Ultra-high efficiency simultaneous nitritation, denitritation and phosphorus removal from digestate centrate using calcium-enhanced aerobic granular sludge

This study demonstrates a calcium-enhanced aerobic granular sludge (AGS) strategy to achieve ultra-high simultaneous nitritation, denitritation, and phosphorus removal (SNDPR) from high-strength digestate centrate under severe carbon limitation. In a long-term (320 d) operation, an AGS reactor successfully treated centrate containing ∼950 mg/L NH₄⁺-N and 150 mg/L PO₄³⁻-P at an extremely low C/N ratio of 0.2. Temporary Ca²⁺ addition (200 mg/L) improved granule integrity, enriched functional microbial groups, and enabled combined biological and chemical phosphorus removal. During Ca²⁺ supplementation, the system achieved 99% ammonia removal, 97% total inorganic nitrogen (TIN) removal, and over 80% phosphorus removal. Enhanced activities of ammonia oxidizing bacteria (AOB) and denitrifying phosphorus accumulating organisms (DPAOs), together with increased tightly bound EPS, supported stable granulation and nutrient removal under high loading. Microbial community analysis revealed selective enrichment of Dechloromonas, Nitrosomonas, Thauera, and Paracoccus, underpinning efficient nitrogen and phosphorus cycling. Importantly, after Ca²⁺ withdrawal, the reactor was operated for an additional 130 days. During this period, it maintained excellent nitrogen removal (>97%) and sustained improved phosphorus uptake (20%, 2.5-fold higher than baseline), demonstrating the lasting benefits of short-term Ca²⁺ exposure. These results highlight a practical and scalable approach for sidestream wastewater treatment. Short-term Ca²⁺ augmentation provides a simple, low-cost tool to enhance granule stability, enrich DPAOs, and achieve robust nutrient removal from high-strength, low C/N ratio wastewaters.