Analysis of the Current Status and Prospects of Research on Residual

Sludge as a Carbon Source

Chengxuan Liu

, Xindong Wei

, Hongjie Sun

, Junwen Ma

and Yubo Cui

2,* e

College of Municipal and Environmental Engineering, Jilin University of Construction, 130118 Changchun, China

Key Laboratory of Biotechnology and Bioresources Utilization, Ministry of Education, Dalian Minzu University,

116600 Dalian, China

Corresponding author

Keywords: Residual Aludge, Hydrolytic Acidification, Carbon Source.

Abstract: In recent years, with the improvement of people's living standard, the nitrogen and phosphorus content in

domestic and industrial wastewater has increased significantly, resulting in a lower carbon to nitrogen ratio

of wastewater, and at the same time, a large amount of residual sludge is generated in wastewater treatment

plants which is difficult and costly to treat. The pretreatment of residual sludge as a carbon source for reuse

in wastewater treatment systems to enhance the efficacy of wastewater denitrification and phosphorus removal

can realize the resource utilization of residual sludge and solve the problem of low carbon to nitrogen ratio of

biological denitrification and phosphorus removal at the same time, which meets the needs of water

environment in China at this stage. This paper describes the current research status of residual sludge as a

carbon source and the mechanism of sludge hydrolysis and acidification and analyzes the application

prospects of this technology.

1 INTRODUCTION

China's domestic wastewater treatment plants have

low influent COD concentrations, low C/N in the

effluent, and a lack of carbon sources for the

denitrification process. To achieve the effluent

discharge standard, wastewater treatment plants

usually solve the problem by adding carbon sources

(such as methanol, acetic acid, glucose, etc.) (Li

2016), which greatly increases the operating cost of

wastewater plants.

Sludge is a byproduct of wastewater treatment

and is an easily decayed and odorous biosolids

produced during the biochemical treatment of

wastewater. The cost of sludge treatment and disposal

is high, reaching 1000-2000 Yuan/t (in terms of dry

sludge), accounting for 20%-50% of the operating

costs of wastewater treatment plants (Yu, Zhang, Li

2013). The residual sludge contains a large amount of

organic matter, which can be used as a carbon source

https://orcid.org/0000-0001-9220-2088

https://orcid.org/0000-0002-3096-5343

https://orcid.org/0000-0002-7201-0764

for wastewater denitrification, but the complex

structure of organic matter in sludge is difficult to be

used directly by microorganisms, and some

pretreatment measures are needed to enhance the

release of biodegradable organic matter.

In recent years, researchers have greatly improved

the dissolved chemical oxygen demand (SCOD)

concentration in sludge by releasing the embedded

carbon from the residual sludge into the solution by

cracking. The cracked sludge is returned to the

bioreactor, which can provide an endogenous carbon

source for the biological treatment of wastewater and

improve the removal rate of nitrogen from

wastewater so that the wastewater can meet the

standard discharge. In the process, partial reduction

of residual sludge can also be achieved. There is a

growing interest in related research.

https://orcid.org/0000-0001-5716-9446

https://orcid.org/0000-0001-8950-5889

Liu, C., Wei, X., Sun, H., Ma, J. and Cui, Y.

Analysis of the Current Status and Prospects of Research on Residual Sludge as a Carbon Source.

DOI: 10.5220/0011157000003444

In Proceedings of the 2nd Conference on Artiﬁcial Intelligence and Healthcare (CAIH 2021), pages 18-22

ISBN: 978-989-758-594-4

2 STATUS OF RESEARCH

In the activated sludge treatment process, the organic

matter content in the residual sludge accounts for

about 40% to 60% of the total sludge production

(Moreno Caselles, Prats, Moral 2006). The organic

matter in the residual sludge is degraded by anaerobic

hydrolytic acidification to transform the large

molecule organic matter in microbial cells into small-

molecule organic matter suitable as a carbon source.

