Autotrophic Denitrifiers Research Articles

The nitrogen cycle and mitigation strategies for nitrogen loss during organic waste composting: A review

Composting is a promising technology to decompose organic waste into humus-like high-quality compost, which can be used as organic fertilizer. However, greenhouse gases (N2O, CO2, CH4) and odorous emissions (H2S, NH3) are major concerns as secondary pollutants, which may pose adverse environmental and health effects. During the composting process, nitrogen cycle plays an important role to the compost quality. This review aimed to (1) summarizes the nitrogen cycle of the composting, (2) examine the operational parameters, microbial activities, functions of enzymes and genes affecting the nitrogen cycle, and (3) discuss mitigation strategies for nitrogen loss. Operational parameters such as moisture, oxygen content, temperature, C/N ratio and pH play an essential role in the nitrogen cycle, and adjusting them is the most straightforward method to reduce nitrogen loss. Also, nitrification and denitrification are the most crucial processes of the nitrogen cycle, which strongly affect microbial community dynamics. The ammonia-oxidizing bacteria or archaea (AOB/AOA) and the nitrite-oxidizing bacteria (NOB), and heterotrophic and autotrophic denitrifiers play a vital role in nitrification and denitrification with the involvement of ammonia monooxygenase (amoA) gene, nitrate reductase genes (narG), and nitrous oxide reductase (nosZ). Furthermore, adding additives such as struvite salts (MgNH4PO4·6H2O), biochar, and zeolites (clinoptilolite), and microbial inoculation, namely Bacillus cereus (ammonium strain), Pseudomonas donghuensis (nitrite strain), and Bacillus licheniformis (nitrogen fixer) can help control nitrogen loss. This review summarized critical issues of the nitrogen cycle and nitrogen loss in order to help future composting research with regard to compost quality and air pollution/odor control.

A composite Fe–C/layered double oxides (Fe–C/LDO) carrier fabrication and application for enhanced removal of nitrate and phosphate from polluted water with a low carbon/nitrogen ratio

In this study, a multifunctional biofilm carrier (Fe–C/LDO) integrating Fe–C micro-electrolysis with Mg–Al layered double oxides (LDO) was developed and applied in a biofilter aimed to remove nitrogen and phosphorus simultaneously from slightly polluted water with a low carbon/nitrogen (C/N) ratio. Four types of carriers were prepared under different conditions and their performance in nitrate reduction and phosphorus removal were compared. Results showed that the composite Fe–C/LDO carrier prepared with polyethylene glycol as the pore-forming agent demonstrated the highest nitrate reduction rate and phosphorate adsorption capacity (2.09 mg PO43--P·g−1). A biofilter packed with the Fe–C/LDO carrier was investigated under heterotrophic conditions (C/N = 4.13) and complete autotrophic conditions (C/N = 0) during a long-term operation. Good removal of NH4+-N (96–98%), TN (92–96%) and PO43--P (50–58%) were achieved in the biofilter under both heterotrophic and autotrophic conditions. High throughput pyrosequencing analysis revealed that a large population of autotrophic denitrifying bacteria Thiobacillus and Thioalkalispira were enriched in the biofilm. Metagenomic analysis was performed to analyze functional genes related to nitrogen metabolism and carbon cycle, and it confirmed the presence of a whole denitrification and Calvin-Benson-Bassham carbon fixation pathway in biofilm. Finally, the possible mechanisms for nitrogen and phosphorus removal in the biofilter were proposed. The Fe–C/LDO carrier not only enriched abundant autotrophic denitrifying bacteria, but also provided numerous Fe–C galvanic cells for nitrate reduction through micro-electrolysis. In addition, the Mg–Al LDO in combination with iron hydroxides generated during micro-electrolysis may contribute greatly to phosphorus removal. Biofilter packed with the multifunctional Fe–C/LDO carrier could be a promising technology for simultaneous nitrogen and phosphorus removal from slightly polluted water.

