[Enhanced nitrogen removal by bioelectrochemical coupling anammox and characteristics of microbial communities]

To investigate the bioelectrochemical enhanced anaerobic ammonia oxidation (anammox) nitrogen removal process, a bioelectrochemical system with coupled anammox cathode was constructed using a dual-chamber microbial electrolysis cell (MEC). Specifically, a dark incubation batch experiment was conducted at 30 ℃ with different influent total nitrogen concentrations under an applied voltage of 0.2 V, and the enhanced denitrification mechanism was investigated by combining various characterization methods such as cyclic voltammetry, electrochemical impedance spectroscopy and high-throughput sequencing methods. The results showed that the total nitrogen removal rates of 96.9%±0.3%, 97.3%±0.4% and 99.0%±0.3% were obtained when the initial total nitrogen concentration was 200, 300 and 400 mg/L, respectively. In addition, the cathode electrode biofilm showed good electrochemical activity. High-throughput sequencing results showed that the applied voltage enriched other denitrifying functional groups, including Denitratisoma, Limnobacter, and ammonia oxidizing bacteria SM1A02 and Anaerolineaceae, Nitrosomonas europaea and Nitrospira, besides the anammox bacteria. These electrochemically active microorganisms comprised of ammonium oxidizing exoelectrogens (AOE) and denitrifying electrotrophs (DNE). Together with anammox bacteria Candidatus Brocadia, they constituted the microbial community structure of denitrification system. Enhanced direct interspecies electron transfer between AOE and DNE was the fundamental reason for the further improvement of the total nitrogen removal rate of the system.

[The toxicity of ZnO and CuO nanoparticles on biological wastewater treatment and its detoxification: a review]

The wide use of ZnO and CuO nanoparticles in research, medicine, industry, and other fields has raised concerns about their biosafety. It is therefore unavoidable to be discharged into the sewage treatment system. Due to the unique physical and chemical properties of ZnO NPs and CuO NPs, it may be toxic to the members of the microbial community and their growth and metabolism, which in turn affects the stable operation of sewage nitrogen removal. This study summarizes the toxicity mechanism of two typical metal oxide nanoparticles (ZnO NPs and CuO NPs) to nitrogen removal microorganisms in sewage treatment systems. Furthermore, the factors affecting the cytotoxicity of metal oxide nanoparticles (MONPs) are summarized. This review aims to provide a theoretical basis and support for the future mitigating and emergent treatment of the adverse effects of nanoparticles on sewage treatment systems.

Simultaneous nitrogen removal via heterotrophic nitrification and aerobic denitrification by a novel Lysinibacillus fusiformis B301

Simultaneous nitrogen removal via heterotrophic nitrification and aerobic denitrification (HN-AD) has received widespread attention in biological treatment of wastewater. This study reported a novel Lysinibacillus fusiformis B301 strain, which effectively removed nitrogenous pollutants via HN-AD in one aerobic reactor with no nitrite accumulated. It exhibited the optimal nitrogen removal efficiency under 30°C, citrate as the carbon source and C/N ratio of 15. The maximum nitrogen removal rates were up to 2.11 mgNH₄ ⁺ -N/(L·h), 1.62 mgNO₃ ^- -N/(L·h), and 1.41 mgNO₂ ^- -N/(L·h), respectively, when ammonium, nitrate, and nitrite were employed as the only nitrogen source under aerobic conditions. Ammonium nitrogen was preferentially consumed via HN-AD in the coexistence of three nitrogen species, and the removal efficiencies of total nitrogen were up to 94.26%. Nitrogen balance analysis suggested that 83.25% of ammonium was converted to gaseous nitrogen. The HD-AD pathway catalyzed by L. fusiformis B301 followed NH 4 + → N H 2 OH → NO 2 - → NO 3 - → NO 2 - → N 2 , supported by the results of key denitrifying enzymatic activities. PRACTITIONER POINTS: The novel Lysinibacillus fusiformis B301 exhibited the outstanding HN-AD ability. The novel Lysinibacillus fusiformis B301 simultaneously removed multiple nitrogen species. No nitrite accumulated during the HN-AD process. Five key denitrifying enzymes were involved in the HN-AD process. Ammonium nitrogen (83.25%) was converted to gaseous nitrogen by the novel strain.

[N2O Emission from the Processes of Wastewater Treatment: Mechanisms and Control Strategies]

As a direct carbon emission source, the amount of nitrous oxide (N₂, which is actually caused by AOB denitrification. To control the N₂O emission during biological N-removal, complete HND and NO₂^- accumulation for AOB denitrification should be avoided to a large extent. For this purpose, DO in aerobic tanks should be controlled at a normal level (approximately 2 mg·L^-1), and solid retention time (SRT) should be extended, up to 20 d, which would avoid accumulating N₂O for AOB denitrification. Additionally, external carbon should be supplemented in time to promote HDN approaching the end, N₂. This review summarizes the mechanisms of all the mentioned N₂O emission pathways and discusses the control strategies of N₂O emission according to the associated mechanisms.

Selection, Identification and Functional Performance of Ammonia-Degrading Microbial Communities from an Activated Sludge for Landfill Leachate Treatment

The increasing amounts of municipal solid waste and their management in landfills caused an increase in the production of leachate, a liquid formed by the percolation of rainwater through the waste. Leachate creates serious problems to municipal wastewater treatment plants; indeed, its high levels of ammonia are toxic for bacterial cells and drastically reduce the biological removal of nitrogen by activated sludge. In the present work, we studied, using a metagenomic approach based on next-generation sequencing (NGS), the microbial composition of sludge in the municipal wastewater treatment plant of Porto Sant'Elpidio (Italy). Through activated sludge enrichment experiments based on the Repetitive Re-Inoculum Assay, we were able to select and identify a minimal bacterial community capable of degrading high concentrations of ammonium (NH₄⁺-N ≅ 350 mg/L) present in a leachate-based medium. The analysis of NGS data suggests that seven families of bacteria (Alcaligenaceae, Nitrosomonadaceae, Caulobacteraceae, Xanthomonadaceae, Rhodanobacteraceae, Comamonadaceae and Chitinophagaceae) are mainly responsible for ammonia oxidation. Furthermore, we isolated from the enriched sludge three genera (Klebsiella sp., Castellaniella sp. and Acinetobacter sp.) capable of heterotrophic nitrification coupled with aerobic denitrification. These bacteria released a trace amount of both nitrite and nitrate possibly transforming ammonia into gaseous nitrogen. Our findings represent the starting point to produce an optimized microorganisms's mixture for the biological removal of ammonia contained in leachate.

