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.
Highlights
The driving force for climate change has been the emission of greenhouse gases (GHGs) since the industrial revolution (Hansen and Lacis, 1990; Lashof and Ahuja, 1990; Montzka et al, 2011), and wastewater treatment plants (WWTPs) are known to significantly contribute to these emissions (Koutsou et al, 2018; Nguyen et al, 2019)
We found that the removal of biochemical oxygen demand (BOD) was always accompanied by the removal of refractory COD (rCOD)
The BOD loss can be explained by the catabolism and anabolism of microbes, whereas the accompanying rCOD loss may be explained by excessive sludge digestion or advanced elimination of pollutants to meet strict Chemical oxygen demand (COD) discharge standards
Summary
The driving force for climate change has been the emission of greenhouse gases (GHGs) since the industrial revolution (Hansen and Lacis, 1990; Lashof and Ahuja, 1990; Montzka et al, 2011), and wastewater treatment plants (WWTPs) are known to significantly contribute to these emissions (Koutsou et al, 2018; Nguyen et al, 2019). Since the environmental protection departments have established strict water quality standards for effluent from WWTPs (Union and Parliament, 2013), to satisfy the environmental standards, organic matter and nutrients are removed from the wastewater (Ra et al, 2000; Zhang et al, 2020) This process produces methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O), which are emitted to the atmosphere and contribute to climate change (Gupta and Singh, 2012; Poulsen and Hansen, 2009). It is estimated that the global emissions of CH4 and N2O from WWTPs will exceed 600 million tons and 100 million tons of CO2 equivalent (CO2e) by 2030, respectively (Ragnauth et al, 2015) This indicates the urgency of reducing GHG emissions from wastewater treatment
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