Enhanced nitrogen removal of constructed wetlands by coupling with the bioelectrochemical system under low temperature: Performance and mechanism

Constructed wetland (CW) integrated with the bioelectrochemical system (BES-CW) is a novel and efficient low-strength wastewater treatment technology. In this study, the integrated CW (E-CW), open-circuit BES-CW (O-CW) and conventional CW (Control) were operated to investigate the performance and mechanism of nitrogen removal in E-CW under low temperature (10 °C). Results showed that low temperature nearly completely inhibited nitrate removal in Control and O-CW, but improved ammonia oxidation efficiencies (99.46% and 99.63%, respectively) due to high dissolved oxygen (DO) concentration in cold influent and enhanced ammonia oxidation archaea (AOA) growth. However, nitrite oxidation became the limited step of nitrification and partial nitrification and denitrification (PND) played the main role in TN removal under low temperature in Control and O-CW. On the contrary, nitrate was almost completely removed in E-CW (98.12%) on the 2nd day even at 10 °C due to its enhanced cold-resistant autotrophic denitrifying bacteria growth by applying low voltage and high DO consumption rate, and accordingly further improved its TN removal efficiency (82.26%), while the average nitrate and TN removal efficiencies in Control were only 11.10% and 37.26%, respectively. Thermomonas, Arenimonas, Gallionella and Thiobacillus were the main denitrifying bacteria attributed to nitrate removal in E-CW under low temperature involving autotrophic denitrification and cathode-driven bioelectrochemical denitrification. Moreover, extremely lower N2O emission from E-CW (0.81%) was obtained than the Control (12.76%), which indicated that the integration of CWs and the bioelectrochemical system is a viable technology for mitigation of greenhouse gas emission from CWs.

Simultaneous removal of tetrachloroethylene and nitrate with a novel sulfur-packed biocathode system: The synergy between bioelectrocatalytic dechlorination and sulfur autotrophic denitrification

The co-contamination of tetrachloroethylene (PCE) and nitrate is ubiquitous in groundwater, posing a serious threat to human health. The lack of a sustainable electron donor source and the inhibitory effect of nitrate on dechlorination make it difficult to achieve simultaneous removal of PCE and nitrate. Here, a novel process coupling bioelectrocatalytic dechlorination and elemental sulfur (S0)-based autotrophic denitrification (eBD-SAD) was established for efficient simultaneous removal of PCE and nitrate. The accelerated nitrate removal in the S0-packed biocathode system mitigated the competition for electrons between reductive dechlorination and denitrification. The 95.9% removal of nitrate within 12 h, 87.2% removal of PCE within 48 h, and stable pH of 7.05–7.49 were achieved at an optimized voltage of 0.5 V. The potential dechlorinators (e.g., Pseudomonas, Chryseobacterium, Desulfovibrio, Mesorhizobium, Geobacter) was significantly enriched in the biocathode biofilm. The S0 particle biofilm was mainly dominated by denitrifiers (e.g., Arcobacter, Pseudomonas, Azospira, Ralstonia). The positive interactions between electroactive and dechlorination bacteria in the cathode biofilm, as well as the S0 autotrophic denitrifiers and non-S0-respiring bacteria in the S0 particle biofilm, might play important roles in the simultaneous removal of PCE and nitrate. This study proposes a novel method for accelerated electro-biodechlorination by alleviating nitrate competition with the SAD process and opens an appealing avenue for the efficient remediation of groundwater co-contaminated with chlorinated aliphatic hydrocarbons and nitrate.