[Promoting Nitrogen Removal in ANAMMOX Biofilm Reactor by Fe2+ Under Low Nitrogen Concentration]

The feasibility for nitrogen removal in a two-stage ANAMMOX biofilm reactor promoted by Fe²⁺ under low nitrogen concentration was investigated. The results showed that the ANAMMOX reaction could be effectively promoted by a ρ(Fe²⁺) of 5, 10, and 15 mg·L^-1. A ρ(Fe²⁺) of 10 mg·L^-1 presented the highest promotion for the ANAMMOX reaction, with the highest nitrogen removal efficiency (NRE) of 81.71% under a ρ(TN) of 150 mg·L^-1and a nitrogen loading rate (NLR) of 0.62 kg·(m³·d)^-1. Fe²⁺ promoted the secretion of extracellular polymeric substance (EPS) and the synthesis of heme c in the ANAMMOX system. Batch test results further verified the positive effects by Fe²⁺on the activity of anaerobic ammonium oxidizing bacteria (AnAOB). The specific ANAMMOX activity (SAA) of 10 mg·L^-1 ρ(Fe²⁺) was 3.6 times as high as that of the control group[ρ(Fe²⁺)=0 mg·L^-1], whereas the activity of AnAOB was significantly inhibited with ρ(Fe²⁺) increased to 20 mg·L^-1. High-throughput sequencing results showed that the addition of Fe²⁺ increased the abundance of Candidatus_Kuenenia. When ρ(Fe²⁺) was 10 mg·L^-1, the relative abundance of Candidatus_Kuenenia in reactor 1 and reactor 2 increased to 16.18% and 4.22%, respectively. The stable operation of the two-stage ANAMMOX biofilm process promoted by Fe²⁺provides an alternative technology for low-strength nitrogen wastewater.

A potential zero water exchange system for recirculating aquarium using a DHS-USB system coupled with ozone

Partial water exchange is one of the most common conventional methods used to maintain water quality and aesthetic beauty in recirculating aquarium systems (RASs). However, this method uses substantial amount of water. The ozone-down-flow hanging sponge-up-flow sludge blanket (ozone-DHS-USB) system attempts to be a more responsible method for aquarium maintenance. It eliminates the necessity for water exchange in aquarium by maintaining nitrogen concentrations at a safe level and by reducing yellow substances. Also, the impact of O₃ on the DHS-USB system was investigated. The system was assayed using an on-site freshwater aquarium influenced by ambient temperature ranging from 23°C to 34°C. During ozonation Phases 1 and 3, the colour of the water in the aquarium was successfully maintained below 10 colour units. The 16S rRNA gene analysis of microorganisms in the DHS revealed that constant application of O₃ has caused a decrease in nitrite-oxidizing bacteria (NOB). Nevertheless, NH₃ and NO2- were maintained within 0.1 mg N L^-1, and NO3- was maintained at 14.6 ± 9.20 mg N L^-1 throughout the study. Carps survived for 425 days without any water exchange performed. Our study supports that the ozone-DHS-USB system has a high potential towards creating low-maintenance aquaria.

[Advances in denitrification microorganisms and processes]

Denitrification is an indispensable part of most sewage treatment systems. The biological denitrification process has attracted much attention in the past decades due to the advantages such as cost-effectiveness, process simplicity, and absence of secondary pollution. This review summarized the advances on biological denitrification processes in recent years according to the different physiological characteristics and denitrification mechanisms of denitrification microorganisms. The pros and cons of different biological denitrification processes developed based on nitrifying bacteria, denitrifying bacteria, and anaerobic ammonia-oxidizing bacteria were compared with the aim to identify the best strategy for denitrification in a complex wastewater environment. The rapid development of synthetic biology provides possibilities to develop highly-efficient denitrifying strains based on mechanistic understandings. Combined with the applications of automatic simulation to obtain the optimal denitrification conditions, cost-effective and highly-efficient denitrification processed can be envisioned in the foreseeable future.

[Treatment of Medium Ammonium Wastewater by Single-stage Partial Nitritation-ANAMMOX SMBBR]

A SMBBR was established to treat medium ammonium under room temperature. Results showed that TN load can reach 0.16 kg·(m³·d)^-1, and the average TN removal efficiency was (51.58±6.80)% in the SMBBR with an influent ammonia concentration of 100 mg·L^-1 and DO of 0.4-0.7 mg·L^-1. AOB, ANAMMOX, and NOB activity reached (2253.21±502.10) mg·(m²·d)^-1, (4847.46±332.89) mg·(m²·d)^-1, and (1455.17±473.83) mg·(m²·d)^-1, and ANAMMOX and AOB bacteria were found to develop a good collaborative relationship. Quantitative PCR results showed that the relative abundance of ANAMMOX, AOB and NOB were 11.57%, 1.01% and 0.94%, respectively. The stable operation of single stage partial nitritation-ANAMMOX process provide an alternative technology for medium ammonia wastewater.

