Acidogenic sludge fermentation to recover soluble organics as the carbon source for denitrification in wastewater treatment: Comparison of sludge types

Highlights

CEPS sludge was compared with conventional primary and secondary sludge for the VFAs yield.

Fe-based CEPS sludge exhibited the highest efficiency of organic recovery.

Fermented CEPS sludge liquor provided a sufficient carbon source for denitrification.

99% of nitrate removal was achieved based on the Fe-CEPS and sludge fermentation.

Abstract

For biological nitrogen (N) removal from wastewater, a sufficient organic carbon source is requested for denitrification. However, the organic carbon/nitrogen ratio in municipal wastewater is becoming lower in recent years, which increases the demand for the addition of external organic carbon, e.g. methanol, in wastewater treatment. The volatile fatty acids (VFAs) produced by acidogenic fermentation of sewage sludge can be an attractive alternative for methanol. Chemically enhanced primary sedimentation (CEPS) is an effective process that applies chemical coagulants to enhance the removal of organic pollutants and phosphorus from wastewater by sedimentation. In terms of the chemical and biological characteristics, the CEPS sludge is considerably different from the conventional primary and secondary sludge. In the present study, FeCl₃ and PACl (polyaluminum chloride) were used as the coagulants for CEPS treatment of raw sewage. The derived CEPS sludge (Fe-sludge and Al-sludge) was then processed with mesophilic acidogenic fermentation to hydrolyse the solid organics and produce VFAs for organic carbon recovery, and the sludge acidogenesis efficiency was compared with that of the conventional primary sludge and secondary sludge. The results showed that the Fe-sludge exhibited the highest hydrolysis and acidogenesis efficiency, while the Al-sludge and secondary sludge had lower hydrolysis efficiency than that of primary sludge. Utilizing the Fe-sludge fermentation liquid as the carbon source for denitrification, more than 99% of nitrate removal was achieved in the main-stream wastewater treatment without any external carbon addition, instead of 35% obtained from the conventional process of primary sedimentation followed by the oxic/anoxic (O/A) treatment.

Graphical abstract

Keywords

Sewage sludge / Chemically enhanced primary sedimentation (CEPS) / Acidogenic fermentation / Organic carbon recovery / Nitrogen removal

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Acknowledgements

This research was supported by grants 17204914, 17261916, C7044-14G and T21-711/16-R from the Research Grants Council (RGC) of the Hong Kong SAR Government, and Project JCYJ20150630170146833 from Shenzhen Municipal Science and Technology Innovation Council of Shenzhen Government, China. The technical assistance of Mr. Keith C.H. Wong is greatly appreciated.

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