Small-sized salt-tolerant denitrifying and phosphorus removal aerobic granular sludge cultivated with mariculture waste solids to treat synthetic mariculture wastewater

The mariculture waste solids generated in mariculture system were harmlessly reused to cultivate salt-tolerant aerobic granular sludge to further treat synthetic mariculture wastewater. Salt-tolerant aerobic granules with small size (~350 µm) were obtained after 45 days in an anaerobic/aerobic/anoxic sequencing batch reactor. The functional microorganisms were efficiently enriched as high nitrogen and phosphate removal efficiencies of 96.1% and 97.2% respectively were obtained after granulation. Loosely bound protein was conducive to form initial granules. EPS barely changed after granulation. The microbial richness and diversity of mariculture waste solids decreased during the granulation process. Dominant microorganisms were different between young granules and matured granules. Many genera which proliferated in young granules were uncompetitive and reduced or eliminated after granulation. Genera of unclassified_f __Rhodobacteraceae, unclassified_f __Flavobacteriaceae and Ilumatobacter related to phosphorus removal process. Denitrification was mainly performed by denitrifying phosphorus accumulating organisms, denitrifying glycogen accumulating organisms and autotrophic denitrifiers after granulation under saline condition.

High treatment effectiveness for secondary effluent in Fe–C microelectrolysis constructed wetlands with electron donor supplementation

The deep purification of secondary effluent from wastewater treatment plants (WWTPs) is essential for protecting the receiving water environment. Recently, iron-carbon materials have attracted more and more attention for treating secondary effluent. In this study, lab-scale iron-carbon microelectrolysis constructed wetlands (Fe–C CWs) filled with iron scraps (ISs) and granular biochar were established to purify secondary effluent with C/N ratios of 0.5–5. Walnut shells or pyrite were amended as organic or inorganic electron donors to enhance nutrient removal and reduce iron consumption. The IS substrate substantially promoted nutrient removal when the C/N ratio ≤2 (20.3–36.5% for nitrogen and 10.4–21.8% for phosphorus) by supplying Fe2+/H2 for autotrophic denitrification and iron compounds for phosphorus sequestration. Walnut shell supplementation further strengthened nutrient removal, achieving excellent effluent water quality. Fe substrate consumption was alleviated by electron donor supplementation. The IS substrate decreased CO2 emissions while increasing CH4 and N2O fluxes of the biochar-based CWs, and pyrite addition mitigated the global warming potential (GWP) of the Fe–C CWs. When the C/N ratio changed from 5.0 to 0.5, the abundance of autotrophic denitrifiers and Fe cycle-related populations dominated by Dechloromonas, Ferritrophicum and Thiobacillus increased notably in ISs-amended CWs, and they were responsible for the efficient nitrogen removal. Fe–C CW supplemented with organic solid wastes presents high potential for advanced purification of low C/N nitrified effluent.

Cold temperature mediated nitrate removal pathways in electrolysis-assisted constructed wetland systems under different influent C/N ratios and anode materials

Electrolysis had proven to be useful for the enhanced performance in constructed wetlands (CWs). While at cold temperature, the nitrate removal pathways, plant physiological characteristics and microbial community structure in electrolysis-assisted CWs were unclear. Therefore, the purification performance of three electrolysis-assisted horizontal subsurface-flow constructed wetlands (E-HSCWs) with different anodes and a control system in cold seasons were evaluated in this study. E-HSCWs showed a 2.02–83.21% increase of total nitrogen (TN) removal when compared to control, and the gaps were enlarged with increasing C/N (chemical oxygen demand/total nitrogen, COD/TN) ratios. Nitrite accumulation in E-HSCWs presented a first increase then went down trend with increasing C/N ratios, compared to a steady increase in control system. The optimum C/N ratio was 8 in E-HSCWs for both TN and COD removal. Moreover, Ti|IrO2–Ta2O5 (Ti) anode showed the highest potential for TN and COD removal. Less root weight, shorter root length and reduced TN and total phosphorus (TP) contents in roots were observed in wetland plants (Iris sibirica) of E-HSCWs. In E-HSCWs with Fe and C anodes, the nitrate removal was mainly accomplished by autotrophic denitrifier Hydrogenophaga. While in E-HSCWs with Ti anode, the synergistic effect of autotrophic denitrifier Hydrogenophaga and heterotrophic denitrifiers Acidovorax, Simplicispira, Zoogloea accounted for the nitrate removal. These results showed that E-HSCWs at proper C/N ratio of 8 would be promising for nitrate removal at cold temperature.