Comparative Genome-Centric Analysis of Freshwater and Marine ANAMMOX Cultures Suggests Functional Redundancy in Nitrogen Removal Processes

There is a lack of understanding of the interaction between anammox bacteria and the flanking microbial communities in both freshwater (non-saline) and marine (saline) ecosystems. Here, we present a comparative genome-based exploration of two different anammox bioreactors, through the analysis of 23 metagenome-assembled genomes (MAGs), 12 from freshwater anammox reactor (FWR), and 11 from marine anammox reactor (MWR). To understand the contribution of individual members to community functions, we applied the index of replication (iRep) to determine bacteria that are actively replicating. Using genomic content and iRep information, we provided a potential ecological role for the dominant members of the community based on the reactor operating conditions. In the non-saline system, anammox (Candidatus Brocadia sinica) and auxotrophic neighboring bacteria belonging to the phyla Ignavibacteriae and Chloroflexi might interact to reduce nitrate to nitrite for direct use by anammox bacteria. Whereas, in the saline reactor, anammox bacterium (Ca. Scalindua erythraensis) and flanking community belonging to phyla Planctomycetes (different than anammox bacteria)—which persistently growing in the system—may catabolize detritus and extracellular material and recycle nitrate to nitrite for direct use by anammox bacteria. Despite different microbial communities, there was functional redundancy in both ecosystems. These results signify the potential application of marine anammox bacteria for treating saline N-rich wastewaters.

Effects of DO on N2O emission during biological nitrogen removal using aerobic granular sludge via shortcut simultaneous nitrification and denitrification

Dissolved oxygen (DO) is an important factor influencing biological nitrogen removal. This study investigated the effects of different DO concentrations (4, 2, 1 mg/L) on nitrous oxide (N₂O) production and nitrogen removal via shortcut simultaneous nitrification and denitrification by aerobic granular sludge (SND_AG) using a sequencing bath reactor. The results showed that N₂O production was highest (127.6 mg/m³) at a DO concentration of 2 mg/L; this was 24.17 and 2.90 times the production at DO concentrations of 4 and 12 mg/L, respectively. The removal efficiency of total nitrogen also was the highest (61.68%) when the DO concentration was 2 mg/L, compared to 35.22% and 50.65% at DO concentrations of 4 and 1 mg/L, respectively. The efficiency of the SND_AG process reached 53.86% at a DO concentration of 2 mg/L, which was 1.33 and 1.67 times the efficiencies at DO concentrations of 4 and 1 mg/L, respectively. Therefore, reducing the DO concentration benefited the SND_AG process, but increased the emission of N₂O.

[Effect of an Aerobic Unit and a Sedimentation Unit on Sludge and Nitrogen Removal in an Anoxic Unit in a Continuous-flow System]

Using an A²/O process with three dissolved oxygen (DO) levels[3.0-3.5 mg·L^-1(Ⅰ stage), 2.0-2.5 mg·L^-1(Ⅱ stage), 1.5-2.0 mg·L^-1(Ⅲ stage)], the sludge and denitrification characteristics of its aerobic unit and sedimentation unit were investigated and compared with that of an anoxic-aerobic (A/O) process with a DO content of 1.5-2.0 mg·L^-1. The results showed that denitrification in the sedimentation unit was accomplished with both internal and external carbon sources, but sludge's denitrification was more efficient with the use of external carbon sources. Nitrate reductase activity and denitrification activity in the sludge in sedimentation unit were highest when DO content was 1.5-2.0 mg·L^-1 under aerobic conditions, and the denitrification efficiency of the A²/O process was greatest under anoxic conditions. The residual PHB in the aerobic A/O process was higher than that in the A²/O process with experimental sludge loading. The denitrification activity of the sludge in the A/O process was higher, and the nitrate reductase activity was 1.08 times higher than that in the A/O process. After returnning of the sludge, denitrification in the anoxic A/O process was poor, although the removal of nitrate nitrogen was sufficient. In comparison, denitrification in the anoxic unit of the A²/O process was better. Denitrification of the sludge in the sedimentation unit was directly related to denitrification in the anoxic unit. Therefore, to ensure that denitrification in sedimentation unit does not seriously affect the separation of sludge and water, appropriate control of the aerobic operation and the maintenance of denitrification in the sedimentation unit will contribute more to the denitrification efficiency of the system rather than simply controlling the level of anoxia.

[Advances in heterotrophic nitrification-aerobic denitrifying bacteria for nitrogen removal under extreme conditions]

Heterotrophic nitrification-aerobic denitrification (HN-AD) is an enrichment and breakthrough theory of traditional autotrophic nitrification heterotrophic denitrification. Heterotrophic nitrification-aerobic denitrifiers with the feature of wide distribution, strong adaptability and unique metabolic mechanism have many special advantages, including fast-growing, rapid biodegradability and long lasting activity, which can rapidly remove ammonia nitrogen, nitrate nitrogen (NO₃⁻-N) and nitrite nitrogen (NO₂⁻-N) under aerobic conditions simultaneously. Therefore, HN-AD bacteria show the important potential for denitrification under extreme conditions with high-salt, low-temperature or high-ammonia nitrogen environment, and HN-AD bacteria attract extensive attention in the field of biological denitrification of wastewater. In this review, we first introduce the previously reported HN-AD bacterial species which have denitrification performance in the extreme environments and state their typical metabolic mechanism. Then, we systematically analyze the nitrogen removal characteristics and potential under extreme conditions. We also briefly describe the progress in the application of HN-AD bacterial. Finally, we outlook the application prospects and research directions of HN-AD denitrification technology.

