Ammonium removal by partial nitritation and Anammox processes from wastewater with increased salinity

A review of microbial nitrogen transformations and microbiome engineering for biological nitrogen removal under salinity stress

The osmotic stress caused by salinity exerts severe inhibition on the process of biological nitrogen removal (BNR), leading to the deterioration of biosystems and the discharge of nitrogen with saline wastewater. Feasible strategies to solve the bottleneck in saline wastewater treatment have attracted great attention, but relevant studies to improve nitrogen transformations and enhance the salt-tolerance of biosystems in terms of microbiome engineering have not been systematically reviewed and discussed. This work attempted to provide a more comprehensive explanation of both BNR and microbiome engineering approaches for saline wastewater treatment. The effect of salinity on conventional BNR pathways, nitrification-denitrification and anammox, was summarized at cellular and metabolic levels, including the nitrogen metabolic pathways, the functional microorganisms, and the inhibition threshold of salinity. Promising nitrogen transformations, such as heterotrophic nitrification-aerobic denitrification, ammonium assimilation and the coupling of conventional pathways, were introduced and compared based on advantages and challenges in detail. Strategies to improve the salt tolerance of biosystems were proposed and evaluated from the perspective of microbiome engineering. Finally, prospects of future investigation and applications on halophilic microbiomes in saline wastewater treatment were discussed.

Candidatus Scalindua, a Biological Solution to Treat Saline Recirculating Aquaculture System Wastewater

Recirculating aquaculture systems (RAS) are promising candidates for the sustainable development of the aquaculture industry. A current limitation of RAS is the production and potential accumulation of nitrogenous wastes, ammonium (NH4+), nitrite (NO2−) and nitrate (NO3−), which could affect fish health and welfare. In a previous experiment, we have demonstrated that the marine anammox bacteria Candidatus Scalindua was a promising candidate to treat the wastewater (WW) of marine, cold-water RAS. However, the activity of the bacteria was negatively impacted after a direct exposure to RAS WW. In the current study, we have further investigated the potential of Ca. Scalindua to treat marine RAS WW in a three-phase experiment. In the first phase (control, 83 days), Ca. Scalindua was fed a synthetic feed, enriched in NH4+, NO2− and trace element (TE) mix. Removal rates of 98.9% and 99.6% for NH4+ and NO2−, respectively, were achieved. In the second phase (116 days), we gradually increased the exposure of Ca. Scalindua to nitrogen-enriched RAS WW over a period of about 80 days. In the last phase (79 days), we investigated the needs of TE supplementation for the Ca. Scalindua after they were fully acclimated to 100% RAS WW. Our results show that the gradual exposure of Ca. Scalindua resulted in a successful acclimation to 100% RAS WW, with maintained high removal rates of both NH4+ and NO2− throughout the experiment. Despite a slight decrease in relative abundance (from 21.4% to 16.7%), Ca. Scalindua remained the dominant species in the granules throughout the whole experiment. We conclude that Ca. Scalindua can be successfully used to treat marine RAS WW, without the addition of TE, once given enough time to acclimate to its new substrate. Future studies need to determine the specific needs for optimal RAS WW treatment by Ca. Scalindua at pilot scale.

Saline short-term shock and rapid recovery on anammox performance

Anaerobic ammonia oxidation (anammox) is an environmental-friendly biological nitrogen removal process, which has been developed as a promising technology in industrial wastewater treatment. However, anammox nitrogen removal under high saline conditions still faces many challenges. This study investigated the performance of anammox sludge under saline short-term shock and the strategy of rapid recovery. Salinity concentration, saline exposure time, and NaCl/Na₂SO₄ ratio were selected as three critical factors for short-term shock. The activity inhibition of anammox sludge were tested by using response surface methodology (RSM). Our results showed that, compared with the NaCl/Na₂SO₄ ratio, the salinity concentration and saline exposure time were the significant factor causing the anammox inhibition. The addition of glycine betaine (GB) in moderate amounts (0.1-5 mM) was found to help anammox to resist in relative low saline shock intensities (e.g., IC₂₅ and IC₅₀), with the activity retention rate of 94.7%. However, glycine betaine was not worked effectively under relatively high saline shock intensities (e.g., complete inhibition condition). Microbial community analysis revealed that Brocadiaceae accounted for only about 7.6%-13.2% at inhibited conditions. Interestingly, 16S rRNA analysis showed that the abundance of activated Brocadiaceae remarkably decreased with time after high-level saline shock. This tendency was consistent with the results of qPCR targeted hzsA gene. Finally, based on quorum sensing, the anammox activity was recovered to 93.5% of original sludge by adding 30% original sludge. The study realized the rapid recovery of anammox activity under complete inhibition, promoting the development and operation of salt-tolerant anammox process.

