The changing nature of the water–energy nexus in urban water supply systems: a critical review of changes and responses

This paper provides a review of the changing nature of the water–energy nexus in urban water supply systems (UWSSs) due to the primary long-term drivers of climate change, population growth and technological development from the ‘energy for water’ perspective. We identify both the physical changes in UWSSs, as well as the changes in the attributes of the system, both of which contribute to the changing nature of the water–energy nexus. We provide an overview of responses to this change in the water–energy nexus through the lens of four application areas, namely long-term planning, system design, system operation and system rehabilitation, based on the review of 52 papers

Nitrous oxide emissions during microalgae-based wastewater treatment: current state of the art and implication for greenhouse gases budgeting

Microalgae can synthesise the ozone depleting pollutant and greenhouse gas nitrous oxide (N2O). Consequently, significant N2O emissions have been recorded during real wastewater treatment in high rate algal ponds (HRAPs). While data scarcity and variability prevent meaningful assessment, the magnitude reported (0.13–0.57% of the influent nitrogen load) is within the range reported by the Intergovernmental Panel on Climate Change (IPCC) for direct N2O emissions during centralised aerobic wastewater treatment (0.016–4.5% of the influent nitrogen load).

Evaluating the potential impact of energy-efficient ammonia control on the carbon footprint of a full-scale wastewater treatment plant

An assessment was performed for elucidating the possible impact of different aeration strategies on the carbon footprint of a full-scale
wastewater treatment plant. Using a calibrated model, the impact of different aeration strategies was simulated. The ammonia controller
tested showed its ability in ensuring effluent ammonia concentrations compliant with regulation along with significant savings on aeration
energy, compared to fixed oxygen set point (DOsp) control strategies. At the same time, nitrous oxide emissions increased due to accumulation of nitrification intermediates. Nevertheless, when coupled with the carbon dioxide emissions due to electrical energy consumption for aeration, the overall carbon footprint was only marginally affected. Using the local average CO2 emission factor, ammonia control slightly reduced the carbon footprint with respect to the scenario where DOsp was fixed at 2 mg·L1. Conversely, no significant change could be detected when compared against the scenarios where the DOsp was fixed. Overall, the actual impact of ammonia control on the carbon footprint compared to other aeration strategies was found to be strictly connected to the sources of energy employed, where the larger amount of low CO2-emitting energy is, the higher the relative increase in the carbon footprint will be.

Denitrification as an N2O sink

The strong greenhouse gas nitrous oxide (N2O) can be emitted from wastewater treatment systems as a byproduct of ammonium oxidation and as the last intermediate in the stepwise reduction of nitrate to N2 by denitrifying organisms. A potential strategy to reduce N2O emissions would be to enhance the activity of N2O reductase (NOS) in the denitrifying microbial community. A survey of existing literature on denitrification in wastewater treatment systems showed that the N2O reducing capacity (VmaxN2O/N2) exceeded the capacity to produce N2O (VmaxNO3/N2O) by a factor of 2e10. This suggests that denitrification can be an effective sink for N2O, potentially scavenging a fraction of the N2O produced by ammonium oxidation or abiotic reactions.

Assessment of future water availability under climate change, considering scenarios for population growth and ageing infrastructure

Climate change is likely to cause higher temperatures and alterations in precipitation patterns, with potential impacts on water resources. One important issue in this respect is inflow to drinking water reservoirs. Moreover, deteriorating infrastructures cause leakage in water distribution systems and urbanization augments water demand in cities. In this paper, a framework for assessing the combined impacts of multiple trends on water availability is proposed.

Adapting to urban flooding: a case of two cities in South Asia

Cities in South Asia are experiencing storm water drainage problems due to a combination of urban sprawl, structural, hydrological, socioeconomic and climatic factors. The frequency of short duration, high-intensity rainfall is expected to increase in the future due to climate change. Given the limited capacity of drainage systems in South Asian cities, urban flooding and waterlogging is expected to intensify. The problem gets worse when low-lying areas are filled up for infrastructure development due to unplanned urban growth, reducing permeable areas.

A knowledge discovery framework to predict the N2O emissions in the wastewater sector

Data Analytics is being deployed to predict the dissolved nitrous oxide (N2O) concentration in a full-scale sidestream sequence batch reactor (SBR) treating the anaerobic supernatant. On average, the N2O emissions are equal to 7.6% of the NH4eN load and can contribute up to 97% to the operational carbon footprint of the studied nitritation-denitritation and via-nitrite enhanced biological phosphorus removal
process (SCENA). The analysis showed that average aerobic dissolved N2O concentration could significantly vary under similar influent loads, dissolved oxygen (DO), pH and removal efficiencies. A combination of density-based clustering, support vector machine (SVM), and support vector regression (SVR) models were deployed to estimate the dissolved N2O concentration and behaviour in the different phases
of the SBR system.

Urban water management and climate change adaptation: A self-assessment study by seven midsize cities in the North Sea Region

Cities are experiencing the impacts of climate change through water-related disasters, while the sustainable management of water resources remains crucial for urban climate resilience. Accordingly, frameworks that integrate urban water management with climate change adaptation become increasingly relevant. The Water Sensitive Cities (WSC) framework is built on three pillars that address cities’ role as communities and networks, water catchments, and providers of ecosystem services. A major element of the framework is the WSC index, which can be applied to assess cities by using multiple indicators under each pillar. With the objectives of adjusting and testing the WSC index in the context of the North Sea Region through a transdisciplinary knowledge co-production process, this paper presents the results from a self-assessment conducted by seven midsize cities. Insights gained from the comparison of self-assessment results are twofold. Firstly, the cities need to value the benefits of ecosystem services for climate change adaptation, and integrate climate change into other sectors. Secondly, the cities differ regarding the priority of climate change, and even the cities that aspire to be frontrunners have weaknesses. The application of the self-assessment process also creates an exchange and mutual learning platform for cities, and increases their awareness on climate resilience.

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