Mechanisms of Granule Detachment and Impact on Nitrifier Growth and Migration to Flocs in Aerobic Granular Sludge

Granule detachment in aerobic granular sludge (AGS) systems is considered an important aspect for both granulation and nitrification performance, yet it has received little attention. This study aimed to identify the dominant detachment mechanisms (erosion, abrasion, sloughing, breakage) and quantify their respective kinetics under operational shear conditions. Ultimately, the impact of detachment on nitrifier migration and growth conditions in flocs was also investigated. Granules from two full-scale AGS wastewater treatment plants (WWTP) were exposed to operational shear (157 s^-1) for up to one week in a Couette-Taylor reactor. The distribution of size-classes in terms of total suspended solids (TSS) and nitrifiers was measured, and detachment kinetics and solids retention time (SRT) in flocs, accounting for detachment, were estimated. Under the tested shear rate, shear-induced erosion from the granule surface was the main detachment mechanism with a median detachment rate of 0.017 gTSS_detach gTSS_granule^-1 d^-1 (25th-75th percentile: 0.014-0.027 gTSS_detach gTSS_granule^-1 d^-1). In contrast, granule breakage was not observed within 15 detachment tests and therefore does not appear to be a continuous process, indicating that re-growth on breakage debris is not a main driver of granulation as previously proposed in literature. Nitrifiers were found to migrate almost exclusively to biomass <0.2 mm, i.e., flocs. While the migration rates of ammonia- and nitrite-oxidizing bacteria (AOB and NOB) were similar in autumn, distinct values were quantified in winter, with two to six-fold lower NOB migration rates, likely reflecting a temperature-dependent stratification of nitrifiers within granules. Consequently, the influence of nitrifier migration on the growth conditions in flocs varies across nitrifying groups and seasons. The floc SRT of AOB was estimated to be above their minimum SRT throughout the year, indicating migration is not critical for AOB to persist in flocs. In contrast, the minimum SRT of NOB increases significantly at low temperature, such that migration becomes crucial to prevent NOB washout. Migration of NOB from granules to flocs may sufficiently increase the SRT of NOB in flocs, if they are predominantly enriched in granules rather than in flocs. Overall, flocs provide favourable growth conditions for AOB throughout the year, whereas this is only intermittently the case for NOB, which ultimately rely on migration from granules, particularly during cold winter months.