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

01 Sewage treatment is an important carbon emission industry

At present, the global carbon emissions are close to 60 billion tons, CO2, CH4, N2O three greenhouse gases accounted for more than 95% of the total carbon emissions, of which CO2 accounted for about 75% of the total emissions, CH4 accounted for about 17% of the total emissions, N2O accounted for about 6% of the total emissions. The emissions of these gases are mainly involved in four major industry categories: energy utilization, agriculture, forestry and animal husbandry, industrial production and waste disposal. Energy use accounts for as much as 70% of total emissions, while waste disposal accounts for about 3.2%. The 3.2% of waste disposal includes garbage treatment, industrial wastewater treatment and domestic sewage treatment, which means water supply and sewage treatment. CO2 emissions mainly come from energy utilization, and CH4 and N2O emissions come from the direct emissions of the other three categories, of which the drainage and sewage treatment industry is the main driver. It is worth mentioning that the global warming potential of CH4 and N2O is much higher than that of CO2, which means that although the emissions of CH4 and N2O in the sewage treatment process are not large, their total carbon emissions are still considerable due to their high global warming potential.

02 Carbon emissions in sewage treatment process

Carbon emissions in the whole sewage treatment process are divided into direct emissions and indirect emissions, of which direct emissions account for more than 60% of the total emissions. In general, there are three ways to directly discharge CO2 in the sewage treatment process: first, the aerobic degradation of organic matter in the influent; Second, the endogenous respiration of microorganisms; The third is the denitrification process [1]. It is worth noting that the IPCC released the latest greenhouse gas guideline "IPCC Guidelines for National Greenhouse Gas Inventories" in 2006, which listed CH4 and N2O as greenhouse gases directly emitted by municipal sewage treatment plants, while direct emissions of CO2 were not included [2]. The reason is that this part of carbon is generally considered to come from plant photosynthesis, which is the organic carbon formed when CO2 is absorbed and fixed in the atmosphere. This part of carbon is biogenic, and it is the CO2 that originally exists in the atmosphere. However, the water in the sewage treatment plant is not only biogenic carbon, and a large number of detergents, cosmetics and drugs used in life are originally sourced from petrochemical products, not original natural raw materials, and this part of carbon is non-biogenic. Therefore, it is difficult to distinguish biogenic carbon and non-biogenic carbon in the carbon dioxide emitted during sewage treatment. Some scholars have calculated the non-biological carbon CO2 emissions of sewage treatment plants, accounting for 29.59%-51.80% of the total CO2 emissions. This shows that if the direct emission of fossil carbon is ignored, the calculation of greenhouse gas carbon footprint will be missing.

The traditional activated sludge denitrification process goes through the process of influent ammonia nitrogen nitrification and denitrification, which are also the main ways to produce N2O. In this process, nitrifying bacteria oxidize NH2OH to NO2-, which in turn produces N2O as a byproduct. At the same time, N2O is also produced by the chemical decomposition of nitrite or hydroxylamine, an intermediate product of ammonia oxidation. Therefore, anaerobic tank, anaerobic tank, aerobic tank and sludge thickening tank are the main sources of N2O release in the sewage treatment process. CH4 in sewage comes from the anaerobic degradation of organic matter by methanogens, which are generally obligate anaerobic bacteria, so CH4 may be produced in an anaerobic environment. In the sewage collection and transportation pipeline, the sewage is in an anaerobic environment, which creates conditions for methanogenic bacteria to anaerobic degrade organic matter, so that the sewage treatment plant carries a large amount of dissolved CH4 in the water, which may be discharged in the subsequent mixing and aeration process.

Indirect carbon emissions from sewage treatment plants come from electricity consumption and drug consumption. The main power-consuming equipment of the sewage treatment plant is aeration equipment, sludge treatment equipment, lifting pumps and other equipment. Aeration equipment is the largest source of electricity consumption in sewage treatment plants, accounting for 49% to 60% of the total electricity consumption, sludge enrichment process accounts for 11%, anaerobic digestion accounts for 9%, and lifting pumps account for 8%. Overall, more than half of the carbon emissions from wastewater treatment plants can be attributed to electricity consumption [4]. Drug consumption comes from additional carbon sources, flocculants and coagulants, liquid chlorine, and alkali to control pH consumption. Each agent also has greenhouse gas emissions during its production and transportation, which are measured by its corresponding carbon emission coefficient.

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.

Second, improve the efficiency of energy recovery in sewage

There are a lot of organic matter in the effluent of the municipal sewage treatment plant, which needs to be removed in order to make the effluent meet the discharge standard. In the traditional activated sludge process, these organic matter are converted to CO2 through an aerobic process. However, these organics actually contain a lot of chemical energy, which is better converted into biogas (such as CH4 or H2) for use than being removed by "energy dissipation". CH4 is a clean energy source with a high heat of combustion, with a calorific value of 8.4×104kJ/kg, which can provide both electrical energy and heat energy during combustion. At present, the influent organic matter concentration of the sewage treatment plant is 400mgCOD/L, and the initial sludge and the remaining sludge are anaerobic digested to produce CH4 and implement cogeneration, and the energy output is much higher than the energy input of the anaerobic digestion tank.

The process of anaerobic digestion to recover biogas from organic matter has been widely used in Europe and the United States. The Strass sewage treatment plant in Austria produces CH4 anaerobic by using the residual sludge, and the operation energy balance rate can reach 108%, that is, the sewage treatment plant can meet its entire energy consumption only by recycling the organic energy in the residual sludge. Although the core technology of sludge anaerobic digestion energy recovery to produce CH4 has been developed and mature, it is only economically feasible to install in large and medium-sized sewage treatment plants, and less than 5% of urban sewage treatment plants in China currently use sludge anaerobic digestion systems.

Third, find other external renewable energy sources

Sewage treatment plants require a certain amount of land, which can be filled with solar panels or with external wind power. For example, if photovoltaic panels are spread over the entire sewage treatment plant, nearly 10% of the operating energy consumption can be covered.

05 Outlook

Sewage is rich in fresh water, energy and nutrients, rather than "waste". With the advancement of technology, the idea of achieving carbon neutrality in wastewater treatment in the future, or even becoming an "energy factory" is not a dream.