Abstract
Chemical oxygen demand (COD) is widely used as an organic pollution indicator in wastewater treatment plants. Large amounts of organic matter are removed during treatment processes to meet environmental standards, and consequently, substantial greenhouse gases (GHGs) such as methane (CH4) are released. However, the COD indicator covers a great amount of refractory organic matter that is not a pollutant and could be a potential carbon sink. Here, we collected and analysed COD data from 86 worldwide municipal wastewater treatment plants (WWTPs) and applied a model published by the Intergovernmental Panel on Climate Change to estimate the emission of CH4 due to recalcitrant organic compound processing in China’s municipal wastewater treatment systems Our results showed that the average contribution of refractory COD to total COD removal was 55% in 86 WWTPs. The amount of CH4 released from the treatment of recalcitrant organic matter in 2018 could have been as high as 38.22 million tons of carbon dioxide equivalent, which amounts to the annual carbon sequestered by China’s wetlands. This suggests that the use of COD as an indicator for organic pollution is undue and needs to be revised to reduce the emission of GHG. In fact, leaving nontoxic recalcitrant organic matter in the wastewater may create a significant carbon sink and will save energy during the treatment process, aiming at carbon neutrality in the wastewater treatment industry.
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Acknowledgements
We thank Dr. Daliang Ning, Dr. Bing Zhang, Prof. Jizhong Zhou and Prof. Yunfeng Yang for their support with the GWMC dataset. This work was supported by the National Basic Science Center of National Natural Science Foundation of China, National Key R&D Program (Grant No. 2016YFA0601400), and the National Natural Science Foundation of China (Grant Nos. 91751207 and 41861144018).
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Lv, Z., Shan, X., Xiao, X. et al. Excessive greenhouse gas emissions from wastewater treatment plants by using the chemical oxygen demand standard. Sci. China Earth Sci. 65, 87–95 (2022). https://doi.org/10.1007/s11430-021-9837-5
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DOI: https://doi.org/10.1007/s11430-021-9837-5