefficient (Yx/s) of 0.45 g biomass/g PHB, the
summarized denitrification equation including biomass formation when PHB is used as the carbon
source can be expressed as (Boley et al., 2000):
0.494[𝐶4 𝐻6𝑂2 ] + 𝑁𝑂3 −
→ 0.415𝑁2 + 𝐻𝐶𝑂3 − + 0.130𝐶𝑂2 + 0.169[𝐶5 𝐻7𝑂2 𝑁]
(2)
+ 0.390𝐻2 𝑂
where C5H7O2N is the molecular formula of the microbial cells.
Because the molecular formulae of commonly used synthetic SCSs (PCL, PBS, PLA, and
PHAs) can be represented as CxHyOz, the basic denitrification reaction for synthetic SCSs is as
follows:
5[𝐶𝑥 𝐻𝑦 𝑂𝑧 ] + (4𝑥 + 𝑦 − 2𝑧)𝑁𝑂3 −
→ (2𝑥 +
𝑦
−
− 𝑧)𝑁2 + (4𝑥 + 𝑦 − 2𝑧)𝐻𝐶𝑂3 + (𝑥 − 𝑦 + 2𝑧)𝐶𝑂2
2
(3)
+ (2𝑦 − 2𝑥 + 𝑧)𝐻2𝑂
Using Equation (3), the calculated mass of PHB required to reduce the unit mass of nitratenitrogen is 2.92 g PHB/g NO3--N when biomass formation is excluded and 3.03 g PHB/g NO3--N
when biomass formation is considered. Based on theoretical calculations, the masses of glucose,
methanol, and ethanol required to reduce the unit mass of nitrate-nitrogen when biomass formation
is excluded are 2.68, 1.90, and 1.37 g/g NO3--N, respectively. When biomass formation is considered,
the required masses of methanol and ethanol are 2.47 and 2.01 g/g NO3--N, respectively (Mateju et
al., 1992).
2.3 Comparison of raw and synthetic SCSs
Compared with liquid carbon sources, SCSs can be used as biofilm carriers for microorganisms,
which are more convenient for transportation, operation and storage (Boley et al., 2000). In previous
studies, most solid carbon sources can release organic matter needed in denitrification systems,
especially in low C/N wastewater treatment.
In terms of material sources, raw and synthetic SCSs come from multiple sources. Cellulosic
agricultural and forestry wastes are the main components of raw SCSs. Synthetic SCSs are mainly
produced by petrochemical engineering, while some studies focus on the utilization of endogenous
accumulation of PHAs by microorganisms (He et al., 2018, Wu et al., 2021). With the development
of material science, more and more new materials are used in sewage treatment. BDPs with excellent
biocompatibility, biodegradability and non-toxicity, such as PCL and PBS, are ideal materials for
environmental protection (Wu et al., 2012).
The hydrolytic ability of carbon sources is regarded as a key factor of denitrification (Hang et
al., 2016). Denitrification rate is inhibited when available carbon from carbon source is insufficient,
while an over-release of carbon may on the contrary, cause carbon loss and organic pollution (Healy
et al., 2012). Regarding the characteristics of carbon release, first-order kinetics can basically
describe the carbon release process of all SCSs. Raw SCSs can release more organic matter, and
synthetic SCSs have an advantage in the sustainability of carbon release. In order to take advantage
of the advantages of both kinds of SCSs in terms of carbon release, some carbon sources mixed with
raw and synthetic SCSs have been prepared (Jiang et al., 2020, Liu et al., 2018b, Xiong et al., 2019) .
The hydrolysate of raw and synthetic SCSs are quite different. Raw SCSs often contain
multiple components. Taking lignocellulosic materials as an example, they are mainly comprised of
lignin, cellulose and hemicellulose. Among them, cellulose can be easily used by microorganisms
and hydrolyzed into glucose, hemicellulose can also be used after enzymatic hydrolysis into small
molecular organic matter, and lignin is difficult to be degraded (Forrest et al., 2010, Zhong et al.,
2020). It has been reported that the denitrification efficiency of woody biomass is lower than that
of herbaceous biomass due to its lignin content and natural structure in woody biomass (Kim et al.,
2016).
The hydrolysate of the synthetic SCSs is related to its own structure, and its degradation
involves the joint effect of several processes. Taking PHB as an example, PHB can directly undergo
abiotic hydrolysis in water because it contains -COO- group as a polyester. When PHB is used as
SCS in a biological denitrification system, biodegradation of PHB also plays a significant role.
Enzymes in the intracellular and extracellular matrix can disrupt long-chain polymer chains and
hydrolyze oligomers, and these oligomers will be further hydrolyzed into polymer monomers after
a short period of time (dos Santos et al., 2018, Kessler et al., 2014). Water temperature, pH value,
microbial community and the supplement of nutrients will all affect the degradation process of SCSs.
In terms of denitrification, both SCSs can promote denitrification in low C/N wastewater
treatment. Due to the difference in carbon release patterns between the two types of carbon sources,
and considering the sustainability of carbon release, synthetic SCSs are ideal slow-release carbon
sources for low TN water treatment (such as groundwater and drinking water), raw SCSs may be a
more ideal carbon source to enhance the denitrification of secondary wastewater (Chu and Wang,
2013, Xiong et al., 2020).
3 Application of SCSs in different target water bodies
Currently, the target water bodies for SCS-based denitrification technologies include
wastewater (Duan et al., 2016, Xu et al., 2018a, Yang et al., 2020b) (including aquaculture
wastewater (Gutierrez-Wing et al., 2012, Zhang et al., 2016b)), agricultural subsurface drainage,
(Christianson et al., 2016, Chun et al., 2010), surface water(Feng et al., 2019), and groundwater
(Zhang et al., 2018).
