Photosynthetically fixed carbon neutralizes nitrous oxide and methane emissions for a carbon-negative algal-bacterial aerobic granular sludge process

Algal-bacterial aerobic granular sludge (AGS) holds great potential for reducing greenhouse gas (GHG) emissions compared to energy-intensive activated sludge. However, nitrous oxide (N₂O) and methane (CH₄) emissions remain poorly understood in the algal-bacterial AGS process. This study aimed to investigate N₂O, CO₂ and CH₄ emission characteristics, and explore non-CO₂ GHG emission mitigation potential using photosynthetically fixed carbon in algal-bacterial AGS systems. Results indicate that N₂O emission factors fluctuated significantly, ranging from 0.4 % to 3.0 % throughout the experimental period. Furthermore, N₂O emission factors exhibited no correlation with algae contents, extracellular polymeric substance contents, or nutrient removal efficiencies, indicating multiple and interdependent biochemical pathways for N₂O production. The reduced CO₂ emission shows a strong positive correlation (R² = 0.88) with Chlorophyll-a content, primarily due to enhanced photosynthetic activity. When Chlorophyll-a achieved 11.1 mg/g-MLVSS, the associated CO₂ fixation can offset average N₂O emissions (1.4 %) and CH4 emissions (<25 mg-CH₄/kg-COD). However, as algae content increased, the algal-bacterial granules experienced two instances of structural disintegration possibly due to decreasing CO₂ availability, involving a shift from Chlorophyta-dominated granules to Cyanobacteria-dominated granules. This study enhances understanding of balancing GHG emissions and granular stability, highlighting the need to mitigate non-CO₂ GHG emissions in algal-bacterial AGS systems. © 2025 Elsevier Ltd.