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Researchers at Colorado State University (CSU) have found that switching from waste greywater treatment facilities to green solutions, such as wetlands, could save $15.6bn and 29.8 million tonnes of CO2-equivalent emissions per year. A grey infrastructure system includes water treatment facilities such as sewage plants, which remove problem nutrients before releasing water back into circulation. These facilities require substantial power to run and, as such, release emissions into the atmosphere. According to researchers at CSU, existing facilities already account for 2% of all energy use in the US and 45 million tonnes of CO2 emissions. Grey infrastructure treatment facilities also address other forms of freshwater contamination, such as fertiliser run-off from agriculture entering rivers, pollution from wildfires or urban development. Rather than building more grey infrastructure treatment facilities to address these increasing sources, a research team at CSU has explored the potential of transitioning to green wastewater treatment approaches in the US and the economic trade-offs that could result from it. These nature-based solutions include the construction of wetlands or reforestation. Using data collected at over 22,000 facilities, the researchers explored data on impaired waters, treatment technologies and life-cycle greenhouse gas emissions. They also looked at the relationship among emissions, costs and treatment capabilities of traditional treatment technologies compared with alternative green infrastructure. The findings show that a transition to green infrastructure could potentially save $15.6bn, 21.2TWh of electricity and 29.8 million tonnes of CO2-equivalent emissions over 40 years. "These findings draw a line in the sand that shows what the potential for adopting green approaches in this space is - both in terms of money saved and total emissions reduced," said Braden Limb, report co-author and PhD student in the department of systems engineering at CSU. "It is a starting point to understand what routes are available to us now and how financing strategies can elevate water treatment from a somewhat local issue into something that is addressed globally through market incentives." The researchers also found that green infrastructure solutions may have the potential to generate $679m annually in carbon financing or carbon credit revenue (at $20 per credit), which provides an opportunity to help accelerate water quality trading. In a bid to incentivise sustainable activities that reduce emissions, companies voluntarily buy "credits" on an open market. This credit offsets the institution's emissions from operations as it aims to reach sustainability goals. While there are similar financing markets for water, the problem is initially more localised than it is for air quality and carbon. The research suggests that carbon markets could also be leveraged to change, for example, some of the financial incentives farmers have around water treatment and impacts from their activity. "This is the first time we are considering air and water quality simultaneously - water is local and carbon is global," said Jason Quinn, co-author of the study and mechanical engineering professor at CSU. "But by bringing these market mechanisms together we can capitalise on a window of opportunity to accelerate the improvement of America's rivers as we transition to a renewable energy and restored watershed future."