Accelerated algal-bacterial granulation through synergistic physical-ionic stimulation: start-up efficiency and...

Algal-bacterial granular sludge (ABGS) represents a sustainable paradigm for carbon-neutral wastewater treatment, yet its industrial deployment is severely bottlenecked by the protracted granulation period and the inherent structural instability of fledgling aggregates. Existing enhancement strategies, primarily relying on single-factor interventions or exogenous additives, often struggle to achieve a cost-effective balance between rapid start-up and long-term structural robustness. Herein, we demonstrate that coupling a weak static magnetic field (SMF, 5 mT) with Mg²⁺ substantially accelerates ABGS formation and modulates the physicochemical pathways governing algal-bacterial aggregation. The coupled strategy shortened the granulation time to 14 days (a 44.0% reduction compared to the control), while achieving stable removal of chemical oxygen demand (COD, 95.1%), total nitrogen (TN, 78.6%), and phosphate-phosphorus (PO₄^3--P, 84.3%). The resulting granules exhibited increased average diameter (2.8 mm) and improved settleability (SVI₃₀ = 55.2 mL/g). Mechanistic analyses revealed that SMF-Mg²⁺ coupling reshaped extracellular polymeric substances (EPS) production, microbial community assembly, and surface interaction energetics. Enhanced protein-dominated EPS production and the enrichment of functional bacteria (such as Thauera and Comamonadaceae) promoted intercellular adhesion. Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory analysis showed that the coupled stimulation significantly reduced surface free energy and interaction energy barriers, thereby facilitating algal-bacterial aggregation and rapid granule maturation. Techno-economic assessment further confirmed that this synergistic strategy could reduce operational expenditure (OPEX) by 59%. This study validates that physical-ionic stimulation enables rapid ABGS granulation while achieving high nutrient removal and stable settleability-offering a scalable strategy to overcome the start-up bottleneck of ABGS systems and facilitating the sustainable transition of wastewater treatment paradigm.