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The Metric

Jan 17, 2014

The waste reuse-energy nexus

Settlement ponds are an effective part of the wastewater treatment process. Image Credit: MihailDechev/iStock/Thinkstock.

Linkages between environmental phenomena mean that policy actions do not have impacts in isolation. In some cases, strong performance on one indicator may lead to a worse score on another indicator. One example of such a trade-off is the relationship between wastewater treatment and per capita carbon dioxide emissions.

Wastewater treatment rate is a valuable measure of water quality. However, wastewater treatment is generally an energy-intensive process, and in a world where energy consumption is dominated by fossil fuels, the intensive use of energy implies heavy emissions of carbon dioxide. According to the U.S. Environmental Protection Agency, drinking water and wastewater systems in the United States account for approximately 3 to 4 percent of energy use, and they are responsible for over 45 million tons of greenhouse gases each year. This is equivalent to the emissions of roughly 9 million passenger vehicles.  

Generally speaking, countries devote their limited resources to the problems with the highest materiality and relevance to their specific circumstances. Many Middle Eastern and Northern Africa countries treat a large percentage of wastewater collected for reusing. Thus the Middle East and Northern Africa region has the highest wastewater treatment rate in the developing world. (The Arab Water Council reported that up to 83 percent of the treated wastewater in the Arab Region was reused in the agricultural sector in 2011). The higher treatment rate correlates with higher energy input and emissions, exacerbating a trend of poor climate and energy performance in the region. Five out of the 10 countries with the highest per capita carbon dioxide emissions in 2010 were in the Middle East and Northern Africa, with Qatar topping the charts. The fact that this region’s countries are willing to spend a vast amount of energy to treat a high percentage of their wastewater for reuse reflects the region’s energy-rich, water-stressed reality.

Trade-offs like these may seem to prevent countries from performing well on the EPI. Fortunately, there are ways to minimize the conflict between responsible water treatment and climate protection. The most straightforward and cost-effective solution is increased energy efficiency within wastewater treatment plants. Studies have estimated readily achievable, cost-saving energy reductions of 15 to 30 percent in water and wastewater plants. Such efficiency improvements would pay for themselves, generating significant financial returns.  

A second, larger-scale solution is the increased incorporation of renewable, carbon-free energy sources into the electricity grid. While this fix would not reduce the energy consumption of wastewater plants, it would mitigate their carbon emissions.  

Lastly, some researchers have suggested that municipalities could ultimately take advantage of the organic and thermal content of wastewater, converting the treatment process into a net energy generator.5 While such a result would require the further development of new membrane processes and the complete anaerobic treatment of wastewater, the net generation of energy from wastewater treatment ought to serve as a goal toward which all governments strive.