Enhanced nitrite accumulation under mainstream conditions by a combination of free ammonia-based sludge treatment and low dissolved oxygen: reactor performance and microbiome analysis

Enhanced nitrogen removal via partial nitrification/denitrification coupled Anammox using three stage anoxic/oxic biofilm process with intermittent aeration

Pre-capturing organics in municipal wastewater for biogas production, combined with Anammox-based nitrogen removal process, improves the sustainability of sewage treatment. Thus, enhancing nitrogen removal via Anammox in mainstream wastewater treatment becomes very crucial. In present study, a three-stage anoxic/oxic (AO) biofilm process with intermittent aeration was designed to strengthen partial nitrification/denitrification coupling Anammox (PNA/PDA) in treatment of low C/N wastewater, which contained chemical oxygen demand (COD) of 79.8 mg/L and total inorganic nitrogen (TIN) of 58.9 mg/L. With a hydraulic retention time of 8.0 h, the process successfully reduced TIN to 10.6 mg/L, achieving a nitrogen removal efficiency of 83.3 %. The 1st anoxic zone accounted for 32.0 % TIN removal, with 10.3 % by denitrification and 21.7 % by PDA, meanwhile, the 2nd and 3rd anoxic zones contributed 19.4 % and 4.5 % of TIN removal, primarily achieved through PDA (including endogenous PD coupling Anammox). The 1st and 2nd intermittent zones accounted for 27.2 % and 17.0 % of TIN removal, respectively, with 13.7 %-21.3 % by PNA and 3.2 %-5.3 % by PDA. Although this process did not pursue nitrite accumulation in any zone (< 1.5 mg-N/L), PNA and PDA accounted for 35.1 % and 52.1 % of TIN removal, respectively. Only 0.21 % of removed TIN was released as nitrous oxide. The AnAOB of Candidatus Brocadia was enriched in each zone, with a relative abundance of 0.66 %-2.29 %. In intermittent zones, NOB had been partially suppressed (AOB/NOB = 0.73-0.88), mainly due to intermittent aeration and effective nitrite utilization by AnAOB since its population size was much greater than NOB. Present study indicated that the three-stage AO biofilm process with intermittent aeration could enhance nitrogen removal via PNA and PDA with a low N2O emission factor.

Optimizing ciprofloxacin removal through regulations of trophic modes and FNA levels in a moving bed biofilm reactor performing sidestream partial nitritation

The performance of partial nitritation (PN)-moving bed biofilm reactor (MBBR) in removal of antibiotics in the sidestream wastewater has not been investigated so far. In this work, the removal of ciprofloxacin was assessed under varying free nitrous acid (FNA) levels and different trophic modes. For the first time, a positive correlation was observed between ciprofloxacin removal and FNA levels, either in the autotrophic PN-MBBR or in the mixotrophic PN-MBBR, mainly ascribed to the FNA-stimulating effect on heterotrophic bacteria (HB)-induced biodegradation. The maximum ciprofloxacin removal efficiency (∼98 %) and removal rate constant (0.021 L g-1 SS h-1) were obtained in the mixotrophic PN-MBBR at an average FNA level of 0.056 mg-N L-1, which were 5.8 and 51.2 times higher than the corresponding values in the autotrophic PN-MBBR at 0 mg FNA-N L-1. Increasing FNA from 0.006 to 0.056 mg-N L-1 would inhibit ammonia oxidizing bacteria (AOB)-induced cometabolism and metabolism from 10.2 % and 6.9 % to 6.2 % and 6.4 %, respectively, while HB-induced cometabolism and metabolism increased from 31.2 % and 22.7 % to 41.9 % and 34.5 %, respectively. HB-induced cometabolism became the predominant biodegradation pathway (75.9 %-85.8 %) in the mixotrophic mode. Less antimicrobial biotransformation products without the piperazine or fluorine were newly identified to propose potential degradation pathways, corresponding to microbial-induced metabolic types and FNA levels. This work shed light on enhancing antibiotic removal via regulating both FNA accumulation and organic carbon addition in the PN-MBBR process treating sidestream wastewater.

