image: An intercept-free least-square regression identifies a strong relationship between COD and rCOD that is robust to cross-validation. (a) rCOD5 contributed to 56% of COD in influent, (c) 74% of COD in effluent, and (b) 55% of COD removal in municipal WWTPs. (d) rCODu accounted for 33% of the influent, (f) 61% of the effluent, and (e) 32% of removal COD. view more
Credit: ©Science China Press
This study is led by Professor Nianzhi Jiao (State Key Laboratory for Marine Environmental Science, Institute of Marine Microbes and Ecospheres, College of Ocean and Earth Sciences, Xiamen University), Professor Yao Zhang, and Dr. Zongqing Lv. In previous study, Jiao et al. found that the use of chemical oxygen demand (COD) in natural water bodies as a monitoring indicator of organic matter is misleading, and could judge refractory dissolved organic matter (RDOM) in water bodies as pollutants. Later, Zongqing Lv found that in the wastewater treatment industry, to meet the emission COD standards, a large amount of RDOM may have been removed, which promotes the emission of greenhouse gases (GHGs).
Zongqing Lv and Xiaoyu Shan, together with Professor Jiao and Professor Zhang, sought to estimate the amount of refractory organic matter that may be removed by the wastewater treatment industry due to the application of the COD standard, as well as the emissions amount of GHGs that these organic matters may be converted into.
The researchers collected COD and biochemical oxygen demand (BOD) data of influent and effluent wastewater of 86 wastewater treatment plants from five continents around the world, and using (COD-BOD) as an indicator of refractory organic matter, estimated the proportions of refractory organics in in influent, removal and effluent. They found that 55% of the organic matter removed during the wastewater treatment was contributed by refractory organic matter (Image below).
Then the team estimated the total annual removal of refractory organic matter from China's domestic wastewater treatment plants, and further estimated the maximum amount of GHG emissions using a model of organic matter conversion into GHGs. They found that the amount of methane released from the treatment of recalcitrant organic matters 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 GHGs.
COD is a monitoring indicator that has been used worldwide for hundreds of years. However, it has the risk of reversing the carbon sink of water bodies. The authors believe that leaving the 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. This paper proposes a new perspective on the carbon neutrality of the wastewater treatment industry.
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See the article:
Lv Z, Shan X, Xiao X, Cai R, Zhang Y, Jiao N. 2021. Excessive greenhouse gas emissions from wastewater treatment plants by using the chemical oxygen demand standard. Science China Earth Sciences, https://doi.org/10.1007/s11430-021-9837-5