Water-energy trajectories for urban water and wastewater reveal the impact of city strategies

The global water industry has a greater emphasis on energy management than ever before. The confluence of rising energy demand and costs, and net-zero greenhouse gas emission targets means the sector must rapidly transition to a new ‘energy future’. Yet, few cities have assessed the long-term energy use of their water and wastewater systems. Here, we undertake a novel and integrated assessment of the historical trends of energy use for water and wastewater in three Australian and two US cities, collectively 17 million people. The key research question is what were the historical trends of energy use for water supply and wastewater treatment, and what can we understand about the drivers? The research contributes a first systematic time-series assessment of energy trajectories of both water and wastewater, across multiple cities. Uniquely, it integrates long-term (up to 20 years) energy dynamics in a comparative analysis. The work also contributes a qualitative analysis of driving factors behind the observed energy variations. The time-series analysis (2001−2020) identifies how energy use is evolving through time in widely differing climate, urban and water infrastructure conditions. The cities studied demonstrated downward trends in water use by 30–42% and wastewater collected by 5–30%, primarily due to water conservation and drought-related restrictions. Annual per-capita energy use for water supply reduced in Los Angeles (−58%, from 276 to 116.5 kWh/p/a), San Diego (−59%, from 503.7 to 204.2 kWh/p/a), Sydney (−26%, from 40.6 to 30.1 kWh/p/a) and increased in Melbourne (+859%, from 15.7 to 150.6 kWh/p/a) and Perth (+139%, from 118.1 to 281.9 kWh/p/a). Compared to water supply, energy use for wastewater was far more stable (it varied between 45 and 85 kWh/p/a), and not the crucial contributor to overall energy use dynamics. The significant increase in seawater desalination is identified as the primary driver of increased energy use in the three Australian cities. To offset this huge demand, developing renewable energy generation emerged as the key strategy. It causes high fluctuation of renewable energy use shares (Sydney: 317 GWh, accounting for 48.5% energy use for water and wastewater in 2011; compared to 19 GWh, accounting for only 2.5% in 2008). In contrast, both Los Angeles and San Diego managed to considerably reduce energy use by decreasing their imported water volume and energy intensity (the result of an adjusted supply portfolio). However, the absence of consistently comprehensive water/energy/renewable energy data remains a significant hurdle for a thorough quantitative analysis of drivers. Given these observations, it is evident that detailed quantitative analysis for influencing factors (e.g. water use, climate, infrastructure upgrading, sustainability targets), requires separately reported energy use for both water supply and wastewater, reported water from categorized sources and significant other data. By addressing these issues, we can have a clear path towards an energy-efficient, sustainable urban water system with net-zero emissions.