Enhancing mainstream nitrogen removal by employing nitrate/nitrite-dependent anaerobic methane oxidation processes

Due to serious eutrophication in water bodies, nitrogen removal has become a critical stage for wastewater treatment plants (WWTPs) over past decades. Conventional biological nitrogen removal processes are based on nitrification and denitrification (N/DN), and are suffering from several major drawbacks, including substantial aeration consumption, high fugitive greenhouse gas emissions, a requirement for external carbon sources, excessive sludge production and low energy recovery efficiency, and thus unable to satisfy the escalating public needs. Recently, the discovery of anaerobic ammonium oxidation (anammox) bacteria has promoted an update of conventional N/DN-based processes to autotrophic nitrogen removal. However, the application of anammox to treat domestic wastewater has been hindered mainly by unsatisfactory effluent quality with nitrogen removal efficiency below 80%. The discovery of nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) during the last decade has provided new opportunities to remove this barrier and to achieve a robust system with high-level nitrogen removal from municipal wastewater, by utilizing methane as an alternative carbon source. In the present review, opportunities and challenges for nitrate/nitrite-dependent anaerobic methane oxidation are discussed. Particularly, the prospective technologies driven by the cooperation of anammox and n-DAMO microorganisms are put forward based on previous experimental and modeling studies. Finally, a novel WWTP system acting as an energy exporter is delineated.

[Nitrogen Removal Characteristics and Analysis of Microbial Community Structure in an IEM-UF Simultaneous Separation and Denitrification System]

A system that combines an ion exchange membrane and ultrafiltration membrane (IEM-UF) to form a simultaneous separation and denitrification system was proposed for domestic sewage with a low carbon/nitrogen ratio. The removal of nitrogen and COD in the system was studied under a three phase operating condition. The characteristics of the microbial community in each reactor were analyzed using metagenomics. The results show that, the average rate of ammonia nitrogen enrichment in the separator reached above 116.1% when the current intensity was 0.2 A. When the system was at C/N 2.80 and operating well, the average removal rates of COD and TN reached above 90% and 50%, respectively. The maximum removal rate of TN was above 65.4%. The results of metagenomics showed a genus of phylum Nitrospirae (Nitrospira) and a genus of phylum Proteobacteria (Nitrosomonas), with the proportions of 12.23% and 2.31%, respectively. In the denitrifying reactor, Dechloromonas, Thauera, and Azospira were detected in the proportions 4.57%, 1.76%, and 1.03%, respectively. These proportions were far larger than those of other bacteria in this reactor. Meanwhile, the presence of iron autotrophic denitrifying bacteria increased the denitrification efficiency of the system.

13C Incorporation as a Tool to Estimate Biomass Yields in Thermophilic and Mesophilic Nitrifying Communities

Current methods determining biomass yield require sophisticated sensors for in situ measurements or multiple steady-state reactor runs. Determining the yield of specific groups of organisms in mixed cultures in a fast and easy manner remains challenging. This study describes a fast method to estimate the maximum biomass yield (Ymax), based on 13C incorporation during activity measurements. It was applied to mixed cultures containing ammonia oxidizing bacteria (AOB) or archaea (AOA) and nitrite oxidizing bacteria (NOB), grown under mesophilic (15-28°C) and thermophilic (50°C) conditions. Using this method, no distinction could be made between AOB and AOA co-existing in a community. A slight overestimation of the nitrifier biomass due to 13C redirection via SMP to heterotrophs could occur, meaning that this method determines the carbon fixation activity of the autotrophic microorganisms rather than the actual nitrifier biomass yield. Thermophilic AOA yields exceeded mesophilic AOB yields (0.22 vs. 0.06-0.11 g VSS g-1 N), possibly linked to a more efficient pathway for CO2 incorporation. NOB thermophilically produced less biomass (0.025-0.028 vs. 0.048-0.051 g VSS g-1 N), conceivably attributed to higher maintenance requirement, rendering less energy available for biomass synthesis. Interestingly, thermophilic nitrification yield was higher than its mesophilic counterpart, due to the dominance of AOA over AOB at higher temperatures. An instant temperature increase impacted the mesophilic AOB yield, corroborating the effect of maintenance requirement on production capacity. Model simulations of two realistic nitrification/denitrification plants were robust toward changing nitrifier yield in predicting effluent ammonium concentrations, whereas sludge composition was impacted. Summarized, a fast, precise and easily executable method was developed determining Ymax of ammonia and nitrite oxidizers in mixed communities.

[Biological Nitrogen Removal Process in a Microbubble-aerated Biofilm Reactor Treating Low C/N Wastewater]

The microbubble-aerated biofilm reactor as a new treatment process combines microbubble aeration technology with aerobic biological treatment. A microbubble aerated biofilm reactor was used in this study to treat low C/N ratio wastewater at a low air/water ratio. The process and performance of biological nitrogen removal were investigated, and the functional bacterial populations for nitrogen removal in the biofilm were analyzed. The results showed that the biological nitrogen removal process was converted from simultaneous nitrification-denitrification to simultaneous partial nitrification, ANAMMOX and denitrification (SNAD) processes when DO concentration was controlled by an air/water ratio of lower than 1:2 and the influent C/N ratio was reduced. As a result, the efficient biological nitrogen removal performance was achieved when treating low C/N ratio wastewater. When the DO concentration was lower than 1.0 mg·L^-1 and the influent C/N ratio was 1:2.8, the SNAD process became dominant for biological nitrogen removal. In this case, the average total nitrogen (TN) removal efficiency was 76.3%, and the average TN loading rate removed was 1.42 kg·(m³·d)^-1. In addition, it was estimated that 86.0% of TN removal was attributed to the ANAMMOX process. The relative abundances of ammonia-oxidizing bacteria populations and ANAMMOX bacteria populations in the biofilm increased gradually, while the relative abundances of nitrite-oxidizing bacteria populations and denitrifying bacteria populations decreased gradually, with a decrease in influent C/N ratio. The variation of functional bacterial populations for nitrogen removal was consistent with the conversion of nitrogen removal process to SNAD process.

