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The implementation of mainstream anammox has gained increasing attention. In this study, the feasibility of using sidestream anammox granules to start up mainstream reactors was investigated by comparing two switching strategies. A maximum nitrogen removal potential of 3.6 ± 0.2 kg N m -3 d -1 was obtained for the reactor after direct switching to mainstream conditions (70 mg TN L -1 , 15 °C). Nevertheless, the reactor preacclimatized to 25 °C (Ma) exhibited a higher nitrogen removal potential of 7.0 ± 0.3 kg N m -3 d -1 at 15 °C, which is the highest volumetric nitrogen removal rate of mainstream anammox reactors to date. Candidatus Kuenenia stuttgartiensis was identified as the dominant anammox bacterium, and its relative abundance in two reactors remained stable throughout the whole operation (200 days). Moreover, with the aid of acclimatization, the activation energy was reduced and the specific growth rate became higher. These results indicated that the physiological evolution of the dominant anammox bacterium instead of interspecies selection was the main reason for the high potential during the switch to mainstream conditions. Therefore, using sidestream anammox granules as seed sludge to start up mainstream reactors was demonstrated to be feasible, and a switching strategy of acclimatization at 25 °C was recommended.

Anaerobic ammonium oxidation (anammox) is a novel process of deammonification that exhibits superior ecological and economic potential compared to that of traditional heterotrophic processes. Although this process has been successfully implemented in treating high‐strength nitrogen‐contaminated wastewater, it still faces many challenges in treating mainstream municipal wastewater. This review aims to provide an overview of the status and challenges of mainstream anammox‐based processes. The different configurations and crucial factors are discussed in this review. Finally, the future needs for feasible application are stated. Practitioner points: Factors restricting mainstream application of anammox‐based processes are reviewed.Control strategies for selecting and maintaining anammox bacteria are discussed.Recent advances in nitrite production via partial nitrification or denitrification are summarized.Future needs for the feasible application of anammox‐based nitrogen removal technology for mainstream municipal wastewater treatment are outlined. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The suppression threshold of Zn(II) on reactor performance was 10 mg L−1 for long-term exposure. • Castellaniella mainly was devoted to denitrification under no or low Zn(II) stress conditions. • Klebsiella was the dominant denitrifying bacteria under high Zn(II) stress conditions. Abstract High-rate denitrification is a popular and efficient process for treatment of nitrate-rich wastewater. Knowing the effect of heavy metals on denitrification is essential for industrial development. In the present study, the long-term impacts of Zn(II) on denitrifying granular sludge were investigated. The suppression threshold of Zn(II) on denitrifying bacteria was 10 mg L−1 for long-term exposure. The nitrogen removal rate was decreased by long-term addition of 10 mg L−1 Zn(II). Castellaniella and Klebsiella were the two dominant genera under Zn(II) stress. The relative abundance of Klebsiella sharply decreased to 4.64% after the addition of 10 mg L−1 Zn(II), whereas Castellaniella was susceptible to 2 mg L−1 Zn(II), revealing that Castellaniella mainly was devoted to denitrification under no or low Zn(II) stress conditions, whereas Klebsiella was effective under high Zn(II) stress. [ABSTRACT FROM AUTHOR]

• The presence of 1 mg L−1 Cu NPs was supposed to be safe in granule-based denitrification systems. • The system lost 51.9% of nitrogen removal capacity under the stress of 5 mg L−1 Cu NPs. • The presence of 5 mg L−1 Cu NPs reduced the gene abundance of nap A, nar G and nos Z. • The introduction of Cu NPs shifted the community composition of denitrifying bacteria. The widespread use of copper nanoparticles (CuNPs) has attracted increasing concern because of their potential effects on biological wastewater treatment. However, their effect on granule-based denitrification systems is unclear. Hence, the effects of CuNPs on denitrifying granules were investigated during long-term operation. The results showed that 51.9% of nitrogen removal capacity was lost after exposure to 5 mg L−1 CuNPs, with the amount of Cu(II) gradually increasing with elevating CuNP levels. Moreover, the relative abundance of denitrifying bacteria (Castellaniella) and denitrifying functional genes (nir K, nap A, nar G and nos Z) obviously decreased. Meanwhile, the specific denitrification activity, the content of extracellular polymeric substances and dehydrogenase activity decreased by 44.0%, 15.2% and 99.9%, respectively, compared to their values in the initial sludge. Considering the downtrend in the abundance of copper resistance genes, it was deduced that the toxicity of CuNPs was mainly or at least partially due to the release of Cu(II). [ABSTRACT FROM AUTHOR]