3.1 Wastewater
Studies on the application of SCSs to wastewater denitrification can be classified into two main
categories; (1) advanced treatment of ordinary wastewater or wastewater treatment plant effluent
and (2) purification of nitrogen pollutants in recirculating aquaculture systems.
3.1.1 Treatment of ordinary wastewater
For wastewater with characteristically low C/N ratios, nitrogen removal is often achieved via
dosing with external carbon sources. Generally, SCSs are used as filling materials and microbial
carriers in fixed-bed reactors for the purification of wastewater by denitrification. In a study by Rout
et al. (Rout et al., 2017), organic solid waste substances were used as carbon sources to investigate
the influence of experimental parameters, such as influent nitrate concentration, hydraulic retention
time (HRT), and bed depth, on denitrification efficiency. The found that a low HRT reduced nitrate
removal efficiency, increased nitrite accumulation, and decreased effluent chemical oxygen demand
(COD). When the influent nitrate concentration was 70, 50, and 30 mg/L, the effluent nitrate
concentration could be maintained at < 10 mg/L for 31, 39, and 49 days, respectively, with the
denitrification process following first-order reaction kinetics. Sun et al. (Sun et al., 2019a) utilized
alkali-pretreated corn cobs as solid carbon sources and biofilm carriers for the removal of nitrates
and refractory organic pollutants from coking wastewater. They found that the treatment process
could concurrently achieve the stable removal of over 90 % of residual nitrate and the degradation
of typical refractory organic matter. In another study, Duan et al. compared the performances of
PBS, PHBV, and PCL as carbon sources for the treatment of nitrified swine wastewater (Duan et al.,
2016). They found that the denitrification reaction time was shortest when PCL was used, with the
nitrate removal rate exceeding 95 % after 20 days of cultivation, and total organic carbon (TOC)
and NH4+-N were absent in the effluent. Xu et al. constructed a packed-bed bioreactor using a
PHBV/PLA blend as a carbon source and biofilm carrier for the removal of ammonia-nitrogen,
nitrite-nitrogen, and nitrate-nitrogen from the effluent of a secondary settling tank in an activated
sludge wastewater plant (Xu et al., 2019b). They found that the nitrogen removal system effectively
removed all three nitrogen pollutants. Furthermore, the nitrogen removal efficiency was influenced
by temperature (the denitrification rate at 30 ºC was five times greater than at 10 ºC); however,
higher temperatures also promoted TOC accumulation. Sun et al. (Sun et al., 2020) also used a
PHBV/PLA blend for the purification of sewage treatment plant effluent, and achieved removal
efficiency of 98.1 ± 2.9, 87.2 ± 6.8, and 89.3 ± 6.3 % for NH4+-N, NO3—N, and TN, respectively.
These results indicate that the reactor system was capable of simultaneous nitrification and
denitrification under appropriate aeration conditions.
Previous studies on SCS-based wastewater treatment utilized both natural and synthetic SCSs
and mainly focused on the impact of the type of carbon source, temperature, HRT, and pH on
treatment efficiency. In light of the objectives of wastewater treatment, a significant number of
relevant studies have also examined nitrite and ammonia-nitrogen accumulation and effluent TOC,
whereas other studies have investigated changes in microbial communities and the abundance of
functional genes.
3.1.2 Recirculating aquaculture systems
Recirculating aquaculture systems have emerged as a novel aquaculture technology that
involve the treatment of aquaculture wastewater and subsequent recycling and reuse of the treated
water. The removal of nitrate-nitrogen represents a key step in the wastewater purification process
(Bao et al., 2019, Podduturi et al., 2020). In a study by Luo et al. (Luo et al., 2019), the denitrification
performance and bacterial properties of recirculating aquaculture systems using PCL and PHBV as
SCSs were compared over a 102 day period. They found that the denitrification rates achieved with
PCL and PHBV under influent nitrate-nitrogen concentrations of 81.1–132.75 mg/L and an influent
flow rate of 1 L/h were 0.27 and 0.19 g·(L·d)-1, respectively. For the removal of the same mass of
nitrate-nitrogen, the mass of PCL consumed was significantly lower than the mass of PHBV, and
the effluent nitrate-nitrogen and ammonia-nitrogen concentrations achieved using PCL were also
lower. Deng et al. investigated the influence of operating conditions such as dissolved oxygen (DO)
concentration and salinity on nitrogen removal performance and microbial communities in a
recirculating aquaculture system that utilized PBS as the carbon source (Deng et al., 2017). They
found that salinity decreased the number and diversity of operational taxonomic units, while DO
had no significant influence on the microbial community. Zhu et al. constructed a denitrification
bioreactor using PBS as the carbon source and compared the denitrification performance using real
and synthetic recirculating aquaculture system wastewater to determine the influence of salinity and
nitrate concentration on heterotrophic denitrification (Zhu et al., 2015). They found that the nitrate
volumetric removal rate increased with influent nitrate loading. Conversely, salinity had little
influence on nitrate removal (an increase of salinity from 0‰ to 25‰ led to an increase of
denitrification rate from 0.53 to 0.66 kg NO3--N·(m3·d)-1); however, it did increase the likelihood of
excessive DOC and ammonia-nitrogen accumulation in the effluent. Li et al. prepared a novel beadshaped SCS using semen litchi (SL), poly (vinyl alcohol) (PVA), and sodium alginate (SA) as raw
materials (Li et al., 2019). They found that the denitrification rate was up to 243.5 ± 7.08 mg
N·(L·d)-1 when the beads were used in the SCS based denitrification of mariculture wastewater.