Effective partial nitrification of ammonia in a fluidized bed bioreactor

A lab-scale fluidized bed bioreactor with high-density polyethylene as biofilm carrier media was operated to study partial nitrification (PN) performance with high ammonia concentrations. The system was run at nitrogen loading rates (NLRs) from 1.2 to 4.8 kg N/(m³ d) with empty bed contact time of 2.0 and 2.7 h and four different influent ammonia concentrations of 100, 200, 300 and 400 mg/L. Dissolved oxygen concentration and temperature were maintained around 1.3 mg/L and 35°C, respectively. Stable PN was successfully achieved during the whole period with low effluent NO₃-N concentration at less than 15 mg/L, due to effective suppression of nitrite-oxidizing bacteria activity at high concentrations of free ammonia (5.3-27.3 mg N/L) and low alkalinity-to-ammonia ratio. At the NLR of 3.6 kg N/(m³ d), NH₄-N conversion and NO₂-N accumulation ratios were 57.8% and 53.9%, respectively, which could be further used in the anaerobic ammonium oxidation process (ANAMMOX) as the effluent NO₂-N/NH₄-N ratio was 1.27.

[High-rate Nitrogen Removal in a Two-stage Partial Nitritation-ANAMMOX Process Under Mainstream Conditions]

A two-stage partial nitritation (PN)-ANAMMOX process was successfully carried out for low-strength NH₄⁺-N (50 mg·L^-1) wastewater treatment at ambient/low temperatures. The results show that an average total nitrogen removal rate and removal efficiency above 0.6 kg·(m³·d)^-1and 80% could be maintained, respectively, at temperatures between 20℃ and 14℃. The two-stage PN-ANAMMOX process was successful both under NO₂^--N-limited and NH₄⁺-N-limited conditions. When the two-stage PN-ANAMMOX process was operated under NH₄⁺-N-limited conditions, the "limit of technology" for nitrogen removal (TN<3 mg·L^-1) could be easily achieved by following anoxic denitrification. Lowering the temperature to 12℃ resulted in the reduction of the total nitrogen removal rate to~0.5 kg·(m³·d)^-1. Due to the low temperature, the anammox reaction became the rate-limiting step for nitrogen removal, while the PN reaction was not impacted. In the temperature range of 10-20℃, the activity-temperature coefficients (θ) of the PN granules and ANAMMOX sludge were 1.056 and 1.172, respectively, suggesting that the anammox bacteria have a higher temperature sensitivity than the ammonium oxidizing bacteria (AOB). Overall, the results clearly indicate that the nitrogen removal loading rate of the two-stage PN-ANAMMOX process is mainly controlled by the activity and quantity of anammox biomass. In contrast, the process nitrogen removal efficiency mainly depends on the performance of the first-stage PN (i.e., effluent NO₂^--N/NH₄⁺-N ratio and NO₃^--N concentration) under a constant nitrogen removal loading rate (no overload). Based on these results, a hierarchically separate control strategy was proposed to obtain a high-rate nitrogen removal during the two-stage mainstream PN-ANAMMOX process.

Metatranscriptomic Investigation of Adaptation in NO and N2O Production From a Lab-Scale Nitrification Process Upon Repeated Exposure to Anoxic-Aerobic Cycling

The molecular mechanisms of microbial adaptation to repeated anoxic-aerobic cycling were investigated by integrating whole community gene expression (metatranscriptomics) and physiological responses, including the production of nitric (NO) and nitrous (N2O) oxides. Anoxic-aerobic cycling was imposed for 17 days in a lab-scale full-nitrification mixed culture system. Prior to cycling, NO and N2O levels were sustained at 0.097 ± 0.006 and 0.054 ± 0.019 ppmv, respectively. Once the anoxic-aerobic cycling was initiated, peak emissions were highest on the first day (9.8 and 1.3 ppmv, respectively). By the end of day 17, NO production returned to pre-cycling levels (a peak of 0.12 ± 0.007 ppmv), while N2O production reached a new baseline (a peak of 0.32 ± 0.05 ppmv), one order of magnitude higher than steady-state conditions. Concurrently, post-cycling transcription of norBQ and nosZ returned to pre-cycling levels after an initial 5.7- and 9.5-fold increase, while nirK remained significantly expressed (1.6-fold) for the duration of and after cycling conditions. The imbalance in nirK and nosZ mRNA abundance coupled with continuous conversion of NO to N2O might explain the elevated post-cycling baseline for N2O. Metatranscriptomic investigation notably indicated possible NO production by NOB under anoxic-aerobic cycling through a significant increase in nirK expression. Opposing effects on AOB (down-regulation) and NOB (up-regulation) CO2 fixation were observed, suggesting that nitrifying bacteria are differently impacted by anoxic-aerobic cycling. Genes encoding the terminal oxidase of the electron transport chain (ccoNP, coxBC) were the most significantly transcribed, highlighting a hitherto unexplored pathway to manage high electron fluxes resulting from increased ammonia oxidation rates, and leading to overall, increased NO and N2O production. In sum, this study identified underlying metabolic processes and mechanisms contributing to NO and N2O production through a systems-level interrogation, which revealed the differential ability of specific microbial groups to adapt to sustained operational conditions in engineered biological nitrogen removal processes.

[Screening and Nitrogen Removing Characteristics of Heterotrophic Nitrification-Aerobic Denitrification Bacteria SLWX2 from Sea Water]

In this study, an efficient heterotrophic nitrifying-aerobic denitrifying bacteria strain SLWX₂ was screened from 7 strains isolated from Stichopus japonicus culture ponds, with removal rates of NH₄⁺-N, NO₂^--N and NO₃^--N up to 100%, 99.5% and 85.6% within 24 h, respectively. Through morphologic observation, physiological characteristics and 16S rDNA sequence analysis, strain SLWX₂ was identified as Bacillus hwajinpoensis. The results of nitrogen removal characterization experiments indicated that, when NH₄⁺-N, NO₂^--N and NO₃^--N existed at the same time, SLWX₂ utilized NH₄⁺-N firstly, then utilized NO₂^--N and NO₃^--N, and removed almost all the inorganic nitrogen within 72 h, suggesting that it could achieve simultaneous nitrification and denitrification itself. The results of nitrogen tolerance examination indicated that strain SLWX₂ showed perfect nitrogen removal ability when the ammonia load was not above 500 mg·L^-1, nitrite load was not above 100 mg·L^-1 and nitrate load was not above 200 mg·L^-1, the maximal removal of ammonia nitrogen, nitrite nitrogen and nitrate nitrogen withinn 96 h reached 180 mg, 30 mg and 120 mg, respectively. Moreover, there was no NO₂^--N accumulation during nitrification. This strain showed great potential in biological nitrogen removal of wastewater with high salt and nitrogen from mariculture and industries.

