Enhanced Nitrate Removal Research Articles

Improvement of bioavailable carbon source and microbial structure toward enhanced nitrate removal by Tubifex tubifex

The research provides new insights into how T. tubifex affects microbe-available organic carbon transformation, microbial community, and related inorganic nitrogen transformation. With different T. tubifex densities under enriched NO3-N concentration with different C/N ratio, the groups with low and high T. tubifex abundance had 17.1% and 27.2% higher TOC concentrations compared to control group under C/N ratio of 3.0 and 2.0, respectively. According to the OC characteristics analyzed by GC–MS and EEM, the group with T. tubifex contained higher dissolved organic carbon (DOC) content and short-chain compounds. As the NO3-N concentration increased and C/N ratio decreased, the proportions of carboxylic acid derivatives (methyl acetate), sulfur-containing compounds, humic-like products and the emission of CO2 were enhanced with T. tubifex, which could be utilized by denitrifiers directly. The OC degradation was highly correlated with the effect of T. tubifex on microbial community and nutrient removal. The T. tubifex can increase OC transformed into sediment and change the microbial community similarity to that in their digestive system. According to the principal component analysis (PCA), the improved proportions of OC that available for microbes and altered microbial community with T. tubifex could enhance denitrification, which experienced a 21% higher denitrifiers and threefold increased NO3-N removal efficiency than that in control group.

Enhanced nitrate removal and high selectivity towards dinitrogen for groundwater remediation using biochar-supported nano zero-valent iron

We prepared nano zero-valent iron/biochar composites (nZVI/BC) to remove nitrate from groundwater with harmless products. Results show nZVI/BCs derived from different mass ratios of Fe(0) to biochar exhibited different nitrate removal efficiencies, and the mass ratio of 1:2 developed the nZVI/BC with optimum nitrate removal. The nZVI/BC maintained preferable nitrate removal (75.0%–97.0%) over a wide pH range 2–12. Nitrate removal amount declined due to serious corrosion and clogging of nZVI/BC’s surface when initial nitrate concentration exceeded 40 mg/L. The nitrate removal process followed a first-order kinetic reaction. As regards real groundwater, the nZVI/BC removed more nitrate than both pure Fe(0) nanoparticles and biochar. Reaction product analysis revealed 60.1% of removed nitrate selectively became dinitrogen. X-ray photoelectron spectroscopy measurements suggested that nitrate-oxidized Fe(0) resulted in significant increase of magnetite on the surface of nZVI/BC. The redox potential and pH of long duration in reaction changed around −210 mV and 8–9, respectively. These changes facilitated the selective reduction of nitrate to dinitrogen. We also proposed that biochar may provide favorable circumstances for nitrate reduction by directly mediating redox potential, pH and electron transfer, which establishes a possible mechanism for the enhanced removal and selective reduction of nitrate. Our study suggests that nZVI/BC would be a promising alternative for the remediation of nitrate-contaminated groundwater.

A horizontal permeable reactive barrier stimulates nitrate removal and shifts microbial ecology during rapid infiltration for managed recharge

We present results from field experiments linking hydrology, geochemistry, and microbiology during infiltration at a field site that is used for managed aquifer recharge (MAR). These experiments measured how a horizontal permeable reactive barrier (PRB) made of woodchips impacted subsurface nitrate removal and microbial ecology. Concentrations of dissolved organic carbon consistently increased in infiltrating water below the PRB, but not in un-amended native soil. The average nitrate removal rate in soils below the PRB was 1.5 g/m2/day NO3-N, despite rapid infiltration (up to 1.9 m/d) and a short fluid residence time within the woodchips (≤6 h). In contrast, 0.09 g/m2/day NO3-N was removed on average in native soil. Residual nitrate in infiltrating water below the PRB was enriched in δ15N and δ18O, with low and variable isotopic enrichment factors that are consistent with denitrification during rapid infiltration. Many putative denitrifying bacteria were significantly enhanced in the soil below a PRB; Methylotenera mobilis and genera Microbacterium, Polaromonas, and Novosphingobium had log2 fold-changes of +4.9, +5.6, +7.2, and +11.8, respectively. These bacteria were present before infiltration and were not enhanced in native soil. It appears that the woodchip PRB contributed to favorable conditions in the underlying soil for enhanced nitrate removal, quantitatively shifting soil microbial ecology. These results suggest that using a horizontal PRB could improve water quality during rapid infiltration for MAR.

