Sustainable carbon management in aerobic granular sludge for municipal wastewater treatment

Aerobic granular sludge (AGS) operated with pre-settled wastewater enables separation of organics with high biomethane potential. However, organic compounds are also needed to support denitrification, and external carbon may be needed to achieve low effluent nitrogen concentrations. This study evaluated the impact of primary sedimentation on carbon management in AGS processes. A pilot-scale reactor reached effluent nitrate concentrations of 2-3 mg NO₃-N/L when fed with pre-settled wastewater with the addition of 0.8 ± 0.2 g COD/g N as methanol in the post-denitrification phase, or when fed with raw wastewater without an external carbon source. The biogas potential of the whole process was 25 % higher with primary sedimentation. A sustainability assessment showed that the benefits of increased biogas production with primary sedimentation could outweigh the drawbacks associated with the use of methanol as external carbon source both in terms of economy and CO₂ emissions, but methane price and biogas yield affect the assessment.