Micro-sized aerobic heterotrophic ammonium assimilation granules and microbial community assembly under varying hydraulic shear force

Heterotrophic ammonium assimilation (HAA) represents an emerging biological nitrogen removal strategy for saline wastewater treatment. Developing halophilic HAA microbiome into granular form enables simultaneously improve the sludge settleability and robustness. However, the regulatory mechanism of hydraulic shear force (HSF) on granule characteristics and microbial community ecology of the HAA microbiome remain unclear. This study investigated the effects of HSF, controlled by reactor height-to-diameter ratios (H/D = 1, 2.5, 5, and 10), on the nitrogen removal performance, granule morphology, and microbial community assembly. The constructed aerobic HAA granules were uniformly micro-sized (230–280 μm) yet exhibited high density (1030–1115 kg/m3) and excellent settleability. Notably, under moderate HSF condition (H/D = 5), the abundance of key HAA-related genes (glnA, gltB, and gdhA) and the enzyme activities of GS, GOGAT, and GDH were maximized, corresponding to highest ammonium removal efficiency. Across all four systems, the constructed aerobic HAA granules strictly performed assimilation function, with no detectable ammonia-oxidizing genes (AMO), nitrifying bacteria, or nitrogen loss. Increasing HSF imposed strong selective pressure on the aerobic HAA granules, resulting in a linear increase in deterministic community assembly while reducing microbial diversity. In system with an H/D of 5, the dominant genera Ponticoccus and Marinobacillus acted as network hubs, maintaining microbial community stability. Overall, this study successfully established micro-sized aerobic HAA granules, and revealed the regulatory effects of HSF on their granule characteristics, microbial community assembly, and nitrogen metabolism. This study provides valuable insights for the design and optimization of stable HAA-based systems for saline wastewater treatment.