Methane and nitrous oxide emissions from municipal wastewater treatment plants in China: A plant-level and technology-specific study

Assessment of methane and nitrous oxide emissions from urban community sewer networks: Field quantification and insights into environmental factors

Sewer networks are essential components of urban infrastructure, yet their contribution to greenhouse gas (GHG) emissions remains poorly understood. In this study, we deployed a new approach of in situ measurements to assess methane (CH4) and nitrous oxide (N2O) emissions across an urban sewer network, which spans 4769.43 m and receives about 750 m3 of domestic sewage per day. By monitoring at 248 and 151 sites for concentrations and fluxes respectively, we confirmed local GHG hotspots. Overall, the sewer network's total GHG emissions were estimated to be 763.3 g CO2eq/h, with CH4 accounting for 99.4 % of the emissions. The mean emission factor was estimated to be 1.05 kg CO2eq/(m·yr). N2O concentrations above the atmospheric background were detected in almost every manhole. Septic tanks (n = 19) were identified as the predominant sources, accounting for 92.5 % of emissions, while sewer pipes (n = 132) contributed the remaining 7.5 %. Emissions exhibited significant spatiotemporal variability, with daily fluctuations in CH4 and N2O ranging from 17- to 138-fold and 3- to 5-fold, respectively. Additionally, strong correlations were observed between CH4 emissions and sewage temperature (R = 0.70, p = 0.017), as well as manhole depth (R = 0.67, p = 0.016). For N2O, its emission strength was mostly related to the sewage temperature (R = 0.67, p = 0.024). These findings indicate that sewage temperature and sewer ventilation are critical factors influencing non-CO2 GHG emissions. This study represents the first direct measurement of GHG emissions from an urban community sewer network in China, providing vital field evidence for regional GHG estimations and further management practices for GHG mitigation.

Neglected role of virus-host interactions driving antibiotic resistance genes reduction in an urban river receiving treated wastewater

Treated wastewater from wastewater treatment plants (WWTPs) is a major contributor to the transfer of antibiotic resistance genes (ARGs) into urban rivers. However, the role of viral communities in this process remains poorly understood. This study focused on North Canal in Beijing, China, which receives over 80 % of its water from treated wastewater, to investigate the impact of viral communities on ARGs transfer. Results showed significant seasonal variation in the abundance and composition of ARGs, with 30 high-risk ARGs detected, accounting for 1.50 % ± 1.28 % of total ARGs. The assembly of ARGs in North Canal followed a stochastic process of homogenizing dispersal, with conjugative mobility playing a key role in horizontal gene transfer with Pseudomonas as primary host for HGT. The potential conjugative mobility of ARGs is significantly higher in wet season (69.4 % ± 17.3 %) compared to dry season (42.9 % ± 17.1 %), with conjugation frequencies ranging from 1.18 × 10-6 to 2.26 × 10-4. Viral species accumulation curves approaching saturation indicated the well captured viral diversity, and no phages carrying ARGs were found among 27,523 non-redundant viral operational taxonomic units. Most of the phages (89.2 % ± 3.8 %) were lytic in North Canal, which were observed to contribute to ARGs reduction by lysing their host bacteria, reflected by higher virus-host ratio and demonstrated by the phage lysis assays in treated wastewater and receiving river. We provided compelling evidence that phage-host interactions can reduce ARGs through host lysis, highlighting their potential role in mitigating ARG transmission in urban rivers receiving treated wastewater.

