Solar PV adoption in wastewater treatment plants: A review of practice in California.

This is the first study to assess the current status of solar photovoltaic (PV) adoption across a range of wastewater treatment plant sizes, and to identify the opportunities for solar PV in the wastewater sector. It quantifies solar PV contributions to the energy demand of the wastewater treatment plants and improves knowledge of sector-specific factors influencing PV uptake. California was used as a case study due to its high commitment to solar power and the high data availability. The study compiled and examined data on multiple wastewater treatment plant attributes from 105 Californian plants, representing 78% of total state flows. The analysis focused on the effect of three sector-specific influencing factors: size of wastewater treatment plant, presence/absence of anaerobic digestion and geographical location (urban vs rural). Solar PV adoption was observed to vary significantly with the size of the wastewater treatment plants. Of the 105 plants analysed, 41 installed a solar PV system. Of these 41, 39 were installed in wastewater treatment plants with a flow rate below 50 mega gallons day-1 (MGD). Only two plants with flow above 50 MGD had solar PV installed. In wastewater treatment plants with a flow rate above 5 MGD, solar PV was primarily installed in hybrid configurations with anaerobic digestion. In these plants, biogas contributed 25-65% to the overall energy demand, while solar provided 8-30%. In wastewater treatment plants with a flow rates below 5 MGD, solar PV often represented the only source of renewable energy, producing 30-100% of the energy demand of these plants. Across all the plants analysed, 1 MW was the most adopted solar installation size and solar PV installations were mostly found in wastewater treatment plants in rural settings. While acknowledging multiple other factors of potential influence, these results demonstrate the role of solar PV in wastewater treatment plants under three sector-specific influencing factors. The results will support the sector in making informed decisions over solar PV investments, helping wastewater utilities to transition towards sustainable management practices.

Opportunities and challenges of tackling Scope 3 "Indirect" emissions from residential hot water.

The water sector could play a major role towards a Net Zero greenhouse gas (GHG) future if Scope 3 emissions were embraced and operationalised. Significant opportunities and challenges exist in tackling Scope 3 emissions including those associated with customer hot water use. Present GHG emission reduction practices predominantly focus on Scope 1 "within utility" and Scope 2 "purchased energy" emissions. In the urban water cycle, Scope 3 "indirect" emissions dominate, and water use is only one example of Scope 3 emissions. Over 90% of all water cycle GHG emissions can be attributed to water use in residential, industrial and commercial premises, collectively some 7% of global GHG emissions. One possibility is for water utilities to actively support efficient hot water use such as new ultra-low flow shower heads. Scope 3 opportunities also offer a range of cost-effective emissions-reduction opportunities, particularly when the wider perspective of "community value" is considered and not just a "business financial perspective". Hot water efficiency is additionally essential to Net Zero carbon futures, even with decarbonised grids, because most major Net Zero roadmaps require energy efficiency gains. Scientific and management advance needed includes: accounting methodologies, clear roles, collaboration, new business models, and clear definitions. The water sector has the opportunity to play a significant role in achieving Net Zero cities. The decision how much is yet to be made.

The connection between water and energy in cities: a review.

We have only rudimentary understanding of the complex and pervasive connections between water and energy in cities. As water security now threatens energy and economic security, this is a major omission. Understanding the water-energy nexus is necessary if we want to contribute to solving water and energy issues simultaneously; if we want to stop moving problems from one resource dimension to another. This is particularly relevant in the Australian context where energy use for water supplies is forecast to rapidly escalate, growing around 300% from 2007 levels, by 2030. This paper presents a literature review with an aim of characterising the research to date with a particular focus on cities, the major centres of consumption and growth. It systematically analyses a wide range of papers and summarises the diverse objectives, dimensions, and scale of the research to-date together with knowledge gaps. There are many major gaps. These include energy use associated with water in industrial and commercial operations as well as socio-political perspectives. A major gap is the lack of a unifying theoretical framework and consistent methodology for analysis. This is considered a prerequisite for quantitative trans-city comparisons.

Urban water metabolism indicators derived from a water mass balance - Bridging the gap between visions and performance assessment of urban water resource management.

Improving resource management in urban areas has been enshrined in visions for achieving sustainable urban areas, but to date it has been difficult to quantify performance indicators to help identify more sustainable outcomes, especially for water resources. In this work, we advance quantitative indicators for what we refer to as the 'metabolic' features of urban water management: those related to resource efficiency (for water and also water-related energy and nutrients), supply internalisation, urban hydrological performance, sustainable extraction, and recognition of the diverse functions of water. We derived indicators in consultation with stakeholders to bridge this gap between visions and performance indicators. This was done by first reviewing and categorising water-related resource management objectives for city-regions, and then deriving indicators that can gauge performance against them. The ability for these indicators to be quantified using data from an urban water mass balance was also examined. Indicators of water efficiency, supply internalisation, and hydrological performance (relative to a reference case) can be generated using existing urban water mass balance methods. In the future, indicators for water-related energy and nutrient efficiencies could be generated by overlaying the urban water balance with energy and nutrient data. Indicators of sustainable extraction and recognising diverse functions of water will require methods for defining sustainable extraction rates and a water functionality index.