Fate of Dissolved Nitrogen in a Horizontal Levee: Seasonal Fluctuations in Nitrate Removal Processes.

Horizontal levees are a nature-based approach for removing nitrogen from municipal wastewater effluent while simultaneously providing additional benefits, such as flood control. To assess nitrogen removal mechanisms and the efficacy of a horizontal levee, we monitored an experimental system receiving nitrified municipal wastewater effluent for 2 years. Based on mass balances and microbial gene abundance data, we determined that much of the applied nitrogen was most likely removed by heterotrophic denitrifiers that consumed labile organic carbon from decaying plants and added wood chips. Fe(III) and sulfate reduction driven by decay of labile organic carbon also produced Fe(II) sulfide minerals. During winter months, when heterotrophic activity was lower, strong correlations between sulfate release and nitrogen removal suggested that autotrophic denitrifiers oxidized Fe(II) sulfides using nitrate as an electron acceptor. These trends were seasonal, with Fe(II) sulfide minerals formed during summer fueling denitrification during the subsequent winter. Overall, around 30% of gaseous nitrogen losses in the winter were attributable to autotrophic denitrifiers. To predict long-term nitrogen removal, we developed an electron-transfer model that accounted for the production and consumption of electron donors. The model indicated that the labile organic carbon released from wood chips may be capable of supporting nitrogen removal from wastewater effluent for several decades with sulfide minerals, decaying vegetation, and root exudates likely sustaining nitrogen removal over a longer timescale.

Alkalinity regulation in a sulfur autotrophic denitrifying filter substantially reduced total dissolved solids and sulfate in effluent

Sulfur autotrophic denitrification (SAD) filters are considered a promising technology due to their stable and excellent performance in nitrogen removal, affordable costs, and operational advantages. In this work, a novel operational strategy that employed sodium bicarbonate as an alkalinity source in the autotrophic denitrification filter (S-SAD) was established. With the sufficient supply of alkalinity, the S-SAD reached an excellent denitrification performance (98.01%±0.43%) with a nitrate concentration of 10 mg/L in influent and hydraulic retention time of 3 hrs. The total dissolved solids increment and sulfate concentration in effluent were significantly reduced by one-third, compared with that of the traditional SAD process under the same conditions. The analysis of microbial community indicated that Thiobacilhus, typical species with the functions of simultaneous sulfur oxidation and denitrification, was evidently enriched in the S-SAD. Thus, this present work demonstrated a feasible, relatively cost-effective and environmentally friendly approach to operate SAD towards further application.

Elucidating the role of carbon shell in autotrophic denitrification driven by carbon-coated nanoscale zerovalent iron

Iron-carbon micro-electrolysis (IC-ME) can promote Fe0 corrosion to provide H2 or e− as electron donors for autotrophic denitrifier under organic-limited wastewater, but also stimulate the chemical reduction of NO3-N to undesired NH4-N. Therefore, it is still a challenge to optimize the eletron pathways provided by Fe0 to improve its eletron selectivity for biodenitrification. Herein, a Carbon-coated Iron (Fe0@C) Autotrophic Denitrification (CCIAD) system was first established to achieve higher removal efficiency of NO3-N (95%) and TN (84%) without NO2-N accumulation, and the role of carbon shell (CS) in Fe0@C was systematically evaluated. Hydrophobic CS could prevent iron oxidation from oxygen and isolate Fe0 from NO3-N to alleviate the competition of biological and chemical processes for reducing nitrate. In addition, the CS promoted e− generation, while inhibited the adsorption of NO3-N, thereby increasing the H2 production. Therefore, the assembly of carbon coated Fe0 regulated the electron pathway so that H2 became the main electron donor and dominated the autotrophic denitrification, thereby improving the electron efficiency of Fe0 (82.7%). The sustained electron release of Fe0@C improved its TN removal capacity (78 mg N/g Fe). This study shed light on the electronic pathways provided by Fe0@C and advanced our understanding of Fe0@C-based bio-denitrification for the purpose of next-generation Fe0 design.

