Most people think as little as possible about the wastewater that is produced daily from their showers, bathtubs, sinks, dishwashers and toilets. But with the right techniques, it can become a valuable resource.
On average, every American uses about 60 gallons of water per day for purposes that include flushing toilets, showering and doing laundry. This figure can easily double if outdoor uses, such as watering lawns and filling swimming pools, are also included.
My research focuses on recovering resources from wastewater. This process is difficult because wastewater contains many different types of contaminants. But researchers in our fields are exploring many creative ways to make valuable products from them.
Energy from organic materials
Diehard wastewater engineers understand the value of wastewater, which they view as an asset rather than a waste. That’s why some of them call it “used water” instead, and refer to what most people call wastewater treatment plants as water resource recovery facilities.
In fact, wastewater can contain more than three times the amount of energy needed to treat it. One simple and mature technique for recovering part of this energy is anaerobic digestion, a natural process in which microorganisms feed on grease and other organic materials in wastewater and produce biogas, just as yeast can eat up barley and spit out beer. Biogas contains roughly 50% methane, which can be used as a renewable fuel for boilers, furnaces and heating systems or to turn turbines and generate electricity.
Inside these anaerobic ‘egg’ digesters at the Deer Island Treatment Plant on Boston Harbor, microbes break down sewage sludge and scum into methane gas, carbon dioxide, water and organic solids that are processed into fertilizer. Frank Hebbert/Wikimedia, CC BY
More advanced techniques, such as hydrothermal processes, take sewage sludge – the solids removed from wastewater during treatment – and convert it into bio-based fuels that can be used to replace gasoline and diesel fuel. This process is currently at the demonstration stage.
Wastewater also contains nutrients like nitrogen and phosphorus, which are essential elements that plants need to grow. In current wastewater treatment processes, we use energy to convert ammonia in the wastewater, which comes mostly from urine, into nitrogen gas. However, industries then use large quantities of natural gas to convert nitrogen gas back into ammonia, predominantly for producing fertiliser, through the Haber-Bosch process.
Sanitised urine also contains other nutrients like phosphorus and potassium. The Rich Earth Institute, a Vermont-based non-profit supported by federal agencies and foundations, is researching ways to turn human urine into fertiliser. The institute is testing harvested urine on real crops, and has found that it works effectively.
Alternatively, we can recover these nutrients as struvite, or magnesium ammonium phosphate, a mineral that contains magnesium, nitrogen and phosphorus. Struvite can naturally form during wastewater treatment processes, but tends to deposit in tanks and pipes and will damage the equipment if left unattended. By controlling the formation of struvite, it can be recovered in separate reactors.
In addition, wastewater is generally warmer than natural water supplies, especially in the winter, so it can serve as a heat source. This technique is well-established and is not limited to commercial scale. You can install drain-water heat recovery systems at home to lower your energy bill.
To me, this is just a beginning. With proper techniques, “wastewater” can offer us much more – and I very much look forward to the day when there is no “wastewater”, just “used water”.
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About Yalin Li
Yalin Li, PhD candidate/research assistant, Colorado School of Mines