Zhang et al. (Zhang et al., 2016b) introduced PHB as a denitrification carbon source into an
aquaculture system in which 120 tilapias were reared. After 120 days of culture without water
exchange, it was found that the nitrate-nitrogen concentration of the system was maintained at a
certain level, thereby effectively avoiding the toxic effects of nitrate-nitrogen on aquatic animals.
Existing studies have indicated that the SCSs used in most recirculating aquaculture systems
mainly consist of synthetic SCSs and other novel SCSs. This is because aquaculture systems are
sensitive to effluent quality, requiring the use of synthetic carbon sources which can provide stable
carbon release rates, less DOC accumulation, and lower effluent color intensity. The main aspects
of interest in relevant studies were the conventional performance indicators of denitrification,
including nitrate concentration, nitrite, and ammonia accumulation; TOC; and microbial community
composition. In certain studies, quality changes in aquatic products (e.g., fishes) reared in systems
with and without denitrification were compared; however, significant differences were not observed
between the experimental and control groups (Boley and Müller, 2005).
3.2 Agricultural subsurface drainage systems
Agricultural subsurface drainage systems are a commonly used agricultural drainage method
in which excess groundwater and surface water is removed through underground (subsurface)
drainage pipes (Schipper et al., 2010). The controlled agricultural drainage enables the elimination
of waterlogging and control of groundwater levels, which are beneficial to the prevention of soil
swamping and salinization, creating favorable conditions for agricultural production. In most
existing subsurface drainage systems in China, water is directly discharged into nearby water bodies,
and has a deleterious impact on the ecological environment (Chun et al., 2009). Consequently, the
installation of denitrification bioreactors prior to drainage discharge has become widely accepted.
The use of corn cobs, corn stover, wheat straw, cardboard fibers, leaf litter composts, tree bark,
wood chips, and almond shells as carbon sources have been reported in previous studies
(Christianson et al., 2016, Chun et al., 2009). Camilo et al. (Krause Camilo, 2016) constructed a
horizontal flow reactor filled with wheat straw and pine bark mulch for the removal of nitratenitrogen and the herbicide agent atrazine (ATR) from subsurface drainage water. At 21 ºC and a
HRT of 0.43 d, the removal rates of nitrate-nitrogen and ATR were 30 g N·(m·d)−1 and 22 mg
ATR·(m·d)−1, respectively. David et al. (David et al., 2016) performed a three-year evaluation of
two wood chip bioreactors and found that nitrate-nitrogen removal requirements could be satisfied
during year one and the early part of year two due to the adequate release of soluble carbon. However,
as operating time increased, temperature became the primary limiting factor of the nitrate-nitrogen
removal rate. In another study, Li et al. (He et al., 2018) utilized wood chips and fly ash in tandem
for the simultaneous removal of nitrate-nitrogen and phosphate pollution from subsurface drainage
water. They found that the nitrate-nitrogen removal efficiency changed significantly with HRT.
However, changes in phosphate removal efficiency with HRT were not significant, and
orthophosphate adsorption by fly ash was far less than the saturated capacity determined from a
previous study.
The use of biomass denitrification beds for the purification of agricultural subsurface drainage
water represents the main practical application of SCS-based denitrification technologies. Research
in this field is also relatively well-established and highly relevant to practical applications. SCSbased denitrification beds located at the end of agricultural subsurface drainage systems mainly
utilize natural SCSs as fillers, with wood chips being one of the most commonly used materials.
Given the heterogeneity in application locations and substantial variations in water quality and
volume, relevant studies have focused on the influence of temperature and HRT on denitrification
and the measures required to overcome denitrification inhibition. Other key research directions
include the age of filling materials (Ghane et al., 2018), decomposition and degradation of fillers
(Seres et al., 2018), and leaching characteristics of DOC (Abusallout and Hua, 2017).
3.3 Surface water
The input of pollutants beyond the carrying capacity has led to the aggravation of surface water
eutrophication. For large water bodies with low pollutant concentrations, SCS-based denitrification
technologies are a promising method to remove excessive nitrate-nitrogen and bring less side effect
to water body (Chang et al., 2016).
By adopting pretreated corn cobs, rice straw, and rice hulls as SCSs, Feng et al. (Xie et al.,
2017) compared the impact of different carbon sources and pretreatment methods on SCS-based
denitrification and analyzed the differences in effluent nitrogen pollutant concentration and
microbial communities. They found that SCSs pretreated with acid or alkali achieved higher
denitrification rates and lower effluent concentrations of ammonia-nitrogen and nitrites. In another
study, Feng et al. (Feng et al., 2019) developed solid-phase denitrification systems using alkalipretreated rice husks, pomelo peels, and durian peels as biodegradable carriers for the simultaneous
nitrification and denitrification of ammonia-nitrogen-polluted wastewater. High nitrogen removal
rates (0.56–0.68 mg NH4+-N·(L·h)-1) and the identification of multiple new aerobic denitrifiers were
realized.
Another typical application of SCS-based denitrification technologies is in the removal of
nitrogen pollutants in constructed wetlands. Jia et al. (Jia et al., 2019) utilized agricultural wastes
(wheat straw) as carbon sources for the removal of nitrogen pollutants in a constructed wetland. The
average dissolved organic carbon release rate was 5.24 mg·(g·d)−1, and three months assessment
revealed TN removal efficiencies of 66.75–93.67 %. The DOM generated from the various
agricultural wastes mainly consisted of humic and fulvic acid-like compounds. In another study,
Shen et al. prepared cornstarch/PCL blends for use as SCSs in constructed wetlands. They found
that the average denitrification rate and nitrate removal efficiency were 0.069 kg·(m3·d)-1 and 98.23 %
(25℃, 72 h HRT), respectively, and the major component of DOM was polysaccharides which
mainly consisted of reducing sugar (Shen et al., 2015). Si et al. selected wheat straw, cotton, PBS,
and newspaper as external carbon sources for the comparison of NO3--N and TN removal rates under
low and high temperatures, and found that newspaper achieved the highest removal rates under all
temperature conditions (Si et al., 2018). Subsequently, 16S rRNA metagenomic sequencing was
employed to investigate the influence of different SCSs on the structure and function of bacterial
communities. Liu et al. constructed a wetland using PBS as the SCS for the treatment of ammonianitrogen-polluted wastewater under aerated conditions (Liu et al., 2018c). They found that TN
removal rates of up to 99 % could be achieved, and simultaneous nitrification and denitrification
was the main microbial nitrogen removal pathway.
