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.