Tuning the Carbon Dial: How to Select Carbon Supplements that Minimize N2O/GHG Emissions in BNR

Presentation Description: Nitrous oxide (N₂O), a potent greenhouse gas with a global warming potential nearly 300 times that of carbon dioxide, has become a critical concern in the sustainability of wastewater treatment processes. N₂O is an inevitable intermediate or byproduct generated during biological nutrient removal (BNR), particularly in nitrification and denitrification stages.
Recent long-term monitoring campaigns have revealed that N₂O emissions from wastewater treatment plants vary widely depending on treatment technologies and operational conditions. One underexplored factor contributing to this variability is the type of carbon source used during denitrification. For carbon-deficient utilities, the selection of carbon type and dosing ratio may play a pivotal role in mitigating N₂O emissions.
To investigate this, our study evaluated the impact of various carbon sources—including commercially available options like MicroC and internally produced fermentate—on N₂O production. We conducted pilot-scale experiments using a sequencing batch reactor (SBR) treating carbon-deficient wastewater. The reactor was operated under low dissolved oxygen (DO) conditions, which have shown conflicting evidence regarding their influence on N₂O emissions. Controlled batch tests were also performed to identify the effects of carbon type and dosing.
Our findings indicate that fermentate, when properly managed, can serve as a viable alternative to external carbon sources, offering reductions in both operational costs and embedded greenhouse gas emissions. However, improper carbon dosing or imbalanced carbon-to-nitrogen (C:N) ratios can exacerbate N₂O emissions.
Importantly, the study found that low DO zones are not necessarily hotspots for N₂O emissions. With appropriate carbon management, emissions in low DO BNR systems were effectively mitigated. These results underscore the need for zone-specific monitoring and control strategies to balance treatment performance with climate impact mitigation.
This work contributes to the growing body of knowledge aimed at decarbonizing water resource recovery facilities. By elucidating the complex interplay between carbon management and N₂O dynamics, the study offers actionable insights for utilities seeking to optimize low DO BNR systems for both energy efficiency and greenhouse gas reduction. It advocates for a holistic approach that integrates carbon sourcing, process control, and emissions monitoring to advance sustainable wastewater treatment practices.