Comprehensive assessment, intelligent prediction, and precise mitigation strategies for greenhouse gas emissions in full-scale wastewater treatment plants

Wastewater treatment plants (WWTPs) are major contributors to global anthropogenic greenhouse gas (GHG) emissions, with China ranks among the leading emitters. In the context of China's "dual-carbon" journey, precision quantification and predictive forecasting of GHG fluxes, particularly methane (CH4) and nitrous oxide (N2O)-are crucial for developing advanced mitigation strategies of WWTPs. To accurately assess GHG emissions, this study firstly introduced customized emission factors (EFs) to precisely evaluate the GHG emissions of a full - scale A2O - based WWTP in Beijing. This approach addressed the overestimation of emissions when using the IPCC's standard EFs. Additionally, the study proposed machine learning (ML) techniques to predict GHG fluxes based on routine wastewater quality parameters. Specifically, Long Short-Term Memory (LSTM) and Random Forest (RF) models showed the strong performance in predicting CH4 and N2O emissions, respectively. Moreover, our findings revealed distinct spatiotemporal patterns of GHG emission: CH4 emissions peak during the summer solstice, while N2O emissions rise during the winter months. For the first time, this study identified the nitrification biofilter in the advanced treatment unit as a significant direct source of N2O emissions. Eventhough, indirect CO2 emissions account for a dominant 57%-90% of the total GHG emissions. Scenario analyses revealed a strategic mitigation approach. Energy conservation emerged as the most effective measure, capable of reducing emissions by 23.41%, followed by heat recovery, which could cut emissions by 10.15%. In practical applications, improving energy efficiency is of utmost importance in real - world mitigation strategies. This highlights the significance of integrated approaches for achieving the sustainable development of WWTPs in the "dual - carbon" background.

A review-based estimation of GHG emissions of China's wastewater management system

Under China's "Dual Carbon Goal", the wastewater treatment system plays a crucial role in the country's efforts to reduce greenhouse gas (GHG) emissions. However, a lack of baseline emissions data poses challenges for decarbonization efforts. This study aims to profile and diagnose the GHG emissions of China's entire wastewater system and identify key contributing factors. Our findings show that China's wastewater system, including wastewater treatment plants (WWTPs) and septic tanks, is responsible for significant emissions, with baseline estimates at 108.26 ± 47.37 Mt CO2-eq/a. Septic tanks and WWTPs emerged as the major GHG hotspots, contributing the most to the total emissions. This study highlights the variability in emission results from previous literature, stressing the need for consistent accounting methods and scientific emission factors. Additionally, current on-site monitoring practices in China show gaps, which hinder the accurate determination of baseline emissions. To guide future emission reduction strategies, regulatory frameworks and improved monitoring practices are recommended for the wastewater sector in China.

Greenhouse Gas Mass-Balance in Conventional Activated Sludge Wastewater Treatment: A Case Study in Mexico for Developing Countries

While numerous studies report methane emissions from wastewater treatment plants (WWTPs) in developed countries, few address emissions from plants in developing countries, where outdated technologies, such as the lack of enhanced primary and sludge treatment, are common. Moreover, these studies often rely on indirect calculations rather than direct measurements. Our study fills this gap by providing unit-process-level direct measurements of methane emissions in a conventional WWTP in Mexico, serving as a case study for developing countries. A standard plant was selected and visited on five occasions. It includes a primary settler, an aerated reactor, and a secondary settler, with no sludge treatment in place. Our findings revealed a CH4 emission factor of 0.396 ± 0.218 g CH4 m-3 of treated water, with the primary settler accounting for 72.3 ± 15.9% of emissions, and the aerated reactor contributing 27.7 ± 15.9%. Notably, the emission factors are comparable to those reported for plants with more advanced treatment technologies, suggesting that technological obsolescence may not significantly enhance CH4 emissions. Methanotrophy in the aerated reactor was a key process, oxidizing 91-98% of the CH4 transported from the primary settler. Additionally, a carbon dioxide (CO2) emission factor of 97.4 ± 34.4 g CO2 m-3 was measured, primarily from the aerated reactor, consistent with the plant's overall treatment efficiency. These findings provide crucial data for understanding greenhouse gas emissions from WWTPs in developing regions and highlight the need for targeted mitigation strategies.