Effects of azo dye on simultaneous biological removal of azo dye and nutrients in wastewater

The potential disrupting effects of Azo dye on wastewater nutrients removal deserved more analysis. In this study, 15 days exposure experiments were conducted with alizarin yellow R (AYR) as a model dye to determine whether the dye caused adverse effects on biological removal of both the dye and nutrients in acclimated anaerobic-aerobic-anoxic sequencing batch reactors. The results showed that the AYR removal efficiency was, respectively, 85.7% and 66.8% at AYR concentrations of 50 and 200 mg l^-1, while higher AYR inlet (400 mg l^-1) might inactivate sludge. Lower removal of AYR at 200 mg l^-1 of AYR was due to the insufficient support of electron donors in the anaerobic process. However, the decolorized by-products p-phenylenediamine and 5-aminosalicylic were completely decomposed in the following aerobic stage at both 50 and 200 mg l^-1 of AYR concentrations. Compared with the absence of AYR, the presence of 200 mg l^-1 of AYR decreased the total nitrogen removal efficiency from 82.4 to 41.1%, and chemical oxygen demand (COD) removal efficiency initially decreased to 68.1% and then returned to around 83.4% in the long-term exposure time. It was also found that the inhibition of AYR, nitrogen and COD removal induced by a higher concentration of AYR was due to the increased intracellular reactive oxygen species production, which caused the rise of oxidation-reduction potential value and decreased ammonia monooxygenase and nitrite oxidoreductase activities.

[Performance of the Removal of Nitrogen During Anaerobic Ammonia Oxidation Using Different Operational Strategies]

The effects of low substrate ratio, cooling methods, and pH on nitrogen removal performance were studied in a laboratory-scale anaerobic ammonium oxidation reactor (ASBR) while treating simulated domestic waste water. The results illustrated that the average removal efficiencies of NH₄⁺-N and NO₂^--N increased from 54.4% and 65.3% to 95.8% and 92.5%, respectively, at a temperature of 30℃ and an influent concentration of NO₂^--N of (30±0.2)mg·L^-1. The substrate ratio (NO₂^--N/NH₄⁺-N) increased from 0.9 to 1.4.However, the removal efficiency of NH₄⁺-N was affected negligibly, and the average removal efficiency of NO₂^--N decreased to 54.6% when the substrate ratio was increased to 1.6, suggesting that the nitrogen removal performance of anaerobic ammonium oxidation was best when the substrate ratio was close to the theoretical value of 1.32.The average removal efficiencies of NH₄⁺-N and NO₂^--N decreased from 97.5% and 98.5% to 35.2% and 40.1%, respectively, when the temperature of the reactor dropped from 30℃ to 15℃ at one time. The average removal efficiencies of NH₄⁺-N and NO₂^--N dropped from 97.7% and 98.6% to 52.7% and 62.4%, respectively, when the ladder cooling method(30℃→25℃→20℃→15℃) was used. The average removal efficiencies of NH₄⁺-N and NO₂^--N increased initially and then decreased when the pH was increased gradually from 7.7 to 8.5.The highest nitrogen removal efficiency was achieved when the pH was controlled at 8.3 with a substrate ratio of NO₂^--N/NH₄⁺-N equal to 1.4.

[Influence of Operating Modes for the Alternating Anoxic/Oxic Process on Biological Nitrogen Removal and Extracellular Polymeric Substances of Activated Sludge]

Nitrogen removal, extracellular polymeric substances (EPS), and the chemical composition (protein (PN), polysaccharide (PS), and DNA) by the aerobic/anoxic (O/A) and the anoxic/aerobic (A/O) modes were studied in a sequencing batch reactor (SBR) fed with domestic wastewater. The results showed that the removal rates of NH₄⁺-N were 97.5% and 98.0% in the two operating modes, respectively, and a removal efficiency of NH₄⁺-N with high efficiency and stability was obtained. The nitrification rate was positively correlated with the nitrogen loading ratio. The influence of operating modes for the alternating anoxic/oxic mode on extracellular polymeric substances of activated sludge was evaluated. The EPS constituent in the A/O mode was slightly higher than the O/A mode. The operating mode had no effect on the contents of PN, PS, and DNA in tightly bound EPS (TB-EPS) and TB-EPS. However, PN and PS in loosely bound EPS (LB-EPS) and LB-EPS in the A/O mode were 1.38 to 1.56 times those of the O/A mode. In the two operating modes, PSs were the main constituents in the TB-EPS and EPS, while PNs were the main constituents in LB-EPS. The EPS content had a good linear correlation with the sludge settling performance.