In addition to investigating nitrate-nitrogen removal, studies with respect to surface water
bodies, including constructed wetlands, have mainly focused on ammonia-nitrogen and TN removal
efficiencies and the simultaneous nitrification and denitrification process in the presence of SCSs.
Furthermore, the impact of different carbon sources, temperatures, and DO levels on removal
efficiencies and the DOM characteristics and functional microorganisms in effluents have also been
frequently studied.
3.4 Groundwater
The pollution of groundwater by nitrate-nitrogen is extremely severe in China (Ma et al., 2012).
With the application of nitrogen fertilizers and the haphazard discharge of domestic sewage,
nitrogen pollutants migrate to groundwater through the infiltration of surface runoff. Consequently,
the adoption of SCS-based denitrification technologies for groundwater purification has received
considerable attention.
Chu et al. used a PHBV and bamboo powder blend as a carbon source and biofilm carrier in a
packed-bed reactor for nitrate removal in groundwater (pump to surface). They found that the
reactors achieved a rapid start-up without external inocula, nitrate removal efficiencies of up to
87.4 %, and less adverse effects in terms of nitrite accumulation (0.5 mg/L) and DOC release (10.5
mg/L) (Chu and Wang, 2016). When Xie et al. utilized a PHA/cellulose blend as a slow-release
carbon material for the removal of nitrates from groundwater, they found that the blend exhibited
excellent nitrate removal efficiency and less adverse effects in terms of nitrite accumulation during
stable operations (Xie et al., 2017). In another study, Ye et al. developed a PHBV and ceramsite
based permeable reactive barrier system, which was used in packed-bed reactors for the treatment
of groundwater polluted with nitrate-nitrogen (Ye et al., 2017). Results of a continuous experiment
conducted over a 35 day period indicated that more than 95 % of nitrate-nitrogen was removed and
a maximum denitrification rate of 241 mg N·(L·d)-1 was achieved. Furthermore, they found that
shortening the HRT significantly reduced the release of DOC. Jin et al. constructed a sawdust/pyrite
mixotrophic denitrification reactor and analyzed the influence of sawdust dosage and HRT on
reactor performance for in situ groundwater remediation (Jin et al., 2019). They found that an overdosage of sawdust increased nitrite-nitrogen and ammonia-nitrogen accumulation, and increasing
the HRT from 12 to 24 h did not significantly enhance removal efficiency.
The main aspects of interest in studies related to SCS-based groundwater treatment are similar
to those of ordinary wastewater treatment and include the influence of the nitrate-nitrogen load,
HRT, temperature, and carbon source dose on denitrification efficiency. Effluent parameters of
interest mainly include nitrogen pollutant concentration, DOC concentration, and microbial
community structure.
3.5 Summary of SCSs applications
Table 2 provides a summary of the applications of SCSs in different target water bodies.
Table 2. Characteristics of studies on the application of SCSs in different target water bodies
Table 2 illustrates that differences in the characteristics and treatment requirements of the various
target water bodies result in different carbon sources, process characteristics, factors of interest, and
effluent parameters. For ordinary wastewater treatment, which is primarily aimed towards pollutant
removal, effluent quality indicators are the primary concern. Furthermore, operating temperature
and HRT and the accumulation of byproducts (e.g., DOC, nitrite-nitrogen, and ammonia nitrogen)
in the effluent should also be examined. Consequently, studies on the application of SCS-based
denitrification to ordinary wastewater treatment have mainly focused on achieving optimal effluent
quality through parameter control. For recirculating aquaculture systems, effluent quality
requirements are more stringent and treatment costs are usually higher compared to ordinary
wastewater treatment. Consequently, synthetic SCSs with a low likelihood of secondary pollution,
rapid carbon release, and high denitrification efficiency are commonly used. The influence of
temperature on denitrification efficiency was rarely investigated with respect to recirculating
aquaculture systems because the temperature is typically maintained within a certain range for
aquatic product survival; however, the aquatic product yield was a key indicator. For agricultural
subsurface drainage systems, natural agricultural and forestry wastes are the most common primary
carbon sources due to the adaptability to local conditions and cost requirements. Research in this
area is relatively well-established, with a key issue being the selection of the appropriate HRT to
address large fluctuations in the quality and volume of drainage water. Because these drainage
systems are mainly located in the open, reducing the constraints imposed by low temperatures on
denitrification efficiency is also a key concern. Other areas of interest include denitrification
efficiency at low HRT, byproduct concentrations in the effluent, carbon source operating life, and
the simultaneous removal of nitrogen and phosphorus. Surface water is characterized by low
pollutant concentrations and high flow rates, limiting microorganism enrichment, the formation of
an adequate supply of localized carbon sources, and the denitrification ability of the microorganisms.
This problem can be effectively resolved through the adoption of SCS-based denitrification
technologies. Most existing studies have focused on conventional influencing factors (e.g. type and
quantity of the carbon source and nitrogen pollutant load) and effluent quality parameters. Given
the geographical and environmental characteristics of surface water, many researchers have also
explored the influence of DO on the performance of carbon sources in surface water denitrification.
