Shouting sewage treatment enterprises: greenhouse gas emission control is coming, are you ready?
Greenhouse gas emission control in the field of sewage treatment has been put on the agenda. In 2023, the Ministry of Ecology and Environment (MEE) and relevant departments issued the Action Plan for Methane Emission Control. This is China's first comprehensive and dedicated methane emission control policy document, which proposes to strengthen the control of methane emissions from sewage treatment.
Not long ago, the National Development and Reform Commission, the Ministry of Housing and Urban-Rural Development, and the Ministry of Ecology and Environment jointly issued the "Implementation Opinions on Promoting the Synergy of Pollution Reduction and Carbon Reduction in Sewage Treatment", proposing to strengthen energy conservation and carbon reduction in sewage treatment and reduce greenhouse gas emissions. Scientifically carry out dredging and management of sewage pipe networks to reduce methane emissions.
However, there are still many difficulties from the release of policies to the implementation of measures. Xu Huaqing and Ma Cuimei of the National Center for Climate Strategy believe that all sectors of society are still relatively unfamiliar with the concept of methane emission control, and the lack of action is relatively insufficient, and there is a lack of technology and talent, so it is necessary to mobilize the whole society to participate.
The greenhouse effect of methane is enormous, and the global warming potential in 20 years is 81 times that of carbon dioxide
Methane (CH4) is second only to carbon dioxide (CO2) is the world's second-largest greenhouse gas, with a lifetime of only about 12 years in the atmosphere, but its warming potential is much higher than that of carbon dioxide.
Wang Hongchen, director of the Low Carbon Water Environment Technology Research Center of Chinese University, told reporters: "According to the latest data released by the IPCC, the warming potential of methane is about 81 times that of carbon dioxide on a 20-year time scale. The potency of methane decreases over time. But even on a 100-year scale, its warming potential is still 27 times that of carbon dioxide. ”
In other words, the same amount of methane can cause the temperature to rise faster than carbon dioxide.
According to statistics, in 2019, global carbon emissions were close to 60 billion tons. Among them, carbon dioxide, methane, nitrous oxide (N2O) The three major greenhouse gases accounted for 97.9% of total carbon emissions, accounting for 74.4%, 17.3% and 6.2% respectively. Although methane accounts for less than 1/4 of carbon dioxide, the impact of the former cannot be ignored when combined with the warming potential.
Where does methane come from in wastewater collection and treatment?
A staff member of the Beijing Drainage Group, which is responsible for the vast majority of sewage collection and treatment in the central urban area of Beijing, told reporters: "Methane emissions in the sewage treatment field mainly come from the sewage pipe network in the sewage collection stage, the sewage and sludge treatment process, and the sludge landfill disposal. "Methane is usually produced in an anaerobic environment and is produced by the decomposition of organic matter in sewage and sludge.
Hao Xiaoguang, an expert member of the Shanxi Provincial Urban Drainage Professional Committee, told reporters: "Most of the sediments in the urban underground pipe network are anaerobic environments, which are easy to induce anaerobic digestion of organic matter and produce methane. Some of it will overflow at the manhole and enter the atmosphere, and the other part will enter the sewage treatment plant with the sewage. The sewage treatment plant lifts the sewage, and during the pretreatment, the water falls and lifts the water, so that some of the methane is released into the air. This is a source of methane discharge from urban sewage. ”
It is worth noting that the generation of methane gas in the underground drainage network is also one of the reasons for the low carbon source concentration of the influent water of the sewage treatment plant, that is, the low COD, especially in summer. Because the organic carbon source in the sewage is converted into methane and released, the concentration of carbon source in the influent water will be reduced, thus affecting the efficiency of sewage treatment.
From the perspective of the existing process, methane will be produced in different links and structures of sewage treatment plants in different degrees of anaerobic environment, including regulating tanks, hydrolysis acidification tanks, primary sedimentation tanks, dead corners of aeration tanks, sludge storage tanks, etc.
Despite this, experts believe that methane emissions in the wastewater treatment sector are smaller and less concentrated than methane emission sources such as coal mines, oil and gas fields, and livestock farms.
