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To investigate the applicability of ratio control strategy to other systems, a continuous floccular sludge reactor was used in this study. It was found that nitrite accumulation was barely detected throughout 70 days’ investigation, being the average concentration in the effluent of 0.7 ± 0.4 mg/L. Batch experiments indicated that low dissolved oxygen (DO < 0.3 mg·L −1 ) greatly repressed the ammonium oxidizing bacteria (AOB) but only slightly inhibited the nitrite oxidizing bacteria (NOB). However, high-throughput sequencing revealed that the ratio of abundance between Nitrospira and Nitrosomonas , being the dominant NOB and AOB respectively, was considerably low (1.2%/18.7%). The weak oxygen gradients in floccular sludge and the selectively enriched K-strategist NOB Nitrospira under oxygen-limited conditions were both contributed to the failure of achieving partial nitritation; therefore, the rapid start-up of partial nitritation process based on proposed ratio control strategy is not feasible for continuous floccular sludge systems treating low-strength wastewater. [ABSTRACT FROM AUTHOR]

This study investigated the performance of a one-stage partial nitritation-anammox (PN/A) system for the treatment of reject water from high-solid-sludge anaerobic digestion with thermal hydrolysis pretreatment (THP-AD). The proportion of reject water was gradually increased (10.8%, 23.2%, 36.4%, 73.7% and 100%) during the 90-day operation. The nitrogen loading rate (NLR) reached 0.44 kg N/m3/d using 73.7% reject water as influent, while it decreased to only 0.15 kg N/m3/d with 100% reject water. Meanwhile, sludge particles with diameters < 180 µm became the largest granulometric fraction. Microbial community structure analysis indicated that the relative abundances of functional microorganisms became extremely low; e.g., 0.05% and 0.5% for Nitrosomonas and Candidatus Brocadia, respectively. An additional 47-day experiment was performed using only the anammox stage to evaluate the inhibition impacts of organics. This was fed with reject water with partial ammonia nitrogen stripping and nitrite nitrogen addition for the first 38 days and PN-treated reject water for days 39–47. Reject water with an 80% raw organic content demonstrated severe inhibition, with the NLR dropping from 1.27 kg N/m3/d to 0.27 kg N/m3/d. This, could be partially restored to 0.93 kg N/m3/d when the influent was pretreated by PN. It appears that the biodegradable organic matter inhibited the one-stage PN/A system more than the non-biodegradable fraction. After removal of biodegradable organics, the NRR of the anammox system recovered, indicating that two-stage PN/A has greater advantages in THP-AD reject water treatment than a one-stage PN/A system. [Display omitted] • Decreasing dilution of THP-AD reject water in the influent reduced one-stage PN/A activity. • Degranulation of granular anammox sludge occurred when treating THP-AD reject water. • The abundance of AOB and AMX bacteria was low when treating undiluted THP-AD. • PN pretreatment alleviated inhibition of anammox activity. • SCOD value couldn't directly explain the inhibition of anammox activity. [ABSTRACT FROM AUTHOR]

Conventional two stages partial nitritation - anammox (C-PN-A) process is an energy-efficient and sustainable technology for sludge anaerobic reject water treatment, which has usually reported with system instability and limited total nitrogen removal rate (TNRR). Overgrowth of flocculent heterotrophic bacteria in Anammox stage induced negative effects on anammox activity. To prevent direct transportation of flocculent heterotrophic bacteria from PN stage to Anammox stage, novel partial nitritation - microfiltration - anammox (PN-MF-A) process was proposed in this research. More stabilized operation was achieved for PN-MF-A process with higher TNRR of 0.80–0.96 kgN/(m3·d) than C-PN-A of 0.53–0.57 kgN/(m3·d). There was a great deal of overlaps in genus level of microbial community structure between floc sludge in PN and Anammox reactor, indicating flocculent microorganism in anammox reactor came from PN reactor, which probably could survive both in aerobic and anaerobic conditions. The in-between membrane of PN-MF-A process removed nearly 100% suspended solid (SS) and 15.31–37.83% COD, accordingly preventing the microorganism transfer from PN stage to Anammox stage and reducing the inhibitory effects of the anaerobic reject water on the anammox. Although same inoculum was used for the two sets anammox reactors, genus level shift was observed that Candidatus Brocadia (abundance 10%) in C-PN-A and Candidatus Kuenenia (1.4%) in PN-MF-A process. Higher anammox activity in PN-MF-A process was illustrated with less abundance (1.4%) than C-PN-A process. This research initially illustrated feasibility and advantages of proposed novel PN-MF-A process for high inhibitory reject water treatment. [Display omitted] • Novel partial nitritation - microfiltration - anammox (PN-MF-A) process was proposed. • More stabilized operation was achieved for PN-MF-A with higher TNRR of 0.80–0.96 kgN/(m3 d). • The in-between membrane removed nearly 100% suspended solid and 15.31–37.83% COD. • PN-MF-A prevented the microorganism transfer from PN stage to Anammox stage and reducing the inhibitory effects. • Although same inoculum was used, anammox genus level shift was observed. [ABSTRACT FROM AUTHOR]