In groundwater, which is characterized by low concentrations of easily oxidizable organic carbon
and low temperatures, denitrification rates under natural conditions are usually lower (Wu, 2002).
The microbial growth induced by the dosing of nutrient solutions in groundwater leads to a reduction
in the pores of the water-containing medium, which consequently results in blockages; hence, the
adoption of SCS-based denitrification technologies (especially denitrification barriers) has provided
a novel means of nutrient supply. Existing studies on groundwater have mainly focused on the
influences of nitrate-nitrogen load, HRT, and carbon source dosage on denitrification efficiency.
Furthermore, the functional genes and microbial community structures related to the denitrification
process have also been explored in research on the various target water bodies.
4 Meta-analysis on synthetic SCS-based denitrification
In order to better understand the effect of different SCSs and reaction conditions on the
denitrification efficiency, we applied meta-analysis on the published literature in the area of solidphase denitrification in recent years. Since the denitrification rates of synthetic carbon sources are
much larger than that of raw carbon sources, in this study, we focus on the application of synthetic
carbon source on nitrate removal.
The data came from published journal articles that presented nitrate removal rates from flowthrough, synthesized SCS-based denitrification bed or lab-scale column reactors with nitrate as the
main target containment. We searched for literature in the database of Web of Science with
synthesized carbon sources (i.e. PBS, PCL, PHA, PHB, PHBV and PLA) and denitrification.
Literature without long-term stable operation and key experimental data were excluded. 23 papers
published focusing on synthesized carbon source based solid phase denitrification in recent three
years were finally analyzed in our study.
The core measure of solid-phase denitrification in our study was denitrification rates (DR) in
the units of nitrate removal per volume of bioreactor per time (gNL-1d-1), and necessary calculation
were applied using other information from papers when DR was not given directly. The data
required for meta-analysis in our study were the mean value of DR and the corresponding standard
deviation (SD).
Based on our former discussion and available data, we chose carbon source type (CS type),
carbon source species (CS species), influent N concentration, HRT and water temperature as target
factors, which were further categorized into two or three levels, adapted from the meta analysis
study on woodchip denitrification reactors (Addy et al., 2016).
CS type is categorized into ‘mix CS’ (mixed synthetic carbon source or the mixture of synthetic
and raw carbon sources) and ‘single CS’; three kinds of synthetic carbon source (i.e. PHBV,
PHBV/PLA, PCL) with abundant experimental data are analyzed; Influent N concentration is
divided into low, intermediate and high categories split by 20 and 50 mgN/L; HRT and water
temperature are categorized in similar manner, split by 2 h and 5 h, 22℃ and 25℃, respectively. The
actual data on mean removal rate, SD, number of studies and the classification of each factors are
listed in Table S1.
The response ratio (lnR) and the response variance (VlnR) were calculated (Addy et al., 2016)
and MetaWin 2.0 was used for the calculation of nitrate removal rate effect size and its standard
deviation. Forest plots of the meta-analysis results were shown in Figure 2.
Figure 2. Mean nitrate removal effect size and 95% bias-corrected confidence interval by different
categories of (a) CS type, (b) CS species, (c) Influent N concentration, (d) HRT and (e) Temperature.
Numbers labeled in the figures are the mean value of effect size, and n represent for the number of studies in
meta-analysis.
There was no significant difference in nitrate removal rates between mix carbon source and
single carbon source (Fig 2a), and while species of CS (PHBV, PHBV/PLA and PCL) were decisive
as shown in Fig 2b. The mixing of carbon sources is generally for two purposes, one is to minimize
the cost while ensuring the denitrification efficiency, and the other is to treat multiple pollutants
simultaneously or in stages (Jiang et al., 2020, Yang et al., 2020a, Yang et al., 2020c). The small
difference in denitrification performance showed that mixing of SCSs under specific water bodies
and specific environments is worth further exploration.
Nitrate removal rates were significantly effected by influent N concentration, as shown in Fig
2c. Reactors with high influent N concentration (>50 mgN/L) obtained higher denitrification rates
than those with intermediate (20-50 mgN/L) and low (<20 mgN/L) influent N concentration. Higher
nitrate concentration always resulted in larger reaction rates, and higher nitrate removal rates could
also be obtained unless the exceeding of maximum denitrification capability of the system (Jiang et
al., 2020, Xu et al., 2018a).
HRT with different levels also significantly influenced nitrate removal rates (Fig 2d). However,
the results showed that higher HRT (>5h) achieved the inferior performance on nitrate removal,
which is contradictory to the results in a certain literature with the consideration of HRT levels (Ding
et al., 2020, Yi et al., 2020, Zhang et al., 2021). This is mainly because, in some studies, researchers
set a relatively long HRT in order to achieve a stable low nitrate effluent concentration (Feng et al.,
2020a, Han et al., 2018, Lan et al., 2020). In fact, synthetic carbon sources often have short lag time
and brilliant carbon release efficiency, and in many cases, HRT of 2h is sufficient for the thorough
removal of nitrate (Fang et al., 2020, Shen et al., 2020).
The nitrate removal rate effect sizes under different temperature were shown in Fig 2e. Low
temperature (<20℃) significantly affect nitrate removal, where less COD release, nitrite
accumulation and shift of denitrifying genus were detected at low-temperature (Shen et al., 2020,
Xu et al., 2019b). Nevertheless, the high and intermediate categories are not significantly different
in nitrate removal. There are two main reasons for this. First, the research literature analyzed mainly
conducted lab-scale column reactors around room temperature (around 25℃), which was reasonable
for the treatment of RAS, municipal waste water and surface water; second, the effect of low
temperature on enzyme activities and microbial community related to carbon hydrolysis and
denitrification are more obvious when it is below 15℃ (Jiang et al., 2020, Shen et al., 2020).
