The benefits of promoting small granules to enhance nitrification in aerobic granular sludge

In aerobic granular sludge (AGS) systems, nitrification occurs across flocs and granules of different sizes, yet maintaining robust nitrification can remain challenging. Small granules (0.2-1 mm) have received little attention in AGS research, although previous observations suggest they provide a favourable niche for nitrifier growth and activity. While small granules naturally exhibit higher per-biomass activity than large granules (>1 mm) due to their higher surface-area-to-volume ratio, it is unclear whether they can outperform flocs. This study investigated whether nitrifier enrichment and activities in small granules can be promoted through targeted solids retention time (SRT) strategies, and how such strategies could be implemented in practice. Lab-scale sequencing batch reactors (SBR) were operated for 3-6 months to assess the effects of decoupling SRTs between flocs and small granules, as well as the influence of floc SRT on the nitrification capacity of small granules. Small granules provided a favourable niche for nitrifiers: ammonia oxidizing bacteria (AOB) thrived in both flocs and granules, irrespective of SRT conditions. In contrast, nitrite oxidizing bacteria (NOB) predominantly enriched in small granules, an effect that was further amplified upon SRT decoupling. Under decoupled SRT operation, median ammonia and nitrite oxidation rates in small granules reached 2.7 and 2.4 mgN gTSS^-1 h^-1, respectively, corresponding to 1.7-3.1-fold higher rates than observed in flocs and around four-fold higher rates than in large granules. These results indicate that SRT decoupling between flocs and small granules is required to fully exploit the nitrification potential of small granules. To evaluate strategies for selectively promoting small granules in full-scale AGS SBR, lab- and full-scale settling and sludge stratification tests were conducted. Flocs and small granules consistently settled together, forming an intertwined sludge bed. Consequently, the selective retention of small granules through settling and sludge stratification alone is not feasible. Alternative operational approaches should therefore be explored to enhance the abundance and activity of small granules in AGS systems.