In China, the carbon emissions from the wastewater treatment industry account for about 1% of the total country’s carbon emissions, which is the largest proportion in the environmental protection industry. Therefore, it is of great importance for the wastewater treatment plants to implement "low-carbon" measures and follow the path of dual carbon transformation. China Environmental Protection Industry Association recently released China's first low-carbon group standard in the field of wastewater treatment - "Technical Specification for Low-Carbon Operation Evaluation of Wastewater Treatment Plants", which came into effect on July 1, 2022. What challenges will the release of this standard solve and how will its implementation play out in the future?

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"Drainage and wastewater treatment are some of the most carbon intense industries," said Wang Hongchen, director of the Research Center for Low-Carbon Water Environment Technology at the Renmin University of China. The three major greenhouse gases, nitrous oxide (N2O), account for 97.9% of total carbon emissions, accounting for 74.4%, 17.3%, and 6.2% respectively. The emissions of these gases mainly involve five major industry categories: energy utilization (coal burning, fuel oil, power generation, etc.), agriculture, forestry and animal husbandry, industrial production, and waste disposal, accounting for 73.2%, 18.4%, 5.2%, and 3.2%, respectively. Waste disposal accounting for 3.2% includes garbage disposal, industrial wastewater treatment, and domestic sewage treatment. CO₂ mainly comes from energy utilization, while CH₄ and N₂O come from the direct emissions of the other three categories.

Of the fact that the industry contributes 1% of the carbon emissions in China and how that may doesn’t seem significant, Wang Hongchen says, "Although the carbon emissions of the drainage and wastewater treatment industry account for only 1%, this can still be reduced by changing the technical route, operation, and completing the low-carbon transformation."

Although wastewater treatment itself already contributes to carbon reduction, its byproduct, the black odor caused by the direct discharge of untreated sewage, leads to an anaerobic process, which will produce more carbon emissions. At present, the wastewater treatment rate in China is relatively high, but the centralized wastewater collection rate is generally low, less than 50% in many cities, and there is a large number of black and odorous water bodies in urban and rural areas. Improving the centralized collection and treatment rate of wastewater and achieving low-carbon wastewater treatment is an important contribution of the wastewater treatment industry to achieving the "dual carbon" goals.

Zheng Jiang, vice president of the China Environmental Protection Industry Association, pointed out: "Because of differences in wastewater quality and wastewater treatment processes, the amount of greenhouse gas emissions produced by wastewater treatment varies widely. The greenhouse gas emissions in the treatment process are measured and the results show that according to the current treatment process and emission standards, the greenhouse gas emissions from the wastewater treatment process are about 600gCO₂/m3. The main emissions are indirect emissions caused by electricity consumption and denitrification processes. The direct emissions of N₂O generated, these two emissions account for 80%-90% of the total emissions; other emissions also include direct CH₄ emissions and indirect emissions from flocculant consumption.”

Why is reducing consumption and using energy-saving technologies a prerequisite for lower carbon emissions in the wastewater treatment industry?

"'Reducing consumption and implementing energy-saving solutions have been emphasized for decades, not only in the wastewater treatment industry but also in other industries and all aspects of life. In the past, it was more to reduce costs, but now, the wastewater treatment industry needs to ensure that the effluent is stable and meets the standard. Wang Hongchen believes that the understanding of the importance of energy-saving and consumption reduction in sewage treatment should be improved.

The electricity consumed in the wastewater treatment process accounts for about 0.5%-1% of the electricity consumption of the whole society. This part of indirect emissions accounts for about 50% of the total greenhouse gas emissions in the wastewater treatment process. Therefore, reducing the power consumption of wastewater treatment is an important way to achieve low-carbon operation.

"At present, the operating units of wastewater treatment facilities mainly adopt integrated plant-network control, smart control (pump stations, precise aeration, etc.), equipment for energy efficiency improvement (high-efficiency equipment), and optimized control strategies (reasonable sludge concentration, dissolution, etc.), methods such as oxygen setting, to name a few.

