As strange as it may seem at first, a sustainable clean energy future may ultimately rely on how well we treat our sludge. Sewage sludge produced in wastewater treatment plants releases the potent greenhouse gas methane which is largely responsible for climate change. Key, however, is how methane can easily be recovered and used to produce renewable natural gas as well as generate electricity for the plant that is converting it. This in turn reduces the plant’s cost of production and bypasses its methane emissions.
Wastewater also provides additional nutrients that can be converted into fuel such as nitrogen and phosphorus which is most often captured and used to produce fertilizer, allowing the water to then be further treated and reused. Surprisingly, the majority of wastewater treatment plants have no processes in place to capture these precious reusable resources and reap their benefits. Luckily that is changing, albeit slowly. Gradually, more wastewater treatment facilities are beginning to notice and implement practices to capture these resources in large part due to the revenue generating opportunities that contribute to ameliorating production costs and meeting sustainability targets.
Recently, scientists have published studies that attempted to determine the potential for recovering these resources from approximately 15,000 municipal wastewater plants in the United States. The hope is to increase the use of resource recovery technologies in the U.S. and to examine the various regions and potential markets to learn how they differ in regards to incorporating these technologies. The results of the studies determined that the potential is high due to the fact that the majority of wastewater resources are produced by very large treatment plants and that a significant portion of the resources could be redistributed based on regional needs. The reports also stated that recovering wastewater resources proved effective in curbing greenhouse gas emissions.
How was the study performed? Researchers assessed the resource recovery potential of a comprehensive list of all the wastewater treatment facilities in the United States. The list was compiled through a thorough examination of any and all pertinent publications and resources. Once a comprehensive data set of wastewater treatment facilities was established, the recovery technologies were applied to each plant to assess their potential for recovering resources. They then compared the applied list against a baseline facility that does not use these technologies. Those recovery technologies included anaerobic digestion with a flare to convert the methane in biogas to carbon dioxide, anaerobic digestion to produce renewable natural gas, and hydrothermal liquefaction to produce renewable diesel.
The findings were eye-opening: while most wastewater resources are produced by very large treatment plants, these facilities make up only eight percent of total treatment plants. When one considers that seventy-seven percent of the total energy, water, and nutrients in wastewater flow is produced at these very large wastewater treatment plants, it becomes clear that initial investments to recover resources require only a small number of large treatment facilities and still capture most of the available resources in their wastewater. The studies go on to show that the methods for energy recovery vary, however anaerobic digestion is the most popular, making up ten percent of treatment facilities using the technology. These facilities can further incorporate anaerobic digestion to produce renewable natural gas or
Crucially, recovering resources from wastewater curbs greenhouse gas emissions and helps meet federal climate change goals which include reaching a net-zero emissions economy by 2050. Still, challenges remain for widescale adoption, among them global economic forces. The technologies may be effective, but they’re not yet cost-effective. As efforts to decarbonize increase, cost-competitive technologies won’t be far behind.
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