Plant-Wide Evaluation of Temperature-Phased anaerobic digestion for internal carbon recovery at a Large-Scale water resource recovery facility

Resource recovery technologies are often evaluated against their own primary performance metric in isolation, without assessing simultaneous consequences across the full plant operational space. This study evaluates temperature-phased anaerobic digestion (TPAD)-derived internal carbon recovery at a full-scale water resource recovery facility (WRRF) and assesses the simultaneous impacts on nitrogen compliance, energy demand, clarifier stability, and biogas recovery across the full operational space. A pilot-scale TPAD system (thermophilic, 55°C; mesophilic, 38°C) was operated for 121 days on primary sludge. Fermentate quality was characterized through physicochemical and respirometric assays. A plant-wide mass balance and calibrated dynamic simulation model assessed trade-offs among fermentate dosing rate, solids retention time (SRT), and seasonal temperature. TPAD achieved effective volatile fatty acid (VFA) accumulation while preserving total methane yields equivalent to single-stage mesophilic digestion, confirming that resource and energy recovery are not mutually exclusive. Plant-wide simulations identified the minimum fermentate dosing requirements and SRT thresholds for nitrogen compliance under cold-weather conditions, quantifying the associated energy costs and clarifier stability boundaries. Temporal variability in fermentate quality was identified as a key implementation challenge with direct consequences for plant energy balance. The resulting decision maps constitute a site-adaptable framework for carbon-limited WRRFs implementing circular carbon management, demonstrating that resource recovery interventions must be evaluated across the full plant key performance indicator (KPI) space to support informed utility decision-making.