The effects of salinity changes on anammox performance: The response rule and tolerance mechanism

Some wastewaters contain high concentrations of ammonia coexisting with large amounts of salt, which might negatively affect the anaerobic ammonium oxidation (anammox) process. In this study, the performance of the anammox process under different saline conditions was investigated using an upflow anaerobic sludge bed-anammox system. After long-term operating for 275 days, the results indicated that the nitrogen removal efficiency remained high under the 0-40 g NaCl/L, and low salinity (15 g NaCl/L) substantially promoted specific anammox activity. Affected by the saline environment, the appearance, color, and shape of sludge notably changed, and the amount of extracellular polymeric substances gradually increased with increasing salinity, which might be one of the reasons for the strong salt tolerance of the system. Chloroflexi and Planctomycetes were the dominant strains under long-term salinity, and Brocadiaceae_g_ unclassified exhibited halophilic characteristics. The redundancy analysis results showed that the concentration of influent NH₄ ⁺ -N and salinity were the main environmental factors affecting the microbial community of the system. PRACTITIONER POINTS: Provides data to support the maximum value for salinity wastewater treatment with anammox processes' tolerance of 40 g NaCl/L. EPS changes may be responsible for the response to salinity challenges and provide direction for high salinity wastewater treatment. Brocadiaceae_g_ unclassified exhibited a halophilic quality. And it can be focused on to improve treatment efficiency.

Can anammox process be adopted for treating wastewater with high salinity exposure risk?

Anammox was a promising technology for nitrogen removal, and has been applied for treating many kinds of nitrogenous wastewaters. Considering the risk in high salinity of the municipal sewage in coastal city, the feasibility of Anammox process for treating low ammonia wastewater (around 50 mg L^-1) with increasing salinity was investigated in this study. The results showed that the salinity in low concentrations (1-5 g L^-1) had slight impact on the nitrogen removal and activity of Anammox bacteria but significantly improved its growth. The moderate salinity (10-40 g L^-1) decreased the specific Anammox activity (SAA) to 8.11 from the initial 13.15 mg N g^-1 SS h^-1, but increased the abundance to 52.3% from 30.1% (Candidatus Kuenenia). High salinity (50-60 g L^-1) performed severe inhibition on activity and abundance both, with the SAA decreased to 0 and abundance to 11.9%. The self-recovery performance was unsatisfactory when salinity was unavailable. A quadratic curve between the SAA and salinity concentration was fitted, and the IC₅₀ was calculated as 42.1 g L^-1 (NaCl). Anammox process could be directly adopted for treating low ammonia sewage with low salinity, whereas activity enhancement or adaption improvement should be pre-presented for treating sewage with moderate or high salinity.

Inhibition of wastewater pollutants on the anammox process: A review

The anaerobic ammonium oxidation (anammox) process has been recognized as an efficient nitrogen removal technology. However, anammox bacteria are susceptible to surrounding environments and different pollutants, which limits the extensive application of the anammox process worldwide. Numerous researchers investigate the effects of various pollutants on the anammox process or bacteria, and related findings have also been reviewed with the focused on their inhibitory effects on process performance and microbial community. This review systemically summarized the recent advances in the inhibition, mechanism and recovery process of traditional and emerging pollutants on the anammox process over a decade, such as organics, metals, antibiotics, nanoparticles, etc. Generally, low-concentration pollutants exhibited a promotion on the anammox activity, while high-concentration pollutants showed inhibitory effects. The inhibitory threshold concentration of different pollutants varied. The combined effects of multipollutant also attracts more attentions, including synergistic, antagonistic and independent effects. Additionally, remaining problems and research needs are further proposed. This review provides a foundation for future research on the inhibition in anammox process, and promotes the proper operation of anammox processes treating different types of wastewaters.

From freshwater anammox bacteria (FAB) to marine anammox bacteria (MAB): A stepwise salinity acclimation process