Bioaugmentation with Diaphorobacter polyhydroxybutyrativorans to enhance nitrate removal in a poly (3-hydroxybutyrate-co-3-hydroxyvalerate)-supported denitrification reactor

A newly isolated and identified Diaphorobacter polyhydroxybutyrativorans strain (SL-205) was employed to enhance the denitrification performance of a laboratory-scale solid-phase denitrification (SPD) reactor using poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) as a carbon source, and dynamic variations in microbial communities in the reactor were investigated. Results indicated that bioaugmentation with strain SL-205 enabled rapid reactor startup and improved denitrification performance relative to the reactor inoculated with activated sludge. Illumina sequencing revealed that bioaugmentation also significantly increased Proteobacteria abundance along with increased influent nitrate loading. Additionally, two genera of PHBV-degrading denitrifers, Diaphorobacter and Acidovorax, exhibited higher abundance, and elevated expression of denitrification-associated genes (narG, nirK, and nirS) was observed following bioaugmentation relative to the control at influent nitrate loading ranging from 1.28 g N/(L·d) to 1.6 g N/(L·d).

Novel bimetallic catalyst supported by red mud for enhanced nitrate reduction

The use of red mud as a support material was evaluated in this study to develop a novel bimetallic catalyst for highly reactive and selective nitrate removal from water and wastewater. The catalyst was optimized for the selection of promoter metal (i.e., Sn, Cu, In, and Zn) and then for noble metal (i.e., Pd, Pt, and Au). Sn-Pd-red mud and In-Pd-red mud catalysts achieved a complete nitrate removal. Sn-Pd-red mud showed 5.1 times higher removal kinetics (k = 11.57 × 10−2 min−1) than that of In-Pd-red mud (k = 2.27 × 10−2 min−1). The results from characterization study confirmed that the enhanced nitrate removal kinetics of Sn-Pd-red mud was mainly attributed to 1) its high affinity towards nitrate, 2) low alloying effect of Sn on Pd, 3) high reduction potential of Sn, and 4) high H2 activation by Pd, as compared to those of other red mud supported bimetallic catalysts. We also found that the interaction between CaO and Fe2O3 was the driving force for the enhanced nitrate reduction and high N2 selectivity in this study. A complete nitrate removal with high N2 selectivity (>88%) was preserved during the 11 times of recycling tests with consistent reduction kinetics (k = 10.87 ± 0.48 × 10−2 min−1), indicating that red mud could be an excellent support material for efficient and durable bimetallic catalyst for the enhanced selective nitrate reduction.

Nitrate Removal from Groundwater by Heterotrophic/Autotrophic Denitrification Using Easily Degradable Organics and Nano-Zero Valent Iron as Co-Electron Donors

Heterotrophic/autotrophic denitrification (HAD) is an effective approach to remove nitrate from contaminated groundwater. To improve its performance, easily degradable organics (methanol, ethanol, oxalic acid, and sodium acetate) and nano-zero valent iron (nZVI) were selected as co-electron donors for HAD, and their effectiveness in enhancing HAD to remove nitrate from simulated groundwater was evaluated. It was found that the removal efficiency of HAD to nitrate was significantly affected by the species of easily degradable organics as their different biological availability. Among the tested organics, ethanol-supported HAD system exhibited a better removal efficiency, and after 10 days reaction, it could achieve a high nitrate removal rate to 85.6% with an initial concentration of 90.94 mg/l, and at the end of the test (27 days), nitrate was almost completely removed in the interaction of heterotrophic denitrification (HD) and autotrophic denitrification (AD), and there was no nitrite and ammonium accumulation (< 0.1 and 1.0 mg/l). Moreover, the initial C/N ratios (0.2, 0.5, 1.0, 2.0, and 4.0) of simulated groundwater had a significant influence on nitrate removal by HAD. Increasing the C/N from 0.2 to 2.0 could markedly enhance nitrate removal efficiency, but continuously increased to 4.0 the removal rate just decreased; nevertheless, the accumulation of nitrite and ammonium were closely related to both the C/N ratios and species of organics. The synergistic effect between HD and AD process plays a vital role in the mixotrophic environment. Therefore, this research provides an effective method for nitrate removal from contaminated water with low organic carbon.

Electrochemical study of enhanced nitrate removal in wastewater treatment using biofilm electrode

Bioelectrochemical enhanced nitrate removal in wastewater with high total nitrogen and low organic carbon was electrochemically investigated focusing on the relationship between biochemical and electrochemical nitrogen cycles. Under optimized external voltage of −0.6 V, apparent nitrate removal rate of bioelectrochemical denitrification was 76% higher than normal biofilm denitrification. And with the introduction of biofilm on the electrode, new reduction peak of N2O, much larger current density, and 0.4 V positively shift of on-set potential of nitrate reduction reaction were observed, suggesting a synergy of electrochemical reaction and biological reaction through enhanced electrochemical reduction of intermediate products from biological process. Oxygen reduction reaction could not be avoided during nitrogen electrochemical reduction reaction since their similar reduction potential. But it led to decrease of oxygen concentration and therefore contribute to biological denitrification. Bacteria community tests also supported a dominant bacteria which could denitrify and use external electron.

RETRACTED: Nitrate removal by combined heterotrophic and autotrophic denitrification processes: Impact of coexistent ions

In this current study, sawdust and zero-valent iron (Fe0) were used as co-electron donors to evaluate the effects of coexistent ions on the combined heterotrophic and autotrophic denitrification (HAD) processes. The results showed that HCO3− and SO42− drastically enhanced nitrate removal. The promotion effect derived from both biological and chemical process by HCO3− and chemical process by SO42−. However, Ca2+ ions would remarkably increase nitrate removal due to promoting the electron transfer and the metabolic activities of bacteria, whereas the Cu2+ ions inhibited the biological process due to the deleterious effect on bacteria. Meanwhile, Fe2+ and Fe3+ ions exhibited inhibition effect firstly because of their toxicity to bacteria and promotion subsequently due to their enhancement on Fe0 chemical denitrification. Moreover, byproducts such as nitrite, ammonium, dissolved organic carbon (DOC), etc. were also influenced by common ions.

Multilayer system of nanoscale zero valent iron and Nano-Fe/Cu particles for nitrate removal in porous media

Abstract This work studies the nitrate removal efficiency in porous media using nanoscale zero valent iron (nZVI) and modified surface of nZVI using Cu metal (known as bimetallic nano-Fe/Cu particles) in a multilayer system through an upflow packed sand column. The effect of multilayer and different thicknesses of nZVI/sand and nano-Fe/Cu particles on the nitrate removal performance was investigated. 10 g of nZVI and nano-Fe/Cu particles were introduced in each column with different layers configuration. The initial nitrate concentration was 45 NO3––N mg/L. The distribution of nitrate trapped inside the column was evaluated by measuring nitrate concentrations, not only at the column inlet and outlet, but also at various intermediate sampling ports along the depth of the column. The results showed that the best condition to remove nitrate among nZVI/sand columns was observed when using a single 10-cm high layer of nZVI/sand and more than 97% of influent nitrate was removed. Whereas in (nano-Fe/Cu)/sand columns, the optimum performance was noted when using double 5-cm high layers of (nano-Fe/Cu)/sand and a complete removal was attained (100%). Furthermore, nano-Fe/Cu particles showed better removal efficiency than nZVI for the treatment of nitrate in simulated groundwater. Based on the results obtained, it was concluded that using nZVI in the full length of porous media or using nano-Fe/Cu particles in multilayer porous media could enhance nitrate removal effectively.

Enhancing Nitrate Removal from Freshwater Pond by Regulating Carbon/Nitrogen Ratio.

Nitrogen accumulation is a serious environmental problem in freshwater ponds, which can lead to massive death of fish and shrimps as well as the eutrophication. The removal of nitrate by regulating the carbon to nitrogen (C/N) ratio and the underlying mechanisms were investigated. The nitrate removal system comprised 530-mL medium containing 5 mg/L -N and 0–66.6 mg/L COD (i.e., C/N ratio of 0–13.3) and 20 g ponds sediments. When the C/N ratio was higher than 8, the nitrate removal efficiency nearly reached 100% during the incubation period and the accumulation of nitrite was negligible. When the C/N ratio was below 8, the nitrate removal efficiency was lower and significant nitrite accumulation occurred. The nitrate removal rate increased with the C/N ratio increased, which was ascribed to the increase in the absolute abundance of denitrifiers (nirS, nirK, and nosZ). Although both nirS-type and nirK-type denitrifiers were found in the sediments of freshwater pond, nirS-type denitrifiers were predominant. Dechloromonas was the major nirS-type denitrifier for nitrate removal in nirS-type with the C/N ratios above 5.33, while the majority of the nirK-type denitrifiers were unclassified. Thus, this study implied that the appropriate C/N ratio played an important role on the removal of excess nitrate from freshwater ponds.

Bacterial community and nitrate removal by simultaneous heterotrophic and autotrophic denitrification in a bioelectrochemically-assisted constructed wetland

To enhance nitrate removal in constructed wetlands (CWs), a bioelectrochemically-assisted CW (BECW) integrating a three-dimensional biofilm-electrode reactor (3D-BER) into the CW was evaluated for the effectiveness of combined autotrophic and heterotrophic denitrification in the presence of organic matter and applied current. The effects of COD/N ratios on nitrate removal were investigated, and the bacterial communities in the granular active carbon (GAC) and graphite felt (GF) in the reactor’s cathode region were compared. The highest NO3−-N and TN removal efficiencies of 91.3±7.2% and 68.8±7.9% were obtained at the COD/N ratio of 5. According to the results of high-throughput sequencing analysis, sample GAC was enriched with a high abundance of Pseudomonas (17.29%) capable of autotrophic and heterotrophic denitrification, whereas autotrophic bacteria Thiobacillus (43.94%) was predominant in sample GF. The synergy between heterotrophic and autotrophic denitrification bacteria is believed to cause the high and stable nitrogen removal performance.

Nitrogen transformations and removal efficiency enhancement of a constructed wetland in subtropical Taiwan

Urbanization condenses reactive nitrogen into cities leaving threats of nitrogen pollution onto nearby environments when sewage is not properly treated. Constructed wetland is an ecological and economical way to remove reactive nitrogen. We investigated the seasonal nitrogen transformations and removal pathways in a surface-flow constructed wetland (93,000m2 with five treatment cells), which treats domestic wastewater in subtropical Taiwan. By using isotopic pairing technique, we found denitrification exceeds anammox dominated the nitrogen removal pathway throughout seasons. The potential denitrification (0.09 to 2.84gNm−2d−1) in the overlying water was in the same magnitude relative to that in sediments (1.26 to 4.14gNm−2d−1). The denitrification rates in sediments were highest in summer followed by autumn and winter. The concentration removal efficiencies of ammonium and dissolved inorganic nitrogen (DIN) were both highest in summer, then decreased significantly in autumn and winter. Temperature is a significant regulator for seasonal nitrogen removal. However, a positive correlation was observed between the potential denitrification rates and the amount of 15NO3− addition, indicating that nitrate addition may still stimulate denitrification under low temperature condition in winter (15.2–16.3°C). Since nitrate concentrations in porewater were much lower than that in water column for autumn and winter, we speculated NOx−-N (nitrite and nitrate) supply to the sediments was a limiting factor for low DIN removal efficiency. We proposed to enhance nitrate removal efficiency via denitrification by physically promoting NOx−-N and oxygen exchanges through the sediment-water interface.

Nitrate removal, spatiotemporal communities of denitrifiers and the importance of their genetic potential for denitrification in novel denitrifying bioreactors

Nitrate treatment performance and denitrification activity were compared between denitrifying biological filters (DNBFs) based on dewatered alum sludge (DAS) and neutralized used acid (NUA). The spatiotemporal distribution of denitrifying genes and the genetic potential associated with denitrification activity and nitrate removal in both DNBFs were also evaluated. The removal efficiency of NUA–DNBF increased by 8% compared with that of DAS–DNBF, and the former NUA–DNBF emitted higher amount of N2O. Analysis of abundance and composition profiles showed that denitrifying gene patterns varied more or less in two matrices with different depths at three sampling times. Burkholderiales, Rhodocyclales, and Rhizobiales were the most commonly detected in both media during stable periods. Denitrification was determined by the abundance of specific genes or their ratios as revealed by controlling factors. The enhanced nitrate removal could be due to increasing qnosZ or decreasing ∑qnir/qnosZ. Furthermore, NUA–DNBF solely reduced nitrate by increasing the denitrification enzyme activity.

Enhanced nitrate removal by novel bimetallic Fe/Ni nanoparticles supported on biochar

In this study, a novel bimetallic iron/nickel nanoparticles supported on biochar (BC@Fe/Ni) was established to degrade nitrate pollution in water. The nanocomposite was prepared from steel pickling waste liquor and sugarcane bagasse, which are made at low cost. The particle sizes of the nanocomposite ranged from about 10 nm to 20 nm and its specific surface area (59.83 m2/g) was about 71% bigger than that of nanoscale zero valent iron prepared from steel pickling waste liquor (S-NZVI), which proved that biochar had an excellent dispersal effect on bimetallic iron/nickel particles. Without controlling the pH, high nitrate removal rates over 93% could be realized in nitrate concentrations below 50 mg/L. The results of batch experiments demonstrated that the kinetics curves fitted the pseudo-first-order reaction well. And it was also found that higher dosages of the nanocomposite, lower initial nitrate concentrations, and acid medium facilitated nitrate degradation. In addition, the observed pseudo-first order rate coefficient in nitrate (20 mg/L) degradation rate by 4 g/L of the bimetallic iron/nickel supported on biochar was 30% faster than that by 2 g/L the bimetallic iron/nickel nanoparticles, due to the existence of biochar. And it was 75% faster than that by 2 g/L the nanoscale zero valent iron due to the existence of both biochar and the nanoscale zero valent nickel catalyst. Furthermore, iron and nickel ion pollution occurred in the denitrification system with bimetallic iron/nickel nanoparticles but not in that with the bimetallic iron/nickel nanoparticles supported on biochar, due to the biochar’s adsorption. After denitrification by the bimetallic iron/nickel nanoparticles supported on biochar, most of nitrate turned into ammonia (17.04 mg/L) and there only was a small amount of nitrite (0.024 mg/L) with high nitrate removal rate (99.5%). And most of ammonia can be absorbed by cation exchange resin with finial ammonia concentration of 0.58 mg/L. At last, the concentrations of nitrate (0.24 mg/L), nitrite (0.021 mg/L), and total nitrogen (0.84 mg/L) were lowest in the nanocomposite system. Overall, the bimetallic iron/nickel nanoparticles supported on biochar exhibited clear advantages over bimetallic iron/nickel nanoparticles and nanoscale zero valent iron with respect to degrading nitrate efficiently, preventing iron and nickel pollution and removing ammonia pollution in combination with cation exchange resin.

Impact of external carbon dose on the removal of micropollutants using methanol and ethanol in post-denitrifying Moving Bed Biofilm Reactors

Addition of external carbon sources to post-denitrification systems is frequently used in wastewater treatment plants to enhance nitrate removal. However, little is known about the fate of micropollutants in post-denitrification systems and the influence of external carbon dosing on their removal. In this study, we assessed the effects of two different types and availability of commonly used carbon sources —methanol and ethanol— on the removal of micropollutants in biofilm systems. Two laboratory-scale moving bed biofilm reactors (MBBRs), containing AnoxKaldnes K1 carriers with acclimated biofilm from full-scale systems, were operated in continuous-flow using wastewater dosed with methanol and ethanol, respectively. Batch experiments with 22 spiked pharmaceuticals were performed to assess removal kinetics. Acetyl-sulfadiazine, atenolol, citalopram, propranolol and trimethoprim were easily biotransformed in both MBBRs (biotransformations rate constants kbio between 1.2 and 12.9 L gbiomass−1 d−1), 13 compounds were moderately biotransformed (rate constants between 0.2 and 2 L gbiomass−1 d−1) and 4 compounds were recalcitrant. The methanol-dosed MBBR showed higher kbio (e.g., 1.5–2.5-fold) than in the ethanol-dosed MBBR for 9 out of the 22 studied compounds, equal kbio for 10 compounds, while 3 compounds (i.e., targeted sulfonamides) were biotransformed faster in the ethanol-dosed MBBR. While biotransformation of most of the targeted compounds followed first-order kinetics, removal of venlafaxine, carbamazepine, sulfamethoxazole and sulfamethizole could be described with a cometabolic model. Analyses of the microbial composition in the biofilms using 16S rRNA amplicon sequencing revealed that the methanol-dosed MBBR contained higher microbial richness than the one dosed with ethanol, suggesting that improved biotransformation of targeted compounds could be associated with higher microbial richness. During continuous-flow operation, at conditions representative of full-scale denitrification systems (hydraulic residence time = 2 h), the removal efficiencies of micropollutants were below 35% in both MBBRs, with the exception of atenolol and trimethoprim (>80%). Overall, this study demonstrated that MBBRs used for post–denitrification could be optimized to enhance the biotransformation of a number of micropollutants by accounting for optimal carbon sources and extended residence time.

Development and reaction mechanism of efficient nano titanium electrode: Reconstructed nanostructure and enhanced nitrate removal efficiency

In this study, an efficient nano titanium electrode (ENTE, treated electrode) was fabricated for electroreduction of nitrate in groundwater. The nano titanium electrode showed a regular reticulate nanostructure by scanning electron microscopy. An acid environment and electrochemical treatment of the nano titanium electrode (NTE, untreated electrode) induced ruptures into its regular nanostructure, which led to improved nitrate electroreduction efficiency. X-ray photoelectron spectra revealed titanium dioxide in the anatase phase at the electrode surface and the treatment did not affect the composition or crystal form. The electroreduction activity of the treated electrode (94.6%) was almost twice as high as that of the untreated electrode (65.8%). In the presence of NaCl, by-products that formed during electroreduction, particularly ammonia, were removed by hypochlorite ions. The optimum NaCl concentration in the electrochemical system was determined as 1.5g/L, which led to a low level of ammonia formation (<1.0%) and an overall efficiency total nitrogen removal of 92.6%.

Intensified nitrate and phosphorus removal in an electrolysis -integrated horizontal subsurface-flow constructed wetland

A novel electrolysis-integrated horizontal subsurface-flow constructed wetland system (E-HFCWs) was developed for intensified removal of nitrogen and phosphorus contaminated water. The dynamics of nitrogen and phosphorus removal and that of main water qualities of inflow and outflow were also evaluated. The hydraulic retention time (HRT) greatly enhanced nitrate removal when the electrolysis current intensity was stabilized at 0.07 mA/cm2. When the HRT ranged from 2 h to 12 h, the removal rate of nitrate increased from 20% to 84%. Phosphorus (P) removal was also greatly enhanced—exceeding 90% when the HRT was longer than 4 h in the electrolysis-integrated HFCWs. This improved P removal is due to the in-situ formation of ferric ions by anodizing of sacrificial iron anodes, causing chemical precipitation, physical adsorption and flocculation of phosphorus. Thus, electrolysis plays an important role in nitrate and phosphorus removal. The diversity and communities of bacteria in the biofilm of substrate was established by the analysis of 16S rDNA gene sequences, and the biofilm was abundant with Comamonadaceae and Xanthomonadaceae bacteria in E-HFCWs. Test results illustrated that the electrolysis integrated with horizontal subsurface-flow constructed wetland is a feasible and effective technology for intensified nitrogen and phosphorus removal.

Enhanced nitrate removal in self-supplying carbon source constructed wetlands treating secondary effluent: The roles of plants and plant fermentation broth

In this study, a self-supplying carbon source constructed wetland (CW) was developed and evaluated. Both the effects of plants (Typha latifolia) and plant fermentation broth on nitrate removal were measured. The results showed that the addition of plant fermentation broth greatly improved the nitrate removal rate. As the ratio of added chemical oxygen demand to influent nitrate (CODAdd/NO3-N ratio) increased from 0 to 3, the nitrate removal rate attributed to the plants increased from 0.09 to 0.29gNm−3d−1, but the proportion of total nitrate removal decreased from 27.3% to 10.7%, and denitrification was always the dominant nitrate removal mechanism. Furthermore, there were strong positive correlations between the CODAdd/NO3-N ratio and the nitrate removal rate, both in unplanted (R2=0.977) and planted (R2=0.996) microcosms. Plant biomass could potentially support a nitrate removal rate of 0.05–0.54gNm−2d−1 in self-supplying carbon source CWs.

Evaluation of sustainable electron donors for nitrate removal in different water media

An external electron donor is usually included in wastewater and groundwater treatment systems to enhance nitrate removal through denitrification. The choice of electron donor is critical for both satisfactory denitrification rates and sustainable long-term performance. Electron donors that are waste products are preferred to pure organic chemicals. Different electron donors have been used to treat different water types and little is known as to whether there are any electron donors that are suitable for multiple applications. Seven different carbon rich waste products, including liquid and solid electron donors, were studied in comparison to pure acetate. Batch-scale tests were used to measure their ability to reduce nitrate concentrations in a pure nutrient solution, light greywater, secondary-treated wastewater and tertiary-treated wastewater. The tested electron donors removed oxidised nitrogen (NOx) at varying rates, ranging from 48 mg N/L/d (acetate) to 0.3 mg N/L/d (hardwood). The concentrations of transient nitrite accumulation also varied across the electron donors. The different water types had an influence on NOx removal rates, the extent of which was dependent on the type of electron donor. Overall, the highest rates were recorded in light greywater, followed by the pure nutrient solution and the two partially treated wastewaters. Cotton wool and rice hulls were found to be promising electron donors with good NOx removal rates, lower leachable nutrients and had the least variation in performance across water types.

Effect of Zero Valent Iron (ZVI) in Wastewater Treatment: A Review

Cathodic hydrogen was produced in the presence of anaerobic zero valent iron (ZVI) corrosion by water. It can enhance microbial denitrification to convert nitrate to N2O and N2. Autotrophic denitrifying growth on ZVI can enhance nitrate removal. Results showed that by increasing nitrate removal rates, innocuous gases (N2O and N2) are produced rather than ammonium. Using steel wool with a small specific surface area instead of powdered ZVI, pH was not significantly increased. Little pH change was caused by corrosion. This is a positive condition for autotrophic denitrifying bacteria. ZVI was used in permeable reactive barriers (PRBs) process under anaerobic conditions. It used sulfate reducing bacteria for immobilization of heavy metals. In fermentation, methanogenesis and sulfate reduction was complete after adding ZVI to mixed anaerobic cultures. It was found that methane production increased and sulfate was reduced. This was due to readily utilizable ZVI which served as a slow-release electron donor for methanogenesis and sulfate reduction. ZVI has potential as a useful material in bioremediation.