Effects of Sulfur (−2) Species on Denitrification by Thiosulfate-Driven Autotrophic Denitrifiers Using S2−, Fes and Scn−

Effects of Sulfur (−2) Species on Denitrification by Thiosulfate-Driven Autotrophic Denitrifiers Using S2−, Fes and Scn−

Responds of Thiosulfate-Driven Autotrophic Denitrifiers to S2−, Fes and Scn− in Autotrophic Denitrification Process: Transformations of Nitrogen and Sulfur

Responds of Thiosulfate-Driven Autotrophic Denitrifiers to S2−, Fes and Scn− in Autotrophic Denitrification Process: Transformations of Nitrogen and Sulfur

Promoting enrichment of sulfur-oxidizing autotrophic denitrifiers via static magnetic fields: Performance and mechanism of magnetic biological effects

Sulfur-driven autotrophic denitrification (SADN) is a promising technology for nitrogen removal from wastewater. In this study, different-strength SMFs (0, 5, 20, 50, 70 mT) were evaluated to investigate the potential of external static magnetic field (SMF) for enriching sulfur-oxidizing autotrophic denitrifiers (SOAD). 50-mT and 70-mT SMFs were most suitable to accelerate the growth of SOAD and the elimination of non-SOAD. The relative abundance of Thiobacillus significantly increased (p < 0.01) from 6.26% in control reactor to 36.15% under 50 mT and 52.51% under 70 mT. Under 50 mT, Thiobacillus denitrificans accumulated most rapidly, with the largest population. Furthermore, functional gene forecast by high-throughput and metagenomic sequencing indicated that SMF changed the two-component system, the adenosine 5‘-monophosphate-activated protein kinase (AMPK) signaling system, the phosphotransferase system (PTS), as well as N/S-related enzymes to regulate stress-response and promote the growth of SOAD. The findings indicated that SMF accelerated the start-up of SADN.

Impact of Periodic Polarization on Groundwater Denitrification in Bioelectrochemical Systems.

Nitrate contamination is a common problem in groundwater around the world. Nitrate can be cathodically reduced in bioelectrochemical systems using autotrophic denitrifiers with low energy investment and without chemical addition. Successful denitrification was demonstrated in previous studies in both microbial fuel cells and microbial electrolysis cells (MECs) with continuous current flow, whereas the impact of intermittent current supply (e.g., in a fluidized-bed system) on denitrification and particularly the electron-storing capacity of the denitrifying electroactive biofilms (EABs) on the cathodes have not been studied in depth. In this study, two continuously fed MECs were operated in parallel under continuous and periodic polarization modes over 280 days, respectively. Under continuous polarization, the maximum denitrification rate reached 233 g NO3--N/m3/d with 98% nitrate removal (0.6 mg NO3--N/L in the effluent) with negligible intermediate production, while under a 30 s open-circuit/30 s polarization mode, 86% of nitrate was removed at a maximum rate of 205 g NO3--N/m3/d (4.5 mg NO3--N/L in the effluent) with higher N2O production (6.6-9.3 mg N/L in the effluent). Conversely, periodic polarization could be an interesting approach in other bioelectrochemical processes if the generation of chemical intermediates (partially reduced or oxidized) should be favored. Similar microbial communities dominated byGallionellaceaewere found in both MECs; however, swapping the polarization modes and the electrochemical analyses suggested that the periodically polarized EABs probably developed a higher ability for electron storage and transfer, which supported the direct electron transfer pathway in discontinuous operation or fluidized biocathodes.