Due to the demand for carbon source by denitrifying

bacteria and phosphorus removal bacteria in the

wastewater treatment process, the original carbon

source in domestic wastewater is insufficient to meet

the demand for a carbon source in wastewater

treatment, which in turn leads to excessive discharge

of nitrogen, phosphorus and other substances in the

effluent of wastewater treatment process (Jin, Wang,

Xing 2020). At the same time, with the input of

additional carbon sources, the efficiency of nitrogen

and phosphorus removal of traditional wastewater

treatment process has been improved, but the

production of residual sludge from biological

treatment of wastewater has also increased

significantly (Wei, Houten, Borger 2003). The

residual sludge is mainly composed of sludge

colloids, inorganic substances, dissolved pollutants,

etc., which contains a large number of pathogenic

microorganisms, heavy metal ions, and other toxic

and harmful substances, so the residual sludge will

cause secondary pollution to the environment if it is

not properly treated

(Xiang, Zhang, Zhuang 2004).

Therefore, the development of wastewater

treatment process with residual sludge as a carbon

source has important guiding significance and

engineering application value for domestic

wastewater treatment technology as well as water

environmental safety issues in China, which is in line

with the needs of China's sustainable development

strategy.

2.1 Mechanism of Residual Sludge

Hydrolysis and Acidification

Anaerobic digestion of sludge is now generally

accepted as a three-stage theory: hydrolysis stage,

acidification stage, and methanation stage (Lu, Lai,

Zhang 2009). The specific process is shown in Figure

1. The first stage is the hydrolysis stage, in which

complex organic substances such as proteins,

polysaccharide carbohydrates, and fatty acids are

degraded into small molecules such as amino acids,

monosaccharides, and fatty acids by anaerobic

hydrolysis and related anaerobic bacteria; the second

stage is the acidification stage, in which the

hydrolysis products of the first stage are further

degraded into small molecules such as hydrogen and

acetic acid by the action of hydrogen and acetic acid-

producing bacteria. The third stage is the methanation

stage, i.e., the products of the second stage are further

degraded by methanogenic bacteria to produce

methane and other gases.

The first stage of hydrolysis is the rate-limiting

stage in the three-stage theory because of the difficult

degradability of sludge cell walls and

macromolecular substances such as EPS

(Extracellular Polymeric Substance). Among them,

the hydrolysis acidification process is a degradation

process in which organic substances are used as

electron acceptors and donors, and fermenting

bacteria convert the small-molecule organic

substances produced by hydrolysis into simpler end

products such as amino acids and fatty acids.

Similarly, the methanation stage uses small

molecules such as fatty acids from the acidification

stage as a substrate for degradation, so the three

stages of anaerobic digestion are interrelated and

affect each other. Therefore, when the utilization rate

of substrate is lower than the substrate yield, the yield

of intermediate products will be accumulated, which

will hinder the completion of the three stages of

anaerobic digestion. In summary, the accumulation of

degradable small molecules such as volatile acids can

be promoted by increasing the rate of microbial cell

lysis in the hydrolysis stage and inhibiting the

methanation stage.

Figure 1: Three stage theory of hydrolysis acidification.

2.2 Technical Method to Promote

Residual Sludge Hydrolysis and

Acidification

Many scholars tend to resource sludge through

anaerobic fermentation for acid production.

However, acidic hydrolysis is the rate-limiting step in

the process of anaerobic fermentation of sludge, so it

Analysis of the Current Status and Prospects of Research on Residual Sludge as a Carbon Source

needs to be pretreated first, and there are three main

types of methods: physical, chemical, and biological.

