Drivers of nitrous oxide accumulation in denitrification biofilters with low carbon:nitrogen ratios

Synergistic removal of bio-recalcitrant organic compounds and nitrate: Coupling photocatalysis and biodegradation to enhance the bioavailability of electron donors

Nitrate pollution poses significant threats to both aquatic ecosystems and human well-being, particularly due to eutrophication and increased risks of methemoglobinemia. Conventional treatment for nitrate-contaminated wastewater face challenges stemming from limited availability of carbon sources and the adverse impacts of toxins on denitrification processes. This study introduces an innovative Intimately Coupled Photocatalysis and Biodegradation (ICPB) system, which utilizes Ag3PO4/Bi4Ti3O12, denitrifying sludge, and polyurethane sponge within an anoxic environment. This system demonstrates remarkable efficacy in simultaneously removing bio-recalcitrant organic compounds (such as sulfamethoxazole) and nitrates, surpassing standalone treatment methods. Optimally, the ICPB achieves complete removal of sulfamethoxazole, along with 87.7 % removal of DOC, and 81.8 % reduction in nitrate levels. Its ability to sustain pollutant removal and biological activity over multiple cycles can be attributed to the special formation of biofilm and mineralization of sulfamethoxazole, minimizing both photocatalytic damage and toxic inhibitory effects on microbes. The dominant microbial genera of ICPB system included Castellaniella, Acidovorax, Raoultella, Giesbergeria, and Alicycliphilus. Additionally, the study sheds light on a potential mechanism for the concurrent treatment of recalcitrant organics and nitrates by the ICPB system, presenting a novel and highly effective approach for addressing biologically resistant wastewater.

Aquatic macrophytes mitigate the conflict between nitrogen removal and nitrous oxide emissions during tailwater treatments

Tailwater from wastewater treatment plants (WWTP) usually reduces the nitrogen (N) removal efficiency while simultaneously elevates nitrous oxide (N2O) emissions due to the low carbon-nitrogen (C/N) ratio. Conflicts between N removal and N2O emissions require mitigation by selecting appropriate aquatic plants for tailwater treatment. In this study, a simulated tailwater mesocosm was established using three aquatic plants including Eichhornia crassipes, Myriophyllum aquaticum and Pistia stratiotes. Results of the 15N isotope mass balance analysis revealed the considerable contributions from plant uptake and benthic retention to overall N removal. It was demonstrated that the N assimilation efficiency of aquatic plants depended more on the root-shoot ratio rather than on growth rate. Furthermore, aquatic plants indirectly influence microbial N removal and N2O emissions by altering the water quality parameters. Additionally, aquatic plants could regulate the N transformation through affecting the structure of bacterial community, including microbial abundance, diversity and association networks. Overall, the study underlined the enormous capacities of E. crassipes and P. stratiotes for N uptake and N2O mitigation in tailwater treatment. Utilizing these two aquatic plants for phytoremediation may help mitigate the conflict between tailwater purification and N2O production.

Genomic synergistic efficient carbon fixation and nitrogen removal induced by excessive inorganic carbon in the anammox-centered coupling system

Given the significance of HCO3- for autotrophic anammox bacteria (AnAOB), excessive HCO3- was always provided in anammox-related systems and engineering applications. However, its impact mechanism on anammox process at genome-level remains unknown. This study firstly established an anammox-centered coupling system that entails heterotrophic partial denitrification (PD) and hydrolytic acidification (A-PDHA) fed mainly with inorganic carbon (high HCO3- concentration and low C/N ratio). Metagenomic binning and metatranscriptomics analyses indicated that high HCO3- concentration enhanced expression of natural most efficient phosphoenolpyruvate (PEP) carboxylase within AnAOB, by up to 30.59 folds. This further induced AnAOB to achieve high-speed carbon-fixing reaction through cross-feeding of phosphate and PEP precursors with heterotrophs. Additionally, the enhanced activity of transporters and catalytic enzymes (up to 4949-fold) induced by low C/N ratio enabled heterotrophs to eliminate extracellular accumulated energy precursors mainly derived from carbon fixation products of AnAOB. This maintained high-speed carbon-fixing reaction within AnAOB and supplemented heterotrophs with organics. Moreover, assimilated energy precursors stimulated nitrogen metabolism enzymes, especially NO2- reductase (968.14 times), in heterotrophs. This established an energy-saving PD-A process mediated by interspecies NO shuttle. These variation resulted in efficient nitrogen removal (>95 %) and reduced external organic carbon demand (67 %) in A-PDHA system. This study unveils the great potential of an anammox-centered autotrophic-heterotrophic coupling system for achieving cost-effective nitrogen removal and enhancing carbon fixation under excessive HCO3- doses.