The effects of influent shift from synthetic wastewater to anaerobically pretreated actual sewage coupling with lowering temperature on microbial community of a two-stage partial nitritation (PN)-anammox process were evaluated through high-throughput sequencing. Venn diagrams and Hill numbers showed the significantly increased bacterial diversity both in the PN and anammox reactor. However, taxonomic analysis indicated that outstanding enrichment of heterotrophic bacteria and reduction of autotrophic species mainly occurred in the PN reactor, while nearly all of the dominant bacteria in the anammox reactor only slightly decreased in abundance. Moreover, immigrant bacteria from the PN reactor to the following anammox reactor had no negative effect on the anammox function. These results implied the positive role of the first-stage PN in maintaining the stability of the following anammox community. Nitrosomonas europaea (17.9–52.9%) and one cluster (19.2–27.7%) within Candidatus Brocadia remained as the dominant functional species in the PN and anammox reactor, respectively. [ABSTRACT FROM AUTHOR]

To investigate the effect of temperature on the recovery of aerobic granule stability, two continuous-flow reactors inoculated with disintegrated partial nitritation granules were operated at different temperatures for organic-free wastewater treatment. Granules disintegration and partial nitritation deterioration further occurred in the reactor (R1) operating at 25–28 °C. However, successful sludge re-granulation with a decrease of sludge volume index (SVI5) from 73 mL/g to 35 mL/g was observed in the reactor (R2) operated with the decreasing temperature from 20 °C to 12 °C. The reduced temperatures decreased the activity and growth rate of nitrifiers (from 47.2 mg/g VSS/h to 21.3 mg/g VSS/h), and selected the extracellular polymeric substance (EPS) producing bacteria (e.g., Flavobacterium and Rhodobacteraceae sp.). Correspondingly, an enhanced secretion of tightly-bound EPS (200.3–224.6 mg/g VSS in R2 vs 166.8–152.5 mg/g VSS in R1), particularly proteins, and an increased expression of quorum sensing-related enzyme genes (2.98% in R2 vs 2.75% in R1) were obtained. All of this contributed to the cold temperature-driven sludge re-granulation. The recovery of granular structure promoted the stable maintenance of efficient partial nitritation at low temperatures. Overall, this study gives insights into the positive role of cold temperatures on the stability of partial nitritation granules. [Display omitted] • Temperature effects on nitrifying granules were studied in continuous-flow reactors. • Granule disintegrated at 25–28 °C, but regranulation occurred at low temperatures. • Cold temperature enriched extracellular polymeric substance-producing microbes. • The mechanism of cold temperature-driven sludge regranulation was proposed. [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]

In this study, a high-rate CANON (Complete Autotrophic Nitrogen-removal Over Nitrite) process was started up successfully by enhancing the in-situ enrichment of anammox bacteria in aerobic granules at conditions relevant for mainstream wastewater treatment. Firstly, to provide nitrite for anammox bacteria growth efficient nitrite-oxidizing bacteria (NOB) repression was rapidly achieved and stably maintained. Both low dissolved oxygen (DO) and ammonium concentrations ratio (DO/NH 4 + <0.15) and selective washing-out of NOB-preferred smaller particles at short hydraulic retention time (HRT, 25-15 min) contributed to the NOB repression. Then the stepwise down-regulating DO concentrations from 2.8 to 1.2 mg/L enhanced the enrichment of anammox bacteria in the aerobic granules. The enriched anammox species was dominated by Ca. Brocadia sapporoensis with the estimated growth rate of 0.008–0.013 d−1 at 15 °C. Chloroflexi and Chlorobi-affiliated bacteria were also significantly enriched in the granules, which may benefit the anammox bacteria activity and growth. At the end of this study, the average total nitrogen removal rate and efficiency of the granular CANON process respectively reached 1.26 kg N·m−3·d−1 and 68% treating low-strength ammonium (∼50 mg N·L−1) wastewater under such aggressive conditions (DO = 0.8–1.5 mg/L, HRT< 1.0 h, and T = 15 °C). Overall, the aerobic granules provided a habitable niche for the proliferation and almost complete retention of the anammox bacteria. This study provides a roadmap for in-situ starting up of high-rate CANON process for mainstream wastewater treatment with aerobic granules as inoculum. • A high-rate CANON process was started up at 15 °C treating low-strength wastewater. • Aerobic granules provided a habitable niche for in-situ enriching anammox bacteria. • Residual ammonium control and small granules wash-out contributed to NOB repression. [ABSTRACT FROM AUTHOR]