Managing Municipal Wastewater Treatment to Control Nitrous Oxide Emissions from Tidal Rivers

Modeling greenhouse gas emissions from riverine systems: A review

Despite the recognized importance of flowing waters in global greenhouse gas (GHG) budgets, riverine GHG models remain oversimplified, consequently restraining the development of effective prediction for riverine GHG emissions feedbacks. Here we elucidate the state of the art of riverine GHG models by investigating 148 models from 122 papers published from 2010 to 2021. Our findings indicate that riverine GHG models have been mostly data-driven models (83%), while mechanistic and hybrid models were uncommonly applied (12% and 5%, respectively). Overall, riverine GHG models were mainly used to explain relationships between GHG emissions and biochemical factors, while the role of hydrological, geomorphic, land use and cover factors remains missing. The development of complex and advanced models has been limited by data scarcity issues; hence, efforts should focus on developing affordable automatic monitoring methods to improve data quality and quantity. For future research, we request for basin-scale studies explaining river and land-surface interactions for which hybrid models are recommended given their flexibility. Such a holistic understanding of GHG dynamics would facilitate scaling-up efforts, thereby reducing uncertainties in global GHG estimates. Lastly, we propose an application framework for model selection based on three main criteria, including model purpose, model scale and the spatiotemporal characteristics of GHG data, by which optimal models can be applied in various study conditions.

Optimal Implementation of Wastewater Reuse in Existing Sewerage Systems to Improve Resilience and Sustainability in Water Supply Systems

A transition from conventional centralized to hybrid decentralized systems has been increasingly advised recently due to their capability to enhance the resilience and sustainability of urban water supply systems. Reusing treated wastewater for non-potable purposes is a promising opportunity toward the aforementioned resolutions. In this study, we present two optimization models for integrating reusing systems into existing sewerage systems to bridge the supply–demand gap in an existing water supply system. In Model-1, the supply–demand gap is bridged by introducing on-site graywater treatment and reuse, and in Model-2, the gap is bridged by decentralized wastewater treatment and reuse. The applicability of the proposed models is evaluated using two test cases: one a proof-of-concept hypothetical network and the other a near realistic network based on the sewerage network in Chennai, India. The results show that the proposed models outperform the existing approaches by achieving more than a 20% reduction in the cost of procuring water and more than a 36% reduction in the demand for freshwater through the implementation of local on-site graywater reuse for both test cases. These numbers are about 12% and 34% respectively for the implementation of decentralized wastewater treatment and reuse.

Minimization of N2O Emission through Intermittent Aeration in a Sequencing Batch Reactor (SBR): Main Behavior and Mechanism

To explore the main behavior and mechanism of minimizing nitrous oxide (N2O) emission through intermittent aeration during wastewater treatment, two lab-scale sequencing batch reactors operated at intermittently aerated mode (SBR1), and continuously aerated mode (SBR2) were established. Compared with SBR2, the intermittently aerated SBR1 reached not only a higher total nitrogen removal efficiency (averaged 93.5%) but also a lower N2O-emission factor (0.01–0.53% of influent ammonia), in which short-cut nitrification and denitrification were promoted. Moreover, less accumulation and consumption of polyhydroxyalkanoates, a potential endogenous carbon source promoting N2O emission, were observed in SBR1. Batch experiments revealed that nitrifier denitrification was the major pathway generating N2O while heterotrophic denitrification played as a sink of N2O, and SBR1 embraced a larger N2O-mitigating capability. Finally, quantitative polymerase chain reaction results suggested that the abundant complete ammonia oxidizer (comammox) elevated in the intermittently aerated environment played a potential role in avoiding N2O generation during wastewater treatment. This work provides an in-depth insight into the utilization of proper management of intermittent aeration to control N2O emission from wastewater treatment plants.