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Freshwater bodies are found as significant sources of greenhouse gas to the atmosphere, but their quantitative significance is not yet clear. The rapid increase in hydropower dams and associated emissions has received much attention from hydropower industries, environmentalists, and policymakers from across the globe. The tropical/subtropical eco-regions are, particularly, considered as hotspots from greenhouse gas production point of view as the emitted gases are playing a critical role in climate change. The present paper reviews the uncertainty and limitations of greenhouse gas measurement from Chinese freshwater bodies based on available data. Literature reported that carbon dioxide emission from Chinese lakes falls in the range of 0.01–2.16 μmol m−2s−1 except for Poyang Lake. The CO 2 emission from Chinese hydropower reservoirs is estimated as 0.15–1.11, while for rivers, it is 0.44–4.12 μmol m−2s−1. The methane and nitrous oxide emissions are, however, found less than carbon dioxide emission. The CO 2 emission falls in the range of −0.17–0.54 and 1.08–1.70, respectively higher than global lakes and rivers, while the global reservoirs have emission in the range of −0.17–2.98 μmol m−2s−1. The paper also reports the challenges in the estimation and prediction of greenhouse gas emissions from hydropower reservoirs along with suggested mitigation measures. Image 10945 • GHG emissions have been limited by data availability and inconsistencies in methodological approach. • Investigate uncertainty and limitation of GHG measurement. • Literature reveals CO 2 emission from Chinese rivers is twice than lakes and four times than hydropower reservoirs. • CH 4 and N 2 O emissions are however found less than CO 2 emission. • Suggested mitigation measures to minimize the impact of GHG on climate change. [ABSTRACT FROM AUTHOR]

Hydropower, which contributes to around 16% of global electricity and more than 72% of renewable electricity, is expected to play an important role in the deep decarbonization of the energy sector. However, the idea that hydropower is a carbon-neutral energy alternative on par with solar and wind is controversial. Research, mainly from limnology and climate modeling, shows that depending on the characteristics of the hydro project, it could be a significant source of GHG emissions. This aspect has been ignored in most life cycle assessment (LCA) studies, affecting the effective use of LCA results, especially in comparative assessments. This paper aims to provide a comprehensive and critical review on this topic by conducting a systematic literature review on hydropower LCA studies published since 2010. We found that there is inconsistency in how LCA is used for hydropower projects. While the emissions associated with the engineering work are well addressed, efforts to accurately estimate and model reservoir GHG emissions are constrained by limited data availability, difficulties in accurately quantifying highly variable carbon fluxes, and inconsistent modeling approaches. A huge range of emissions values is reported in the reviewed literature, from 1.5 to 3747.8 g CO 2 eq per kWh. Reservoir-based hydropower shows high variability, which is mainly dictated by reservoir-related GHG emissions. Reservoir GHG emissions could be more than 90% of the life cycle emissions, especially for hydropower in a tropical region. The regionalized aspect is a key factor to be considered in extrapolating reservoir GHG emissions. • There is inconsistency in how LCA is used for hydropower projects. • A wide range of emission intensities has been reported (1.5–3747.8 g CO 2 eq/kWh). • The high variability in the emissions is due to major differences in key reservoir characteristics. • Data limitations are the main cause for over- or underestimation of hydropower emissions. • Harmonization of data is vital to effectively conduct LCA and to generate useful results. [ABSTRACT FROM AUTHOR]

