Nitrous oxide - a potential energy resource from wastewater

Many people may think of nitrous oxide (N₂O) as a laughing gas or a key character in the Fast and the Furious movie franchise. However, a research team has been working out how its recovery in the wastewater treatment process could benefit Australia.

To disclose the economic value of nitrogen resources contained in wastewater, a research group led by Professor Bing-Jie Ni and Dr Wei Wei from the University of Technology Sydney (UTS) have turned their attention to nitrous oxide (N₂O) recovery during wastewater treatment.

Professor Ni is a worldwide pioneer in modelling and wastewater management. Dr Wei is a young lecturer at the School of Civil and Environmental Engineering and a renowned pioneer in revolutionising the science and practice of urban wastewater/sludge management.

Why focus on nitrous oxide?

“Wastewater treatment plant (WWTP) is an important source of N₂O emissions,” said Lan Wu. Wu is a PhD candidate currently working on renewable energy under the guidance of Professor Ni and Dr Wei at UTS. “The produced N₂O can account for up to 80 per cent of the carbon footprint of a WWTP. Using N₂O as an untapped energy resource is another possible way to achieve sustainable wastewater treatment. This is instead of treating N₂O as an unwanted greenhouse gas (GHG) and devoting intensive efforts to mitigate its emissions.”

The team was inspired by the use of N₂O as an energetic oxidiser in a rocket motor. This team then investigated the feasibility of recovering this potentially valuable gas during the biological removal process. Despite the challenging nature of the research, the team has achieved promising experimental results and demonstrated outstanding research performance.

Several papers concerning N₂O derived from biological nitrogen process are now successfully published by Wu as the leading author in several reputable academic journals. These journals include Environmental Science & Technology, Journal of Hazardous Materials, and Bioresource Technology.

What does this nitrous oxide recovery research mean?

“Energy consumption and the external carbon dosage are the major operational costs for wastewater treatment. Short-cut nitrogen removal is a cost-effective technology that reduces the amount of external carbon dosage. Therefore, this technique is currently applied on a commercial scale widely,” said Wu.

“However, higher N₂O production via a short-cut nitrogen removal process would offset the advantage of this technique, given its potent greenhouse gas effect,” Wu said. “Trying to reclaim this unwanted gas massively during biological nitrogen removal is a pioneering and revolutionary concept. Rather than mitigating the N₂O emissions by sacrificing nitrogen removal efficiencies, this new technique could solve both energy-deficient and low nitrogen removal challenges simultaneously.”

“Multiple organisms involved in the nitrogen removal process can produce N₂O as a by-product,” explained Wu. “Denitrifiers are the main contributors to N₂O emissions. Heterotrophic and autotrophic denitrification are the potential processes for N₂O production.”

Previous nitrous oxide research

Previous researchers have tested the feasibility of producing N₂O abundantly via a heterotrophic denitrification process. That research combusted it as an oxidant with methane to generate higher energy. CANDO, the abbreviation of coupled aerobic anoxic nitrous decomposition operation process, is the term used to describe the N₂O recovery via the heterotrophic denitrification process.

Despite the economic gain of N₂O recovery being improved via CANDO, some external carbon is still needed. Therefore, this team continuously optimises the N₂O reclamation process by eliminating the need for carbon dosage via the autotrophic denitrification process. They have successfully attained high N₂O recovery efficiency (~40 per cent) when using elemental sulphur (S⁰) or thiosulfate (S₂O₃^2-) as the sole electron donor.

“In addition to the promising N₂O recovery efficiency, sulphur- or thiosulfate-based autotrophic denitrification process are more suitable to be applied in the system deficient in carbon. 70 per cent less sludge production and 30 per cent energy consumption could be obtained by replacing heterotrophic denitrification with this energy-saving and cost-effective approach,” said Lan Wu.

“Therefore, I believe the results would have great potential to be applied on a larger scale,” she said. “It could assist in the development of a bio-based economy within Australia. More efforts are required to improve the applicability of this concept-of-proof study.”

Where will the follow-up research focus on?

It is still challenging to harvest N₂O from mixed liquor effectively and economically. Some strategies have been employed to improve N₂O harvest efficiency. This includes applying gas stripping, alternating N₂O solubility, and adopting membrane-contained reactors. However, some drawbacks still lie in the harvesting methodology, including the fouling or clogging of the membrane and the difficulties of selecting the appropriate gas for stripping.

These drawbacks have made the current designed extraction methods less widely applicable and economically viable. Moreover, no integrated tests have been conducted to evaluate the process of N₂O harvesting and subsequent combustion. Therefore, one of the prioritised focuses for this team should be on optimising the N₂O harvesting and combining this harvesting technology with the combustion process to achieve energy autarky during wastewater treatment.