2.2.1 Physical Method

The physical method is to break the sludge by

external energy to increase its solubility, which is

more conducive to anaerobic fermentation and acid

production, including ultrasonic method, grinding

method, hot water solution method, freezing and

melting method, etc. The ultrasonic method uses the

mechanical shear force of water to destroy the cell

walls of microorganisms in sludge in a local high-

temperature and high-pressure environment so that

their contents flow out for subsequent treatment

(Guo, Ma, Liu 2019). The increase in dissolved

chemical oxygen demand (SCOD) after low-intensity

ultrasound treatment of sludge indicates an increase

in the leaching of organic matter from the sludge,

which facilitates the subsequent treatment of the

sludge (Liu 2011). However, the application of

ultrasound technology has the problem of high energy

consumption, and it is usually used in combination

with other methods to reduce costs and increase the

efficiency of sludge cracking. The heating method

also destroys the floc structure of sludge, and treating

sludge in the range of 120-160 ℃ increases the sludge

solubility, leading to an increase in the content of

dissolved proteins and carbohydrates, a significant

increase in the rate of anaerobic digestion, and an

increase in methane production (Xue, Liu, Chen

2015).

2.2.2 Chemical Method

The chemical method involves the addition of various

chemical reagents, mainly oxidants (ozone, Fenton

reagent, ClO2, etc.) and bases [NaOH, Ca(OH)2,

etc.], to the sludge. The oxidizing agent can destroy

the floc structure of sludge, dissolve the cell wall

(membrane) of microorganisms, make the cell

contents leach out, increase the concentration of

SCOD in sludge, and improve the utilization rate of

microorganism. The alkali treatment dissolves the

fibrous components and organic flocs in the sludge,

destroys the cell structure of microorganisms,

releases the dissolved organic matter from the cells,

and increases the content of biodegradable organic

carbon in the sludge

(Xu, Zhuan, Zhang, Chang

2018).

2.2.3 Biological Method

The biological method is mainly used to change the

solubility of sludge by microorganisms or some

enzymes, which facilitates the anaerobic

fermentation process. The microbial method mainly

degrades the organic components of sludge through

the microbial flora contained in the sludge itself or by

adding microorganisms, and is divided into 2 types:

aerobic and anaerobic, usually anaerobic digestion

has a better treatment effect and is more widely used.

After anaerobic digestion of sludge first at high

temperature and then at medium temperature, the

sludge solubility increases, and methane production

rises because all microorganisms contained in itself

can find suitable conditions for growth

(Ge, Jensen,

Batstone 2010). Further, the direct addition of

digestive enzymes, such as protease and α-amylase,

can increase the production of VFAs and achieve

sludge reduction (Luo, Yang, Yu 2011).

3 PROSPECT ANALYSIS

With the rapid development of industry and the

increasing urban population, the discharge of urban

sewage has increased, and in this context, the

development of sewage treatment plants is on the rise.

At present, the number of urban sewage treatment

plants in China has exceeded 2000. During the

operation of the wastewater treatment process, part of

the sludge produced by the process is returned as

reactants for biological reactions, while the remaining

sludge is to be discharged outside the system. The

amount of this remaining sludge is alarming, with its

high-water content, large volume, easy decay, foul

odor, and containing a large number of heavy metals,

germs, and other toxic and harmful substances.

According to the "China Sludge Treatment and

Disposal Deep Research and Investment Strategic

Planning Analysis Report", with the popularity of

sewage treatment facilities, sewage treatment

efficiency, and the deepening of the degree of sewage

treatment, urban sewage treatment plant sludge

production has increased sharply. As China's urban

sewage treatment plant sludge treatment and disposal

capacity is insufficient, means backward, many of

sludge has not been standardized treated and disposal,

directly bring "secondary pollution" to the water, soil,

and atmosphere, not only reduces the effective

treatment capacity of sewage treatment facilities but

also poses a serious threat to the ecological

environment, while also causing a great waste of

resources. A great waste of resources.

In contrast, China's research on wastewater

treatment started very late; in the early 1990s, China's

sludge treatment technology was at a preliminary

stage, and a few wastewater plants were able to treat

CAIH 2021 - Conference on Artiﬁcial Intelligence and Healthcare

sludge simply through mechanical dewatering, thus

reducing the water content of sludge from the original

97%-99% to 75%-80%, but there were still problems

with the storage and reprocessing of treated sludge.