Therefore, realizing high-efficiency denitrification under low temperature conditions (generally
groundwater) is still a topic worthy of continued research.
Synthetic solid-phase carbon sources have good application prospects for solid phase
nitrification. The optimization of operation parameters (especially HRT), the mixing of synthetic
carbon sources for actual complex water bodies and the further design of low temperature-tolerated
reactors are important considerations for future study.
5 Advantage and disadvantage summary
The eutrophication of water bodies remains a serious problem in many parts of the world;
hence, research on the removal of nitrate-nitrogen pollution is extensive. In particular, research has
focused on SCSs due to their ease of transport, low tendency for secondary pollution, long service
life, and ease of management. In existing research, target water bodies for SCS-based denitrification
include wastewater (including aquaculture wastewater), agricultural subsurface drainage, surface
water, and groundwater. Relevant research on practical applications in agricultural subsurface
drainage systems is extensive and well-established, while studies on practical applications in other
water bodies are relatively scarce.
Although SCS-based denitrification technologies have received widespread attention, they also
possess certain shortcomings. Natural SCSs are inexpensive, easily acquirable, and have a long
operating life; however, their applications are often limited due to unstable carbon release rates,
excessive DOC release, and increased color intensity during the early stages of denitrification.
Synthetic SCSs are readily utilized by microorganisms due to their strong bioaffinities, have a low
tendency to cause secondary pollution due to their simple composition, and exhibit rapid carbon
release rates and high denitrification efficiencies. Synthetic SCSs are superior to natural SCSs in
most applications; however, they are expensive, have limited carbon release rates, and their
denitrification performance is strongly influenced by temperature. While these factors currently
limit the broader application of synthetic SCSs, it is anticipated that ongoing research will
successfully address these issues. In conclusion, despite the presence of certain limitations, SCSbased denitrification technologies show promise for applications in many fields due to their superior
advantages.
6 Conclusion
Solid-phase carbon sources have good application prospects for solid phase nitrification.
Research with respect to wastewater treatment has mainly been focused on the removal efficiency
of nitrogen pollutants and DOC accumulation in the effluent, while studies on recirculating
aquaculture systems have focused on product yield and water quality parameters. Agricultural
subsurface drainage system research was extensive, and focused on natural SCSs and the influence
of temperature and HRT on denitrification efficiency. The primary aspect of surface water research
was the influence of DO on denitrification efficiency, while studies on groundwater were mainly
focused on the influence of nitrate-nitrogen load, HRT, and carbon source dosage on denitrification
efficiency. The optimization of operation parameters (especially HRT), the hybrid application of
synthetic carbon sources and the further design of low temperature-tolerated reactors are worthy of
continued study.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Grant No.
51809195), Postdoctoral Science Foundation of China (No. 2018M642083) and National Water
Pollution Control and Treatment Science and Technology Major Project of China (Nos.
2017ZX07204004 and 2017ZX07204002).
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Table
Click here to access/download;Table;Table1.docx
Table 1. Summary of denitrification performance of different solid carbon source (SCSs) reported in previous literature
Nitrate-nitrogen
Denitrification
concentration in
Carbon source
Scale
Form
rate
HRT
Note
Reference
untreated water
[mg/(L·h)]
(mg/L)
Simultaneous removal
of nitrate-nitrogen and
A mix of wheat straw and pine bark mulch
Pilot
100
1.25
0.43 d
(Krause Camilo, 2016)
the herbicide agent
atrazine
Macrophyte residues
Laboratory
Wood chips
Laboratory
Glycyrrhiza glabra and Arunda donax
Laboratory
1.0–5.0 mm
5–8
2.12
50.04
1.49–7.27
(Yao et al., 2019)
Gradient filling
(Zhao et al., 2020)
0.29
100
(Ovez et al., 2006)
0.18
Corn cobs
Laboratory
Approx. 2 cm
24.5–25.5
