Feasibility of continuous-flow algal-bacterial granular sludge bioreactor with no mechanical aeration for recirculating aquaculture wastewater treatment

Recirculating aquaculture system (RAS) can achieve high-density aquatic animal production in advanced and controlled environments. However, its application is still limited due to high resource/operation costs and difficulty in maintaining stable water quality, particularly ammonia and nitrite control. Integrating algal-bacterial aerobic granular sludge (AGS) into RAS may help reduce aeration demand and enable simultaneous C/N/P management and recovery. However, its application in continuous-flow reactors for RAS remains largely unexplored. In this study, a continuous-flow reactor with algal-bacterial AGS and no mechanical aeration was first established as the biological treatment component of RAS. The bioreactor demonstrated an excellent daily NH₄⁺ -N removal capacity of 17–31 mg/g-VSS (volatile suspended solids) with > 77 % of total inorganic nitrogen removal in 4 min per cycle during the whole test period and > 90 % after 18 days’ operation in addition to 28 % N recovery. This observation was initially attributed primarily to nitrification/denitrification and subsequently transitioned to algal assimilation regarding the dominant N removal mechanism. Moreover, no external mechanical aeration was required, since high dissolved oxygen levels can be maintained by photosynthesis and water turbulence, greatly lowering energy demands. The granules also reflected strong structural stability with integrity coefficient < 20 % and good settleability. These results suggest that the proposed continuous-flow algal-bacterial AGS bioreactor is a promising solution for sustainable aquaculture industry.