A new method for treating sewage sludge from a wastewater treatment plant efficiently created renewable natural gas while reducing the cost of the treatment. The work could help communities sustainably clean up waste while getting renewable natural gas for their energy needs.
When workers at Washington State U. pretreated sludge collected from a nearby wastewater facility, they produced 200% more renewable natural gas compared to current practices and reduced the final disposal cost by nearly 50%. Renewable natural gas could be used in the same way as fossil-fuel based natural gas for a wide variety of uses, including for electricity generation, home heating, or for transportation without the same climate effect as fossil fuels.
“This technology basically converts up to 80% of the sewage sludge into something valuable,” said Prof. Ahring in WSU Insider (April 21, 2016). The WSU team added a pretreatment step, treating the sludge at high temperature and pressure with oxygen added before the anaerobic digestion process. The small amount of oxygen under high-pressure conditions acts as a catalyst to break down the long polymer chains in the material. The team showed that their pretreatment resulted in reduced cost to treat the sewage from $494 to $253 per ton of dry solids.
“This approach not only enhances carbon conversion efficiency and methane yield but also enables direct production of pipeline-quality renewable natural gas with minimal CO2 content — addressing two major limitations of existing sludge-to-energy systems into a single, scalable methodology,” said Ahring.
Wastewater treatment facilities use large amounts of electricity to clean up municipal wastewater, making up between 3% and 4% of the total electricity demand in the U.S. They are often the largest user of electricity in a small community. Their treatment processes also contribute to global warming, adding about 21 million metric tons of greenhouse gases to the atmosphere annually.
About half of the 15,000 wastewater treatment plants in the U.S. use anaerobic digestion to reduce sewage waste and make biogas, but the process, in which microbes break down the waste, is inefficient and struggles to break down all the complex molecules in the sludge.
Classroom discussion questions:
- Why is wastewater such an important issue?
- Referring to Supplement 5 in your Heizer/Render/Munson text, how does this support the triple bottom line?