In the last 20 years, sludge treatment technology in

China has made great progress, but the research is not

deep enough, no unified evaluation criteria have been

formed, and there is a lack of reference in the

selection of technical solutions.

To accelerate the speed of sludge treatment,

during the 13th Five-Year Plan, the central

government will invest 200 billion yuan for sludge

treatment in sewage plants, and with the continuous

breakthrough of sludge disposal technology and the

promotion of policies, the sludge treatment, and

disposal industry will soon usher in a blue ocean

market. With the policy guidance, China's sludge

treatment industry market demand has been released.

It is predicted that the sludge treatment market size

will reach about 86.7 billion yuan in 2023 according

to the effective sewage treatment rate to the project.

Therefore, the realization of sewage resource

utilization and effective development of internal

carbon sources to achieve recycling not only make

effective use of sludge but also reduce the generation

of residual sludge, while achieving sludge reduction

and resource utilization, killing two birds with one

stone. Biological methods have a greater prospect of

development because of their low environmental

pollution, low cost, and effective sludge resource

utilization. However, single biological methods often

have limitations such as low removal efficiency and

long duration of action, while synergistic action with

fast-acting chemical or physical methods will

improve the efficiency of anaerobic fermentation

while reducing environmental pollution (Liu, Liu,

Liu 2021). The residual sludge can be used as a high-

quality carbon source to promote the efficacy of

nitrogen and phosphorus removal from wastewater

by cracking and then hydrolytic acidification. Future

research should still focus on how to apply the

laboratory technology for large-scale application,

explore a more energy-efficient cracking method as

well as a suitable dosing ratio, and study a solution

suitable for the actual situation in China.

At present, nearly half of the sludge produced by

the national sewage treatment plants has not been

harmlessly treated, and the progress of investment in

sludge treatment facilities is slow, and the

development of the sludge harmless and resourceful

disposal market still needs a process, through the

active guidance of relevant national policies, the

sludge harmless disposal industry will usher in a

larger development opportunity.

4 CONCLUSIONS

The residual sludge cracking can be used as an

endogenous carbon source for nitrogen removal in

wastewater plants, which can simultaneously achieve

residual sludge reduction and reduce the added

carbon source, thus increasing the total nitrogen

removal rate, improving the effluent quality, and

reducing the treatment cost. A variety of physical,

chemical, and biological methods are available to

achieve effective residual sludge cracking, and the

combined process is more effective. However, the

following problems exist: first, how to achieve

selective cracking, so that the proportion of carbon

source is higher than that of N and P. Second, how to

reduce the cost and achieve economy, since the

release of SCOD from the sludge after cracking is

accompanied by the release of N and P from the

sludge. The research on the above issues may be a hot

topic in the future.

For the study of using cracked sludge as a

denitrification carbon source, it is necessary to further

investigate the mechanism of sludge cracking, to

determine which organic compounds are released

from the floc after sludge cracking, and to explain

their role in biological denitrification and phosphorus

removal. In the process of sludge cracking, the

reaction conditions need to be strictly controlled so

that C, N, and P are released in a proportional manner,

and the released C is much larger than N and P, in

order to avoid additional nitrogen load to the system,

which makes it difficult to remove organic nitrogen

from the system, and also to avoid weakening the

phosphorus removal effect for nitrogen removal.

In addition, when selecting the carbon source, the

endogenous carbon source and the external carbon

source can be complementary to each other, in order

to save the treatment cost and improve the nitrogen

removal effect as much as possible, so as to achieve

a dynamic balance between the cost and the treatment

effect.

ACKNOWLEDGEMENTS

The research was financed by the Natural Science

Foundation of Liaoning, China (2020-MZLH-02) and

Science and Technology Innovation Foundation of

Dalian, China (2018J12SN080).

Analysis of the Current Status and Prospects of Research on Residual Sludge as a Carbon Source

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