Microbial communities in full-scale woodchip bioreactors treating aquaculture effluents

Woodchip bioreactors are being successfully applied to remove nitrate from commercial land-based recirculating aquaculture system (RAS) effluents. In order to understand and optimize the overall function of these bioreactors, knowledge on the microbial communities, especially on the microbes with potential for production or mitigation of harmful substances (e.g. hydrogen sulfide; H2S) is needed. In this study, we quantified and characterized bacterial and fungal communities, including potential H2S producers and consumers, using qPCR and high throughput sequencing of 16S rRNA gene. We took water samples from bioreactors and their inlet and outlet, and sampled biofilms growing on woodchips and on the outlet of the three full-scale woodchip bioreactors treating effluents of three individual RAS. We found that bioreactors hosted a high biomass of both bacteria and fungi. Although the composition of microbial communities of the inlet varied between the bioreactors, the conditions in the bioreactors selected for the same core microbial taxa. The H2S producing sulfate reducing bacteria (SRB) were mainly found in the nitrate-limited outlets of the bioreactors, the main groups being deltaproteobacterial Desulfobulbus and Desulfovibrio. The abundance of H2S consuming sulfate oxidizing bacteria (SOB) was 5–10 times higher than that of SRB, and SOB communities were dominated by Arcobacter and other genera from phylum Epsilonbacteraeota, which are also capable of autotrophic denitrification. Indeed, the relative abundance of potential autotrophic denitrifiers of all denitrifier sequences was even 54% in outlet water samples and 56% in the outlet biofilm samples. Altogether, our results show that the highly abundant bacterial and fungal communities in woodchip bioreactors are shaped through the conditions prevailing within the bioreactor, indicating that the bioreactors with similar design and operational settings should provide similar function even when conditions in the preceding RAS would differ. Furthermore, autotrophic denitrifiers can have a significant role in woodchip biofilters, consuming potentially produced H2S and removing nitrate, lengthening the operational age and thus further improving the overall environmental benefit of these bioreactors.

Hydrolysis Combined with Chemically Enhanced Primary Treatment to Remove COD and Retain Nitrogen in Sewage

AbstractIn autotrophic nitrogen removal, heterotrophic denitrifiers compete for nitrite/nitrate with autotrophic denitrifiers in the presence of organic matter, leading to a decline in autotrophic ...

Investigating the effects of physical properties of sulphur-based carriers on autotrophic denitrification

ABSTRACT In this study, four sulphur-based carriers (C1–C4) which have different mass ratio of sodium silicate to carrier from 30% to 50% (C1–C3) and the existence of water (C4) were prepared in order to evaluate the effect of the different physical properties on denitrification in sulphur-based autotrophic processes. While the apparent density and the compressive strength decreased as the proportion of sodium silicate increased and water was added in the carriers, the average pore size and the porosity increased from 0.43 to 3.13 µm and from 38% to 67%, respectively. The treatment system using the carrier C4 with the highest surface area was stabilized most rapidly and achieved the highest nitrogen removal efficiency of 85.6 ± 5.0% during a relatively short HRT of 3 h. The efficiency of nitrate removal was enhanced by 36.9% due to the increase of the ratio of sodium silicate in the carriers from C1 to C3, and more 4.8% point of removal rate increased in the carrier C4 by adding water to the carrier C3. The sum of Thiobacillus and Sulfurimonas was obtained up to 65.90% among the microbial community in the carrier C4 which has the highest distribution (38.35%) of pore size above 20 µm considered to be favourable for retaining autotrophic denitrifiers. From the above results, it is obvious that the physical properties of the sulphur-based carrier and its ability of denitrification can be influenced significantly by the composition of the carrier.

Co-removal of 2,4-dichlorophenol and nitrate using a palladized biofilm: Denitrification-promoted microbial mineralization following catalytic dechlorination