Nitrite Oxidation in Wastewater Treatment: Microbial Adaptation and Suppression Challenges

Microbial nitrite oxidation is the primary pathway that generates nitrate in wastewater treatment systems and can be performed by a variety of microbes: namely, nitrite-oxidizing bacteria (NOB). Since NOB were first isolated 130 years ago, the understanding of the phylogenetical and physiological diversities of NOB has been gradually deepened. In recent endeavors of advanced biological nitrogen removal, NOB have been more considered as a troublesome disruptor, and strategies on NOB suppression often fail in practice after long-term operation due to the growth of specific NOB that are able to adapt to even harsh conditions. In line with a review of the history of currently known NOB genera, a phylogenetic tree is constructed to exhibit a wide range of NOB in different phyla. In addition, the growth behavior and metabolic performance of different NOB strains are summarized. These specific features of various NOB (e.g., high oxygen affinity of Nitrospira, tolerance to chemical inhibitors of Nitrobacter and Candidatus Nitrotoga, and preference to high temperature of Nitrolancea) highlight the differentiation of the NOB ecological niche in biological nitrogen processes and potentially support their adaptation to different suppression strategies (e.g., low dissolved oxygen, chemical treatment, and high temperature). This review implicates the acquired physiological characteristics of NOB to their emergence from a genomic and ecological perspective and emphasizes the importance of understanding physiological characterization and genomic information in future wastewater treatment studies.

Changes in the Species and Functional Composition of Activated Sludge Communities Revealed Mechanisms of Partial Nitrification Established by Ultrasonication

To achieve energy-efficient shortcut nitrogen removal of wastewater in the future, selective elimination of nitrite-oxidizing bacteria (NOB) while enriching ammonia-oxidizing microorganisms is a crucial step. However, the underlying mechanisms of partial nitrification are still not well understood, especially the newly discovered ultrasound-based partial nitrification. To elucidate this issue, in this study two bioreactors were set up, with one established partial nitrification by ultrasonication while the other didn't. During the operation of both reactors, the taxonomic and functional composition of the microbial community were investigated through metagenomics analysis. The result showed that during ultrasonic partial nitrification, ammonia-oxidizing archaea (AOA), Nitrososphaerales, was enriched more than ammonia-oxidizing bacteria (AOB), Nitrosomonas. The enrichment of microorganisms in the community increased the abundance of genes involved in microbial energy generation from lipid and carbohydrates. On the other hand, the abundance of NOB, Nitrospira and Nitrolancea, and Comammox Nitrospira decreased. Selective inhibition of NOB was highly correlated with genes involved in signal transduction enzymes, such as encoding histidine kinase and serine/threonine kinase. These findings provided deep insight into partial nitrification and contributed to the development of shortcut nitrification in wastewater treatment plants.

Wastewater Primary Treatment Using Forward Osmosis Introduces Inhibition to Achieve Stable Mainstream Partial Nitrification

Achieving stable long-term mainstream nitrite oxidizing bacteria (NOB) suppression is the bottleneck for the novel partial nitrification (PN) process toward energy- and carbon-efficient wastewater treatment. However, long-term PN stability remains a challenge due to NOB adaptation. This study proposed and demonstrated a novel strategy for achieving NOB suppression by the primary treatment of mainstream wastewater with a forward osmosis (FO) membrane process, which facilitated two external NOB inhibition factors (salinity and free nitrous acid, FNA). To evaluate the proposed strategy, a lab-scale sequencing batch reactor was operated for 200 days. A stable PN operation was achieved with a nitrite accumulation ratio of 97.7 ± 2.8%. NOB were suppressed under the combined inhibition effect of NaCl (7.9 ± 0.2 g/L, as introduced by the FO direct filtration) and FNA (0.11 ± 0.02 mg of HNO₂-N/L, formed as a result of the increased NH₄⁺-N concentration after the FO process). The two inhibition factors worked in synergy to achieve a more stable PN operation. The microbial analysis showed that the elevated salinity and accumulation of FNA reshaped the microbial community and selectively eliminated NOB. Finally, an economic and feasibility analysis was conducted, which suggests that the integration of an FO unit into PN/A is a feasible and economically viable wastewater treatment process.

