Mass Transfer Limitations: An Overlooked Factor in the Decline of Anammox Granular Activity for Treating Sulfate-Rich High-Salinity Wastewater

Anammox is a sustainable biotechnology for nitrogen removal from wastewater. The granule-based anammox process provides excellent biomass retention and is widely applied for the treatment of high-strength wastewater, but its efficacy declines significantly when treating sulfate-rich high-salinity wastewater. This study evaluated the impacts of mass transfer limitations caused by sulfate-induced crystallization in the micropores of anammox granules. Long-term bioreactor operation revealed that salts containing chloride and sulfate inhibited total-nitrogen removal rates by 58% more than chloride salts alone. Through a combination of batch activity assays, nanocomputed tomography (nano-CT), Raman spectroscopy, and numerical simulations, we identified the in situ crystallization of Ca3(SO3)2.12(SO4)0.88 as a key stressor in sulfate-containing systems. This crystallization, facilitated by localized Ca2+ enrichment on EPS-polysaccharides and the microporous architecture of granules, preferentially occurred in narrow pores and high-curvature regions. The resulting pore blockage accounted for ∼40% of the observed decline in anammox activity, primarily through restricted substrate diffusion. By highlighting the roles of sulfate and calcium in restricting mass transfer in anammox granules, this work provides a mechanistic basis for optimizing the performance of anammox systems.