The effects of nitrate on 2,4-dichlorophenol (2,4-DCP) dechlorination and biodegradation in a hydrogen (H2)-based palladized membrane biofilm reactor (Pd-MBfR) were studied. The Pd-MBfR was created by synthesizing palladium nanoparticle (Pd0NPs) that spontaneously associated with the biofilm to form a Pd0-biofilm. Without input of nitrate, the Pd-MBfR had rapid and stable catalytic hydrodechlorination: 93% of the 100-μM influent 2,4-DCP was continuously converted to phenol, part of which was then fermented via acetogenesis and methanogenesis. Introduction of nitrate enabled phenol mineralization via denitrification with only a minor decrease in catalytic hydrodechlorination. Phenol-degrading bacteria capable of nitrate respiration were enriched in the Pd0-biofilm, which was dominated by the heterotrophic genera Thauera and Azospira. Because the heterotrophic denitrifiers had greater yields than autotrophic denitrifiers, phenol was a more favorable electron donor than H2 for denitrification. This feature facilitated phenol mineralization and ameliorated denitrification inhibition of catalytic dechlorination through competition for H2. Increased nitrite loading eventually led to deterioration of the dechlorination flux and selectivity toward phenol. This study documents simultaneous removal of 2,4-DCP and nitrate in the Pd-MBfR and interactions between the two reductions.

Enrichment of Autotrophic Denitrifiers From Anaerobic Sludge Using Sulfurous Electron Donors.

This study compared the rates and microbial community development in batch bioassays on autotrophic denitrification using elemental sulfur (S0), pyrite (FeS2), thiosulfate (S2O32–), and sulfide (S2–) as electron donor. The performance of two inocula was compared: digested sludge (DS) from a wastewater treatment plant of a dairy industry and anaerobic granular sludge (GS) from a UASB reactor treating dairy wastewater. All electron donors supported the development of a microbial community with predominance of autotrophic denitrifiers during the enrichments, except for sulfide. For the first time, pyrite revealed to be a suitable substrate for the growth of autotrophic denitrifiers developing a microbial community with predominance of the genera Thiobacillus, Thioprofundum, and Ignavibacterium. Thiosulfate gave the highest denitrification rates removing 10.94 mM NO3– day–1 and 8.98 mM NO3– day–1 by DS and GS, respectively. This was 1.5 and 6 times faster than elemental sulfur and pyrite, respectively. Despite the highest denitrification rates observed in thiosulfate-fed enrichments, an evaluation of the most relevant parameters for a technological application revealed elemental sulfur as the best electron donor for autotrophic denitrification with a total cost of 0.38 € per m3 of wastewater treated.

Enhancing anoxic denitrification of low C/N ratio wastewater with novel ZVI composite carriers

External organic carbon sources are needed to provide electron donors for the denitrification of wastewater with a low COD/NO3−-N (C/N) ratio, increasing the treatment cost. The economic strategy is to enhance the bioactivity and/or biodiversity of denitrifiers to efficiently utilize organic substances in wastewater. In this study, novel zero-valent iron (ZVI) composite carriers were prepared and implemented in a suspended carrier biofilm reactor to enhance the bioactivity and/or biodiversity of denitrifiers. At the influent C/N ratio of 4 (COD was 179.5 ± 5.0 mg/L and TN was 44.2 ± 0.8 mg/L), COD and TN removal efficiencies were 85.1% and 66.4%, respectively, in the reactors filled with 3 wt% ZVI composite carriers. In contrast, COD and TN removal efficiencies were 70.4% and 55.3%, respectively, in the reactor filled with conventional high-density polyethylene (HDPE) biofilm carriers. The biofilm formation on the 3 wt% ZVI composite carriers was optimized due to its higher roughness (surface square roughness increased from 76.0 nm to 93.8 nm) and favorable hydrophilicity (water contact angle dropped to 72.5° ± 1.4° from 94.3° ± 3.2°) compared with the HDPE biofilm carriers. In addition, heterotrophic denitrifiers, Thauera and Dechloromonas, were enriched, whereas autotrophic denitrifiers, Raoultella and Thiobacillus, exhibited high relative abundance in the biofilm of ZVI composite carriers. The coexistence of heterotrophic denitrifiers and autotrophic denitrifiers on the surface of ZVI composite carriers provided mixotrophic metabolism of denitrification (including heterotrophic and iron-based autotrophic), thereby ensuring effective denitrification for wastewater with a low C/N ratio without external organic carbon source addition.