[Substrate Inhibition and Kinetic Characteristics of Marine Anaerobic Ammonium Oxidizing Bacteria Treating Saline Wastewater]

An anaerobic sequencing batch reactor (ASBR) was used to study substrate inhibition and kinetic characteristics of marine anaerobic ammonium oxidizing bacteria (MAAOB) treating saline wastewater. The results indicated that when ammonia increased to 1200 mg·L^-1, the MAAOB still maintained good nitrogen removal capability, though there was a slight inhibitory effect. At the same time, nitrite nitrogen removal efficiency was stable at about 80.70%. When nitrite increased to 265.6 mg·L^-1, the MAAOB were inhibited obviously, and ammonia nitrogen removal efficiency decreased to about 63.01%. When influent nitrite concentration increased to 305.6 mg·L^-1, the removal rate of ammonia nitrogen further decreased to 43.93%. The kinetic characteristics resulting from inhibition of the MAAOB were simulated by the Haldane model and Aiba model. Three parameters, TNRR_max, K_S, and K_i, and the relationship between effluent substrate concentration and total nitrogen loading (TNRR) were evaluated. Based on further analysis, the Haldane model was more suitable for describing dynamic characteristics resulting from NH₄⁺-N inhibition, while the Aiba model was more suitable for describing the dynamic characteristics resulting from NO₂^--N inhibition. The predicted effluent inhibitory concentrations of NH₄⁺-N and NO₂^--N were 3893.625 mg·L^-1 and 287.208 mg·L^-1, respectively. The results could provide a theoretical basis for saline wastewater treatment by MAAOB.

Laboratory-scale assessment of vacuum-degassed activated sludge for improved settling properties

Vacuum degassing of activated sludge was tested at eight different Swedish wastewater treatment plants with laboratory-scale equipment in batch mode in order to evaluate its efficiency on improvement of sludge compaction and settling properties. The results show that the efficiency of the degassing technique is mainly dependent on the initial sludge volume index (SVI) of the target sludge which was found to be related to its process configuration. Facilities with full activated sludge-based nitrogen removal processes, including both nitrification and denitrification, had high SVIs (>300 mL g^-1) and were strongly affected by vacuum degassing with reduction of SVI up to 30%. Nitrogen removal facilities also including biological phosphorus removal showed better compaction and settling properties with relatively lower SVIs and were affected to a lesser extent by degassing with SVI reduction of 10-20%. Wastewater treatment plants without full biological nitrogen removal, lacking either nitrification or denitrification (or both) processes in the activated sludge had the lowest SVIs observed with almost no effect of vacuum degassing.

[Removal of Nitrogen from Alcohol Wastewater by PN-ANAMMOX]

An integrated partial nitrification anaerobic ammonia oxidation reactor was used to explore the feasibility of nitrogen removal from recycled ethanol wastewater. The results show that the integrated partial nitrification-anaerobic ammonia oxidation (PN-ANAMMOX) reactor was started successfully after 40 d under the conditions of pH 7.8±0.5, temperature 30-35℃, and aerobic ORP value 120-150 mV. The total nitrogen removal rate of 0.125 kg·(m³·d)^-1 increased to 0.75 kg·(m³·d)^-1, Inoculation of mature nitrosated biofilms and anaerobic ammonium oxide granules can accelerate the start of the reactor. The effects of alcohol wastewater on the PN-ANAMMOX reactor were mainly caused by biodegradable TOC, The biodegradable TOC concentration of 100mg·L^-1 in alcohol wastewater can reduce the removal rate of total nitrogen from 0.75 kg·(m³·d)^-1 to 0.25 kg·(m³·d)^-1,this inhibition can be restored. Different concentrations of alcohol wastewater were dosed into the PN-ANAMMOX reactor to acclimate the bacteria. The total nitrogen removal rate first decreased and then increased, as the influent concentration gradient increased, which was beneficial for improving the efficiency of nitrogen removal by extending the HRT and increasing the dissolved oxygen in the PN stage. Finally, the nitrogen removal rate stabilized at 0.65 kg·(m³·d)^-1. These results show that PN-ANAMMOX can be used for the treatment of alcohol wastewater.

[Effect of pH Shock on Nitrogen Removal Performance of Marine Anaerobic Ammonium-Oxidizing Bacteria Treating Saline Wastewater]

The effect of pH shock on nitrogen removal performance of marine anaerobic ammonium-oxidizing bacteria (MAAOB) treating saline wastewater was studied by employing an ASBR reactor. Dynamic characteristics of the MAAOB were simulated by the Andrew model and Ratkowsky model. The results indicated that the reactor had the best nitrogen removal efficiency when the pH value was 7-8. The nitrogen removal rate (NRR) was (0.30±0.04) kg·(m³·d)^-1, and the total nitrogen removal efficiency (TNRE) was (76.73±5.74)%. When the pH value was 8.5, FA had a mean concentration of 14.22 mg·L^-1 and little effect on nitrogen removal. The NRR was (0.30±0.02) kg·(m³·d)^-1. However, NO₂^--N accumulated and it was not completely removed. When the pH values were 6.5 and 9, the concentrations of FA were 0.22 mg·L^-1 and 37.84 mg·L^-1, respectively, the NRRs were (0.10±0.02) and (0.15±0.02) kg·(m³·d)^-1, and the TNREs were (23.04±9.88)% and (42.12±5.52)%. The tolerance of the MAAOB in alkaline condition was stronger than that in acidic condition. The Andrew model was modified to determine the relationship between NRR and FA. Other parameters such as NRR_max, k_S, and k_I were also achieved simultaneously. These are key to describing the nitrogen removal process of MAAOB.

Operational strategies and environmental conditions inducing aerobic denitritation in short-cut biological nitrogen removal at side-line treatment

Factors promoting aerobic denitritation in a pilot-scale short-cut biological nitrogen removal (SBNR) process were investigated. The study implemented optimization of nitrogen removal in the anaerobic reject water (ARW) having a low organic C:N ratio ARW was produced in a large-scale municipal wastewater treatment plant (WWTP). Aerobic denitritation occurred consistently during study of a specific period of sequential batch reactor (SBR) where nitrite removal under fully aerobic conditions was obtained with a switch from oxygen to nitrite respiration, creating an aerobic (high oxidation-reduction potential) condition. Specific factors inducing aerobic denitritation were found related to several parameters as ammonium concentration, temperature, feeding mode, duration of the oxic stage and substrate availability due to beta-oxidation of lipid matter. Microbial analyses indicated a higher increase in nitrite reducing than ammonium oxidizing activity, as an evidence for nitrifying denitrifier bacterial dominance in the biomass. The reaction induced a reduction in the inhibitory products of the process as volatile fatty acids (VFAs) and free nitrous oxide (FNA), produced bicarbonate and increased removal efficiency of ammonium and nitrite, thus, total nitrogen. The outcome presents potential ways for further saving on aeration and chemical need via operational means, while taking advantage of the slowly degrading organic matter on SBNR performance.