The green footprint of anammox processes under simulated actual operating conditions: Focusing on the nitrous oxide and methane production

The anammox process has attracted increasing attention due to its advantages of low-carbon and energy-saving, nevertheless, greenhouse gas was still generated during its engineering applications process. Hence, it is vital to comprehensively understand the production characteristics and mechanisms of N2O and CH4 in anammox processes by responding to practical conditions including dissolved oxygen, temperature, and salinity. Results showed that N2O production increased by 192 %-358 %, while nitrogen removal efficiency (NRE) increased by 64.2 %-86.8 % with increasing temperature. The increased salinity inhibits 40.60 %-65.33 % N2O production with a decrease NRE of 7.85 %-18.2 %. CH4 production was the highest at 18-27 °C, reaching 3.07 ± 0.11-4.06 ± 0.16 mg·L-1, which were 1.59-2 and 1.29-1.38 times higher than that at 8-17 °C and 28-37 °C, respectively. Denitratisoma, Thauera, and Nitrosomonas were the main functional microbes for greenhouse gas production in anammox consortia. Notably, H2O2-induced intracellular Fenton reaction may be critical for the CH4 production in anammox consortia. This work provides valuable insights into achieving efficient nitrogen removal and minimizing carbon footprint in anammox systems and provides a theoretical basis for implementing the net-zero emission idea in wastewater treatment plants.

More applicable quantification of non-CO2 greenhouse gas emissions from wastewater treatment plants by on-site plant-integrated measurements

Wastewater treatment is an important source of non-CO2 greenhouse gases (GHGs). However, current quantification of these GHG emissions mainly employs unit-based measurements, where emissions from individual process units are identified, leading to large uncertainties of overall emissions. Here we introduce plant-integrated measurements, where emissions from the whole plant are measured through the off-gas pipelines of the enclosed facility, to quantify methane (CH4) and nitrous oxide (N2O) emissions from an underground municipal wastewater treatment plant (WWTP) in southern China. Our results show that the primary oxic tank contributes the largest in total CH4 and N2O emissions, with an average fraction of over 80 % and over 90 %, respectively. This can be attributed to the vigorous aeration process, which facilitates the transfer of dissolved CH4 and N2O from the liquid phase to the atmosphere through intensive air stripping. The plant-integrated measurements yield around 3-9 times higher emission factors of CH4 and N2O than the unit-based measurements. This difference in emission accounting is attributed to both varying survey durations of the two approaches and the omission of uncertain emission sources during unit-based measurements. The comparison between these two approaches indicates that plant-integrated measurements are more applicable for emission quantification of the whole plant whereas unit-based measurements provide insights into the emission characteristics of individual process units. More plant-integrated measurements are needed in the future for more accurate emission accounting of WWTPs.

Quantifying Methane Influx from Sewer into Wastewater Treatment Processes

Wastewater treatment contributes substantially to methane (CH4) emissions, yet monitoring and tracing face challenges because the treatment processes are often treated as a "black box". Particularly, despite growing interest, the amount of CH4 carryover and influx from the sewer and its impacts on overall emissions remain unclear. This study quantified CH4 emissions from six wastewater treatment plants (WWTPs) across China, utilizing existing multizonal odor control systems, with a focus on Beijing and Guiyang WWTPs. In the Beijing WWTP, almost 90% of CH4 emissions from the wastewater treatment process were conveyed through sewer pipes, affecting emissions even in the aerobic zone of biological treatment. In the Guiyang WWTP, where most CH4 from the sewer was released at the inlet well, a 24 h online monitoring revealed CH4 fluctuations linked to neighborhood water consumption and a strong correlation to influent COD inputs. CH4 emission factors monitored in six WWTPs range from 1.5 to 13.4 gCH4/kgCODrem, higher than those observed in previous studies using A2O technology. This underscores the importance of considering CH4 influx from sewer systems to avoid underestimation. The odor control system in WWTPs demonstrates its potential as a cost-effective approach for tracing, monitoring, and mitigating CH4.