Professor Hao Xiaodi of the Sino-Dutch Future Sewage Treatment R&D Center of Beijing University of Civil Engineering and Architecture told reporters after carbon accounting: "In the total carbon emissions of sewage treatment (2/3 direct carbon emissions + 1/3 indirect carbon emissions), nitrous oxide accounts for about 55%, and methane accounts for only about 5%. ”
Industry experts believe that when it comes to controlling methane emissions, there is a need to consider costs. For example, some abandoned mine pits produce high concentrations of methane, and it is stored at a low cost and with good results. Methane emissions in the wastewater treatment sector are relatively small and at low concentrations. When it comes to emission control, the cost of collection and disposal is relatively high. If the input-output ratio is considered, it may not be optimal.
Wang Hongchen told reporters: "In fact, the anaerobic environment of the sewage treatment plant produces very little methane, and the methane discharge of septic tanks at the front end of sewage collection is worth paying attention to." ”
Septic tank is a treatment facility that uses the principles of anaerobic fermentation and middle-layer manure to remove suspended organic matter in domestic sewage. At present, the most common in rural areas of our country is the three-format septic tank. After the sewage has been settled in the septic tank, it can enter the sewage treatment plant through the drainage pipe.
Wang Hongchen believes that septic tanks should be gradually dismantled to reduce methane emissions in the field of sewage treatment. "Some people think that removing septic tanks can easily cause drainage pipes to clog, which is a false proposition," he said. There are many reasons that lead to the blockage of septic tanks in our country. First of all, many places in China only clean the septic tank once a year, the frequency is low, resulting in more sediment; secondly, China usually has a septic tank per building, and the population density is large, resulting in more slag in the sewage. At present, the level of automatic cleaning of China's drainage pipe network is gradually improving, which is also helpful to solve the problem of drainage pipeline blockage. ”
It is difficult to actually monitor how much methane is emitted, and it can be estimated using a formula
The difficulty of methane emission control lies first in the difficulty of statistical monitoring.
Hao Xiaoguang said: "With clear policy requirements, greenhouse gas emission control in sewage treatment plants has been pushed to the forefront, but the problem now is that some sewage treatment plants lack understanding of methane gas emissions, and even do not know which links produce methane. In the absence of technology and data, it is difficult to establish a methane emission accounting system and report methane emission data on a regular basis. ”
Although China has issued relevant standards for pollutant discharge from urban sewage treatment plants, and put forward relevant quantitative requirements for methane emission control, the implementation and implementation of these standards need to be further strengthened due to the temporary lack of upper-level methods, the lack of high-precision monitoring technology and equipment, and the lack of professional and technical personnel.
Hao Xiaodi told reporters: "Theoretically, the actual detection is the most accurate, but not every sewage treatment plant can achieve it, because it requires investment in manpower, online detection instruments and other material resources to collect and analyze gas, and the cost is high." At present, it is common practice to calculate the emission factors after they have been determined. ”
It is reported that the "Technical Guide for Carbon Accounting and Emission Reduction Paths of Urban Water System", which was compiled by the China Urban Water Supply and Drainage Association and edited by Hao Xiaodi, has listed detailed calculation methods and formulas.
According to Hao Xiaodi, according to estimates, China's sewer carbon emissions are comparable to the total carbon emissions of sewage treatment plants (28 million tons/year, excluding sludge treatment), nearly 20 million tons/year. International studies have also shown that methane emissions from municipal sewer systems are comparable to those from the energy consumption of wastewater treatment plants. Therefore, the problem of methane discharge from sewers should not be underestimated, and measures should be taken to reduce the retention time of sewage in the pipes.
"In fact, in terms of direct carbon emissions from sewage treatment plants, the first priority should be to control nitrous oxide emissions. Methane is produced in small quantities in sewage treatment plants, mainly in waterways, and is difficult to control. Nitrous oxide is mainly produced in the nitrification and denitrification processes, so complete nitrification and denitrification should be ensured as much as possible to avoid intermediate retention of nitrogen conversion. In practice, nitrous oxide production can be prevented to a large extent as long as 1.5 m g/L of dissolved oxygen is maintained in the aeration tank and an adequate carbon source is ensured. ”