8.46
153 L/d
(Xu et al., 2009)
1.22
Pretreated sawdust
100
Laboratory
Pretreated with
(Hu et al., 2019)
1.25
peracetic acid
Bioaugmentation by
PBS
Laboratory
Approx. 3 mm
15
28.04
0.5 h
high-efficiency
(Lu et al., 2017)
denitrifying bacteria
PCL
Laboratory
2.5–3.5 mm
30
19.00
1.5 h
(Ji et al., 2017)
15
8.57
1.5 h
(Liang et al., 2015)
H=12.5 mm,
D=25 mm
PCL
Laboratory
(shaped into
cascade mini
ring)
PCL
Laboratory
PCL
Laboratory
Poly(3-hydroxybutyrate-co-3-
2 × 3 × 4 mm
200
30.3
5.5 h
(Luo et al., 2016)
5h
(Luo et al., 2019)
11.25
2–3 mm
81.1–132.75
Laboratory
7.92
hydroxyvalerate) (PHBV)
911 m2/m3
(Gutierrez-Wing et al.,
PHB
Laboratory
(specific surface
50
2012)
area)
PHBV
Laboratory
4–6.5 mm
44.75–57.25
10.04
2.6 h
(Ye et al., 2017)
15
32.08
0.5 h
(Xu et al., 2019a)
15
27.90
0.5 h
(Yang & Wu, 2014)
50
1.65
H=3 mm D=3
PHBV
Laboratory
mm
H=0.32 cm,
PHBV
Laboratory
D=0.31 cm
PLA
Laboratory
H≈3.02 mm,
(Fan & Wang, 2009)
D=2.22–3.60
mm
PHB
Laboratory
1.49 m2/L
7–41
Specific surface area-
PCL
Laboratory
0.87 m2/L
21–166
related denitrification
Bionolle1
Laboratory
1.22 m2/L
5–40
1.5–10
0.75–1.25 h
(specific surface
Bionolle2
rates: 5–28; 20–160;
(Boley et al., 2000)
1.3–9; 10.5–67
Laboratory
12–77
mg/(m2·h)
area)
H=5 mm D=3
PBS
Laboratory
Approx. 50
22.08
8h
(Zhu et al., 2015)
50
3.64
12 h
(Haihong et al., 2006)
15–18
5–7.5
2–3 h
(Chu & Wang, 2016)
Approx. 100
28.33–34.58
mm
PBS
Laboratory
1 × 2 × 3 mm
H=3.5 mm
PHBV/bamboo powder
Laboratory
D=2.5 mm
PBS/BP
Laboratory
H=10 mm D=10
Freshwater/seawater
(Liu et al., 2018a)
mm
(Shen et
Starch/PCL
Laboratory
3~5 mm
50
2.88
al.,
2015)
PHBV/PLA
Laboratory
2.5–3 mm
PLA/PHBV
Laboratory
2.5–3 mm
15.42
(Xu et al., 2018a)
85
31.28
2.5 h
(Qiu et al., 2017)
100
40.53
2h
(Li et al., 2012)
Oval-shaped
particles with a
PLA/PHBV
Laboratory
specific surface
area of 0.015
m2/g
Starch/polyolefins
Laboratory
1 × 2 × 3 mm
60–80
2.5–4.5
Low temperatures (8–
(Zhenxing & Jianlong,
10 ℃)
2008)
2.3–3.3 h
Table
Click here to access/download;Table;table2.docx
Agricultural
Target water body
Wastewater
Recirculating aquaculture systems
subsurface drainage
Surface water
Groundwater
systems
(Duan et al., 2016;
Rout et al., 2017;
(Boley & Müller, 2005; Deng et al., 2017; Li
(Abusallout & Hua,
Shen et al., 2013; Sun
(Feng et al., 2017;
et al., 2019; Liu et al., 2018a; Luo et al.,
Nitrates
2017; David et al.,
et al., 2019a; Sun et
Jin et al., 2019; Xie et
Shen et al., 2015;
2019; Luo et al., 2016; Ruan et al., 2016;
2016; Krause Camilo,
Zhu et al., 2015),
2016; Li et al., 2018)
al., 2019b; Sun et al.,
Target pollutants
(Chu & Wang, 2016;
al., 2017; Ye et al.,
Yao et al., 2019)
2017)
2020; Xu et al.,
2019b),
Ammonia-
(Sun et al., 2020; Xu
(Feng et al., 2019;
(Ruan et al., 2016)
nitrogen
et al., 2019b)
Liu et al., 2018b),
total nitrogen
(Honda & Osawa,
(Jia et al., 2019)
2002; Yang et al.,
2020)
Other refractory
(Sun et al., 2019a;
organic
Sun et al., 2019b)
substances
Phosphates
(Li et al., 2018),
Herbicides
(Krause Camilo, 2016)
(Rout et al., 2017;
(Abusallout & Hua,
(Feng et al., 2017;
Sun et al., 2019a),
2017; David et al.,
Jia et al., 2019; Yao
Natural
(Jin et al., 2019),
Types of carbon
natural blends (Rout
2016), (Krause
et al., 2019) (Feng et
sources
et al., 2017)
Camilo, 2016),
al., 2019)
(Duan et al., 2016;
(Boley & Müller, 2005; Deng et al., 2017;
Honda & Osawa,
Luo et al., 2019; Luo et al., 2016; Ruan et
Synthetic
(Liu et al., 2018b),
2002; Sun et al.,
al., 2016; Zhu et al., 2015)
2019b; Xu et al.,
2018b; Yang et al.,
2020),
(Sun et al., 2020; Xu
(Chu & Wang, 2016; Ye
et al., 2019b)
et al., 2017)
（Shen et al., 2013;
(Chu & Wang, 2016;
Synthetic blends
Natural and
(Liu et al., 2018a)
synthetic blends
(Shen et al., 2015)
Yang et al., 2020)
Xie et al., 2017)
Natural blends
synthetic with
(Li et al., 2019)
additives
Natural in
Li et al., 2018)
tandem with
other solid
wastes
(Abusallout & Hua,
(Feng et al., 2017;
2017; David et al.,
Feng et al., 2019; Jia
Fragmented
(Rout et al., 2017;
agricultural and
(Jin et al., 2019),
Sun et al., 2019a)
2016; Krause Camilo,
et al., 2019; Yao et
2016; Li et al., 2018)
al., 2019)
forestry wastes
(Duan et al., 2016;
Forms of carbon
Honda & Osawa,
sources
(Boley & Müller, 2005; Deng et al., 2017;
2002; Shen et al.,
(Chu & Wang, 2016;
Liu et al., 2018a; Luo et al., 2019; Luo et al.,
Granules
2013; Sun et al.,
(Li et al., 2018)
2016; Ruan et al., 2016; Zhu et al., 2015),
2019b; Sun et al.,
2019b; Xu et al.,
Xie et al., 2017; Ye et
Shen et al., 2015)
