Effect of carbon to nitrogen ratios of municipal sludge on its dehydration property

ABSTRACT

Municipal sludge is the main by-product in the process of sewage treatment, and the primary problem to be solved for its harmless disposal or energy utilization is its poor dehydration property. In this paper, biological conditioning technology was used to ferment municipal sludge with methanogens under different carbon to nitrogen ratios (C/N) through an anaerobic fermentation system. The mechanism of improving sludge dewatering performance under fermentation and the effect of C/N of sludge on its dehydration property were investigated. The results showed that the flocculent structure of the sludge was destroyed by the fermentation of methanogens, the sludge’s inherent moisture was released, and the dehydration property was promoted. With the change of C/N from 8 to 20, the hydrophilic oxygen-containing functional group content of the sludge decreased first and then increased. When C/N was 16, the content of hydrophilic oxygen-containing functional groups diminished to the lowest level. Meanwhile, the surface of the sludge showed dense microscopic cavities and small holes, indicating that the sludge flocs had been cracked to a large extent, and thus the ability of the sludge to bind water was weakened. The bound water content of the fermented sludge from low to high was 48.9, 59.0, 64.3, and 71.7% respectively, with C/N of 16, 12, 20, and 8. Therefore, within the scope of this experiment, the optimal C/N for the sludge dehydration property could be identified as around 16, which could reduce the bound water content of the sludge from 87% to 48.9%.

KEYWORDS:

• Municipal sludge
• dehydration property
• carbon to nitrogen ratios (C/N)
• flocculent structure
• bound water content
• energy utilization

Acknowledgements

The authors would like to acknowledge the financial supports from the project, No. LZ21E060002, of Zhejiang Provincial Natural Science Foundation of China, and the project, No. 202003N4315 and No. 202003N4312, of Ningbo Natural Science Foundation of China, and the project, No. 51506185, of National Natural Science Foundation of China.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The work was supported by the National Natural Science Foundation of China [No. 51506185]; Zhejiang Provincial Natural Science Foundation of China [No. LZ21E060002]; Ningbo Natural Science Foundation of China [No. 202003N4315 and No. 202003N4312]

Notes on contributors

Yanfeng Jiang

Yanfeng Jiang completed B.E. Energy and Power Engineering from Harbin Engineering University in 2021. Now he is a postgraduate student in power engineering at Zhejiang University. He is interested in the clean utilization of waste and the development of biomass fuels.

Fuyan Gao

Fuyan Gao obtained B.E. from Northeast Electric Power University in 2002 and M.E. from Zhejiang University in 2005. She received Ph.D. in Engineering Thermal Physics from Zhejiang University, Hangzhou, China in 2013. She is working as Associate Professor at NingboTech University from 2015. She is interested in the social concerns towards sustainability.

Guodong Peng

Guodong Peng obtained B.E. from Southeast University in 2019 and M.E. from Zhejiang University in 2022. He is interested in the clean utilization of energy.

Jiadi Ding

Jiadi Ding completed B.E. Energy and Environmental Engineering from NingboTech University in 2022. Now he is a postgraduate student in power engineering and engineering thermal physics at Beijing University of Technology. He is interested in the efficient utilization of medium and low grade energy.