An investigation into the effect of stepwise saline introduction (3-20 g·L^-1 NaCl) on the anaerobic ammonium oxidation (anammox) process in a lab-scale sequencing batch reactor was carried out for 252 days by evaluating the changes in influent and effluent nitrogen concentrations, conductivity, microbial extracellular polymeric substances' (EPS) ionic content, as well as stresses due to salinity, via microbial ATP analysis. It was observed that, effluent nitrogen concentrations remained stable at low saline levels of 3 g·L^-1 to 10 g·L^-1. Nonetheless, midway through 10 g·L^-1 and the preliminary phase of 15 g·L^-1 salinity presented a very unstable, highly fluctuating as well as deteriorating effluent nitrogen concentrations. A more satisfactory nitrogen removal efficiency of 83.7 ± 5.9% was obtained at higher saline concentrations implying that, the adaptation mechanism to tolerate increasing salinity was taking place. Saline induced stress, which measures the variation in viable anammox bacteria, was correlative to the formation of EPS and changes in its cationic contents along the increasing salinity. Although the specific anammox activity (SAA) dropped by approximately 15% from the beginning of the process to the midpoint, the drop in SAA after the midpoint was not as drastic as the initial phase. A change in microbial aggregation and dominance proved the existence of new saline-dependent species that can withstand high saline stresses. Recovery from abrupt high saline shocks in batch experiment was seen to be almost impossible.

Stability of microbial functionality in anammox sludge adaptation to various salt concentrations and different salt-adding steps

The stability of community functioning in anaerobic ammonia oxidation (anammox) sludge adaptation to various salinity changes are concerned but not fully explored. In this study, two anammox reactors were designed in response to different salt levels and salt-adding methods. The reactor PI, run with small stepwise salt increments (0.5%-1.0%), removed >90% of nitrite and ammonium in the influent over the range of 0%-4% salt. By contrast, the reactor SI, run with a sharp salt increment (>2.5%), exhibited a reduced performance (by up to 44%) over the same salt range with a new steady state. The observed resilience times after salt perturbations indicated that the PI reactor recovered substantially and rapidly at all imposed salt levels. Principal coordinates analysis of 16S rRNA gene amplicon sequences revealed that bacterial community structures of the anammox sludge altered conspicuously in response to the salinity changes. However, quantitative PCR analysis showed that the shift in copy number of studied nitrogen-converting genes encoding hydrazine synthase (hzsA), bacterial and archaeal ammonia monooxygenases (amoA), nitrite oxidoreductase (nxrB), nitrite reductase (nirK), and nitrous oxide reductase (nosZ) was not significant (p > 0.05) in anammox sludge across the salt levels of 0.5%-4%, which suggests the stability of microbial community functioning in the osmoadaptation processes. The freshwater anammox Ca. Kuenenia showed high osmoadaptation by potentially adopting both high-salt-in and low-salt-in strategies to dominate in both reactors. The quantitative transcript analysis showed that the active anammox bacteria represented by hzsA transcripts in the SI reactor were approximately two orders of magnitude lower than those in the PI reactor during the long-term exposure to 4% salinity, manifesting the influence by the salt-increasing methods. These results provided new insight into osmo-adaptation of the anammox microbiome and will be useful for managing salinity effects on nitrogen removal processes.

Application of an enrichment culture of the marine anammox bacterium "Ca. Scalindua sp. AMX11" for nitrogen removal under moderate salinity and in the presence of organic carbon

Seawater can be directly used for toilet flushing in coastal areas to reduce our dependence on desalination and freshwater resources. The presence of high-salt content in the generated wastewater from seawater toilet flushing could limit the performance of conventional biological nitrogen removal processes. Anaerobic ammonium oxidation (anammox) process is regarded as one of the most energy-efficient process for nitrogen removal from N-rich waste streams. In this study, we demonstrated the application of a novel marine anammox bacterium (Candidatus Scalindua sp. AMX11) in a membrane bioreactor (MBR) to treat moderate-saline (∼1.2% salinity) and N-rich organic (2 mM acetate) solution, prepared using real seawater. The MBR showed stable performance with nitrogen removal rate of 0.3 kg-N m^-3 d^-1 at >90% N-removal efficiency. Furthermore, results of ¹⁵N stable isotope experiments revealed that anammox bacteria was mainly responsible for respiratory ammonification through NO₃^- reduction to NH₄⁺ via NO₂^-, and the by-products of respiratory ammonification were used as substrates by anammox bacteria. The dominant role of anammox bacteria in nitrogen removal under saline and organic conditions was further confirmed by genome-centric combined metagenomics and meta-transcriptomic approach. Taken together, these results highlight the potential application of marine anammox bacteria for treating saline wastewater generated from seawater toilet flushing practices.

