Nitrate Removal Rate Research Articles

Cyclohexanecarboxylic acid degradation with simultaneous nitrate removal by Marinobacter sp. SJ18.

Naphthenic acid (NA) is a toxic pollutant with potential threat to human health. However, NA transformations in marine environments are still unclear. In this study, the characteristics and pathways of cyclohexanecarboxylic acid (CHCA) biodegradation were explored in the presence of nitrate. The results showed that CHCA was completely degraded with pseudo-first-order kinetic reaction under aerobic and anaerobic conditions, accompanied by nitrate removal rates exceeding 70%, which was positively correlated with CHCA degradation (P < 0.05). In the proposed CHCA degradation pathways, cyclohexane is dehydrogenated to form cyclohexene, followed by ring-opening by dioxygenase to generate fatty acid under aerobic conditions or cleavage of cyclohexene through β-oxidation under anaerobic conditions. Whole genome analysis indicated that nitrate was removed via assimilation and dissimilation pathways under aerobic conditions and via denitrification pathway under anaerobic conditions. These results provide a basis for alleviating combined pollution of NA and nitrate in marine environments with frequent anthropogenic activities.

ZnS quantum dots implanted polyaniline-wrapped corn straw catalysts for efficient photocatalytic nitrate reduction without external addition of hole scavengers

Photocatalytic reduction to harmless nitrogen gas is considered a promising method for the removal of nitrate from water. However, achieving high efficiency of photocatalytic nitrate reduction, especially without the assistance of a hole scavenger, remains a critical issue. Herein, PANI-CS (PC) is prepared by in-situ polymerization of aniline on the surface of biomass corn straw, and then ZnS QDS is loaded to synthesize ZnS QDs-PANI-CS (ZPC) catalysts for photocatalytic reduction of nitrate. The optimized ZPC shows highly selective and efficient photocatalytic reduction of NO3– without extra sacrificial agents, with the 0.8% ZPC delivering an excellent nitrate removal rate of 98.4% and N2 selectivity of 96.8%. Outstanding catalytic performance is attributed to the ZPC having higher electron mobility and longer electron lifetime. Additionally, ZnS QDs induce the conversion of the quinone ring of PANI to the benzene ring, thereby improving the reducibility of the catalyst. Most importantly, The carbon–sulfur double bond on the ZnS quantum dots and the bicarbonate on the surface of the straw are the sources of formic acid, which provides great help for the production of CO2•- with reduction activity. The introduction of ZnS QDs also enriches the Lewis acid sites on the ZPC surface, thereby enhancing the N2 selectivity, and simultaneity, the stability of the composite is consolidated due to the formation of N-Zn covalent bonds, laying the foundation for the commercialization of the catalyst. This work introduces the characteristics of self-produced formic acid into photocatalysts through ingenious design, which provides a new idea for the photocatalytic treatment of nitrate wastewater.

Synergistic removal of nitrate by a cellulose-degrading and denitrifying strain through iron loaded corn cobs filled biofilm reactor at low C/N ratio: Capability, enhancement and microbiome analysis

Optimization of nitrate removal rate under low carbon-to-nitrogen ratio has always been one of the research hotspots. Biofilm reactor based on functional carrier and using interspecific synergic effect of strains provides an insight. In this study, iron-loaded corn cob was used as a functional carrier that can contribute to the cellulose degradation, iron cycling, and collaborative denitrification process of microorganisms. During biofilm reactor operation, the maximum nitrate removal efficiency was 99.30% and could reach 81.73% at no carbon source. Dissolved organic carbon and carrier characterization showed that strain ZY7 promoted the release of carbon source. The crystallinity of cellulose I and II in carrier of experimental group increased by 31.26% and decreased by 21.83%, respectively, in comparison to the control group. Microbial community showed the synergistic effect among different strains. The vitality and metabolic activity of the target microorganisms in bioreactor were increased through interspecific bacterial cooperation.

