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The microbial characterization of three size-fractionated sludge obtained from a suspended-growth anoxic anammox reactor treating low-strength wastewater at low temperatures were investigated by using high-throughput sequencing. Particularly, the spatial variability in relative abundance of microorganisms involved in nitrogen metabolism were analyzed in detail. Results showed that population segregation did occur in the reactor. It was found, for the first time, that the genus Nitrotoga was enriched only in large granules (>400 μm). Three anammox genus including Candidatus Jettenia , Brocadia and Kuenenia were detected. Among them, Candidatus Brocadia and Kuenenia preferred to grow in large-sized granules (>400 μm), whereas Candidatus Jettenia dominated in small- and moderate-sized sludge (<400 μm). The members of genus Candidatus Jettenia appeared to play the vital role in nitrogen removal, since sludge with diameters smaller than 400 μm accounted for 81.55% of the total biomass. However, further studies are required to identify the activity of different-size sludge. [ABSTRACT FROM AUTHOR]

Biomass detachment generally occurred in granular sludge systems. However, little is known about the influence of biomass detachment on the granules performing nitritation. Here, a granular sludge reactor with high loading rates (6.8 ± 0.4 kg N·m−3·d−1) was achieved at mainstream conditions. Though the low ratio control strategy was maintained, the deterioration of nitritation performance was observed after the further increase of air supply rates to 3.4 ± 0.2 L min−1. In parallel with that, the loss of AOB and the proliferation of NOB was observed. Additionally, with the decrease of granules size and biomass concentration, the incomplete stratification of nitrifiers in the granules was confirmed by batch tests. All these results suggested that granules abrasion under the high shear stress conditions caused the detachment of external AOB and hence resulted in the deteriorated stratified structure of nitrifiers, which subsequently contributed to the proliferation of the internal NOB and the deterioration of nitritation. These findings highlight that the granules abrasion should be well controlled in the development of high-rate nitritation process with granular sludge. • High-rate nitritation with granular sludge was achieved at mainstream conditions. • Granules abrasion-based deterioration of nitritation was firstly observed. • Detachment of AOB resulted in incomplete stratification of nitrifiers in granules. • Granules abrasion must be well controlled in high-rate granular nitritation system. [ABSTRACT FROM AUTHOR]

Achieving mainstream nitritation with aerobic granules is attractive based on increasing evidence but generally treating artificial low-ammonium wastewater. Real municipal wastewater is much more complex in composition, the behavior of the nitritation granules would be different when treating real municipal wastewater. Herein, the response of nitritation granules to influent shift from artificial low-ammonium (35–40 mg/L) wastewater to anaerobically pre-treated municipal wastewater (MWW pre-treated) was investigated at low temperatures. Results showed that MWW pre-treated caused the outgrowth of filamentous bacteria on the granule surface and developed into finger-like structures, which in turn resulted in the decrease of the overall granular sludge settleability. Batch-tests and microbial analysis indicated the functional and microbial differentiation between the newly formed fluffy exterior and the original compact granule. The fluffy exterior was dominated by genus Flavobacterium (66.6%) and primarily functioned as COD removal, whereas the nitrifiers (mainly Nitrosomonas) were still located in the compact core and performed nitritation. Moreover, the heterotrophs-dominated fluffy exterior hindered the oxygen transfer towards nitrifiers located in the compact granule and thereby facilitated the stable NOB repression in the granule particularly at low temperatures (<10 °C). Finally, gradual recovery of the granular sludge morphology and settleability occurred after the influent reverted to synthetic low-ammonium wastewater. Overall, this work demonstrated that the feeding of MWW pre-treated only caused morphological changes of the nitritation granules, but its structural and functional stability could be maintained stably. [Display omitted] • Granule-based nitritation was maintained treating anaerobically pretreated sewage. • COD removal occurred primarily in external flocs, while nitritation in compact core. • External flocs promoted NOB repression by increasing oxygen transfer resistance. [ABSTRACT FROM AUTHOR]

To elucidate how high dissolved oxygen (DO) favors the startup of nitritation with aerobic granular sludge, two granular reactors were operated under low (1–2 mg O 2 ·L−1) and high DO (3–5 mg O 2 ·L−1) conditions with similar effluent ammonium concentrations (>20 mg N·L−1). The results showed that though nitritation with an average nitrite accumulation ratio of above 95% was finally achieved in both reactors, a five-fold start-up time (eleven weeks) was required for the low DO reactor compared to the high DO reactor. Moreover, the nitritation performance was positively correlated with the extent of nitrifiers stratification in granules. The faster startup of nitritation under high DO conditions mainly resulted from the faster formation of well-stratified nitrifiers, with ammonium oxidizing bacteria (AOB) dominating granule surface. High DO operation combined with sufficient ammonium supply ensured the faster growth of AOB, which should provide a competitive advantage to AOB in competing for habitable space (i.e., granule surface). Besides, the lower porosity, larger size, and more active extracellular polymeric substances (particularly proteins) production of granules was observed under the high DO condition. Overall, these findings supported the proposition that the switch from mixed to stratified distribution of nitrifiers in granule was primarily driven by their competition for habitable space rather than by oxygen-limitation. Image 1 • Faster startup of granular nitritation under higher DO concentrations. • The startup of nitritation linked with the development of stratified nitrifiers. • Habitable space competition between AOB and NOB led to stratification of nitrifiers. [ABSTRACT FROM AUTHOR]

Biomass segregation between granules/biofilm and flocs is widespread in anammox-based processes. The segregation of biomass allows for easy control of processes stability. The goal of this study is to understand the biomass segregation in two anoxic anammox reactors respectively operated in nitrite-limited (R NO2) and ammonium-limited (R NH4) modes treating low-strength wastewater at 20 °C. Results showed that size-based biomass segregation was developed in both reactors. But the functional and population heterogeneity was more significant in the ammonium-limited anammox reactor. The activity and abundance of anammox bacteria in large granules were significantly higher than that in flocs under the ammonium-limited conditions. The large granules played a major role in nitrogen removal in R NH4. By contrast, both large granules and small flocs contributed significantly to the nitrogen loss in the nitrite-limited anammox reactor, since a large number of anammox bacteria existed in both granules and flocs. Besides, a number of Nitrospira -like NOB were also detected in both anoxic anammox reactors, which primarily inhabited in flocs seemingly droved by the availability of oxygen. But the abundance of Nitrospira in R NH4 was much higher than that in R NO2. All these results suggested that selective flocs removal would be necessary for R NH4 to improve its anammox performance but non-essential for R NO2. The two anammox reactors shared the predominant anammox species with the closest relative to Ca. Brocadia sp. 40 (98%). Unexpectedly, the anammox species grew faster in R NH4. But the microbial diversity and evenness was much greater in R NO2 , suggesting its higher functional stability. • The ammonium-limited and nitrite-limited anammox reactors were compared. • Anammox bacteria mainly enriched in granules, NOB preferred to live in flocs. • Anammox bacteria grew faster in the ammonium-limited reactor. • The nitrite-limited reactor possessed the higher microbial diversity and evenness. [ABSTRACT FROM AUTHOR]