Nitrous oxide (N2O), as a potent greenhouse gas, is receiving increasing attention from the scientific community and policy makers regarding its emissions. The warming potential of N2O is nearly 300 times that of carbon dioxide, and its lifespan in the atmosphere is as long as 120 years. Its impact on climate change cannot be ignored. In today's increasingly frequent human activities, the sewage treatment process, which may seem unrelated to greenhouse gas emissions, has actually become one of the important anthropogenic sources of N2O emissions. So, how does the sewage treatment process contribute to N2O emissions? How can we accurately monitor and effectively control it? This article will delve into the current situation, generation mechanism, accounting management methods, and monitoring technology of N2O emissions from sewage treatment plants, in order to provide reference for achieving green and low-carbon development in the sewage treatment industry.

The sources of nitrous oxide emissions are extensive and can be divided into two categories: natural emissions and anthropogenic emissions. Natural emissions mainly include natural processes such as the release of soil in its original state and the release of N2O in the ocean. And anthropogenic emissions cover multiple fields such as agricultural cultivation, fossil fuel combustion, and pollution control. According to statistics, anthropogenic emissions account for about 40% of the total N2O emissions. It is worth noting that sewage treatment, as an important link in pollution control, has become one of the significant anthropogenic sources of N2O emissions. So, what is the actual N2O emissions from sewage treatment plants?

According to existing research, there are significant differences in N2O release factors (N2O emissions/N load) among different sewage treatment plants. In the denitrification process of large-scale urban sewage treatment plants, 0% to 14.6% of nitrogen may ultimately be converted into N2O and released into the atmosphere. This data indicates that N2O emissions from sewage treatment plants are not a fixed value, but are influenced by multiple factors, and their potential emissions should not be underestimated. For example, some studies have indicated that the N2O emission factor of activated sludge wastewater treatment plants is usually between 0.0016 and 0.0032, but the specific values may vary depending on process conditions, inlet water quality, and other factors. Therefore, accurate assessment and effective control of N2O emissions from sewage treatment plants are of great significance for achieving China's "dual carbon" goals and addressing climate change.

The production of N2O during sewage treatment is a complex biochemical process that mainly involves the nitrification and denitrification of microorganisms. So, what specific pathways lead to the generation of N2O?

N2O is mainly generated in the following processes in wastewater treatment:

1Nitrification of Aerobic Ammonia Oxidizing Bacteria (AOB)

In the process of converting ammonia nitrogen into nitrite, when the dissolved oxygen (DO) concentration is low or the ammonia nitrogen concentration is high, AOB may not completely oxidize some of the ammonia nitrogen, generating N2O as a byproduct.

2Denitrification of AOB

Under certain specific conditions, AOB can also undergo denitrification, reducing nitrite to N2O.

3Denitrification of heterotrophic denitrifying bacteria (HDN)

Heterotrophic denitrifying bacteria gradually reduce nitrate or nitrite to nitrogen (N2) under anaerobic conditions. However, if the denitrification process is not completed, or if the N2O reductase activity of denitrifying bacteria is lost, or if some denitrifying bacteria do not have an N2O reductase system, it may lead to the accumulation and emission of N2O.

4Other ways

In addition to the main pathways mentioned above, there are other factors that may lead to the production of N2O, such as fluctuations in pH, limitations of carbon sources, temperature changes, and the presence of certain inhibitors, all of which may affect the metabolic pathways of microorganisms and promote N2O production.

The complexity of these mechanisms makes N2O production uncertain and poses challenges for precise control. Understanding these mechanisms is key to optimizing wastewater treatment processes and reducing N2O emissions.

It is crucial to scientifically account for and manage greenhouse gas emissions from sewage treatment plants in order to effectively address climate change. So, what are the guidelines and standards in this field in China?

According to the "Technical Guidelines for Coordinated Control of Greenhouse Gas Accounting for Pollutant Removal in Urban Sewage Treatment Plants (Trial)" released by the ecological environment unit, the accounting of greenhouse gases for coordinated control of pollutant removal in urban sewage treatment plants can be divided into the following key steps:

1Determine the accounting boundary

Clearly define the scope of accounting, including the process flow and discharge sources of sewage treatment plants.

2Choose accounting method

Choose appropriate accounting methods based on the actual situation, such as emission factor method, mass balance method, etc.

