Holistic assessment of management strategies and technological solutions handling anaerobic digester supernatants in wastewater treatment plants

Effective handling and treatment of anaerobic digestion supernatants (AnDS) in wastewater treatment plants (WWTP) is essential for maintaining effluent quality, minimizing operational costs, and reducing environmental impacts. This study aims to provide a holistic framework to assess management strategies and technological solutions for handling AnDS within the broader context of wastewater operations. The International Water Association (IWA) Benchmark Simulation Model No 2 (BSM2) is used as a testing platform. A new set of models are developed (and adapted) to predict the behavior of storage tanks (ST) with variable volume, aerobic granular sludge reactors (AerGSR) and struvite precipitation units (SPU). Innovative control strategies are implemented and tested under dynamic conditions. Effluent quality (EQi), operational cost (OCi) and greenhouse gas emissions (GHGi) indices are the selected key performance indicators (KPI). Results show that daytime storage of AnDS has a limited impact on plant performance (< 5 % variation in most KPI). The addition of PN/ANX improves nitrogen (N) and phosphorus (P) removal by 20 % but substantially increases total GHGi emissions due to high N₂O production (Emission factors (EF) range between 0.24 % to 3.9 %). The study also reveals that aeration patterns play a crucial role in those emissions and smart dissolved oxygen (DO) control strategies will determine the sustainability of N removal intensification processes. SPU obtained the best P effluent values. Exploration by simulation shows how to balance EQi, OCi and GHGi simultaneously by means of changing the crystallizer dosage (Mg, Na) strategy. Indeed, P recovery can reach up to 30 %. The study reveals synergies and trade-offs between intensified biological nutrient removal and emission minimization, limitations of model-based evaluations and how virtual tools can help narrow down the number of design and operational options at an early stage.