Improved insights into the adaptation and selection of Nitrosomonas spp. for partial nitritation under saline conditions based on specific oxygen uptake rates and next generation sequencing

Partial nitritation (PN) is a bioprocess that is essential for developing cost-effective biological nitrogen removal processes. Understanding the abundant bacterial communities responsible for nitrification under salt stress conditions is important to achieve a stable PN system for treating saline wastewater. Therefore, in this study, we identified the core nitrifying communities and investigated their correlations with the process parameters in a nitrifying bioreactor that was used for treating saline high-strength ammonia wastewater. A PN system worked efficiently under saline conditions with varying operational factors, such as temperature, dissolved oxygen (DO), and alkalinity. Interestingly, the specific oxygen uptake rate (SOUR) became similar under salt-free and saline media after the salt adaption. Next generation sequencing results suggested that the inactivation of Nitrobacter winogradskyi was a key factor for the PN reaction under salt stress conditions. We also found that Nitrosomonas europaea, a freshwater type ammonia-oxidizing bacteria (AOB), was predominantly found under both salt-free and saline conditions, whereas other halotolerant or halophilic AOB species, including Nitrosomonas nitrosa and Nitrosomonas mobilis, became selectively abundant under saline conditions. This implies that adaptation (training of N. europaea) and selection (presence of N. nitrosa and N. mobilis) were simultaneously attributed to selective ammonia conversion for the PN reaction. The redundancy analysis showed that the salinity and ammonia loading rates were statistically significant process parameters that determined the nitrifying bacterial community, suggesting that these parameters drive the adaptation and selection of the core AOB species during the PN reaction. Furthermore, the correlation analysis revealed that the abundance of N. nitrosa and N. mobilis was critically correlated with the specific oxygen uptake rates in saline media containing ammonia.

Comparison of the Efficiency of Deammonification under Different DO Concentrations in a Laboratory-Scale Sequencing Batch Reactor

The efficiency of deammonification depends on the cooperation of ammonium oxidizing bacteria and archaea (AOB/AOA), anaerobic ammonium oxidizing bacteria (AnAOB) and the effective suppression of nitrite oxidizing bacteria (NOB) that compete with AnAOB for nitrite (NO2-N). One of the effective NOB suppression strategies is intermittent aeration. However, it is important to have a good understanding of the optimum dissolved oxygen (DO) value in the aeration period and optimize the non-aeration time used during the reaction phase. This study comprised the investigation of the effect of different DO set points (0.4, 0.7, 1.0 and 1.5 mg O2/L) under the same aeration length off/on (12/3 min). Moreover, three different intermittent aeration modes (9/3, 6/3, 3/3) under the same DO set point (0.7 mg O2/L) were more investigated. The experiment was conducted for 6 months (180 days) in a laboratory-scale sequencing batch reactor (SBR) with a working volume of 10 L. The results indicated that a high N removal efficiency was achieved 74% at the DO set point = 0.7 mg O2/L during aeration strategy off/on (6/3 min) due to the low nitrate production rate (NPR) 0.9 mg N/g VSS/h and high ammonium utilization rate (AUR) 13 mg N/g VSS/h (NPR/AUR = 0.06). Mathematical modeling results confirmed that the feasible DO set point 0.7 and intermittent aeration mode off/on (6/3 min) were especially suitable for the optimal balance between the NOB suppression and keeping high activities of AOB and anammox in the system.

Oxygen control and stressor treatments for complete and long-term suppression of nitrite-oxidizing bacteria in biofilm-based partial nitritation/anammox

Mainstream nitrogen removal by partial nitritation/anammox (PN/A) can realize energy and cost savings for sewage treatment. Selective suppression of nitrite oxidizing bacteria (NOB) remains a key bottleneck for PN/A implementation. A rotating biological contactor was studied with an overhead cover and controlled air/N₂ inflow to regulate oxygen availability at 20 °C. Biofilm exposure to dissolved oxygen concentrations < 0.51 ± 0.04 mg O₂ L^-1 when submerged in the water and < 1.41 ± 0.31 mg O₂ L^-1 when emerged in the headspace (estimated), resulted in complete and long-term NOB suppression with a low relative nitrate production ratio of 10 ± 4%. Additionally, weekly biofilm stressor treatments with free ammonia (FA) (29 ± 1 mg NH₃-N L^-1 for 3 h) could improve the NOB suppression while free nitrous acid treatments had insufficient effect. This study demonstrated the potential of managing NOB suppression in biofilm-based systems by oxygen control and recurrent FA exposure, opening opportunities for resource efficient nitrogen removal.