3Collect activity level data and determine emission factors

Collect activity data on sewage treatment capacity, inlet and outlet water quality, energy consumption, etc., and calculate them based on relevant emission factors.

4quality control

Quality assurance and quality control shall be carried out on the data and methods used in the accounting process to ensure the accuracy of the accounting results.

5Form accounting report

Prepare detailed greenhouse gas emission accounting reports to provide a basis for management decisions.

In addition, the country and industry have also issued a series of relevant standards and guidelines, such as the "Technical Standards for Monitoring Greenhouse Gas Emissions from Sewage Treatment Plants" (draft for comments), which provide specific technical specifications for the accounting of greenhouse gas emissions from sewage treatment plants. The development of these standards and guidelines aims to promote the refined management of greenhouse gas emissions in the sewage treatment industry, thereby better serving the national carbon peak and carbon neutrality goals. Through standardized accounting and management, sewage treatment plants can have a clearer understanding of their carbon emissions status, identify emission reduction potential, and develop effective emission reduction strategies, thereby playing a positive role in environmental protection and addressing climate change.

Accurate monitoring of N2O during wastewater treatment is the foundation for understanding its emission patterns, optimizing process parameters, and achieving effective emission reduction. So, what are the main monitoring methods currently available, and how can equipment be used to achieve precise monitoring?

The traditional N2O monitoring methods mainly include:

◆Airbag methodBy collecting gas samples and sending them to the laboratory for analysis, the operation is simple, but real-time continuous monitoring cannot be achieved.

◆Static box methodPlace static boxes in specific areas, collect gas inside the boxes, take regular samples for analysis, suitable for small-scale, short-term flux observation.

◆Floating gas flux hoodCombined with the principle of static box, it is used for gas flux observation at the water gas interface, but there is also a problem of discontinuity.

◆Gas Chromatography (GC)As the main means of laboratory analysis, it has high accuracy but slow response speed, and is not suitable for online real-time monitoring.

With the advancement of technology, online continuous monitoring technology plays an increasingly important role in N2O emission monitoring. Online monitoring can provide real-time, continuous, and high-precision data, which helps to detect emission anomalies in a timely manner and guide process adjustments. For example, the ERUN-QB9610S developed and produced by Yingrun GroupPortable greenhouse gas flux analyzerThis provides a solution for precise monitoring of N2O in sewage treatment plants. This device can be paired with a static floating box and is specifically designed for flux observation of CO2, CH4, N2O, and H2O at the water gas interface.

The outstanding features of the ERUN-QB9610S analyzer include:

★Wide measurement rangeThe measurement range of N2O can reach 0-1000ppm.

★High measurement accuracyN2O accuracy can reach ± 2%, CO2 accuracy<300ppb, CH4 accuracy<10pppb, H2O accuracy<2%, achieving ppb level accuracy to ensure data reliability.

★quick responseThe measurement rate is 1Hz, which can capture instantaneous emission changes.

★Excellent repeatabilityThe repeatability is better than 1%, ensuring the stability of the monitoring results.

★Intelligent functionsEquipped with functions such as control measurement, storage, and data processing, supporting online viewing and analysis of measurement results. By combining temperature, humidity, and other sensor monitoring data controlled by oneself, raw data can be obtained and stored simultaneously. In addition, it also supports data transmission through the network, making remote control easy and meeting the scientific research needs of long-term stable measurement deployment in the field.

By introducing such online monitoring equipment, sewage treatment plants can achieve refined management of N2O emissions, provide accurate and real-time activity data for emission accounting, and provide scientific basis for process optimization, thereby effectively reducing N2O emissions and helping to achieve green and low-carbon operation.

In summary, the issue of nitrous oxide emissions from sewage treatment plants cannot be ignored. Its impact on climate change and the pressure to reduce emissions under China's "dual carbon" goals have made precise monitoring and effective control of N2O an important issue facing the current sewage treatment industry. A deep understanding of the mechanism of N2O production and standardized accounting and management based on national guidelines is the foundation for achieving emissions reduction. The online monitoring technology represented by the ERUN-QB9610S portable greenhouse gas flux analyzer provides real-time, accurate, and efficient monitoring methods for sewage treatment plants, making it possible to finely manage N2O emissions.