Performance of Anammox Processes for Wastewater Treatment: A Critical Review on Effects of Operational Conditions and Environmental Stresses

The anaerobic ammonium oxidation (anammox) process is well-known as a low-energy consuming and eco-friendly technology for treating nitrogen-rich wastewater. Although the anammox reaction was widely investigated in terms of its application in many wastewater treatment processes, practical anammox application at the pilot and industrial scales is limited because nitrogen removal efficiency and anammox activity are dependent on many operational factors such as temperature, pH, dissolved oxygen concentration, nitrogen loading, and organic matter content. In practical application, anammox bacteria are possibly vulnerable to non-essential compounds such as sulfides, toxic metal elements, alcohols, phenols, and antibiotics that are potential inhibitors owing to the complexity of the wastewater stream. This review systematically summarizes up-to-date studies on the effect of various operational factors on nitrogen removal performance along with reactor type, mode of operation (batch or continuous), and cultured anammox bacterial species. The effect of potential anammox inhibition factors such as high nitrite concentration, high salinity, sulfides, toxic metal elements, and toxic organic compounds is listed with a thorough interpretation of the synergistic and antagonistic toxicity of these inhibitors. Finally, the strategy for optimization of anammox processes for wastewater treatment is suggested, and the importance of future studies on anammox applications is indicated.

Two-stage granular sludge partial nitritation/anammox process for the treatment of digestate from the anaerobic digestion of the organic fraction of municipal solid waste

The increasing amount of source separated organic fraction of municipal solid wastes (OFMSW) treated by anaerobic digestion for energy recovery requires the implementation of cost-efficient processes for the treatment of the produced digestate, especially in terms of nitrogen removal. The autotrophic nitrogen removal process, based on the coupling of two biological processes, partial nitritation (PN) and anammox (A), appears as a suitable solution due to important savings in operational costs compared to conventional treatment processes. However, its application could be hampered by the high salinity and inhibitory potential of this kind of digestate. In this contribution, two lab-scale granular sludge reactors performing the PN and anammox processes, respectively, were used to treat (opportunely diluted) real OFMSW digestate originating from full-scale biogas plants with the aim of assessing their treatment feasibility in a two-stage PN/A configuration. The PN process was implemented in an air-lift granular sludge reactor and was able to treat a nitrogen loading rate of about 1 g N L^-1 d^-1 at 30 ± 0.5 °C; moreover, its effluent was suitable for the subsequent anammox treatment, with an appropriate effluent NO₂^-/NH₄⁺ ratio and marginal inhibiting effects. In the anammox granular sludge reactor, the anammox activity was affected by high salinity levels, nonetheless a stable reactor performance at a nitrogen removing rate of 0.83 ± 0.20 and 0.31 ± 0.04 g N L^-1 d^-1 at 35 ± 0.5 °C, were achieved when treating 50% and 30% diluted real wastewaters at a conductivity in the reactor of 9.1 and 11.2 mS cm^-1, respectively.

Nitritation-anammox process - A realizable and satisfactory way to remove nitrogen from high saline wastewater

In this study, acclimation of freshwater nitritation-anammox sludge to remove nitrogen in high saline and hypersaline wastewater was evaluated, during which the microbes activity and microbial community revolution were revealed to understand the fate of a nitritation-anammox process (SNAP) in response to increasing salt stress. By enhanced aeration, the SNAP system can treat saline (3%) ammonium-rich (185 mg/L) wastewater after gradual adaption. Hypersalinity (5%) caused final deterioration of the SNAP system due to a severe inhibition on anammox activity. Genera Kuenenia (anammox), Nitrosomonas (AOB) and Nitrosovibrio (AOB) bacteria were salt adaptable microbes, while genus Nitrospira (NOB) bacteria were sensitive to salinity. Under the enhanced aeration, AOB bacteria could bear 3% salinity with possible enriched ammonia monooxygenase to stimulate the conversion of ammonium to nitrite by producing more intermediate-hydroxylamine, which could alleviate the negative effect of insufficient hydroxylamine oxidase members in AOB bacteria.

Is anammox a promising treatment process for nitrogen removal from nitrogen-rich saline wastewater?

Rapidly growing discharge of nitrogen-rich saline wastewater has significantly affect environment. However, due to the inhibition resulting from high salinity on microbes, it is still a challenge to treat nitrogen-rich saline wastewater efficiently. Anammox process, as a cost-effective and environment-friendly nitrogen removal approach, has shown a potential in treating nitrogen-rich saline wastewater. This review is conducted from a critical perspective and provides a comprehensive overview on the performance of anammox process treating nitrogen-rich saline wastewater. Two strategies including freshwater-derived anammox bacteria acclimatization and marine anammox bacteria enrichment are evaluated. Second, effects resulting from salinity on the performance of anammox reactor, the microbial communities and sludge characteristics are discussed. Third, salinity-tolerant mechanism of anammox bacteria is analyzed. This review also reveals some critical knowledge gaps and future research needs, which benefits application of anammox process to treat nitrogen-rich saline wastewater.

Influence of biomass acclimation on the performance of a partial nitritation-anammox reactor treating industrial saline effluents

The performance of the partial nitritation/anammox processes was evaluated for the treatment of fish canning effluents. A sequencing batch reactor (SBR) was fed with industrial wastewater, with variable salt and total ammonium nitrogen (TAN) concentrations in the range of 1.75-18.00 g-NaCl L^-1 and 112 - 267 mg-TAN L^-1. The SBR operation was divided into two experiments: (A) progressive increase of salt concentrations from 1.75 to 18.33 g-NaCl L^-1; (B) direct application of high salt concentration (18 g-NaCl L^-1). The progressive increase of NaCl concentration provoked the inhibition of the anammox biomass by up to 94% when 18 g-NaCl L^-1 were added. The stable operation of the processes was achieved after 154 days when the nitrogen removal rate was 0.021 ± 0.007 g N/L·d (corresponding to 30% of removal efficiency). To avoid the development of NOB activity at low salt concentrations and to stabilize the performance of the processes dissolved oxygen was supplied by intermittent aeration. A greater removal rate of 0.029 ± 0.017 g-N L^-1 d^-1 was obtained with direct exposure of the inoculum to 18 g-NaCl L^-1 in less than 40 days. Also, higher specific activities than those from the inoculum were achieved for salt concentrations of 15 and 20 g-NaCl L^-1 after 39 days of operation. This first study of the performance of the partial nitritation/anammox processes, to treat saline wastewaters, indicates that the acclimation period can be avoided to shorten the start-up period for industrial application purposes. Nevertheless, further experiments are needed in order to improve the efficiency of the processes.

Performance and microbial features of the partial nitritation-anammox process treating fish canning wastewater with variable salt concentrations

The partial nitritation-anammox (PN-AMX) process applied to wastewaters with high NaCl concentration was studied until now using simulated media, without considering the effect of organic matter concentration and the shift in microbial populations. This research work presents results on the application of this process to the treatment of saline industrial wastewater. Obtained results indicated that the PN-AMX process has the capability to recover its initial activity after a sudden/acute salt inhibition event (up to 16 g NaCl/L). With a progressive salt concentration increase for 150 days, the PN-AMX process was able to remove the 80% of the nitrogen at 7-9 g NaCl/L. The microbiological data indicated that NaCl and ammonia concentrations and temperature are important factors shaping PN-AMX communities. Thus, the NOB abundance (Nitrospira) decreases with the increase of the salt concentration, while heterotrophic denitrifiers are able to outcompete anammox after a peak of organic matter in the feeding.

Inhibition on anammox bacteria upon exposure to digestates from biogas plants treating the organic fraction of municipal solid waste and the role of conductivity

The aim of this research was to evaluate the applicability of the anammox process for removing nitrogen from the supernatant originating from the anaerobic digestion of the organic fraction of municipal solid waste (OFMSW). The short term inhibitory potential of this concentrated wastewater was evaluated by means of batch tests in terms of maximum specific anammox activity reduction. A total of 20 real wastewater samples were tested originating from 4 different full scale anaerobic digestion plants treating OFMSW. Activity reduction between 73% and 89% was observed in the presence of undiluted real wastewaters. The specific activity remained stable for 6-7days after the initial reduction, thus suggesting its treatability even without dilution. The inhibitory effect of both the real and synthetic saline media tested could be modelled as a function of conductivity. IC50 of 6.1mS/cm was obtained for exposure to the tested liquid fraction of biowaste digestate.

Achieving nitritation in a continuous moving bed biofilm reactor at different temperatures through ratio control

A ratio control strategy was implemented in a continuous moving bed biofilm reactor (MBBR) to investigate the response to different temperatures. The control strategy was designed to maintain a constant ratio between dissolved oxygen (DO) and total ammonia nitrogen (TAN) concentrations. The results revealed that a stable nitritation in a biofilm reactor could be achieved via ratio control, which compensated the negative influence of low temperatures by stronger oxygen-limiting conditions. Even with a temperature as low as 6°C, stable nitritation could be achieved when the controlling ratio did not exceed 0.17. Oxygen-limiting conditions in the biofilm reactor were determined by the DO/TAN concentrations ratio, instead of the mere DO concentration. This ratio control strategy allowed the achievement of stable nitritation without complete wash-out of NOB from the reactor. Through the ratio control strategy full nitritation of sidestream wastewater was allowed; however, for mainstream wastewater, only partial nitritation was recommended.

Single-stage PN/A technology treating saline ammonia-rich wastewater: finding the balance between efficient performance and less N2O and NO emissions

N₂O emission from a one-stage PN/A process was studied for the first time with elevated salinity.