Flow analysis and hydraulic performance of denitrifying bioreactors under different carbon dosing treatments

Denitrifying bioreactors are an effective approach for removing nitrate from a variety of non-point wastewater sources, including agricultural tile drainage. However, compared to alternate mitigation approaches such as constructed wetlands, nitrate removal in bioreactors may decline with time and low temperature, resulting in poor long-term nitrate removal rates. To address the low nitrate removal rates in bioreactors, the addition of an external carbon source has been found to be an effective method for enhancing and maintaining nitrate removal rates. While carbon dosing has led to a significant improvement in nitrate removal, some of the possible adverse effects of carbon dosing, such as clogging and reduction in hydraulic efficiency, remain unknown and need to be investigated. Using observations from both field and mesocosm trials, we compared the hydraulic performance of bioreactors with and without carbon dosing. The pilot-scale field bioreactor (58 m3 total woodchip volume, 25 m3 saturated volume, referred to as field bioreactor in this work) treated drainage water from a paddock of a dairy farm. The bioreactor received an exogenous carbon dose of 8% methanol (v/v) at 10 mL min−1 and 5 mL min−1 in the 2020 and 2021 drainage seasons, respectively. The field bioreactor had a statistically higher hydraulic conductivity in 2018 when not carbon-dosed of 4601 m day−1, reducing to 1600 m day−1 in 2021 which was the second year of carbon dosing. Field observations could not establish whether the addition of liquid carbon could affect the bioreactor's internal hydraulics performance, such as actual hydraulic retention time (AHRT), despite a significant decline in hydraulic conductivity in the field bioreactor. Separate experiments on replicated bioreactor mesocosms were conducted to investigate the effects of carbon dosing on the internal hydraulic parameters of bioreactors. These mesocosm bioreactors had previously been used to study the long-term effects of methanol dosing on bioreactor performance, such as nitrate removal under steady-state conditions. The mesocosm and field bioreactors shared some characteristics, such as the use of methanol as an external carbon source, but the mesocosm experiments were hydrologically controlled contrary to the field bioreactor's transient operating conditions. We found that methanol dosing in either carbon or nitrate limiting conditions had no significant effects (p-value >0.05) on internal hydraulic parameters (e.g., effective utilization of media) when compared to control bioreactors. The present study offers insight into the long-term hydraulic performance of bioreactors and may help develop small-footprint bioreactors that incorporate external carbon dosing.

Effect of hydraulic retention time on performance of autotrophic, heterotrophic, and split-mixotrophic denitrification systems supported by polycaprolactone/pyrite: Difference and potential explanation.

Biological denitrification is still the most important pathway to purifying nitrate-containing wastewater. In this study, pyrite (FeS2 ) and polycaprolactone (PCL) were used as electron donors to construct sole or combined denitrification systems, that is, pyrite-based autotrophic denitrification (PAD) system, PCL-supported heterotrophic denitrification (PHD) system, and split-mixotrophic denitrification system (combined PAD + PHD), all of which were operated under five different hydraulic retention times (HRTs) for 150 days. The results showed that the removal rates (RE) of nitrate (NO3 - -N) and inorganic phosphorus (PO4 3- -P) by PAD were 91% and 94%, respectively, but the effluent sulfate (SO4 2- ) concentration was as high as 168.2mg/L; the removal rate of NO3 - -N by PHD was higher than 99%, but the PO4 3- -P could not be removed ideally; the removal rates of NO3 - -N and PO4 3- -P by PAD + PHD were higher than 95% and 99%, respectively, and the effluent SO4 2- concentration was only 7.2mg/L. Through the analysis of the surface scanning electron microscope (SEM) images of the two kinds of media before and after use, it was found that the coupled mode of PAD + PHD was more favorable for biofilm formation than the sole PAD or PHD process, and the microorganisms in the PAD + PHD mode made more full use of electron donors. Moreover, the biomass of the PAD + PHD mode was lower than that of the PAD or PHD process, but the denitrification efficiency of the coupled mode was more efficient, indicating that the functional microorganisms in the PAD + PHD mode might have a certain synergistic effect. PRACTITIONER POINTS: Removal rates of NO3 -, PO4 3 -, and SO4 2 - by PAD were 91%, 94%, and -233%, respectively. Removal rate of NO3 - by PHD exceeded 99%, but PO4 3 - could not be removed ideally. Removal rates of NO3 -, PO4 3 -, and SO4 2 - by PAD + PHD were 95%, 99%, and 86%, respectively. The coupled mode was more favorable for biofilm formation than the sole PAD or PHD. The coupled mode had lower biomass but got more excellent denitrification efficiency.