A comprehensive literature mining and analysis of nitrous oxide emissions from different innovative mainstream anammox-based biological nitrogen removal processes

The biological nitrogen removal (BNR) process in wastewater treatment plants generates a substantial volume of nitrous oxide (N2O), which possesses a potent greenhouse gas effect. A limited number of studies have systematically investigated the N2O emissions of anammox-based systems with different BNR processes under mainstream conditions. Based on extensive big data statistical analysis, it had been revealed that simultaneous nitritation, anammox and denitrification (SNAD), partial nitritation anammox (PNA) and partial denitrification anammox (PDA), exhibit significantly lower N2O emission factors when compared to traditional BNR processes. The median values for N2O emission factors were determined to be 1.01 %, 1.15 % and 1.43 % for SNAD, PNA and PDA, respectively. Based on nitrogen removal data and N2O emission factors, the N2O emissions from PNA, SNAD and PDA processes were calculated to be 0.016 g·d-1, 0.037 g·d-1 and 0.008 g·d-1, respectively. Furthermore, the machine learning models (SVM and ANN) exhibited excellent predictive performance for N2O emissions in the BNR processes. However, after removing environmental factors, the R2 value of the SVM model sharply decreased. The SHAP feature analysis demonstrated the significant impact of environmental factors on the accuracy of predictive performance in machine learning models. Spearman correlation analysis was employed to investigate the relationship between N2O emissions and operational factors as well as microbial communities. The results demonstrated a negative correlation between HRT, temperature and C/N with N2O emissions. Moreover, strong associations were observed between Nitrosomonas, Nitrospira, Denitratisoma, Thauera species and N2O emissions. The contribution of N2O production via AOB pathways played a key role that was quantitatively calculated to be 93 %, 80 % and 48 % in the PNA, SNAD and PDA processes, respectively. These findings highlight the potential of these innovative BNR processes in mitigating N2O emissions.

Nitrogen removal by algal-bacterial consortium during mainstream wastewater treatment: Transformation mechanisms and potential N2O mitigation

This work investigated nitrogen transformation pathways of the algal-bacterial consortium as well as its potential in reducing nitrous oxide (N₂O) emission in enclosed, open and aerated reactors. The results confirmed the superior ammonium removal performance of the algal-bacterial consortium relative to the single algae (Chlorella vulgaris) or the activated sludge, achieving the highest efficiency at 100% and the highest rate of 7.34 mg N g MLSS^-1 h^-1 in the open reactor with glucose. Enhanced total nitrogen (TN) removal (to 74.6%) by the algal-bacterial consortium was achieved via mixotrophic algal assimilation and bacterial denitrification under oxygen-limited and glucose-sufficient conditions. Nitrogen distribution indicated that ammonia oxidation (∼41.8%) and algal assimilation (∼43.5%) were the main pathways to remove ammonium by the algal-bacterial consortium. TN removal by the algal-bacterial consortium was primarily achieved by algal assimilation (28.1-40.8%), followed by bacterial denitrification (2.9-26.5%). Furthermore, the algal-bacterial consortium contributed to N₂O mitigation compared with the activated sludge, reducing N₂O production by 35.5-55.0% via autotrophic pathways and by 81.0-93.6% via mixotrophic pathways. Nitrogen assimilation by algae was boosted with the addition of glucose and thus largely restrained N₂O production from nitrification and denitrification. The synergism between algae and bacteria was also conducive to an enhanced N₂O reduction by denitrification and reduced direct/indirect carbon emissions.

Sustainable nitrogen removal in anammox-mediated systems: Microbial metabolic pathways, operational conditions and mathematical modelling

Anammox-mediated systems have attracted considerable attention as alternative cost-effective technologies for sustainable nitrogen (N) removal from wastewater. This review comprehensively highlights the importance of understanding microbial metabolism in anammox-mediated systems under crucial operation parameters, indicating the potentially wide applications for the sustainable treatment of N-containing wastewater. The partial nitrification-anammox (PN-A), simultaneous PN-A and denitrification (SNAD) processes have demonstrated sustainable N removal from sidestream wastewater. The partial denitrification-anammox (PD-A) and denitrifying anaerobic methane oxidation-anammox (DAMO-A) processes have advanced sustainable N removal efficiency in mainstream wastewater treatment. Moreover, N2O production/emission hotspots are extensively discussed in anammox-based processes and are related to the dominant ammonia-oxidizing bacteria (AOB) and denitrifying heterotrophs. In contrast, N2O is not produced in the metabolism pathways of AnAOB and DAMO-archaea; Moreover, the actual contribution of N2O production by dissimilatory nitrate reduction to ammonium (DNRA) and DAMO-bacteria in their species remains uncertain. Thus, PD-A and DAMO-A processes would achieve reduction in greenhouse gas production, as well as energy consumption for the reliability of N removal efficiencies. In addition to reaction mechanisms, this review covers the mathematical models for simultaneous anammox, partial nitrification and/or denitrification (i.e., PN-A, PD-A, and SNAD). Promising NO3- reduction technologies by endogenous PD, sulfur-driven autotrophic denitrification, and DNRA by anammox are also discussed. In summary, this review provides a better understanding of sustainable N removal in anammox-mediated systems, thereby encouraging future investigation and exploration of the sustainable N bio-treatment from wastewater.