The connection between aeration regimes and EPS composition in nitritation biofilm

The effects of aeration regimes (intermittent and continuous aeration) on nitritation performance and biofilm EPS composition were evaluated in moving bed biofilm reactors (MBBRs), and a hypothesis that the aeration regimes affect EPS composition by affecting the microbial activity and sludge discharge content was proposed. The effluent NO₂^-/NH₄⁺ ratio corresponded to that of an anammox reaction (1.07 ± 0.20) for the MBBR with continuous aeration (MBBR_con.), while that in the MBBR with intermittent aeration (20 min on/15 min off) (MBBR_int.) was relatively lower (0.75 ± 0.19). Furthermore, the activity of ammonia-oxidizing bacteria in MBBR_con. was 0.4-7.9 mg-N·L^-1·h^-1 more than that in MBBR_int., which was consistent with the lower proportion of dead cells in MBBR_con. compared with MBBR_int. (9.4% vs. 31.8%). The higher microbial activity in MBBR_con. led to more sludge discharge than MBBR_int., which was reflected in the higher biofilm detachment rate in MBBR_con. compared with MBBR_int. (0.15 ± 0.02 vs. 0.11 ± 0.02 g m^-2·d^-1). The ratio of humic substances to polysaccharides in the EPS was high (0.96 ± 0.08) in the detachment biomass, while the ratios in the nitritation biofilm on carriers from MBBR_con. and MBBR_int. were 0.52 ± 0.13 and 0.72 ± 0.16, respectively. We hypothesized that biofilm matrix with high ratios of humic substances to polysaccharides are structurally unstable and prone to fall off. In addition, the higher proportion of dead cells in MBBR_int. made the proportion of humic substances in EPS high. Meanwhile, less sludge discharge in MBBR_int. than MBBR_con. caused more humic substances to accumulate in the biofilm. These was responsible for the higher ratio of humic substances to polysaccharides in MBBR_int. compared with MBBR_con. The findings elucidate the connection between aeration regimes and biofilm EPS composition, and guide the choice of aeration regimes in the design of biofilm reactors for partial nitritation.

Defining Culture Conditions for the Hidden Nitrite-Oxidizing Bacterium Nitrolancea

Nitrification is a key process for N-removal in engineered and natural environments, but recent findings of novel nitrifying microorganisms with surprising features revealed that our knowledge of this functional guild is still incomplete. Especially nitrite oxidation - the second step of nitrification - is catalyzed by a phylogenetically diverse bacterial group, and only recently bacteria of the phylum Chloroflexi have been identified as thermophilic nitrite-oxidizing bacteria (NOB). Among these, Nitrolancea hollandica was isolated from a laboratory-scale nitrifying bioreactor operated at 35°C with a high load of ammonium bicarbonate. However, its distribution remains cryptic as very few closely related environmental 16S rRNA gene sequences have been retrieved so far. In this study, we demonstrate how such thermophilic NOB can be enriched using modified mineral media inoculated with samples from a wastewater side-stream reactor operated at 39.5°C. Distinct cultivation conditions resulted in quick and reproducible high enrichment of two different strains of Nitrolancea, closely related to Nl. hollandica. The same cultivation approach was applied to a complex nitrite-oxidizing pre-enrichment at 42°C inoculated with biomass from a geothermal spring in the Copahue volcano area in Neuquen, Argentina. Here, an additional distinct representative of the genus Nitrolancea was obtained. This novel species had 16S rRNA and nitrite oxidoreductase alpha subunit (nxrA) gene sequence identities to Nl. hollandica of 98.5% and 97.2%, respectively. A genomic average nucleotide identity between the Argentinian strain and Nl. hollandica of 91.9% indicates that it indeed represents a distinct species. All Nitrolancea cultures formed lancet-shaped cells identical to Nl. hollandica and revealed similar physiological features, including the capability to grow at high nitrite concentrations. Growth was optimal at temperatures of 35-37°C and was strongly enhanced by ammonium supplementation. Genomic comparisons revealed that the four Nitrolancea strains share 2399 out of 3387 orthologous gene clusters and encode similar key functions. Our results define general growth conditions that enable the selective enrichment of Nitrolancea from artificial and natural environments. In most natural habitats these NOB apparently are of low abundance and their proliferation depends on the balanced presence of nitrite and ammonium, with an optimal incubation temperature of 37°C.