Greenhouse Gas Emissions Accounting And Carbon Neutrality Strategies For Wastewater Treatment Plants

The environmental problems caused by global warming have become increasingly prominent in recent years,and the reduction of greenhouse gas emissions has become one of the most important topics of concern for all mankind.As an important energy-intensive infrastructure of cities,the greenhouse gas emissions from the operation of wastewater treatment plants are gradually gaining international attention and are considered to be one of the important anthropogenic carbon emissions.The number of wastewater treatment plants and the volume of water treated in China are increasing rapidly,and the pressure and task of reducing their greenhouse gas emissions is enormous.It is significant to accurately account the greenhouse gas emissions from wastewater treatment and to propose corresponding carbon-neutral measures.This paper analyzed the main factors affecting the greenhouse gas emissions of sewage treatment plants,collects the emission factors reported by current domestic and foreign researches.Using the localized emission factor method based on the amount of pollutant removal to calculate the urban sewage treatment plants in China from 2008 to2017 and the spatial and temporal distribution of greenhouse gas emissions,the direct emissions from 2017 to 2021 in the Dianchi Lake Basin in Kunming City,and the emissions from the No.1 Sewage Treatment Plant in Yuxi City.According to the accounting results and the current research status,proposing the carbon-neutral strategy of the sewage treatment plant.The main conclusions are as follows.（1）Part of CO₂ directly emitted by sewage treatment plants comes from fossil products.These fossil carbons exist in every processes of sewage treatment and have the same biodegradation pathway as biological carbon and can be converted into CO₂ finally.Therefore,in the calculation of greenhouse gas emissions are not negligible.（2）The emission of greenhouse gases from sewage treatment plants is affected by many basic operating parameters such as pH value,influent C/N,sludge residence time,dissolved oxygen concentration,and nitrite concentration.Under the premise of not changing the hardware facilities of the sewage treatment plant,appropriately increasing the DO concentration in the anoxic tank and intermittently adding nitrite at a concentration of about 100 mg/L can effectively inhibit the generation of CH₄;the amount of N₂O generated varies with the concentration of DO and The increase of SRT decreases;the amount of N₂O generated will increase with the increase of nitrite concentration,so if the CH₄ generation is controlled by adding nitrite,the increase of N₂O must be considered at the same time.By comprehensively optimizing these operating parameters,the aim of emission reduction low-costly and high-effectly can be achieved.（3）Calculated the greenhouse gas emissions of urban sewage in China from 2008to 2017 by the localized emission factor method based on the removal of pollutants,the direct emissions of greenhouse gases from sewage treatment plants in Kunming Dianchi Lake Basin from 2017 to 2021,and the amount of greenhouse gases emitted by No.1Sewage Treatment Plant in Yuxi City in 2020.The results show that:（1）From 2008 to2017,China’s urban sewage treatment plant greenhouse gas emissions increased by9,297,850 tons,in which indirect emissions increased by 5,454,880 tons;GHGs emission intensity per unit of treated water increased year by year,from 308 g/m³ in2008 to 378 g/m³in 2017;the total emissions in space are generally higher in the east and lower in the west,and higher in the north and lower in the south.geographical disparity.（2）The higher the GDP per capita,the higher the daily emission intensity of greenhouse gases per capita.The emission intensity per unit of water treated is related to the treatment process,and the actual treatment scale of the treatment plant has little effect on the emission intensity of the unit treated water.（3）Indirect emissions are mainly concentrated in the secondary biochemical pool stage,accounting for about 60%of indirect emissions;direct emissions are mainly N₂O,accounting for about 70%of direct emissions.（4）By optimizing of core operating parameters,adopting advanced technologies such as carbon redirection technology,sludge incineration coupled heat and power technology,photovoltaic power generation and sewage source heat pump technology,the emission of greenhouse gases from sewage treatment plants can be effectively reduced or offset.Early achievement the carbon-neutral targets for wastewater treatment plants.