Consideration on carbon emission and carbon emission reduction of sewage treatment system

In countries with full coverage of sewage treatment, carbon emissions from the drainage and sewage treatment industry account for about 1% of the total carbon emissions of the whole society. Although the proportion of carbon emissions in the drainage and sewage treatment industry is small, carbon emissions can be reduced by changing the technical route, changing the operation mode, and supplemented by appropriate low-carbon transformation. Compared with other industries, carbon reduction benefits are greater.

Whether the carbon emission of 01% of the industry is worthy of attention

The main principle of sewage treatment is the aerobic decomposition of organic matter, and the direct discharge of untreated sewage will lead to black odor, which is an anaerobic process that will emit CH4 and N2O, which in turn produces more carbon emissions. At present, the global sewage treatment rate is only 20%, and 80% of sewage is in direct discharge water bodies. Although China's statistical sewage treatment rate is high, the centralized sewage collection rate is generally low, less than 50% in many cities, and there are a lot of black and smelly water bodies in urban and rural areas. Therefore, it should be recognized that sewage treatment itself is a carbon emission reduction process, a process of both pollution reduction and carbon reduction, along the low-carbon technology route to speed up the construction of facilities, improve the centralized sewage collection and treatment rate, to achieve low-carbon sewage treatment, is the industry's greatest contribution to the "carbon peak, carbon neutral" strategy.

04 From "Energy Neutral" to "Carbon neutral"

In recent years, the concept of "Energy neutrality" has been mentioned by a growing number of wastewater treatment plants and has been one of the hot topics of academic research. At the same time, sewage treatment to achieve "carbon neutral" is also the trend of The Times.

Are energy neutral and carbon neutral synonymous and can they be compared? Energy neutralization, as the name suggests, means that the sewage treatment plant reduces its own energy consumption and can recover or generate one or more clean energy sources inside and outside the plant, which can directly (electricity and heat for self-use) or indirectly (generating energy connected to the grid) make up for the sewage treatment plant's own energy consumption, so as to achieve energy self-sufficiency without relying on fossil energy sources (electricity and heat) for sewage treatment. The concept of carbon neutrality in wastewater treatment plants is more straightforward. It refers to the sewage treatment plant through its own energy saving or consumption reduction or increase of its own capacity, or increase of carbon sink, so that the carbon reduction of the sewage treatment plant and carbon emissions offset each other. Among them, the carbon emissions of sewage treatment plants are the direct carbon emissions and indirect carbon emissions mentioned above. Obviously, energy neutrality in wastewater treatment plants is not the same as carbon neutrality. Energy neutrality only means that the sewage treatment plant is self-sufficient in energy consumption, and only offsets the energy consumption carbon footprint of indirect carbon emissions, while the carbon footprint of drug consumption in indirect carbon emissions and the carbon emissions of greenhouse gases such as N2O, CH4 and VOCs in direct carbon emissions are not offset. However, if a wastewater treatment plant is carbon neutral, it can basically be considered energy neutral. For example, sewage residual temperature heat energy has great potential, but it is a low-grade energy that can not be directly used for power generation, and can only be used as heat/cold output heating or cooling, and sewage treatment plants still need to rely on external power; This low-grade energy (hot/cold clean energy) can replace/make up for the use of high-grade energy (electricity, natural gas, etc.) after being used by the society outside the plant, thereby reducing a large amount of carbon emissions in the society, and these carbon emissions can be completely used to offset the carbon footprint of the sewage treatment plant's own power consumption.

To achieve energy neutrality, and further carbon neutrality, the following measures can be taken:

First, reduce the energy consumption of sewage treatment

The traditional activated sludge method converts organic matter in sewage into biomass and CO2 through microbial metabolism, and the result is "energy dissipation" and "pollution transfer". Although the traditional activated sludge process was promoted and applied from the early 20th century, and has been the mainstream technology of the sewage treatment industry for more than 100 years, it is clearly contrary to the concept of sustainable development that is now pursued. In the future, the sewage treatment will be gradually transformed from activated sludge method to A new AB process, that is, section A is responsible for efficient carbon capture, so that the organic matter in the sewage is captured before being used by microorganisms, and then used for energy recovery. The organic matter content of the sewage treated in section A is low, so new technologies (such as anaerobic ammonia oxidation technology) with low carbon source demand will be implemented in section B. Further removal of pollutants from sewage.