al., 2017)
spherical gel (Li et al., 2019)
2020; Xu et al.,
(Liu et al., 2018b;
2018b; Yang et al.,
2020)
Batch
(Rout et al., 2017)
experiments
(Honda & Osawa,
2002; Shen et al.,
2013; Sun et al.,
Process
Up flow fixed2019a; Sun et al.,
characteristics
(Boley & Müller, 2005; Deng et al., 2017; Li
(Feng et al., 2017;
bed
(Chu & Wang, 2016;
2019b; Sun et al.,
et al., 2019; Liu et al., 2018a; Luo et al.,
Feng et al., 2019;
denitrification
Jin et al., 2019)
2020; Xu et al.,
reactors
2019b; Xu et al.,
2018b; Yang et al.,
2020)
2019; Luo et al., 2016; Zhu et al., 2015),
Yao et al., 2019)
Airlift inner-loop
(Ruan et al., 2016)
reactors
Horizontal flow
(Abusallout & Hua,
fixed-bed
2017; David et al.,
(Xie et al., 2017)
denitrification
2016; Krause Camilo,
reactors
2016; Li et al., 2018)
(Jia et al., 2019; Liu
Constructed
et al., 2018b; Shen
wetlands
et al., 2015)
Ractive barriers
Nitrate
(Xie et al., 2017)
(Rout et al., 2017;
Influencing factors
(Zhu et al., 2015),
concentration
Shen et al., 2013)
HRT
(Rout et al., 2017; Xu
(Xie et al., 2017),
of interest
(Deng et al., 2017)
(Li et al., 2018)
(Jin et al., 2019; Ye et
et al., 2018b),
al., 2017)
(Shen et al., 2013; Xu
(David et al., 2016;
et al., 2019b)
Krause Camilo, 2016)
Temperature
Initial pH
(Shen et al., 2013),
(Feng et al., 2019;
DO
(Deng et al., 2017; Luo et al., 2016)
Liu et al., 2018b;
Yao et al., 2019),
Deng et al., 2017; Liu et al., 2018a; Zhu et
salinity
al., 2015)
Feng et al., 2017;
Type of carbon
(Duan et al., 2016;
(Jin et al., 2019; Ye et
(Li et al., 2019; Luo et al., 2019)
source
Feng et al., 2019; Jia
Yang et al., 2020),
al., 2017)
et al., 2019)
Pretreatment of
(Sun et al., 2019a)
(Feng et al., 2017)
carbon sources
(Duan et al., 2016;
Rout et al., 2017;
Shen et al., 2013; Sun
(Chu & Wang, 2016;
(Boley & Müller, 2005; Li et al., 2019; Luo
Nitrate
concentration
Jin et al., 2019; Xie et
Shen et al., 2015;
al., 2017; Ye et al.,
2020; Xu et al.,
interest
Krause Camilo, 2016;
al., 2019b; Sun et al.,
2016),
parameters of
(Feng et al., 2017;
et al., 2019a; Sun et
et al., 2019; Luo et al., 2016; Ruan et al.,
Effluent
(David et al., 2016;
Li et al., 2018)
Yao et al., 2019), ,
2017), ,
2019b; Xu et al.,
2018b)
Denitrification
(Li et al., 2019; Liu et al., 2018a; Zhu et al.,
rate
2015)
(Duan et al., 2016;
Nitrite and
(Chu & Wang, 2016;
Shen et al., 2013; Sun
(Liu et al., 2018a; Ruan et al., 2016; Zhu et
(Feng et al., 2017)
ammonia-
Jin et al., 2019; Xie et
et al., 2019b; Sun et
al., 2015), (Liu et al., 2018a; Luo et al.,
(Feng et al., 2017;
nitrogen
al., 2017; Ye et al.,
al., 2020; Yang et al.,
2019; Ruan et al., 2016; Zhu et al., 2015)
Feng et al., 2019)
concentration
2017), (Jin et al., 2019)
2020)
(Honda & Osawa,
(Feng et al., 2019;
2002; Sun et al.,
TN concentration
(Luo et al., 2016),
Jia et al., 2019; Liu
2020; Yang et al.,
et al., 2018b)
2020),
(Duan et al., 2016;
(Jia et al., 2019;
TOC
Shen et al., 2013; Sun
(Liu et al., 2018a; Ruan et al., 2016; Zhu et
(Abusallout & Hua,
(Chu & Wang, 2016;
Shen et al., 2015;
accumulation
et al., 2020; Xu et al.,
al., 2015),
2017)
Ye et al., 2017)
Yao et al., 2019)
2019b; Yang et al.,
2020)
Changes in mass
(Honda & Osawa,
and form of
2002; Xu et al.,
carbon source
2018b)
Eemoval rates of
(Li et al., 2018),
(Jin et al., 2019), , (Jin
(Krause Camilo, 2016)
et al., 2019)
(Sun et al., 2019b)
other pollutants
Material
(Li et al., 2019; Zhu et al., 2015) (Boley &
characteristics,
Müller, 2005) (Boley & Müller, 2005) (Deng
pH changes,
et al., 2017)
weight gain in
(Jin et al., 2019)
fish, microbial
community
structures
cidovorax (Duan et
Acidovorax (Feng et
al., 2016; Shen et al.,
al., 2017),
Acidovorax (Xie et al.,
2013),
Bacillus (Shen et al.,
2017),
Bacillus (Rout et al.,
Acidovorax (Luo et al., 2019), Azoarcus
2015),
Azospira (Xie et al.,
2017),
(Ruan et al., 2016), Bdellovibrio (Luo et al.,
Bosea (Feng et al.,
2017), Crotonatovorans
Comamonas (Duan et
2019), Denitratisoma (Luo et al., 2019),
2017),
(Chu & Wang, 2016),
al., 2016),
Simplicispira (Ruan et al., 2016)
Dechloromonas
Firmicus (Chu & Wang,
Dechloromonas
(Feng et al., 2017),
2016), Rhizomicrobium
(Duan et al., 2016),
Thauera (Shen et al.,
(Xie et al., 2017)
Diaphorobacter (Shen
2015), Simplicispira
Microorganisms
et al., 2013),
Ochrobactrum (Rout
et al., 2017),
Stenotrophomonas
(Rout et al., 2017)
(Feng et al., 2017)
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*Declaration of Interest Statement
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: