A novel coupling process with partial nitritation-anammox and short-cut sulfur autotrophic denitrification in a single reactor for the treatment of high ammonium-containing wastewater

A novel coupling process to replace the traditional multi-stage anammox process-sulfur autotrophic denitrification coupled anammox system

A novel coupling process to replace the traditional multi-stage anammox process-sulfur autotrophic denitrification (SAD) coupled anaerobic ammonium oxidation (anammox) system was designed, which solved problems of nitrate produced in anammox process and low nitrate conversion rate caused by nitrite accumulation in SAD process. Different filter structures (SAD filter and anammox granular sludge) were investigated to further explore the excellent performance of the novel integrated reactor. The results of sequential batch experiments indicated that nitrite accumulation occurred during SAD, which inhibited the conversion of nitrate to dinitrogen gas. When SAD filter and anammox granular sludge were added to packed bed reactor simultaneously, the nitrate removal rate increased by 37.21% and effluent nitrite concentration decreased by 100% compared to that achieved using SAD. The stratified filter structure solved groove flow. Different proportion influence of SAD filter and anammox granular sludge on the stratified filter structure was evaluated. More suitable ratio of SAD filter to anammox granular sludge was 2:1. Proteobacteria (57.26%), Bacteroidetes (20.12%) and Chloroflexi (9.95%) were the main phyla. The dominant genera of denitrification functional bacteria were Thiobacillus (39.80%), Chlorobaculum (3.99%), norank_f_PHOs-HE36 (2.90%) and Ignavibacterium (2.64%). The dominant genus of anammox bacterium was Candidatus_Kuenenia (3.05%).

Influence of temperature on performance and mechanism of advanced synergistic nitrogen removal in lab-scale denitrifying filter with biogenic manganese oxides

Temperature is a significant operational parameter of denitrifying filter (DF), which affects the microbial activity and the pollutants removal efficiency. This study investigated the influence of temperature on performance of advanced synergistic nitrogen removal (ASNR) of partial-denitrification anammox (PDA) and denitrification, consuming the hydrolytic and oxidation products of refractory organics in the actual secondary effluent (SE) as carbon source. When the test water temperature (TWT) was around 25, 20, 15 and 10 °C, the filtered effluent total nitrogen (TN) was 1.47, 1.70, 2.79 and 5.52 mg/L with the removal rate of 93.38%, 92.25%, 87.33% and 74.87%, and the effluent CODcr was 8.12, 8.45, 10.86 and 12.29 mg/L with the removal rate of 72.41%, 66.17%, 57.35% and 51.87%, respectively. The contribution rate of PDA to TN removal was 60.44%∼66.48%, and 0.77-0.96 mg chemical oxygen demand (CODcr) was actually consumed to remove 1 mg TN. The identified functional bacteria, such as anammox bacteria, manganese oxidizing bacteria (MnOB), hydrolytic bacteria and denitrifying bacteria, demonstrated that TN was removed by the ASNR, and the variation of the functional bacteria along the DF layer revealed the mechanism of the TWT affecting the efficiency of the ASNR. This technique presented a strong adaptability to the variation of the TWT, therefore, it has broad application prospect and superlative application value in advanced nitrogen removal of municipal wastewater.

S0-driven partial denitrification coupled with anammox (S0PDA) enables highly efficient autotrophic nitrogen removal from wastewater

This study proposed a novel strategy that integrates S0 particles (diameter: 2-3 mm) and granular sludge to establish S0-driven partial denitrification coupled with anammox (S0PDA) process for autotrophic nitrogen removal from NH4+- and NO3--containing wastewaters. This process was evaluated using an up-flow anoxic sludge bed bioreactor, operating continuously for 240 days. The influent concentrations of NH4+ and NO3- were 29.9 ± 2.7 and 50.2 ± 2.7 mg-N/L, respectively. Throughout the operation, the hydraulic retention time was shortened from 4.0 h to 2.0 h, while the effluent concentrations of NH4+ and NO3- were maintained at a desirable level of 1.45-1.51 mg-N/L and 4.46-6.52 mg-N/L, respectively. Despite an autotrophic process, the nitrogen removal efficiency and rate reached up to 88.5 ± 2.0 % and 1.75 ± 0.07 kg-N/(m3·d), respectively, indicating the remarkable robustness of the S0PDA process. Autotrophic anammox and sulfur-oxidizing bacteria (Candidatus Brocadia and Thiobacillus) were the predominant bacterial genera involved in the S0PDA process. Candidatus Brocadia was primarily enriched in the granular sludge, with a relative abundance of 6.70 %. Thiobacillus occupied a unique niche on the S0 particles, with a relative abundance as high as 57.6 %, of which Thiobacillus thioparus with partial denitrification function (reducing NO3- to NO2- without further reduction to N2) accounted for 78.0 %. These findings challenge the stereotype of low efficiency in autotrophic nitrogen removal from wastewater, shedding fresh light on the applications of autotrophic processes.

Temperature-resilient superior performances by coupling partial nitritation/anammox and iron-based denitrification with granular formation

Partial nitritation-anammox (PN/A), an energy-neutral process, is widely employed in the treatment of nitrogen-rich wastewater. However, the intrinsic nitrate accumulation limits the total nitrogen (TN) removal, and the practical application of PN/A continues to face a significant challenge at low temperatures (<15 °C). Here, an integrated partial nitritation-anammox and iron-based denitrification (PNAID) system was developed to address the concern. Two up-flow bioreactors were set up and operated for 400 days, with one as the control group and the other as the experiment group with the addition of Fe0. In comparison to the control group, the experiment group with the Fe0 supplement showed better nitrogen removal during the entire course of the experiment at different temperature levels. Specifically, the TN removal efficiency of the control group decreased from 82.9 % to 53.9 % when the temperature decreased from 30 to 12 °C, while in stark contrast, the experiment group consistently achieved 80 % of TN removal in the same condition. Apart from the enhanced nitrogen removal, the experiment group also exhibited better phosphorus removal (10.6 % versus 74.1 %) and organics removal (49.5 % versus 65.1 %). The enhanced and resilient nutrient removal performance of the proposed integrated process under low temperatures appeared to be attributed to the compact structure of granules and the increased microbial metabolism with Fe0 supplement, elucidated by a comprehensive analysis including microbial-specific activity, apparent activation energy, characteristics of granular sludge, and metagenomic sequencing. These results clearly confirmed that Fe0 supplement not only improved nitrogen removal of PN/A process, but also conferred a certain degree of robustness to the system in the face of temperature fluctuations.

The evolution of nitrogen transformation microorganism consortium under continued manganese domestication conditions

Manganese redox-stimulated bioremediation of nitrogen wastewater is receiving increasing attention. However, the nitrogen metabolic capacity and community evolution during manganese-mediated nitrogen transformation process under continued manganese domestication conditions are ambiguous. In this study, nitrogen- metabolizing microbial consortiums were incubated with synthesized Mn-humic acid complex (Mn-HA) for one month (M1), three months (M2) and six months (M3), respectively. During the Mn-HA incubation period, Bio-MnOx accompanying with bacterial consortiums (MnOB consortiums) with high TIN removal capacities were obtained. The TIN removal rates in M1, M2 and M3 were 0.220, 1.246 and 4.237 mg·L-1·h-1, respectively, which were 15.961, 90.006 and 1550.006 times higher than CK (Control Check group, no Mn-HA added group) (0.014 mg·L-1·h-1), respectively. Functional genes (amoA, AMX and narG) were most abundant in M3, which was associated with the highest nitrogen removal rate in M3. MnOB1 (bacterial consortium in M1), including Geobactor, Geothrix, Anaeromyxobacter and Bacillus, may be responsible for the Mnammox-NDMO (MnOx reduction coupled to ammonium oxidation - nitrate/nitrite-dependent low-valent Mn oxidation) process. MnOB3 (bacterial consortium in M2) enriched nitrifying bacteria Ellin6067, and denitrifying bacteria Denitratisoma, which dominated nitrogen transformation. MnOB6 (bacterial consortium in M3) enriched denitrifiers Denitratisoma, nitrifiers Ellin6067 and potential anammox bacteria SM1A02, Candidatus_Brocadia. Combined with the reduced abundance of Nitrospirae, a short-cut partial nitrification and denitrification (PND) or partial nitrification, denitrification and anammox (PNDA) could occurred in M2 and M3. It is suggested that community may have evolved into an energetically efficient short-cut nitrification, denitrification and anammox consortium to replace the full-range nitrification and denitrification community in M1 and CK under the continued manganese domestication conditions. Enhanced metabolic pathways of hydroxylamine oxidation and the nitric oxide reduction may confirm that PND or PNDA occurred in M2 and M3.

Anaerobic starvation realizes partial nitrification and starts anammox bacteria self-enrichment in mainstream municipal sewage treatment in a low filling ratio sequencing batch reactor

The initiating and stable preservation of partial nitrification (PN) and achievement of anammox bacteria self-enrichment in domestic sewage is a purposeful subject. In this article, an originality tactics of anaerobic starvation for 100 days was adopted for rapidly achieving PN in actual wastewater, the nitrite accumulation rate (NAR) improved from 4.95% to 81.73% in 18 days. After anaerobic starvation was stopped, the stable PN effect furnished enough stroma for the growth of anammox bacteria. The abundance of Candidatus Brocadia grew from 0% to 0.42% in floc sludge and 0.43% in blank biofilm, which promoted nitrogen removal effect. Anaerobic starvation continuing 74 days generated further decrease in the abundance of Nitrobacter and Nitrospira of nitrite-oxidizing bacteria (NOB), indicating that anaerobic starvation can restore the destroyed partial nitrification. In conclusion, this article furnished a low-cost method for achieving anammox bacteria self-enrichment in mainstream municipal wastewater in 10% filling ratio without chemicals addition.

Sulfur autotrophic denitrification as an efficient nitrogen removals method for wastewater treatment towards lower organic requirement: A review

The sulfur autotrophic denitrification (SADN) process is an organic-free denitrification process that utilizes reduced inorganic sulfur compounds (RISCs) as the electron donor for nitrate reduction. It has been proven to be a cost-effective and environment-friendly approach to achieving carbon neutrality in wastewater treatment plants. However, there is no consensus on whether SADN can become a dominant denitrification process to treat domestic wastewater or industrial wastewater if organic carbon is desired to be saved. Through a comprehensive summary of the SADN process and extensive discussion of state-of-the-art SADN-based technologies, this review provides a systematic overview of the potential of the SADN process as a sustainable alternative for the heterotrophic denitrification (HD) process (organic carbons as electron donor). First, we introduce the mechanism of the SADN process that is different from the HD process, including its transformation pathways based on different RISCs as well as functional bacteria and key enzymes. The SADN process has unique theoretical advantages (e.g., economy and carbon-free, less greenhouse gas emissions, and a great potential for coupling with novel autotrophic processes), even if there are still some potential issues (e.g., S intermediates undesired production, and relatively slow growth rate of sulfur-oxidizing bacteria [SOB]) for wastewater treatment. Then we present the current representative SADN-based technologies, and propose the outlooks for future research in regards to SADN process, including implement of coupling of SADN with other nitrogen removal processes (e.g., HD, and sulfate-dependent anaerobic ammonium oxidation), and formation of SOB-enriched biofilm. This review will provide guidance for the future applications of the SADN process to ensure a robust-performance and chemical-saving denitrification for wastewater treatment.

Multi-chambers of pilot-scale reactor enhanced partial nitritation performance

Nowadays, applying anammox to treat high nitrogenous side-stream wastewater has taken a step forward. However, the partial nitritation process is sensitive to the ammonium concentration and the nitrogen loading rate, which significantly influences the nitrogen removal performance. This study investigated the performance of a novel nitritation pilot-scale reactor which was divided into four chambers. The nitrite accumulation efficiency reached more than 90 % in the rural wastewater treatment process. As the reactor was divided into four chambers, the comprehensive statistical results showed that the concentration of free ammonium in the front chambers had been effectively improved. The proportion of free ammonium concentration (>0.1 mg NH₃·L^-1), which could inhibit the activity of nitrite oxidizing bacteria, in first chamber (PN1) was 2 times higher than in the last chamber (PN4). Meanwhile, Nitrosomonas, responsible for ammonium oxidation to nitrite, was highly enriched in the first two chambers even though the dissolved oxygen was maintained at 1.5 ± 0.3 mg·L^-1. Compare to conventional reactor, the resistance of the novel reactor to volumetric shock loading has been enhanced. Even though the ammonium loading rate fluctuated greatly, the effluent was still stable and could meet the demand following the anammox process. This study demonstrated that the reactor with multi-chambers could effectively improve the nitrite accumulation efficiency in the partial nitritation process and thus provide a new perspective on the partial nitritation process in a single reactor and further promote the anammox performance in the wastewater treatment process.

Integration of the sulfate reduction and anammox processes for enhancing sustainable nitrogen removal in granular sludge reactors

The Anammox and Sulfate Reduction Ammonium Oxidation processes were compared in two granular sequencing batch reactors operated for 160 days under anammox conditions. It was hypothesized that increasing the concentration of SO₄^2- may positively influence the rate of N removal under anaerobic conditions and it was tested whether SO₄^2- reduction and anammox occur independently or are related to each other. The cooperation of N-S cycles by increasing the concentration of influent SO₄^2- to 952 mg S/L in the second reactor, a higher ammonium utilization rate and sulfate utilization rate was achieved compared to the first reactor, i.e., 2.1-fold and 15-fold, respectively. Nitrosomonas played the dominant role in the N metabolism, while Thauera - in the S metabolism. This study highlights the benefits of linking the N-S cycles as an effective approach for the treatment of NH₄⁺ and SO₄^2- - rich wastewater, including lower substrate removal cost and reduced energy consumption.

Advanced synergetic nitrogen removal of municipal wastewater using oxidation products of refractory organic matters in secondary effluent by biogenic manganese oxides as carbon source

Due to the high operational cost and secondary pollution of the conventional advanced nitrogen removal of municipal wastewater, a novel concept and technique of advanced synergetic nitrogen removal of partial-denitrification anammox and denitrification was proposed, which used the oxidation products of refractory organic matters in the secondary effluent of municipal wastewater treatment plant (MWWTP) by biogenic manganese oxides (BMOs) as carbon source. When the influent NH₄⁺-N in the denitrifying filter was about 1.0, 2.0, 3.0, 4.0, 5.0 and 7.0 mg/L, total nitrogen (TN) in the effluent decreased from about 22 mg/L to 11.00, 7.85, 6.85, 5.20, 4.15 and 2.09 mg/L, and the corresponding removal rate was 49.15, 64.82, 69.40, 76.70, 81.36 and 90.58%, respectively. The proportional contribution of the partial-denitrification anammox pathway to the TN removal was 12.00, 26.45, 39.70, 46.04, 54.97 and 64.01%, and the actual COD_cr consumption of removing 1 mg TN was 0.75, 1.43, 1.26, 1.17, 1.08 and 0.99 mg, respectively, which was much lower than the theoretical COD_cr consumption of denitrification. Furthermore, COD_cr in the effluent decreased to 8.12 mg/L with a removal rate of 72.40%, and the removed organic matters were mainly non-fluorescent organic matters. Kinds of denitrifying bacteria, anammox bacteria, hydrolytic bacteria and manganese oxidizing bacteria (MnOB) were identified in the denitrifying filter, which demonstrated that the advanced synergetic nitrogen removal was achieved. This novel technology presented the advantages of high efficiency of TN and COD_cr removal, low operational cost and no secondary pollution.

Deciphering the role of granular activated carbon (GAC) in anammox: Effects on microbial succession and communication

Anaerobic ammonium oxidation (anammox) offered an energy-efficient option for nitrogen removal from wastewater. Granular activated carbon (GAC) addition has been reported that improved biomass immobilization, but the role of GAC in anammox reactors has not been sufficiently revealed. In this study, it was observed that GAC addition in an upflow anaerobic sludge blanket (UASB) reactor led to the significantly shortened anammox enrichment time (shortened by 45 days) than the reactor without GAC addition. The nitrogen removal rate was 0.83 kg N/m³/day versus 0.76 kg N/m³/day in GAC and non-GAC reactors, respectively after 255 days' operation. Acyl-homoserine lactone (AHL) quorum sensing signal molecule C8-HSL had comparable concentrations in both anammox reactors, whereas the signal molecule C12-HSL was more pervasive in the reactor containing GAC than the reactor without GAC. Microbial analysis revealed distinct anammox development in both reactors, with Candidatus Brocadia predominant in the reactor that did not contain GAC, and Candidatus Kuenenia predominant in the reactor that contained GAC. Denitrification bacteria likely supported anammox metabolism in both reactors. The analyses of microbial functions suggested that AHL-dependent quorum sensing was enhanced with the addition of GAC, and that GAC possibly augmented the extracellular electron transfer (EET)-dependent anammox reaction.

Establishment of anammox coupled with sulfide-depending autotrophic denitrification process and its efficient pollutants removal performance

Nitrogen and sulfur pollutants coexist in many industrial wastewaters, which may cause serious water pollution issues. In this study, Anammox coupled with sulfide-depending autotrophic denitrification process (coupling process) was established by adding sulfide to an Anammox system in a membrane bioreactor. Variations in nitrogen and sulfur removal performance, extracellular polymeric substances (EPS), key enzyme activities, and microbial components were analyzed. The sulfide in 25.0 mg L^-1 successfully induced denitrification, and then helped establish the coupling process. This process achieved 96.1% TN removal and complete sulfide removal when the sulfide was increased to 100.0 mg L^-1. The protein and polysaccharide in EPS gradually increased to 2.0 and 4.9 mg g^-1 SS, respectively. The hydroxylamine oxidoreductase activity, Heme-c content, nitrite reductase activity, and nitrate reductase activity slightly decreased to 19.1 EU g^-1 SS, 0.001 mmol g^-1 SS, 0.002 μg min^-1 mg^-1 protein, and 0.005 μg min^-1 mg^-1 protein, respectively, indicating the slight suppression of sulfide in high concentration on the coupling process. However, after acclimatization, the Anammox and denitrifying bacteria interacted and cooperatively contributed to the simultaneous nitrogen and sulfur removal, with relative abundances of Thiobacillus-denitrifying bacteria and Candidatus Kuenenia-Anammox bacteria of 31.7% and 9.0%, respectively. The establishing strategy was proposed and then verified in another Anammox system, in which the coupling process was also established, with TN removal increasing from 73.4% to 82.5%.

Coupling sulfur-based denitrification with anammox for effective and stable nitrogen removal: A review

Anoxic ammonium oxidation (anammox) is an energy-efficient nitrogen removal process for wastewater treatment. However, the unstable nitrite supply and residual nitrate in the anammox process have limited its wide application. Recent studies have proven coupling of sulfur-based denitrification with anammox (SDA) can achieve an effective nitrogen removal, owing to stable provision of substrate nitrite from the sulfur-based denitrification, thus making its process control more efficient in comparison with that of partial nitrification and anammox process. Meanwhile, the anammox-produced nitrate can be eliminated through sulfur-based denitrification, thereby enhancing SDA's overall nitrogen removal efficiency. Nonetheless, this process is governed by a complex microbial system that involves both complicated sulfur and nitrogen metabolisms as well as multiple interactions among sulfur-oxidising bacteria and anammox bacteria. A comprehensive understanding of the principles of the SDA process is the key to facilitating the development and application of this novel process. Hence, this review is conducted to systematically summarise various findings on the SDA process, including its associated biochemistry, biokinetic reactions, reactor performance, and application. The dominant functional bacteria and microbial interactions in the SDA process are further discussed. Finally, the advantages, challenges, and future research perspectives of SDA are outlined. Overall, this work gives an in-depth insight into the coupling mechanism of SDA and its potential application in biological nitrogen removal.

A novel sulfur-driven autotrophic denitrification coupled with bio-cathode system for bioelectricity generation and groundwater remediation

This study explored the feasibility of treating wastewater using sulfur-driven autotrophic denitrification (SAD) coupled with the bio-cathode of microbial fuel cell (MFC), focusing on simultaneous bioelectricity generation, denitrification, and desulphurization. A maximum output voltage of 360 mV was obtained with a power generation cycle of 25 h when simulated wastewater with 100.0 mg/L of each NO₃^--N and S^2--S was employed as the influent in the SAD-BMFC. Compared with solo SAD or MFC, SAD-BMFC obtained a higher NO₃^--N removal rate (E_{12 h} = 87.7%, E_{24 h} = 100%), and less NO₂^--N accumulation. S^2--S of the influent was almost completely removed, oxidized to S⁰-S (88.6-90.2 mg/L) and SO₄^2--S (9.8-11.4 mg/L). The reaction system achieved self-balance of acidity-alkalinity (pH 7.05-7.35). The SAD process was the main pathway for NO₃^--N removal (80.2%) and a smaller proportion of electrons came from the bio-cathode. This study effectively combined SAD with a bio-cathode system for simultaneous energy harvest and bio-enhanced remediation of groundwater contaminated by both NO₃^--N and S^2--S.

Denitrification Performance in Packed-Bed Reactors Using Novel Carbon-Sulfur-Based Composite Filters for Treatment of Synthetic Wastewater and Anaerobic Ammonia Oxidation Effluent

To avoid nitrate pollution in water bodies, two low-cost and abundant natural organic carbon sources were added to make up the solid-phase denitrification filters. This study compared four novel solid-phase carbon-sulfur-based composite filters, and their denitrification abilities were investigated in laboratory-scale bioreactors. The filter F₄ (mixture of elemental sulfur powder, shell powder, and peanut hull powder with a mass ratio of 6:2.5:1.5) achieved the highest denitrification ability, with an optimal nitrate removal rate (NRR) of 723 ± 14.2 mg NO₃^–-N⋅L^–1⋅d^–1 when the hydraulic retention time (HRT) was 1 h. The HRT considerably impacted effluent quality after coupling of anaerobic ammonium oxidation (ANAMMOX) and solid-phase-based mixotrophic denitrification process (SMDP). The concentration of suspended solids (SS) of the ANAMMOX effluent may affect the performance of the coupled system. Autotrophs and heterotrophs were abundant and co-existed in all reactors; over time, the abundance of heterotrophs decreased while that of autotrophs increased. Overall, the SMDP process showed good denitrification performance and reduced the sulfate productivity in effluent compared to the sulfur-based autotrophic denitrification (SAD) process.

Effect of S2O32--S addition on Anammox coupling sulfur autotrophic denitrification and mechanism analysis using N and O dual isotope effects

Anaerobic ammonia oxidation (Anammox) coupling sulfur autotrophic denitrification is an effective method for the advanced nitrogen removal from the wastewater with limited carbon source. The influence of S₂O₃^2--S addition on Anammox coupling sulfur autotrophic denitrification was investigated by adding different concentrations of S₂O₃^2--S (0, 39, 78, 156 and 312 mg/L) to the Anammox system. The contribution of sulfur autotrophic denitrification and Anammox to nitrogen removal at S₂O₃^2--S concentrations of 156 mg/L was 75% ∼83% and 17%∼25%, respectively, and the mixed system achieved completely nitrogen removal. However, Anammox bioactivity was completely inhibited at S₂O₃^2--S concentrations up to 312 mg/L, and only sulfur autotrophic denitrification occurred. The isotopic effects of NO₂^--N (δ¹⁵N_NO2 and δ¹⁸O_NO2) and NO₃^--N (δ¹⁵N_NO3 and δ¹⁸O_NO3) during Anammox coupling sulfur autotrophic denitrification showed a gradual decrease trend with the increase of S₂O₃^2--S addition. The ratios of δ¹⁵N_NO2:δ¹⁸O_NO2 and δ¹⁵N_NO3:δ¹⁸O_NO3 was maintained at 1.30-2.41 and 1.36-2.52, respectively, which revealed that Anammox was dominant nitrogen removal pathway at S₂O₃^2--S concentrations less than 156 mg/L. Microbial diversity gradually decreased with the increase of S₂O₃^2--S. The S₂O₃^2--S addition enhanced the S₂O₃^2--driven autotrophic denitrification and weakened the Anammox, leading to a gradually decreasing trend of the proportion of Candidatus Brocadia as Anammox bacteria from the initial 27% to 4% (S₂O₃^2--S of 156 mg/L). Yet Norank-f-Hydrogenophilaceae (more than 50%) and Thiobacillus (54%) as functional bacteria of autotrophic denitrification obviously increased. The appropriate amount of S₂O₃^2--S addition promoted the performance of Anammox coupling sulfur autotrophic denitrification achieved completely nitrogen removal.

Denitrification performance of sulfur-based autotrophic denitrification and biomass‑sulfur-based mixotrophic denitrification in solid-phase denitrifying reactors using novel composite filters

Both the elemental sulfur-based autotrophic denitrification (ESAD) and the biomass‑sulfur-based mixotrophic (simultaneous autotrophic and heterotrophic) denitrification processes (BSMD) are efficient methods for removing nitrate from wastewater. However, a comparative analysis of the denitrification capacity of the BSMD and ESAD in the packed bed reactors is still lacking. In this paper, corncob powder was selected as the biomass source to prepare biomass‑sulfur-based composite filter (BSCF) for the BSMD process. The denitrification performances of the three identical lab-scale bioreactors packed with varying elemental sulfur-based composite filters (ESCFs) were compared under varying loading conditions, and the optimal ESCF of the ESAD system was 2:1 by weight ratio of sulfur powder to shell powder. In pilot-scale experiments, the results showed that BSCF could decrease the sulfate productivity and gave better denitrification performance than the ESCF with the optimal nitrate removal rate (NRR) of 504 ± 12.3 mg NO₃^--N·L^-1·d^-1. In addition, the two-stage flushing strategy (for the removal of aged sludge) can effectively improve the denitrification capacity, while the denitrification will be inhibited when the influent dissolved oxygen concentration was higher than 4.5 mg L^-1. Moreover, the heterotrophs and autotrophs were abundant in the reactors. Over time, the abundance of autotrophs increased while that of heterotrophs decreased. Overall, BSCF could be a promising and economic technology to improve the effluent quality.

Biochar enhances partial denitrification/anammox by sustaining high rates of nitrate to nitrite reduction

To examine the short-term effects of biochar on the partial-denitrification anammox (PD/A) process, the adsorption kinetics, nitrogen degradation, electron transfer properties, and microbial community succession of wastewater treatment systems with and without biochar added were monitored and characterized. The results showed that biochar increased nitrate reduction rates, which enhanced total nitrogen (TN) removal of the system by about 10%. The findings attributed improved TN removal to biochar's influence in accelerating electron transfer rather than its adsorption properties. Analysis of the nitrogen transfer pathway showed that when sufficient substrate was available, the anammox and denitrification reactions simultaneously removed nitrogen. When the nitrite supply was insufficient, the anammox reaction outcompeted the denitrification reaction for regenerated nitrite. Integrated microbial community and functional protein analyses indicated that biochar addition increased the abundance of Ca. Kuenenia and Pseudomonas. Meanwhile, biochar modulates denitrifying cellular metabolism by inducing protein changes.

Modeling of sulfur-driven autotrophic denitrification coupled with Anammox process

While sulfur-driven autotrophic denitrification (SDAD) occurring in the anoxic reactor of the sulfate reduction, autotrophic denitrification and nitrification integrated (SANI) system has been regarded as the main nitrogen removal bioprocess, little is known about the accompanying Anammox bacteria whose presence is made possible by the co-existence of NH₄⁺ and NO₂^-. Therefore, this work firstly developed an integrated SDAD-Anammox model to describe the interactions between sulfur-oxidizing bacteria and Anammox bacteria. The model was subsequently used to explore the impacts of influent conditions on the reactor performance and microbial community structure of the anoxic reactor. The results revealed that at a relatively low ratio of <1.5 mg S/mg N, Anammox bacteria could survive and even take a dominant position (up to 58.9%). Finally, a modified SANI system configuration based on the effective collaboration between SDAD and Anammox processes was proposed to improve the efficiency of the treatment of sulfate-rich saline sewage.

Comparison of Different Carriers to Maintain a Stable Partial Nitrification Process for Low-Strength Wastewater Treatment

Practical application of the partial nitritation–anaerobic ammonium oxidation (anammox) process has attracted increasing attention because of its low operational costs. However, the nitritation process, as a promising way to supply nitrite for anammox, is sensitive to the variations in substrate concentration and dissolved oxygen (DO) concentration. Therefore, a stable supply of nitrite becomes a real bottleneck in partial nitritation–anammox process, limiting their potential for application in mainstream wastewater treatment. In this study, five 18-L sequencing batch reactors were operated in parallel at room temperature (22°C ± 4°C) to explore the nitritation performance with different carrier materials, including sepiolite-nonwoven carrier (R1), zeolite-nonwoven carrier (R2), brucite-nonwoven carrier (R3), polyurethane carrier (R4), and nonwoven carrier (R5). The ammonia oxidation rate (AOR) in R1 reached the highest level of 0.174 g-N L−1 d−1 in phase II, which was 1.4-fold higher than the control reactor (R4). To guarantee a stable supply of nitrite for anammox process, the nitrite accumulation efficiency (NAE) was always higher than 77%, even though the free ammonia (FA) decreases to 0.08 mg-N/L, and the pH decreases to 6.8 ± 0.3. In phase V, the AOR in R1 reached 0.206 g-N L−1 d−1 after the DO content increase from 0.7 ± 0.3 mg/L to 1.7 ± 0.3 mg/L. The NAE in R1 was consistently higher than 68.6%, which was much higher than the other reactor systems (R2: 43.8%, R3: 46.6%, R4: 23.7%, R5: 22.7%). Analysis of 16S rRNA gene sequencing revealed that the relative abundance of Nitrobacter and Nitrospira in R1 was significantly lower than other reactors, indicating that the sepiolite carrier plays an important role in the inhibition of nitrite-oxidizing bacteria. These results indicate that the sepiolite nonwoven composite carrier can effectively improve the nitritation process, which is highly beneficial for the application of partial nitritation–anammox for mainstream wastewater treatment.

Transforming heterotrophic to autotrophic denitrification process: Insights into microbial community, interspecific interaction and nitrogen metabolism

For investigating the microbial community, interspecific interaction and nitrogen metabolism during the transform process from heterotrophic to synergistic and autotrophic denitrification, a filter was built, and carbon source and sulfur concentration were changed to release the transformation process. The results demonstrated that the transformation process was feasible to keep nitrate nitrogen (NO₃^--N) discharge concentration lower than 15 mg L^-1, however, nitrite nitrogen (NO₂^--N) accumulation and its rate reached 7.85% at initial stages. The dominant denitrification gunes were Methylophilaceae, Thiovulaceae and Hydrogenophilaceae for three processes, respectively, and the microbial interspecific interaction of heterotrophic denitrification was more complex than others. NO₂^--N accumulation was confirmed by the low abundance of EC1.7.7.1 and EC1.7.2.1, and the dominance degree of dark oxidation of sulfur compounds and dark sulfide oxidation improved in synthesis and autotrophic denitrifications.

Efficient nitrite accumulation and elemental sulfur recovery in partial sulfide autotrophic denitrification system: Insights of seeding sludge, S/N ratio and flocculation strategy

Partial sulfide autotrophic denitrification (PSAD) has been proposed as a promising process to achieve elemental sulfur recovery and nitrite accumulation, which is required for anaerobic ammonium oxidation reaction. This study investigated the effect of seeding sludge on the start-up performance of PSAD process, with different sludge taken from the oxidation zone (S-o) of wastewater treatment plants, partial denitrification reactor (S-_PD), and anoxic/oxic reactor (S-_A/O). The results showed that the PSAD process could be achieved rapidly in three systems on day 22, 29 and 26, respectively. In particular, the S-_O system completed the start-up in the shortest time of 22 d, with NO₃^--N and S^2- removal efficiency of 85.3% and 99.3%, respectively. Selected the S-_O system to operate long term, the nitrite (NO₂^--N) and biological elemental sulfur (S⁰) accumulation efficiencies were systematically investigated under different S/N ratios (in a range of 0.71-1.2). The maximum NO₂^--N and S⁰ accumulation efficiencies were 85.2% and 73.5%, respectively, at the S/N ratio of 1.1. In addition, the separation and recovery of S⁰ in effluent was achieved by employing polyaluminum chloride (PAC) as a flocculant. Using 2D Gaussian function as quadratic model for the maximizing of S⁰ flocculant efficiency (SFR), an optimal condition of PAC dosage 7.92 mL/L and pH 5.14 was obtained, and the SFR reached 94.1%, under such conditions. The findings offered useful information to facilitate the application of the PSAD process.

Exploration and verification of the feasibility of the sulfur-based autotrophic denitrification integrated biomass-based heterotrophic denitrification systems for wastewater treatment: From feasibility to application

The sulfur-based autotrophic denitrification (SAD) and the solid organic carbon-based denitrification processes are both efficient techniques to remove nitrate from wastewater, and the hydrogen ions generated by the SAD process would be consumed in the heterotrophic denitrification process. Therefore, it is possible to improve the denitrification capacity when the solid organic carbon was added into a SAD reactor. In this study, corncob powder and sawdust powder were selected as solid organic carbon sources, and the sulfur-based autotrophic denitrification integrated biomass-based heterotrophic denitrification system was formed (SBD). The laboratory and field experiments showed that SBD could shorten the start-up period, decrease the sulfate productivity, and maintain a good denitrification performance when treated wastewater. According to the field experiment results, when the HRT was 1 h, the effluent total nitrogen (TN) concentration was always lower than 15 mg L^-1. In addition, nitrite inhibition was observed when the concentration of nitrite in the reactors reached above 30 mg L^-1. The mixture of elemental sulfur powder, shell powder, corncob powder, and sawdust powder with a mass ratio of 6:2:1:1 was the optimal filter for the SBD system, with an average nitrate reduction rate (NAR) of 420 mg NO₃^-N·L^-1·d^-1 obtained at the end of the study. During the whole operation, the major autotrophs in the SBD systems were Thermomonas, Ferritrophicum, and Thiobacillus, while the major heterotrophs were Saprospiraceae, Ferruginibacter, Dokdonella, and Simplicispira. Overall, the SBD system was a feasible and practically favorable way to remove nitrate from wastewater.

Rapidly achieving and optimizing simultaneous partial nitrification denitrification and anammox integrated process by hydroxylamine addition for advanced nitrogen removal from domestic wastewater

The achievement and stable maintenance of partial nitrification and partial anammox process for municipal sewage is a challenging research topic at present. In this study, a novel strategy of hydroxylamine (NH₂OH) addition under low DO condition was adopted for rapidly achieving simultaneous partial nitrification denitrification and anammox process (SPNDA) to deal with domestic wastewater, the nitrite accumulation ratio (NAR) increased from 1% to 82% in the first 4 days. After the addition of NH₂OH was stopped, the PN effect of SPNDA process remained relatively stable within 100 days. During the stable operation period with aerobic HRT of 5 h, the nitrogen removal efficiency was 87.9 ± 4.2%. Moreover, the abundance of denitrifying bacteria and Candidatus Brocadia increased from 1.79% and 0.062% to 22.49% and 0.38% respectively, which promoted nitrogen removal effect. Overall, this study provided a quickly way for achieving the cost-effective SPNDA process to enhance nitrogen removal with NH₂OH addition.

Insights into complete nitrate removal in one-stage nitritation-anammox by coupling heterotrophic denitrification

Nitritation-anammox has been considered to be the most promising process for nitrogen (N) removal from wastewater. However, the anammox reaction still produces an amount of nitrate, which cannot be removed further. This study hypothesizes that heterotrophic denitrification can be an appealing option to remove the residual nitrate in the one-stage nitritation-anammox process. Through monitoring N-removal performance and microbial community succession of a laboratory microaerobic reactor, the effect of four different levels of oxygen supply on nitrate removal was investigated. The reactor was continuously fed with real manure-free piggery wastewater containing ~240 mg NH₄⁺-N/L and chemical oxygen demand (COD)/total nitrogen (TN) ratio of less than 1 for 180 days. With a high influent loading rate of 0.7 kg N/(m³·d), efficient total nitrogen removal (>80 %) was achieved during stable operation of dissolved oxygen (DO) concentrations between 0.3 and 0.6 mg O₂/L, indicating N-removal via the nitritation-anammox pathway in the low-carbon wastewater treatment. At the same time, the effluent nitrate reduced with decreased oxygen supply and completely depleted at DO of 0.3 ± 0.1 mg O₂/L. In addition to oxygen, preventing ammonia nitrogen from falling to very low levels (<10 mg/L) could be also useful for the complete nitrate removal and stable nitritation-anammox. 16S rRNA gene-based analyses confirmed a complex microbial community including nitrifiers, denitrifiers and anammox bacteria in the biomass of the reactor. Collectively, this study provides new insights into high-level N-removal of a nitritation-anammox process by complete nitrate depletion.

Simultaneous ammonium and sulfate biotransformation driven by aeration: Nitrogen/sulfur metabolism and metagenome-based microbial ecology

The present study aimed to clarify the effect of oxygen respiration on biotransformation of alternative electron acceptors (e.g., nitrate and sulfate) underlying the simultaneous removal of ammonium and sulfate in a single aerated sequencing batch reactor. Complete nitrification was achieved in feast condition, while denitrification was carried out in both feast and famine conditions when aeration intensity (AI) was higher than 0.22 L/(L·min). Reactors R1 [0.56 L/(L·min)], R2 [0.22 L/(L·min)], and R3 [0.08 L/(L·min)] achieved 72.39% sulfate removal efficiency in feast condition, but H₂S release occurred in R3. Following exogenous substrate depletion, sulfate concentration increased again and exceeded the influent value in R1, indicating that sulfate transformation was affected by oxygen intrusion. Metagenomic analysis showed that a higher AI promoted sulfate reduction by switching from dissimilatory to assimilatory pathway. Lower AI-acclimated microorganisms (R3) produced H₂S and ammonium, while higher AI-acclimated microorganisms (R1) accumulated nitrite, which confirmed that biotransformation of N and S was strongly regulated by redox imbalance driven by aeration. This implied that respiration control, a microbial self-regulation mechanism, was linked to the dynamic imbalance between electron donors and electron acceptors. Aerobic nitrate (sulfate) reduction, as one of the effects of respiration control, could be used as an alternative strategy to compensate for dynamic imbalance, when supported by efficient endogenous metabolism. Moderate aeration induced microorganisms to change their energy conservation and survival strategy through respiration control and inter-genus protection of respiratory activity among keystone taxa (including Azoarcus in R1, Thauera in R2, and Thiobacillus, Ottowia, and Geoalkalibacter in R3) to form an optimal niche in response to oxygen intrusion and achieve benign biotransformation of C, N, and S without toxic intermediate accumulation. This study clarified the biotransformation mechanism of ammonium and sulfate driven by aeration and provided theoretical guidance for optimizing existing aeration-based techniques.

Coupling oxidation of acid volatile sulfide, ferrous iron, and ammonia nitrogen from black-odorous sediment via autotrophic denitrification-anammox by nitrate addition

The coupling removal of acid volatile sulfide (AVS), ferrous iron, and ammonia nitrogen has been applied for black-odorous sediment remediation. In this study, calcium nitrate with different N/(S + Fe) ratios (0.45, 0.90, 1.20 and 1.80) was added into black-odorous sediment in four systems named R1, R2, R3, and R4. Results showed that the removal rate of AVS was 76.40% in the R1, which was lower compared with rates in R2-R4 around 96.70%. The ferrous oxidation rate was approximately 87.00% in R2-R4, which was considerably higher than that in the R1 (24.62%). And the ammonia was reduced by 81.02%, 88.00%, 100%, and 57.18% in R1, R2, R3 and R4, respectively. During the reaction, nitrite accumulation was observed, indicating partial denitrification. Moreover, microbes related to autotrophic denitrification (e.g., genus Thiobacillus, Dok59, GOUTA19, Gallionella, with the highest abundance of 15.40%, 13.21%, 8.79%, 9.44%, respectively) were detected in all systems. Furthermore, the anammox bacteria Candidatus_Brocadia with the highest abundance of 3.44% and 4.00% in R2 and R3, respectively was also found. These findings confirmed that AVS, ferrous iron, and ammonia nitrogen could be simultaneously removed via autotrophic denitrification coupled with anammox in black-odorous sediment by nitrate addition.

Multiple electron acceptor-mediated sulfur autotrophic denitrification: Nitrogen source competition, long-term performance and microbial community evolution

Sulfur-driven autotrophic denitrification (SDAD) is feasible for the treatment of low-C/N-ratio and sulfur-laden wastewaters. The nitrite accumulated in SDAD will affect the performance and stability of the system but can be a potential electron acceptor. Thus, single- and multiple-electron acceptor-mediated SDAD systems were investigated. Batch assays revealed that nitrite and nitrate were the preferential options in the SDAD system with single and multiple electron acceptors, respectively. Synchronous nitrogen and sulfur removal was successfully achieved in continuous flow experiments with multiple electron acceptors, and the system could adapt well to high concentrations of sulfide, nitrate and nitrite (i.e., 720, 108 and 64.8 mg L^-1, respectively), with the predominant genera shifting from Thiobacillus (48.88%) at the initial stage to unclassified_p_Firmicute (34.24%) and Syner-01 (12.31%) at the last stage. This work provides a fundamental basis for applying and regulating SDAD with multiple electron acceptors for the remediation of nitrogen- and sulfide- laden wastewaters.

Formation Mechanism of hydroxyapatite encapsulation in Anammox-HAP Coupled Granular Sludge

The potential of the formation of anammox-hydroxyapatite (HAP) granule composites as a cost-effective approach to removing nitrogen and phosphorus in the treatment of wastewater has been recently reported. Before these annamox granules, which consist of an anammox biofilm layer and an HAP crystallizing layer, can be used in applications, the formation mechanism of hydroxyapatite (HAP) encapsulation in the granules needs to be further studied. In this work, the role of extracellular polymeric substance (EPS) secreted by microorganisms and HAP core in Ca and P removal in anammox-HAP coupled granular sludge was investigated. According to the Lamer model, it is possible that the nucleation time of the granules becomes shorter as the crystal seeds. The enhanced buffering capacity of the granules was 0.08 mmol-H⁺ SS-g^-1 with the pH kept above 6.5 for a comfortable environment for anammox. The results of this study show that ion competition and exchange, mainly between cations of Ca²⁺ and Mg²⁺ and between anions of PO₄^3- and CO₃^2-, affects the precipitation process. The results of this study indicate that the addition of granule crystal seeds can be used as a strategy to hasten the anammox process, and therefore accelerate the overall process.