Wang Hongchen emphasized that "N2O emission reduction is the key." The release, detection, and control of N2O in wastewater treatment should be strengthened. At present, the release mechanism of N₂O in the wastewater treatment and its influencing factors are still unclear, and the detection and online monitoring methods are not yet developed, with no corresponding evaluation methods and control strategies, especially on how to achieve the control of N₂O and improve the nitrogen removal rate.

Furthermore, the development and use of new sources of renewable energy are crucial for carbon neutrality. Zheng Jiang put it in these words: "The wastewater contains a lot of organic matter and heat, while the wastewater treatment facilities cover a large area. These characteristics can be used to develop new energy in the wastewater treatment plant. The main methods currently used include: using sludge to dissipate energy. The oxygen digestion process produces biogas, and co-generation is carried out in the wastewater treatment plant to achieve efficient utilization of biogas resources; distributed photovoltaic power generation equipment is installed above the wastewater treatment facility to use solar energy to generate electricity and reuse it in the wastewater treatment process. Another method is to extract the heat from the sewage by water source heat pump; in some areas with terrain conditions (such as Chongqing), the tailwater discharge is used for energy recovery. In general, the new energy development of wastewater treatment plants has included biomass energy and thermal energy, to name a few.

"Carbon sequestration" is another method that also has a lot of room for development in carbon emission reduction of wastewater treatment plants. The wastewater treatment plant uses the reclaimed water produced as an important water source for environmental recharge, helps to build a good water environment, and can produce a certain carbon sink effect. Furthermore, the fully stabilized sludge product can be used in restoration projects such as sandy wasteland and mines. On this basis, the development of forestry carbon sink projects can also achieve better ecological and environmental effects.

What carbon emission evaluation methods are used in the wastewater treatment industry?

Due to the differences in influent water quality, treatment process, treatment scale, and effluent water quality standards, it is difficult to evaluate the carbon emissions and emission intensity of the wastewater treatment process.

The "Specification" clarifies the emissions of N₂O, CH₄, fossil fuel combustion, power consumption, heat consumption, and material consumption in wastewater treatment, thereby helping water companies to fully understand their total carbon emissions and their composition, and to formulate targeted carbon emission reduction strategies and measures.

Secondly, the "Specification" stipulates the carbon emission evaluation method. It fully considers the differences in wastewater treatment and proposes a normalized carbon emission evaluation method and evaluation process. Each operating unit can use the correction coefficient of total nitrogen removal rate, the correction coefficient of treatment scale, etc. to correct its carbon emission or carbon emission intensity, to facilitate the comparison of emissions in the same industry. Each operating unit can also clarify the level of its carbon emissions in the entire industry.

Finally, "Specification" proposes low carbon development in wastewater treatment. It proposes 12 low-carbon behaviors in four categories. Sewage treatment facility operators can combine their characteristics and focus on the transformation of key energy-consuming equipment such as lift pumps and aeration fans, the optimized operation of biological systems and the precise dosing of chemicals, the construction of clean energy recovery and utilization projects, and the implementation of monitoring and accounting, and carbon emissions in the production process.

Guo Chengzhan, President of the China Environmental Protection Industry Association, says, "The ’Specification’ provides an important methodological basis for management departments, operating units, and third-party agencies to evaluate the low-carbon behavior of wastewater treatment plants. The certification issued by the China Environmental Protection Association (Beijing) Certification Center for low-carbon achievements in wastewater treatment plants will be allocated based on the evaluation method proposed by this standard. The aim is to incentivize wastewater treatment plant managers to implement the new standards. In addition to the "Technical Specifications for Evaluation of Low-Carbon Operation of Sewage Treatment Plants", the association focuses on carbon dioxide online monitoring, and has recently released the "Technical Specifications for the Setting of Monitoring Points for Waste Gas Discharge Outlets from Fixed Pollution Sources" and "Continuous Carbon Dioxide Emissions from Fixed Pollution Sources", “Monitoring System Technical Requirements" and "Technical Specifications for Continuous Monitoring of Carbon Dioxide Emissions from Stationary Pollution Sources". As a complete set of standards to support the online monitoring of carbon dioxide in key industries, the formulation of these three standards has received strong support from the Ministry of Ecology and Environment. The performance testing and certification work have been carried out at the China Environmental Monitoring Station and the China Environmental Protection Industry Association Certification Center."