[Influence of Biological Activated Carbon on Simultaneous Nitrification and Denitrification in Inflow with Different C/N Ratios]

Influence of biological activated carbon (BAC) on simultaneous nitrification and denitrification (SND) in inflow with different C/N ratios was investigated with continuous operation of BAC reactor and SBR. Methanol was added as carbon source and the inflow C/N ratio was set to 3, 5, 8 and 10 to run for about 120 cycles, under conditions of indoor temperature (15-27℃), initial DO 2-3mg·L^-1. The TN removal efficiency and stability of two reactors were compared. The results showed that, BAC reactor had a higher TN removal efficiency than SBR at different C/N ratios. TN removal rate of BAC reactor was 44.88%, 58.07% and 66.64%, when the C/N ratio was 3, 5 and 8 respectively. After increasing the C/N ratio to 10, the BAC reactor could maintain TN removal rate of 63.65%, but the SBR showed sludge bulking. BAC provided various DO environments for microorganisms in a vessel, which was beneficial to SND. BAC could reduce the influence of excessive carbon source on the nitrification system, enlarge the application range and improve the stability of reactor at different C/N ratios, and increase the nitrogen removal capacity of organic matter. BAC provided condition for efficient nitrogen removal.

Evaluating the effect of biofilm thickness on nitrification in moving bed biofilm reactors

This study evaluates the effect of biofilm thickness on the nitrifying activity in moving bed biofilm reactors (MBBRs) in a controlled environment. In-depth understanding of biofilm properties in MBBRs and their effect on the overall treatment efficiency is the key to optimizing process stability and efficiency. However, evaluating biofilm properties in continuously operated MBBRs can be extremely challenging. This study uses a carrier design which enables comparison of four different biofilm thicknesses, in otherwise equally operated lab-scale MBBRs. The results show that within the studied range (200-500 µm) and specific operation conditions, biofilm thickness alone had no significant effect on the overall ammonium removal. The nitrate production, however, decreased with a decreasing biofilm thickness, and the ratio between nitrite and ammonia-oxidizing activity decreased both with increasing load and decreasing oxygen concentration for all thicknesses. The suggestion that nitratation is disfavoured in thin biofilms is an interesting contribution to the current research being performed on nitrite-oxidizing bacteria inhibition for deammonification applications. By indicating that different groups of bacteria respond differently to biofilm thickness, this study accentuates the importance of further evaluation of these complex systems.

N2O emission from nitrogen removal via nitrite in oxic-anoxic granular sludge sequencing batch reactor

Bionitrification is considered to be a potential source of nitrous oxide (N2O) emissions, which are produced as a by-product during the nitrogen removal process. To investigate the production of N2O during the process of nitrogen removal via nitrite, a granular sludge was studied using a lab-scale sequence batch reactor operated with real-time control. The total production of N2O generated during the nitrification and denitrification processes were 1.724 mg/L and 0.125 mg/L, respectively, demonstrating that N2O is produced during both processes, with the nitrification phase generating larger amount. In addition, due to the N2O-N mass/oxidized ammonia mass ratio, it can be concluded that nitrite accumulation has a positive influence on N2O emissions. Results obtained from PCR-DGGE analysis demonstrate that a specific Nitrosomonas microorganism is related to N2O emission.

Biological nutrients removal from the supernatant originating from the anaerobic digestion of the organic fraction of municipal solid waste

This study critically evaluates the biological processes and techniques applied to remove nitrogen and phosphorus from the anaerobic supernatant produced from the treatment of the organic fraction of municipal solid waste (OFMSW) and from its co-digestion with other biodegradable organic waste (BOW) streams. The wide application of anaerobic digestion for the treatment of several organic waste streams results in the production of high quantities of anaerobic effluents. Such effluents are characterized by high nutrient content, because organic and particulate nitrogen and phosphorus are hydrolyzed in the anaerobic digestion process. Consequently, adequate post-treatment is required in order to comply with the existing land application and discharge legislation in the European Union countries. This may include physicochemical and biological processes, with the latter being more advantageous due to their lower cost. Nitrogen removal is accomplished through the conventional nitrification/denitrification, nitritation/denitritation and the complete autotrophic nitrogen removal process; the latter is accomplished by nitritation coupled with the anoxic ammonium oxidation process. As anaerobic digestion effluents are characterized by low COD/TKN ratio, conventional denitrification/nitrification is not an attractive option; short-cut nitrogen removal processes are more promising. Both suspended and attached growth processes have been employed to treat the anaerobic supernatant. Specifically, the sequencing batch reactor, the membrane bioreactor, the conventional activated sludge and the moving bed biofilm reactor processes have been investigated. Physicochemical phosphorus removal via struvite precipitation has been extensively examined. Enhanced biological phosphorus removal from the anaerobic supernatant can take place through the sequencing anaerobic/aerobic process. More recently, denitrifying phosphorus removal via nitrite or nitrate has been explored. The removal of phosphorus from the anaerobic supernatant of OFMSW is an interesting research topic that has not yet been explored. At the moment, standardization in the design of facilities that treat anaerobic supernatant produced from the treatment of OFMSW is still under development. To move toward this direction, it is first necessary to assess the performance of alternative treatment options. It study concentrates existing data regarding the characteristics of the anaerobic supernatant produced from the treatment of OFMSW and from their co-digestion with other BOW. This provides data documenting the effect of the anaerobic digestion operating conditions on the supernatant quality and critically evaluates alternative options for the post-treatment of the liquid fraction produced from the anaerobic digestion process.