Correction: Zajac et al. Changes in Nitrification Kinetics and Diversity of Canonical Nitrifiers and Comammox Bacteria in a Moving Bed Sequencing Batch Biofilm Reactor—A Long-Term Study. Water 2024, 16, 534

There was an error in the original publication [1]. The inaccuracies are found in Section 3.3. Nitrification Kinetics and Nitrifier Diversity in the Moving Bed Sequencing Batch Biofilm Reactor and pertain to references to the authors’ previous work.

A correction has been made to Section 3.3. Nitrification Kinetics and Nitrifier Diversity in the Moving Bed Sequencing Batch Biofilm Reactor, paragraph 2:

The analysis of the data collected from batch tests shows that changes introduced in MBSBBR operation parameters had a greater effect on the activity of ammonia-oxidizing bacteria. Previous studies also reported that the activity of ammonia oxidizers was more susceptible to changes in system operation than those capable of oxidizing nitrite [17,18]. It is worth emphasizing that in that case, the biofilm came from a hybrid reactor, where, next to this form of biomass, also activated sludge developed. This study investigated the system with pure moving bed technology. Nevertheless, a similar trend was observed. Furthermore, the batch test results showed that the activity of bacteria capable of oxidizing ammonia nitrogen changed in a different way than that of those capable of oxidizing nitrite. However, the results of qPCR showed a similar direction of change in the amount of AOB and NOB bacteria except S.II.4. For example, after changing the aeration strategy from continuous to intermittent (R = 1/4) (S.I.2.), the abundance of all studied bacteria increased by 6.9-fold, 8.1-fold, and 2.7-fold, for AOB, NOB, and Comammox bacteria, respectively. The opposite trend was observed in the biofilm from the aforementioned hybrid reactor, where, upon introducing intermittent aeration in the same regime, the abundance of these bacteria decreased [17].

A correction has also been made to Section 3.3. Nitrification Kinetics and Nitrifier Diversity in the Moving Bed Sequencing Batch Biofilm Reactor, paragraph 10:

In addition to the canonical nitrifiers that oxidize ammonia nitrogen to nitrites and nitrites to nitrates, referred to as AOB and NOB, Comammox bacteria were detected in the MBSBBR. These microorganisms are believed to be genetically adapted to carry out the full nitrification process because their genome contains a full set of genes enabling the oxidation of both ammonia and nitrite nitrogen [46]. Biofilm is considered a favorable environment for the development of these microorganisms due to the low growth rate of Comammox Nitrospira [47]. Comammox Nitrospira may even dominate nitrification in biofilm under certain conditions [48,49]. Although it has been 7 years since the discovery of these microorganisms in wastewater treatment systems, the conditions conducive to their development are still not well defined. The results of the qPCR showed that the frequency of occurrence of Comammox bacteria was mostly affected by the transition from continuous to intermittent aeration (S.I.1.–S.I.2.), as well as by reducing the DO. Next to the obvious decrease in DO concentration in MBSBBR, the introduction of IA also resulted in the biomass being periodically subjected to lower oxygen concentrations. According to Roots et al. (2019), lower DO is one of the factors considered favorable for Comammox bacteria [50]. Other studies have shown, however, a lack of significance in the relationship between DO and the occurrence of Comammox bacteria [51]. For example, in analogous research conducted for a hybrid system, a reduction in DO did not yield any discernible impact on the abundance of Comammox bacteria in the biofilm [17]. In contrast to this study, a significant decrease in this group of microorganisms occurred when continuous aeration was switched to intermittent aeration. Zhao et al. (2022), conducting research in the MBBR reactor, observed selective enrichment of Comammox bacteria while the system operated at a DO concentration above 6 mgO₂/L [15]. In contrast to the findings of Zhao et al. (2022) [15], this study determined the highest abundance of Comammox bacteria during the series, with the highest number of subphases without aeration during the aerobic phase (R = 1/2) and the lowest concentration of DO (3.5 mgO₂/L). The observed differences, could among, others result from continuous feeding of the MBBR in the cited paper with synthetic wastewater containing only ammonia and no organic carbon. Moreover, unlike this work, they did not use IA. These discrepancies indicate the need for further research on different systems that may, in the future, clearly determine how the concentration of DO affects Comammox bacteria.

The authors state that the scientific conclusions are unaffected. This correction was approved by the Academic Editor. The original publication has also been updated.