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As a by-product of industry, waste iron scraps (WIS) are low-cost and widely available, which was potential for the development of iron-assisted anammox. In this study, the feasibility of adding WIS to enhance the nitrogen removal of the anammox process (also called WIS-assisted anammox) was demonstrated. Results indicated that the WIS-assisted anammox reactors performed a 15-35% higher nitrogen removal efficiency than that of the control. Compared to the sludge from the control, the sludge from the WIS-assisted anammox reactors had a higher iron content (78-113 g kg -1 SS) and a better specific anammox activity (10.8-15.5 mg N g -1 VSS h -1 ). The enhanced growth of the anammox bacteria (related to Ca. Kuenenia stuttgartiensis with 99% similarity) in the WIS-assisted anammox reactors was also confirmed by high-throughput sequencing and qPCR. Furthermore, the functional genes predicted by PICRUSt2 revealed a higher level of hydroxylamine oxidoreductase (hao)-like proteins expression of the biomass from the WIS-assisted anammox reactors, implying that the hydroxylamine-related anammox pathway was promoted. Additionally, the observation of cytoplasmic nitrate reductase (narG), copper-containing nitrite reductase (nirK), and nitric oxide reductase (norB) suggested that the introduction of WIS might promote the denitrification ability. This was correlated to the lower ΔNO 3 - /ΔNH 4 + ratio observed in these WIS-assisted anammox reactors. Overall, the WIS-assisted anammox offers a sustainable nitrogen removal process for wastewater treatment with waste iron recycling., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

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., Competing Interests: Declaration of competing interest All authors declare that they have no conflict of interest., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

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]

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]