Constant carbon dosing of a pilot-scale denitrifying bioreactor to improve nitrate removal from agricultural tile drainage

Denitrifying bioreactors are effective tools for removing nitrate from agricultural drainage water. However, as woodchips age with time, a shortage of labile carbon supply limits nitrate removal by microbial denitrification when high nitrate pulses occur in drainage waters. In this study, we investigated the potential of methanol dosing at a constant rate to increase nitrate removal rates in a 58 m3 pilot-scale bioreactor (25 m3 saturated volume) installed on a dairy farm for two consecutive drainage seasons. The drainage water in the bioreactor had a mean hydraulic retention time (HRT) of 12.7 days and 13.5 days in the 2020 and 2021 drainage seasons. With 14.4 L of methanol solution (8% v/v) added per day, the seasonal nitrate removal rates were 8.6 g N m−3 d−1 in 2020 and 5.1 g N m−3 d−1 in 2021 when the methanol dosing rate was halved. Both rates (2020 and 2021) were enhanced as compared to seasonal rates of 0.67–1.60 g N m−3 d−1 in previous years (2017 and 2018) when the bioreactor was not dosed. When there were very large pulses of nitrate into the bioreactor, which exhausted added methanol, nitrate was observed at concentrations above limiting ranges (> 3 mg N L−1) at the outlet on several occasions in 2021. Cumulative nitrate load reductions of 1859 g N and 1620 g N occurred in 2020 and 2021, respectively, resulting in overall nitrate removal efficiencies of 85 and 73%, respectively.Methanol concentrations decreased by order of magnitude along the bioreactor length, from mean inlet concentrations of 327 mg CH3OH-C L−1 in 2020 (higher dosing rate) to concentrations of <50 mg CH3OH-C L−1 at the outlet. The mean methanol removal rates of 106 g CH3OH-C m−3 d−1 in 2020 and 109 g CH3OH-C m−3 d−1 in 2021. A 90% (2020) and 100% (2021) overall methanol removal efficiency was calculated. With methanol dosing, sulfate removal occurred in the bioreactor, with an average sulfate removal rate of 8.5 g SO42−-S m−3 d−1 in 2020 and 0.5 g SO42−-S m−3 d−1 in 2021. This work demonstrated that methanol additions to bioreactors could enhance denitrification rates even when nitrate was limiting without considerable losses of methanol being released to the receiving waters.

Domesticating a Halotolerant Bacterium of Vibrio sp. LY1024 with Heterotrophic Nitrification–Aerobic Denitrification Property for Efficient Nitrogen Removal in Mariculture Wastewater Treatment

Dealing with mariculture wastewater that contains high nitrogenous compounds with efficient biological nitrogen removal technology is challenging but meaningful. The key lies in developing an active microorganism that can spontaneously complete the nitrification–denitrification processes in the marine environment. Herein, a halotolerant heterotrophic nitrification–aerobic denitrification (HN-AD) bacterium of Vibrio sp. LY1024 with good nitrogen removal capacity is domesticated to achieve the aforementioned goal. As a result, ammonium (NH4+-N) and nitrate (NO3−-N) removal rates of almost 100% and 98.5% are detected over Vibrio sp. LY1024 at the salinity of 3.5%, even further increasing the salinity of wastewater to 5.5%. Its removal capacity towards both NH4+-N and NO3−-N can still maintain at almost 100% and 94.7%, respectively. Further combining these results with those of intermediate product determination, it can be speculated that the ammonium removal is according to the pathway of NH4+-N → NH2OH → NO3−-N → N2O → N2. Moreover, the influence of wastewater temperature on the nitrogen removal efficiency of Vibrio sp. LY1024 is also considered. The NH4+-N and NO3−-N removal efficiency over Vibrio sp. LY1024 at a relatively low temperature of 15 °C is still up to 97.3% and 76.4%, respectively. Our work provides a promising halotolerant and low-temperature resistance microorganism for the treatment of mariculture wastewater.

Insight into characteristics of sulphur-based autotrophic denitrifying microbiota in the nitrate removal

ABSTRACT Mariculture wastewater is characterized by low organic carbon to nitrogen ratio (C/N) but high nitrate concentration, which makes it difficult to remove nitrate by the completely heterotrophic denitrification. However, high nitrate discharge poses a threat to the natural environment and human health. Thus, we enriched sulphur-based autotrophic denitrifying (SAD) microbiota and optimized the nitrate removal under different environmental factors and electron donor conditions. The results showed that the dominant genera in the enriched microbiota were previously confirmed autotrophic denitrifiers, Sulfurovum, Thioalkalispira-Sulfurivermis, and Sedimenticola, with a high relative abundance of 41.14%, 21.01%, and 6.17%. Among the environmental factors, pH was the key factor affecting SAD microbiota, and pH 7–9 favoured nitrate removal. However, high pH led to nitrite accumulation (e.g. 10 mg/L at pH = 9), which should be strictly avoided. With regard to electron donors, the optimal concentrations of thiosulphate and nitrate were 50 and 5 mg/L, respectively. The best organic carbon is acetate with an optimal concentration of 10 mg/L. Meanwhile, the initial concentration of thiosulphate was proportional to the nitrate removal rate, while higher concentrations of organic carbon stimulated the heterotrophic denitrification potential of microbiota and thus benefited to dentrification. This study showed that the enriched SAD microbiota was able to achieve efficient nitrate removal under suitable environmental conditions and mixed electron donors and thus presented the potential for application in the treatment of mariculture wastewater.

Evaluation of electroactive denitrifiers at different potentials, temperatures and buffers based on microcalorimetry

Electroactive denitrifiers contribute to the nitrate removal in a bioelectrochemical system, but their metabolism and growth parameters remain vague. In this study, microcalorimetry as a suitable method was used to evaluate the metabolism and growth parameters of electroactive denitrifiers at different cathode potentials, temperatures and buffer solutions. The suitable cathode potential and temperature for electroactive denitrifiers were deemed as −100 mV and 30 °C, respectively. The suitable buffer was found to be phosphate buffer solution but can be replaced by bicarbonate buffer solution. When cultivated with bicarbonate buffer solution at −100 mV and 30 °C, electroactive denitrifiers achieved a specific nitrate removal rate of 2.20 ± 0.08 × 10−10 mg NO3−-N·(min·cell)−1 and two growth rate constants (k1 = 0.0051 ± 0.0004 min−1, k2 = 0.0030 ± 0.0004 min−1), with gaseous nitrogen as the end product. The bioelectrochemical denitrification behaved as a two-step process, in which the nitrite reduction to gaseous nitrogen was the rate-limiting step.

Effects of culture conditions on denitrification activity by Magnetospirillum gryphiswaldense strain MSR-1

The application of denitrification function of Magnetospirillum gryphiswaldense strain MSR-1 has not received attention. Here, we studied the effects of different culture conditions, including carbon (C) and nitrogen (N) sources, C/N ratio, iron source species and addition, initial pH, and temperature on the growth, magnetic production, and denitrification activity and application potential of MSR-1 under anoxic and nutrient environments similar to that of urban domestic sewage. Sodium lactate remains the best carbon source for denitrification by MSR-1, although acetic acid is also promising. Nitrate is the best nitrogen source, where addition and the ratio to nitrite should be >3 mM and 1, respectively. At C/N between 3 and 7, initial pH between 6.5 and 7.5, and culture temperature from 25 to 35 °C, nitrate content (84 mg N/L) decreased to the lowest value in 24 h, concomitant with the lowest residual nitrite. A maximum nitrate removal rate of 88.22 % was observed in 16 h by the optimization of response surface methodology with C/N = 5.13, initial pH = 6.83, and culture temperature = 30.3 °C. MSR-1 effectively removed nitrate, with the advantage of a low C/N demand; furthermore, optimal pH and temperature for denitrification are similar to those used in the sewage biological treatment system. Here, the consistency between optimal denitrification conditions of MSR-1 and actual wastewater conditions was considered for the first time, demonstrating the application potential of MSR-1 for wastewater denitrification and providing a theoretical basis for the application of the strain in wastewater treatment.

Heterotrophic denitrification: An overlooked factor that contributes to nitrogen removal in n-DAMO mixed culture

Nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) has been recognized as a sustainable process for simultaneous removal of nitrogen and methane. The metabolisms of denitrifying anaerobic methanotrophs, including Candidatus Methanoperedens and Candidatus Methylomirabilis, have been well studied. However, potential roles of heterotrophs co-existing with these anaerobic methanotrophs are generally overlooked. In this study, we pulse-fed methane and nitrate into an anaerobic laboratory sequencing batch bioreactor and enriched a mixed culture with stable nitrate removal rate (NRR) of ∼28 mg NO3−-N L−1 d−1. Microbial community analysis indicates abundant heterotrophs, e.g., Arenimonas (5.3%–18.9%) and Fimbriimonadales ATM1 (6.4%), were enriched together with denitrifying anaerobic methanotrophs Ca. Methanoperedens (10.8%–13.2%) and Ca. Methylomirabilis (27.4%–34.3%). The results of metagenomics and batch tests suggested that the denitrifying anaerobic methanotrophs were capable of generating methane-derived intermediates (i.e., formate and acetate), which were employed by non-methanotrophic heterotrophs for denitrification and biomass growth. These findings offer new insights into the roles of heterotrophs in n-DAMO mixed culture, which may help to optimize n-DAMO process for nitrogen removal from wastewater.

Effect of sulfur particle morphology on the performance of element sulfur-based denitrification packed-bed reactor

The effect of particle morphology on denitrification performance in element sulfur-based denitrification (ESDeN) packed-bed process is a gap. In this study, three different types of commercial sulfur particles were selected to build the ESDeN reactors. The results showed the reactors filled with rougher sulfur particles took shorter time to reach stable denitrification performance in the start-up stage. The reactors filled with cap-shape sulfur particles received the maximum nitrate removal rate of 849.49 ± 79.29 g N m−3 d−1 at empty bed contact time of 0.50 h, which was 2.34 times higher than that with ball-shape sulfur particles in the steady stage. The superior denitrification performance in the cap-shape particles set linked to its larger effective volumetric surface area (ωe, 1.67 times larger) and to the longer actual hydraulic retention time (AHRT, 1.80 times longer). This study extends the knowledge of the dependency of sulfur particle properties on denitrification performance in ESDeN packed-bed reactor.

Green preparation of nano-zero-valent iron-copper bimetals for nitrate removal: Characterization, reduction reaction pathway, and mechanisms

• A successful preparation of green nano-zero-valent iron-copper bimetals by a simple method. • The removal rate of nitrate by GT-nZVI/Cu under optimal conditions can reach 99.00%. • The prepared GT-nZVI/Cu has good anti-oxidation and anti-agglomeration properties. • Most of the reaction products for the removal of nitrate by GT-nZVI/Cu are N 2 . • The mechanism of GT-nZVI/Cu denitrification is complexation, redox and adsorption co-precipitation. In this study, green tea was used as a reducing agent to prepare GT-nZVI/Cu nano-zero-valent iron-copper bimetallic materials for nitrate degradation. Through single factor experiments, the process conditions for nitrate removal by GT-nZVI/Cu were optimized. By measuring the contents of nitrate, nitrite, and ammonia nitrogen in the reaction products, the reduction pathway of nitrate-nitrogen in GT-nZVI/Cu was studied. In addition, the removal mechanism of nitrate by GT-nZVI/Cu was also analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and kinetic experiments. The results showed that the size of GT-nZVI/Cu particles is between 50 and 70 nm, and they are irregular spherical shapes. The polyphenol antioxidants in green tea make GT-nZVI/Cu have stronger antioxidant properties, and the denitration products are mostly nitrogen. The addition of copper ions accelerates the reaction time and reaction rate. The maximum removal rate of nitrate by GT-nZVI/Cu within 75 min can be up to 99 %. After three repeated experiments, the removal rate of GT-nZVI/Cu can still reach 70 %. The removal mechanism of GT-nZVI/Cu has enhanced nitrate removal through complexation, redox, and adsorption co-precipitation. This study provides a new route for the green preparation and efficient application of nZVI.

Combining biological denitrification and electricity generation in methane-powered microbial fuel cells

Methane has been demonstrated to be a feasible substrate for electricity generation in microbial fuel cells (MFCs) and denitrifying anaerobic methane oxidation (DAMO). However, these two processes were evaluated separately in previous studies and it has remained unknown whether methane is able to simultaneously drive these processes. Here we investigated the co-occurrence and performance of these two processes in the anodic chamber of MFCs. The results showed that methane successfully fueled both electrogenesis and denitrification. Importantly, the maximum nitrate removal rate was significantly enhanced from (1.4 ± 0.8) to (18.4 ± 1.2) mg N/(L·day) by an electrogenic process. In the presence of DAMO, the MFCs achieved a maximum voltage of 610 mV and a maximum power density of 143 ± 12 mW/m2. Electrochemical analyses demonstrated that some redox substances (e.g. riboflavin) were likely involved in electrogenesis and also in the denitrification process. High-throughput sequencing indicated that the methanogen Methanobacterium, a close relative of Methanobacterium espanolae, catalyzed methane oxidation and cooperated with both exoelectrogens and denitrifiers (e.g., Azoarcus). This work provides an effective strategy for improving DAMO in methane-powered MFCs, and suggests that methanogens and denitrifiers may jointly be able to provide an alternative to archaeal DAMO for methane-dependent denitrification.

Enhanced nitrate removal and side effects of methanol dosing in denitrifying bioreactors

The nitrate removal efficiency of denitrifying bioreactors decreases when the carbon supply from woodchips is insufficient, particularly during large nitrate pulses. This study aimed to assess the effects of methanol dosing, as an external carbon source, on nitrate removal rates in mesocosm-scale bioreactors while monitoring the secondary effects of dosing that may occur. A secondary goal was to quantify sulfate reduction rates and methanol consumption in the presence and absence of nitrate to aid in determining the dosing load to minimize undesirable effects such as methanol entering receiving waters and minimizing sulfate reduction. We continuously dosed the bioreactors with nitrate (∼20 mg NO3−-N L−1), methanol (∼35 mg CH3OH-C L−1), and sulfate (∼9 mg SO42−-S L−1), which was already present in the tap water. In a long-term controlled mesocosm experiment, we established three bioreactor treatments to investigate nitrate, methanol, and sulfate removal rates with and without the presence of nitrate and methanol. Compared to the woodchip control treatment, methanol dosing resulted in an approximately fourfold increase in nitrate removal rates from 7 to 27 g N m−3 day−1. Methanol dosing increased sulfate removal rates, from average sulfate removal rates of 1.5 g SO42−-S m−3 day−1 under nitrate-prevailing conditions to 5.5 g SO42−-S m−3 day−1 removal rates under nitrate-limiting conditions compared to woodchip control removal of 0.3 g SO42−-S m−3 day−1. Mean methanol removal rates were 23 g CH3OH-C m−3 day−1 under nitrate-prevailing conditions compared to 18 g CH3OH-C m−3 day−1 in the woodchip control experiment. Improved nitrate removal rates, as well as methanol consumption and sulfate removal rates, might be leveraged to develop innovative low-footprint bioreactors.

High-rate ex situ and in situ treatment system for groundwater denitrification via membrane-based bacterial macro-encapsulation

The main aim of this study was to develop a new biotreatment process for denitrification by using a small bioreactor platform (SBP) for macro-encapsulation of a bacterial consortium equipped with a slow-release organic matter core for ex situ and in situ treatment configurations. The biotreatment system in the continuous ex situ mode achieved a maximal nitrate removal load of 38.9 g nitrate/L/day with a short hydraulic retention time (HRT) of 1.3 h. The in situ configuration showed high performance for >50 days, with 85 % nitrate removal from an initial concentration of 110 mg/L. The capsules continuously removed nitrate at the same rate even when the organic matter level in the internal capsule was low. After 30 days of in situ experiments, the bacterial population composition set inside the SBP capsules presented dominant populations of well-known heterotrophic denitrifying bacteria. Thus, the encapsulated process not only demonstrated a short HRT necessary for high nitrate removal rates in ex and in situ configurations, but also potentially lower operational costs and no secondary environmental contamination caused by excess organic matter.

Highly effective reduction of phosphate and harmful bacterial community in shrimp wastewater using short-term biological treatment with immobilized engineering microalgae

Shrimp farming wastewater includes high amounts of phosphate and microbiological contaminants, necessitating further treatment before release into receiving water bodies. After 24 h of shrimp wastewater treatment, alginate beads containing the blue-green algal Synechocystis strain lacking the phosphate regulator gene (mutant strain ΔSphU) at 150 mg L−1 reduced phosphate content from 17.5 mg L−1 to 5.0 mg L−1, representing 71.5% removal efficiency, with phosphate removal rate reaching 6.9 mg gDW−1 h−1 during photobioreactor operation. For short-term treatment, removal rates of nitrate, ammonium and nitrite were 42.7, 48.5 and 92.9%, respectively. Microalgal encapsulated beads also impacted the bacterial community composition dynamics in shrimp wastewater. Next-generation sequencing targeting the V3–V4 region of the 16S rDNA gene showed significant differences in bacterial community composition after 24 h of treatment. Proteobacteria are the most abundant phylum in shrimp wastewater. After 24 h of bioremediation, reductions of harmful bacteria in the Cellvibrionaceae and Pseudomonadaceae families were recorded at 5.85 and 3.18%, respectively. Engineered microalgal immobilization under optimal conditions can be applied as an alternative short-term bioremediation strategy to remove phosphate and other harmful microbial contamination from shrimp farming wastewater.

Element sulfur-based autotrophic denitrification constructed wetland as an efficient approach for nitrogen removal from low C/N wastewater

Constructed wetlands (CWs) integrated with sulfur autotrophic denitrification to stimulate high-rate nitrogen removal from carbon-limited wastewater holds particular application prospect due to no excessive carbon source addition, high efficiency, and good stability. In this study, we conducted elemental sulfur-based constructed wetland (SCW) and traditional constructed wetland (CW) under different C/N (2, 1, and 0.5) to explore the feasibility and mechanisms for nitrogen removal from low C/N wastewater. Compared with CW, SCW was demonstrated more robust in nitrogen removal in the case of low C/N influent. When the influent C/N control was at 0.5, SCW observed total nitrogen (TN) and nitrate removal efficiency of 69.36 ± 3.96% and 81.71 ± 3.96%, with the corresponding removal rate of 1.18 ± 0.66 and 1.70 ± 0.92 g-N·m–2·d–1, which were 2.11 and 10.03 times of CW, respectively. The nitrate removal rate constant k in the SCW was 1.05, 3.83, and 10.33 times higher than the CW with C/N of 2, 1 and 0.5. Furthermore, 14.40, 54.51, and 79.82% of nitrogen were removed by the sulfur autotrophic denitrification (SAD) in SCW, which also contributed 43.89, 73.68, and 71.70% of sulfate production. Moreover, the combined system of CW-SCW is proved be an efficient operation mode for simultaneously removing total ammonia nitrogen (TAN) and nitrate. In the SCW, the richness of the microbial community was improved and sulfur-oxidizing genera (e.g. Thiobacillus, Sulfurimonas) was selectively enriched, which affect the performance the elemental sulfur-based denitrification process. The nitrate reduction pathway was overwhelmed by denitrification and the dissimilatory nitrate reduction process. These findings offer elemental sulfur-based autotrophic denitrification constructed wetland has excellent potential to enhance nitrogen removal from carbon-limited wastewater.

Performance and mechanism of nitrate removal by the aerobic denitrifying bacterium JI-2 with a strong autoaggregation capacity

Here, a new strain JI-2 of the strongly autoaggregating aerobic denitrifying bacteria was screened. The nitrate removal ability and autoaggregation mechanism of JI-2 were analyzed using the nitrogen balance and genomics technology. The nitrate removal rate was 27.05 mg N/(L·h) at pH 9.0 and C/N 8.0. The strain JI-2 removes nitrate via the aerobic denitrification and dissimilation pathways and removes ammonium via the assimilation pathway. 66.81 % nitrate was converted to cellular components under aerobic conditions. Complex nitrogen metabolism genes were detected in strain JI-2. C-di-GMP mediates the motility behavior of JI-2 by binding the FleQ and PilZ proteins, and regulating the expression of PslA. Furthermore, the mechanism of autoaggregation was verified by extracellular polymeric substance analysis. Meanwhile, the nitrate removal rates of strain JI-2 was 11.13–12.50 mg N/(L·h) in wastewater. Thus, strain JI-2 has good prospects for application in the treatment of nitrate wastewater.

Unravelling Mass Transport Effects in Electrochemical Nitrate Reduction on Titanium

Nitrate is a prevalent waterborne pollutant that threatens the health of humans and aquatic systems. By selectively producing ammonia, electrochemical nitrate reduction reaction (NO3RR) can directly transform nitrate pollutants into widely used commodity chemicals and fertilizers, thus balancing the nitrogen cycle while reducing energy consumption from the traditional Haber-Bosch process.Although research efforts to date have primarily focused on the development of efficient nitrate reduction catalysts, the electrolyte environment has been found to substantially influence NO3RR activity and selectivity. Specifically, the composition of the first ten nanometers of electrolyte away from the electrode surface dictates the immediate environment within which reactions take place. As has been demonstrated in the electrochemical CO2 reduction reaction, this interfacial reaction microenvironment comprised of near-surface electrolyte and electrocatalyst influences reaction activity and selectivity. In electrochemical NO3RR, where an anionic species reacts at a negatively charged electrode, the reaction rate can often be limited by the mass transport of the reactant nitrate. Additionally, mass transport phenomena define the electrolyte side of the reaction microenvironment by influencing other interfacial properties (i.e., electric potential, pH and ion concentrations). Therefore, we investigated the influence of mass transport phenomena on NO3RR activity and selectivity.In this study, we used a membrane-separated flow cell as a representative and translatable reactor and titanium foil as a common ammonia-selective NO3RR electrode. By varying the electrolyte flow rate, we controlled the mass transport phenomena in the flow cell and empirically determined the diffusion layer thickness. We found that NO3RR activity generally increased with increasing electrolyte flow rate, as predicted from the decreasing diffusion layer thickness, whereas NH3 selectivity slightly decreased, showing an activity-selectivity trade-off.To unravel the origins of the experimentally observed mass transport effects, simulations using the 1D generalized modified Poisson-Nernst-Planck (GMPNP) model were conducted to spatially resolve interfacial properties. Interfacial cation concentration and pH were the two properties that changed most significantly with electrolyte flow rate. Informed by simulation results, we controlled the bulk electrolyte composition to modify the interfacial cation concentration and pH and examine their impacts on NO3RR activity and selectivity. Nitrate removal rate decreased by half when the bulk cation concentration was lowered to 1/100 of the original bulk concentration but the product distribution was similar. Meanwhile, in phosphate buffered electrolytes with different bulk pH values, NH3 selectivity decreased as bulk pH increased, with NO3RR activity remaining largely unchanged. Therefore, we attributed the influence of mass transport on NO3RR selectivity to changes in the interfacial pH during reaction, likely as the result of the varying NO3RR activity under different flow rates. By comparing the nitrogen species product distribution in different electrolytes, we inferred that the interfacial pH increased rapidly to basic in non-buffered systems regardless of their bulk pH.In summary, we systematically investigated the underexplored effects of mass transport on NO3RR and found that it influenced the reaction activity and selectivity by influencing interfacial properties. This study underscores the importance of scrutinizing the interfacial electrolyte environment in electrocatalyst development and provides insight to optimize NH3 production by engineering the bulk electrolyte.