Impact of soluble organic matter and particulate organic matter on anammox system: Performance, microbial community and N2O production

In this study, the effects of soluble readily biodegradable COD (sCOD) and particulate slowly biodegradable COD (pCOD) on anammox process were investigated. The results of the long-term experiment indicated that a low sCOD/N ratio of 0.5 could accelerate the anammox and denitrification activity, to reach as high as 84.9%±2.8% TN removal efficiency. Partial denitrification-anammox (PDN/anammox) and denitrification were proposed as the major pathways for nitrogen removal, accounting for 91.3% and 8.7% of the TN removal, respectively. Anammox bacteria could remain active with high abundance of anammox genes to maintain its dominance. Candidatus Kuenenia and Thauera were the predominant genera in the presence of organic matter. Compared with sCOD, batch experiments showed that the introduction of pCOD had a negative effect on nitrogen removal. The contribution of denitrification to nitrogen removal decreased from approximately 14% to 3% with increasing percentage of pCOD. In addition, the analysis result of the process data using an optimized ASM1 model indicated that high percentage of pCOD resulted in serious N₂O emission (the peak value up to 0.25 mg N/L), which was likely due to limited mass diffusion and insufficient available carbon sources for denitrification. However, a high sCOD/N ratio was beneficial for alleviating N₂O accumulation.

Comparison of primary and secondary sludge carbon sources derived from hydrolysis or acidogenesis for nitrate reduction and denitrification kinetics: Organics utilization and microbial community shift

Seeking available and economical carbon sources for denitrification process is an intractable issue for wastewater treatment. However, no study compared different types of waste sludge as carbon source from denitrification mechanism, organics utilization and microbial community aspects. In this study, primary and secondary sludge were pretreated by thermophilic bacteria (TB), and its hydrolysis or acidogenic liquid were prepared as carbon sources for denitrification. At C/N of 8-3, the variations of NO₃^--N and NO₂^--N were profiled in typical cycles and denitrification kinetics was analyzed. Primary sludge achieved a competitive NO_X-N removal efficiency with less dosage than secondary sludge. Fourier transform infrared (FTIR) spectroscopy was introduced to analyze organic composition from functional-group perspective and the utilization of organic matters in different sludge carbon sources was investigated. To further analyze the microbial community shift in different denitrification systems, high-throughput sequencing technology was applied. Results showed that denitrifier Thauera, belonging to Proteobacteria, was predominant, and primary sludge acidogenic liquid enriched Thauera most intensively with relative abundance of 47.3%.

Mainstream double-anammox driven by nitritation and denitratation using a one-stage step-feed bioreactor with real municipal wastewater

A novel double-anammox process for advanced mainstream nitrogen removal was established using step-feed sequencing batch reactor (SBR) system with integration of suspend sludge and biofilms. Following optimization of influent distribution ratio, the effluent total inorganic nitrogen (TIN) was < 10.2 mg N/L, with influent TIN of 43.4 mg N/L, and anammox contributed 71.4% to TIN removal. Biological processes and batch tests revealed that gradient C/N reduction promoted denitratation/anammox in anoxic stage, and simultaneous nitritation and anammox were achieved in oxic stage. Specially, anammox maintained on biofilms with abundance over 10⁹ copies/ (g dry sludge). High-throughput sequencing revealed that Thauera and Nitrosomonas were enriched in flocs. Furthermore, metagenomic sequencing confirmed that Thauera owns narG and napA (NO₃^-→NO₂^-) and Nitrosomonas owns amoA (NH₄⁺→NO₂^-), support stable NO₂^- supply for double-anammox. This mainstream anammox-dominant process could potentially be used for stable nitrogen removal in municipal wastewater treatment plants.

A predictive model for N2O production in anammox-granular sludge reactors: Combined effects of nitrite/ammonium ratio and organic matter concentration

Once the use of anammox reactors has been increasing on a global scale, it is important to understand the mechanisms of N₂O emissions and how to minimise the emissions by optimising the operating conditions. In this study, the influence of chemical oxygen demand (COD) (from 0 mgO₂ L^-1 to 100 mgO₂ L^-1) and nitrite/ammonium ratio from 0.79 to 2.21 (maintaining ammonium at 100 mgN L^-1 and varying nitrite from 79 mgN L^-1 to 221 mgN L^-1) in the N₂O emissions from anammox-granular sludge reactor was investigated in two steps. Step 1 consisted of batch tests, using central composite design, and Step 2, long-term operation of a 6.5 L continuous up-flow reactor. The results showed that the N₂O emissions were minimized by controlling, in the influent, the NO₂^--N/NH₄⁺-N ratio from 1.1 to 1.3 and maintaining the COD concentration below 100 mgO₂ L^-1. TN removal efficiencies were higher than 70% in all conditions tested".

Bioaugmented constructed wetlands for efficient saline wastewater treatment with multiple denitrification pathways

Six laboratory-scale constructed wetlands (CWs) were used to quantify the nitrogen removal (NR) capacity in the treatment of saline wastewater at high (6:1) and low (2:1) carbon-nitrogen ratios (C/N), with and without bioaugmentation of aerobic-denitrifying bacterium. Sustained high-efficiency nitrification was observed throughout the operation. However, under different C/N ratios, although the bioaugmentation of aerobic-denitrifying bacterium promoted the removal of NO₃^--N and TN, there were still great differences in denitrification. Molecular biology experiments revealed ammonia-oxidizing archaea, together with the Nitrosomonas and Nitrospira, led to highly efficient nitrification. Furthermore, aerobic-denitrifying bacterium and sulfur-driven denitrifiers were the core denitrification groups in CWs. By performing these combined experiments, it was possible to determine the optimal CW design and the most relevant NR processes for the treatment of salty wastewater. The results suggest that the bioaugmentation of salt-tolerant functional bacteria with multiple NR pathways are crucial for the removal of salty wastewater pollutants.

Correlation of extracellular polymeric substances and microbial community structure in denitrification biofilm exposed to adverse conditions

Microbial community may respond to different adverse conditions and result in the variation of extracellular polymeric substances (EPS) in denitrification biofilm; this study discovered the role of EPS in accordance with the analysis of cyclic diguanylate (c-di-GMP) and electron equilibrium (EE) under low organic loading rate, shock organic loading rate and low temperature conditions. Good nitrate removal performance could be achieved under shock organic loading rate and low temperature conditions; however, owing to the low organic loading rate, the carbon source was preferentially utilized for biomass growth. Tightly bound EPS (TB-EPS) contents progressively increased and facilitated cell adhesion and biofilm formation. The stable TB protein (TB-PN) content in TB-EPS built a cross-linked network to maintain internal biofilm structure and led to the rapid biosynthesis of polysaccharides, which could further enhance microbial adhesion and improve nitrate removal. C-di-GMP played an important role in biomass retention and biofilm formation, based on the correlation analysis of c-di-GMP and EPS. TB polysaccharide (TB-PS) contents presented a significant positive correlation with c-di-GMP content, microbial adhesion and biofilm stabilization was further enhanced through c-di-GMP regulation. In addition, a remarkable negative correlation between electron deletion rate (EDR) and TB-PN and TB-PS was discovered, and TB-PS was required to serve as energy source to enhance denitrification according to EE analysis. Surprisingly, dynamic microbial community was observed due to the drastic community succession under low temperature conditions, and the discrepancy between the dominant species for denitrification was found under shock organic loading rate and low temperature conditions. The notable increase in bacterial strains Simlicispira, Pseudomonas and Chryseobacterium was conducive to biofilm formation and denitrification under shock organic loading rate, while Dechloromonas and Zoogloea dramatically enriched for nitrate removal under low temperature conditions. The high abundance of Dechloromonas improved the secretion of EPS through the downstream signal transduction, and the c-di-GMP conserved in Pseudomonas concurrently facilitated to enhance exopolysaccharide production to shock organic loading rate and low temperature conditions.

Recent advances in nitrous oxide production and mitigation in wastewater treatment

Nitrous oxide (N₂O) emitted from wastewater treatment plants has caused widespread concern. Over the past decade, people have made tremendous efforts to discover the microorganisms responsible for N₂O production, elucidate metabolic pathways, establish production models and formulate mitigation strategies. The ultimate goal of all these efforts is to shed new light on how N₂O is produced and how to reduce it, and one of the best ways is to find key opportunities by integrating the information obtained. This review article critically evaluates the knowledge gained in the field within a decade, especially in N₂O production microbiology, biochemistry, models and mitigation strategies, with a focus on denitrification. Previous research has greatly deepened the understanding of the N₂O generation mechanism, but further efforts are still needed due to the lack of standardized methodology for establishing N₂O mitigation strategies in full-scale systems. One of the challenges seems to be to convert the denitrification process from a net N₂O source into an effective sink, which is recommended as a key opportunity to reduce N₂O production in this review.

A stable simultaneous anammox, denitrifying anaerobic methane oxidation and denitrification process in integrated vertical constructed wetlands for slightly polluted wastewater

Anaerobic ammonium oxidation (anammox), denitrifying anaerobic methane oxidation bacteria (DAMO) have received great attention for their excellent performance in nitrogen removal. However, not much study focused on the co-existence of anammox, DAMO, and denitrification in constructed wetlands, not to mention the advantage of their application in mitigating the necessary byproduct nitrous oxide (N₂O), methane (CH₄) from the biodegradation process. In this study, the result indicated the construction of integrated vertical constructed wetlands (IVCWs) contributed to the high-efficient stable simultaneous anammox, DAMO and denitrification (SADD) process for the nutrients removal, with denitrification being the least contributor to nitrogen reduction. Besides the succession of SADD process was largely the driver for the variation of N₂O, CH₄ emission. The structural equation method (SEM) further suggested that the three biological pathways of qnorB/bacteria, archaea/qnorB, and anammox/nirK accounted for the N₂O production, as were top-controlled by mcrA/DAMO in IVCWs. Besides the anammox-associated nitrifier denitrification was the main source for N₂O production. And that the trade-off effect between the CH₄ and N₂O production was exerted by the DAMO, while the influence was far from satisfactory under the methane constraints.

Catechol biodegradation by a novel hybrid anoxic biofilter

In this study, simultaneous catechol biodegradation and nitrate removal were successfully achieved in a hybrid anoxic biofilter (HAB). The maximum biodegradation concentration of catechol by HAB, was determined 1,000 mg/L. However, optimum conditions of HAB for biodegradation of catechol were determined in catechol and nitrate concentrations of 750 mg/L and 140 mg/L-N, respectively, and a hydraulic retention time of 18 h, which corresponded to organic loading rate of 1.89 kg COD/m³. d and COD/N of 2.32. The low COD/N ratio is related to nitrogen cycling and reflected N₂O flux within denitrification biofilters. Due to the use of a biofilter in this process, despite the concentration of MLSS above 3000 mg/L in the suspended growth section, HAB was capable of producing effluent with low turbidity (0.8 ± 0.5 NTU), without the need for any secondary sedimentation basin. Therefore, HAB process offers an efficient way for wastewater treatment with high concentration of aromatic compounds and nitrate in anoxic conditions and providing environmental standards for wastewater discharge.

Optimal regulation of N/P in horizontal sub-surface flow constructed wetland through quantitative phosphorus removal by steel slag fed

High concentration of nitrogen and phosphorus and imbalance of N/P can lead to the formation of water and the malignant proliferation of toxic microalgae. This study put forward the advanced nutrient removal with the regulation of effluent N/P as the core in order to restrain the eutrophication and growth of poisonous algae. According to the preliminary study and review, the optimal N/P for non-toxic green algae was 50:1. The horizontal sub-surface flow constructed wetland was filled with steel slag and ceramsite to achieve the regulation of effluent N/P. The results showed that steel slag had the stable P removal capacity when treating synthetic solution with low P concentration and the average removal rate for 1.5, 1.0, and 0.5 mg/L synthetic P solution was 2.98 ± 0.20 mg kg^-1/h, 2.26 ± 0.15 mg kg^-1/h, and 1.11 ± 0.10 mg kg^-1/h, respectively. Combined with P removal rate and P removal task, the filling amount of steel slag along the SSFCW (sub-surface flow constructed wetland) was 3.22 kg, 4.24 kg, and 4.31 kg. In order to ensure the stability of dephosphorization of steel slag, the regeneration of P removal capacity was investigated by switching operation of two parallel SSFCW in 20 days for cycle. The N removal was limited for the deficiency of carbon source (COD (chemical oxygen demand)/TN = 3-4), and was stable at 18.5-31.9% which was less affected by temperature. Therefore, by controlling the process of quantitative P removal of steel slag, the effluent N/P in SSFCW can be stable at 40-60:1 in the whole year, so as to inhibit the malignant proliferation of toxic algae.

Multifunctional modified polyvinyl alcohol: A powerful biomaterial for enhancing bioreactor performance in nitrate, Mn(II) and Cd(II) removal

The co-existence of multiple pollutants in wastewater such as nitrate and heavy metal, is of high concern due to the potential environmental impact. In this study, a novel biomaterial PPy@Fe₃O₄/PVA was synthesized as a multifunctional bacteria immobilized carrier, to enhance simultaneous denitrification, Cd(II) and Mn(II) removal efficiency in bioreactor environments. The morphology and main components of the PPy@Fe₃O₄/PVA material were characterized by SEM and XRD. Using PPy@Fe₃O₄/PVA as a carrier, the maximum removal efficiencies for nitrate (0.207 mg L^-1·h^-1), Mn(II) (90.98%) and Cd(II) (98.78%) were increased by 27.05%, 30.27%, and 16.48%, respectively, compared to in the absence of PPy@Fe₃O₄/PVA. Regeneration experiments were performed, demonstrating the excellent stability and reusability of the PPy@Fe₃O₄/PVA material. Furthermore, effects of key factors were investigated on the performance of the PPy@Fe₃O₄/PVA bioreactor in simultaneous denitrification, Mn(II) and Cd(II) removal. Experimental results indicate that the highest nitrate, Mn(II) and Cd(II) removal efficiencies were obtained under the conditions of HRT of 10 h, initial Mn(II) concentration of 40 mg/L and initial Cd(II) concentration of 10 mg/L. Gas chromatography analysis indicated that N₂ was the mainly final gaseous product. Moreover, the bioreactor community diversity was markedly influenced by the initial concentration of Cd(II) and Pseudomonas sp. H117 played a primary role in the process of simultaneous denitrification, Mn(II) and Cd(II) removal.

Mixture of different Pseudomonas aeruginosa SD-1 strains in the efficient bioaugmentation for synthetic livestock wastewater treatment

Strains selection for inoculation is the key to the successful construction of a bioaugmentation system, a promising strategy for specific pollutant removal. Pseudomonas aeruginosa SD-1 wild-type (WT) strain exhibited high capacity for biofilm formation but low efficiency for nitrate (NO₃^-) removal. Meanwhile, quorum sensing deficient strain ΔlasR showed excellent efficiency for NO₃^- removal but poor capability for colonization in activated sludge. The opposite effect of biofilm formation and NO₃^- removal exist in WT or ΔlasR, which limits the construction of bioaugmentation system of strain SD-1 and its application. To solve this issue, a mixture of WT and ΔlasR (v/v = 1:1) was used to construct a bioaugmentation system. Compared with the inoculation of WT or ΔlasR alone, the mixed inoculation not only was beneficial for activated sludge development but also for pollutant removal. The indicators for activated sludge including the abundance of P. aeruginosa, the sludge volume index and the average particle size in mixed inoculated reactors were close to those of reactors with single and repeated inoculation of WT. The effluent of chemical oxygen demand (COD) and NO₃^--N were stable at 3.9-22.6 mg L^-1 and 0-5.53 mg L^-1 after d 3, respectively. This study presents a detailed case on the ecological tradeoff of colonization and pollutant removal of inoculated strains during bioaugmentation. The results provide information on the appropriate conditions for application of P. aeruginosa SD-1 for livestock wastewater treatment and further enrich our ecological understanding of bioaugmentation.

Nitrous oxide emissions from pilot scale three-stage constructed wetlands with variable nitrogen loading

Pilot scale three-stage surface flow constructed wetlands (SFCWs) were constructed to study nitrous oxide (N₂O) emissions from swine wastewater with different nitrogen levels. The SFCWs had mean total nitrogen (TN) removal efficiencies and removal rates of 84.6-97.1% and 0.6-2.4 g N m^-2 d^-1 respectively. The N₂O emissions and nitrate nitrogen (NO₃^--N) concentration both peaked at a TN value of approximately 100 mg N L^-1. N₂O emissions had a positive correlation with NO₃^--N concentration (p < 0.001). This correlation suggests that the effect of TN loading on N₂O emissions may be related to NO₃^--N in aquatic environment. Significant correlation was observed between N₂O emission and the gene abundance of N₂O reductase (nosZ; p < 0.05). The general linear model revealed that TN loading affected nosZ gene abundance. These results suggest that pollution loading should be considered to balance nitrogen removal and N₂O emissions when designing constructed wetlands.

Does rice straw application reduce N2O emissions from surface flow constructed wetlands for swine wastewater treatment?

Rice straw was applied often as a carbon source to improve nitrogen removal; however, few studies have considered the effect of rice straw on nitrous oxide (N₂O) emission during nitrogen removal in constructed wetlands (CWs). We constructed eighteen combined systems, consisting of rice straw ponds and surface flow CWs to investigate the effect of rice straw application on N₂O emission in three strengths of swine wastewater treatments. The results showed rice straw (RS) treatment increased 131.5% of N₂O emission factor from low strength CWs, but decreased 37.2-43.7% of N₂O emission factors for medium and high strengths compared with no rice straw (NRS) treatment. The RS application led to an average 10.7% increase in the potential denitrification rate, and simultaneously enhanced gene abundances of the total bacteria (16S rRNA), ammonia-oxidising archaea, ammonia-oxidising bacteria, nitrate reductase, and N₂O reductase (nosZ) for all strengths CWs. The multiple regression model revealed N₂O emissions were strongly related to water temperature, nitrate, chemical oxygen demand, and denitrification genes. The proportion of nosZ gene abundance in 16S rRNA was higher in RS (0.7-1.3%) than NRS (0.4-0.9%) for medium and high strengths, while an opposite trend was observed for low strength. The discrepancy was responsible for increasing or decreasing N₂O emission by RS application among different strengths. These findings indicated the effectiveness of RS application to control N₂O emissions from the surface flow CWs was related to the pollution level of wastewater.

Effects of Afforestation Restoration on Soil Potential N2O Emission and Denitrifying Bacteria After Farmland Abandonment in the Chinese Loess Plateau

Denitrification is a critical component of soil nitrogen (N) cycling, including its role in the production and loss of nitrous oxide (N2O) from the soil system. However, restoration effects on the contribution of denitrification to soil N2O emissions, the abundance and diversity of denitrifying bacteria, and relationships among N2O emissions, soil properties, and denitrifying bacterial community composition remains poorly known. This is particularly true for fragile semiarid ecosystems. In order to address this knowledge gap, we utilized 42-year chronosequence of Robinia pseudoacacia plantations in the Chinese hilly gullied Loess Plateau. Soil potential N2O emission rates were measured using anaerobic incubation experiments. Quantitative polymerase chain reaction (Q-PCR) and Illumina MiSeq high-throughput sequencing were used to reveal the abundance and community composition of denitrifying bacteria. In this study, the afforestation practices following farmland abandonment had a strong negative effect on soil potential N2O emission rates during the first 33 years. However, potential N2O emission rates steadily increased in 42 years of restoration, leading to enhanced potential risk of greenhouse gas emissions. Furthermore, active afforestation increased the abundance of denitrifying functional genes, and enhanced microbial biomass. Actinobacteria and Proteobacteria were the dominant denitrifying bacterial phyla in the 0 to 33-years old sites, while the 42-years sites were dominated by Planctomycetes and Actinobacteria, implying that the restoration performed at these sites promoted soil microbial succession. Finally, correlation analyses revealed that soil organic carbon concentrations had the strongest relationship with potential N2O emission rates, followed by the abundance of the nosZ functional gene, bulk density, and the abundance of Bradyrhizobium and Variovorax across restoration stages. Taken together, our data suggest above-ground restoration of plant communities results in microbial community succession, improved soil quality, and significantly altered N2O emissions.

Microbial nitrate removal in biologically enhanced treated coal gasification wastewater of low COD to nitrate ratio by coupling biological denitrification with iron and carbon micro-electrolysis

Mixotrophic denitrification coupled biological denitrification with iron and carbon micro-electrolysis (IC-ME) is a promising emerging bioprocess for nitrate removal of biologically enhanced treated coal gasification wastewater (BECGW) with low COD to nitrate ratio. TN removal efficiency in R1 with IC-ME assisted was 16.64% higher than R2 with scrap zero valent iron addition, 23.05% higher than R3 with active carbon assisted, 30.51% higher than R4 with only active sludge addition, 80.85% higher than R5 utilizing single IC-ME as control. Fe²⁺ generated from IC-ME decreased the production of N₂O and enriched more Nitrate-reducing Fe(Ⅱ) oxidation bacteria (NRFOB) Acidovorax and Thiobacillus, which could convert nitrate to nitrogen gas. And the presence of Fe³⁺, as the Fe²⁺ oxidation product, could stimulate the growth of Fe(III)-reducing strain (FRB) that indicated by redundancy analysis. Microbial network analysis demonstrated FRB Geothrix had a co-occurrence relationship with other bacteria, revealing its dominant involvement in nitrate removal of BECGW.

Biological groundwater denitrification systems: Lab-scale trials aimed at nitrous oxide production and emission assessment

Bio-trenches are a sustainable option for treating nitrate contamination in groundwater. However, a possible side effect of this technology is the production of nitrous oxide, a greenhouse gas that can be found both dissolved in the liquid effluent as well as emitted as off gas. The aim of this study was to analyze NO₃^- removal and N₂O production in lab-scale column trials. The column contained olive nut as organic carbon media. The experimental study was divided into three phases (I, II and III) each characterized by different inlet NO₃^- concentrations (30, 50, 75mgNO₃-NL^-1 respectively). Sampling ports deployed along the length of the column allowed to observe the denitrification process as well as the formation and consumption of intermediate products, such as nitrite (NO₂^-) and nitrous oxide (N₂O). In particular, it was observed that N₂O production represent only a small fraction of removed NO₃^- during Phase I and II, both for dissolved (0.007%) and emitted (0.003%) phase, and it was recorded a high denitrification efficiency, over 99%. Nevertheless, significantly higher values were recorded for Phase 3 concerning emitted phase (0.018%). This fact is due to increased inlet concentration which resulted in a carbon limitation and in a consequent decrease in denitrification efficiency (76%).

Reactor staging influences microbial community composition and diversity of denitrifying MBBRs- Implications on pharmaceutical removal

The subdivision of biofilm reactor in two or more stages (i.e., reactor staging) represents an option for process optimisation of biological treatment. In our previous work, we showed that the gradient of influent organic substrate availability (induced by the staging) can influence the microbial activity (i.e., denitrification and pharmaceutical biotransformation kinetics) of a denitrifying three-stage Moving Bed Biofilm Reactor (MBBR) system. However, it is unclear whether staging and thus the long-term exposure to varying organic carbon type and loading influences the microbial community structure and diversity. In this study, we investigated biofilm structure and diversity in the three-stage MBBR system (S) compared to a single-stage configuration (U) and their relationship with microbial functions. Results from 16S rRNA amplicon libraries revealed a significantly higher microbial richness in the staged MBBR (at 99% sequence similarity) compared to single-stage MBBR. A more even and diverse microbial community was selected in the last stage of S (S3), likely due to exposure to carbon limitation during continuous-flow operation. A core of OTUs was shared in both systems, consisting of Burkholderiales, Xanthomonadales, Flavobacteriales and Sphingobacteriales, while MBBR staging selected for specific taxa (i.e., Candidate division WS6 and Deinococcales). Results from quantitative PCR (qPCR) showed that S3 exhibited the lowest abundance of 16S rRNA but the highest abundance of atypical nosZ, suggesting a selection of microbes with more diverse N-metabolism (i.e., incomplete denitrifiers) in the stage exposed to the lowest carbon availability. A positive correlation (p < 0.05) was observed between removal rate constants of several pharmaceuticals with abundance of relevant denitrifying genes, but not with biodiversity. Despite the previously suggested positive relationship between microbial diversity and functionality in macrobial and microbial ecosystems, this was not observed in the current study, indicating a need to further investigate structure-function relationships for denitrifying systems.

Removal Characteristics of Dissolved Organic Nitrogen and Its Bioavailable Portion in a Postdenitrifying Biofilter: Effect of the C/N Ratio

Addition of external carbon sources to postdenitrification biofilters (DNFs) is frequently used in municipal wastewater treatment plants to enhance dissolved inorganic nitrogen removal. However, little is known about its influence on the removal of dissolved organic nitrogen (DON). This study investigated the effect of the carbon-to-nitrogen (C/N) ratio (3, 4, 5, and 6) on the removal characteristics of DON and bioavailable DON (ABDON) in the pilot-scale DNFs treating real secondary effluent. Results showed that DNFs effluent DON accounted for 31.2-39.8% of the effluent total nitrogen. The maximum effluent DON and ABDON concentrations both occurred in DNF operated at a C/N ratio of 3. There was no significant difference in effluent DON concentrations in DNFs at C/N ratios of 4, 5, and 6; however, effluent ABDON and DON bioavailability significantly decreased with C/N ratios (p < 0.05, t-test). According to the chemical composition analysis, effluent DON at high C/N ratios tends to contain less % molecular weight < 1 kDa nitrogenous organic compounds and proteins/amino sugars-like nitrogenous organic formulas, which is likely responsible for its low bioavailability. Overall, this study indicates the benefit of a high C/N ratio during the DNF process in terms of controlling the DON forms that readily stimulate algal growth.

Correlation between nitrous oxide (N2O) emission and carbon to nitrogen (COD/N) ratio in denitrification process: a mitigation strategy to decrease greenhouse gas emission and cost of operation

The reliability and accuracy of in-situ ion selective electrode and ultraviolet (NO_x) probes have been investigated at four different treatment plants with different operational conditions. This study shows that the mentioned probes tend to compromise their accuracy and trending stability at lower NO_x of <1.0 mg N/L, which if used as a measuring variable for PI feedback controller for denitrification (biological reduction of nitrate to nitrogen gas), would cause overfeeding the external carbon source. In-situ Clark-type N₂O sensors, recently introduced for industrial scale use (Unisense Environment) could potentially open a new horizon in the automation of biological processes and particularly denitrification. To demonstrate the applicability of such probes for automation, two in-situ N₂O probes were used in two treatment plants in parallel with NO_x-N probes. The effects of operational conditions such as COD/N ratios and the correlation between NO_x and N₂O were investigated at those plants. N₂O production at non-detect dissolved oxygen concentrations and pH of 7-7.2 were found to be a function of influent nitrogen load or the ratio of COD/N_INFLUENT. Finally, using an N₂O probe as a proxy sensor for nitrates is proposed as a measured variable in the PI feedback in the automation of the denitrification process with a NO_x set point of <1.2 mg N/L).

Assessing membrane biofouling and its gel layer of anoxic/oxic membrane bioreactor for megacity municipal wastewater treatment during plum rain season in Yangtze River Delta, China

This study assessed membrane biofouling and its gel layer of anoxic/oxic membrane bioreactor (A/O-MBR) for megacity municipal wastewater treatment during plum rain season, which was continuous rainy weather, in Yangtze River Delta, China. A laboratory-scale A/O-MBR was operated to treat the municipal wastewater from Quyang wastewater treatment plant, which located at the typical megacity of Shanghai in Yangtze River Delta, from April to July accompanying with plum rain season. As reactor performance showed, COD_Cr, NH₄⁺-N, TN, TP of the influent gradually decreased during plum rain season, and inhibited pollutant removal due to organic carbon shortage. However, dissolve inorganic carbon and inorganic components in mixed liquid had an obvious increase under rainy weather. Membrane filtration results indicated that plum rain season enhanced pore blocking behavior, further leading to the serious membrane biofouling but inhibiting gel layer formation. Additionally, gel layer analysis predicted that plum rain season led to plenty of inorganic components and precipitate flew into A/O-MBR reactor. Inorganic components with elements of Ca, Mg Ba, Fe, Al and Si seriously blocked membrane pores. Those components also accumulated into gel layer in the form of SiO₂, CaCO₃, CaSiO₃, MgNH₄PO₄, BaCO₃, AlPO_4, etc. Consequently, plum rain season enhanced pore blocking behavior and led to severe membrane biofouling but with the inhibition of gel layer formation.