A pilot study in the Chemical Engineering Journal evaluated a new pretreatment for sewage sludge that could change how wastewater solids are handled. The team pretreated sludge from a nearby treatment plant before standard anaerobic digestion and reported 200% more renewable natural gas and a reduction in final disposal cost by nearly 50%. The researchers say the technology can convert up to 80% of the sewage sludge into a valuable product.
The study highlights broader impacts. Wastewater treatment facilities account for between 3% and 4% of total electricity demand in the US and add about 21 million metric tons of greenhouse gases each year. Of roughly 15,000 US treatment plants, about half use anaerobic digestion, which often leaves complex polymers in the sludge and produces biogas that contains carbon dioxide and methane with limited use.
The new pretreatment exposes sludge to high temperature and pressure with a small amount of oxygen added before anaerobic digestion. The oxygen under high pressure acts as a catalyst to break long polymer chains. The team reported that pretreatment lowered the cost to treat sewage from $494 to $253 per ton of dry solids. After pretreatment they used a novel bacterial strain to upgrade the biogas: the strain converts carbon dioxide with hydrogen into methane, producing renewable natural gas measured as 99% pure methane.
The researchers have patented the bacterial strain and are working with Washington State University’s Office of Innovation and Entrepreneurship and an industrial partner to scale the process. Additional contributors include Pacific Northwest National Laboratory and Clean-Vantage LLC. The work was funded by the US Department of Energy Bioenergy Technologies Office.
Difficult words
- pretreatment — treatment applied before the main treatment processThe new pretreatment
- sludge — thick wet waste from wastewater treatmentsewage sludge
- anaerobic digestion — biological process that breaks down waste without oxygen
- renewable natural gas — methane gas produced from organic renewable sources
- biogas — gas produced by decomposition, including methane and carbon dioxide
- polymer — long molecule made of many repeating unitspolymers
- catalyst — substance that speeds a chemical reaction without changing
- patent — obtain legal protection for an inventionpatented
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Discussion questions
- What benefits might come from converting more sewage sludge into renewable natural gas at wastewater plants? Give reasons based on the article.
- What practical challenges could facilities face when trying to scale the high-temperature, high-pressure pretreatment described in the study?
- How could producing 99% pure methane from biogas affect local energy supply and greenhouse gas emissions? Explain using points from the article.
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