Hydrogen (H 2) can play a critical role in global greenhouse gas (GHG) mitigation. Photocatalytic water splitting using solar radiation is a promising H 2 technology. Titanium dioxide (TiO 2) and carbon nitride (g–C 3 N 4)–based photocatalysts are the most widely used photocatalytic materials because of their activity and abundance. Several attempts have been made to improve the photocatalytic performance of these materials in terms of their activity level, life span, response to visible radiation, and stability. However, the environmental impacts of these modifications are often not included in existing studies. This research, therefore, develops a cradle-to-grave life cycle assessment (LCA) framework to evaluate and compare the GHG footprints of four alternative pathways: TiO 2 nanorods and fluorine-doped carbon nitride quantum dots embedded with TiO 2 (CNF: TNR/TiO 2), g-C 3 N 4 , and g-C 3 N 4 /BiOI composite. Unlike most studies that focus only on certain stages such as laboratory-scale photocatalytic fabrication, this study includes utility-scale cell production, assembly, operation, and end of life to give a more accurate and precise environmental performance estimation. The results show that g-C 3 N 4 /BiOI has the lowest GHG footprint (0.38 kg CO 2 eq per kg of H 2) and CNF: TNR/TiO 2 has the lowest energy payback time (0.4 years). In every pathway, energy use in material extraction processes makes up the largest GHG contribution, between 83% and 89%. Sensitivity and uncertainty analyses were conducted under the impact of various input parameters on the life cycle GHG emissions of hydrogen production. Photocatalytic water splitting is highly feasible for adaptation as a mainstream hydrogen production pathway in the future. • LCA of H 2 production via photocatalytic water splitting is conducted. • GHG emissions range from 0.33 to 1.15 kg of CO 2 eq per kg of H 2 produced. • Material extraction has the largest GHG emissions; 56–67% of total emissions. • System is scaled up to produce 5 tonnes of H 2 per day. • H 2 produced has 95% less GHG emissions than steam methane reforming. [ABSTRACT FROM AUTHOR]

Greenhouse gases (GHGs; particularly, CO 2 , CH 4 , and N 2 O) emission from wastewater treatment systems (WWTS) is one of the inevitable concerns for sustainable development. This indicator is directly linked with the carbon footprint and potential impacts of WWTS on climate change. In this view, various modeling, design, and operational tools have been introduced to mitigate the WWTS associated GHGs, at regional and global scales. In this study, authors have critically reviewed the selected potential operational control strategies for GHGs emission, particularly emitted from the operational stages of biological WWTS. The investigated operational control strategies and/or treatment configurations included intermittent aeration, varying dissolved oxygen, enhanced sludge retention time, coupled aerobic–anoxic nitrous decomposition operation, and microalgae integrated treatment process. Based on this analysis and considering the trade-off between treatment performance of WWTS and GHGs control, an integrated framework is also proposed for existing and upcoming WWTS. The findings of this study and proposed framework will play an instrumental role in mitigating the GHGs at various operational stages of WWTS. Future research works in this direction can lead to a better understanding of investigated operational GHGs emission control strategies in WWTS. • GHGs emissions control in WWTS is an inevitable sustainability concern. • Potential operational GHGs control strategies are discussed in this manuscript. • IA, DO, CANDO, microalgae assisted process, and enhanced SRT are investigated in this study. • An integrated framework for GHGs control through operational strategies is suggested. [ABSTRACT FROM AUTHOR]

[Display omitted] • Water pollution severely impacts the GHG emissions from riverine ecosystems. • GHG emissions from Riverine ecosystem follows: CO 2 > CH 4 > N 2 O. • High nutrient loading and eutrophication enhanced the GHG emissions in rivers. • Riverine CTLI values were positively correlated with CO 2 and CH 4 fluxes. Many recent studies have revealed that polluted rivers are emerging as potent emitters of greenhouse gases (GHGs). However, the scientific understanding of GHG emissions is still limited. The present review aims to provide a systematic analysis of GHG emissions from polluted rivers in various eco-regions. The analysis highlighted that high nutrient loadings such as total nitrogen (TN), organic carbon (OC), total phosphorous (TP), and the anoxic condition of rivers are the major influencing factors for CO 2 , N 2 O, and CH 4 emissions. Moreover, a strong positive correlation of trophic level index (TLI) with CH 4 and CO 2 fluxes was observed. The state-of-art suggested that based on land use and water quality, GHG fluxes from different rivers vary considerably. For instance, the reported N 2 O fluxes ranged between 0.007 and 3.721 mmol m-2d-1 for urban-impacted rivers and 0.009–0.049 mmol m-2d-1 for agricultural rivers. The CH 4 fluxes ranged between 0.001 and 458 mmol m-2d-1, 0.05–1.02 mmol m-2d-1, and 1.40–14.30 mmol m-2d-1 for urban impacted, agricultural and natural rivers, respectively. Further reported CO 2 fluxes range between 19.94 and 895 mmol m-2d-1 for urban-impacted rivers. Overall the in-depth analysis of the existing state of the art revealed that water pollution (i.e. eutrophication) enhances the GHG emissions from rivers; therefore, incorporating effective restoration measures can mitigate the intensity of GHG emissions from rivers and assist in achieving sustainable development goals (SDGs). [ABSTRACT FROM AUTHOR]