Impact of organic carbon on sulfide-driven autotrophic denitrification: Insights from isotope fractionation and functional genes

Additional organics are generally supplemented in the sulfide-driven autotrophic denitrification system to accelerate the denitrification rate and reduce sulfate production. In this study, different concentrations of sodium acetate (NaAc) were added to the sulfide-driven autotrophic denitrification reactor, and the S0 accumulation increased from 7.8% to 100% over a 120-day operation period. Batch experiments revealed a threefold increase in total nitrogen (TN) removal rate at an Ac--C/N ratio of 2.8 compared to a ratio of 0.5. Addition of organic carbon accelerated denitrification rate and nitrite consumption, which shortened the emission time of N2O, but increased the N2O production rate. The lowest N2O emissions were achieved at the Ac--C/N ratio of 1.3. Stable isotope fractionation is a powerful tool for evaluating different reaction pathways, with the 18ε/15ε values in nitrate reduction ranging from 0.5 to 1.0. This study further confirmed that isotope fractionation can reveal denitrifying nutrient types, with the 18ε (isotopic enrichment factor of oxygen)/15ε (isotopic enrichment factor of nitrogen) value approaching 1.0 for autotrophic denitrification and 0.5 for heterotrophic denitrification. Additionally, the 18ε/15ε values can indicate changes in nitrate reductase. There is a positive correlation between the 18ε/15ε values and the abundance of the functional gene napA, and a negative correlation with the abundance of the gene narG. Moreover, 18ε and 15ε were associated with changes in kinetic parameters during nitrate reduction. In summary, the combination of functional gene analysis and isotope fractionation effectively revealed the complexities of mixotrophic denitrification systems, providing insights for optimizing denitrification processes.

Establishment and optimization of sulfur-based autotrophic-heterotrophic denitrification biofilters for advanced post-anaerobic treatment of effluent from kitchen wastewater and landfill leachate under low temperature

Landfill leachate treatment is a major challenge in wastewater treatment. In this study, two sulfur-based autotrophic-heterotrophic denitrification biofilters (Ra biofilter with room-temperature molded filler and Rb biofilter with melt molded filler) were used to treat kitchen-landfill leachate at low temperatures. The effects of reflux ratio, concentrations of NaHCO3, and Na2S2O3 on the total nitrogen removal efficiency were analyzed, and based on response surface methodology, the optimum parameters were determined. After optimization, the total nitrogen removal efficiency for the Ra and Rb biofilters increased by 83% and 81%, respectively. Moreover, sulfur-based autotrophic denitrification accounted for more than 70% of the nitrogen removal in both biofilters. Based on high-throughput sequencing results, the functional bacteria exhibited high abundance in the Ra biofilter, indicating that the room-temperature molded filler favored the enrichment of functional bacteria. These findings were important for optimizing the operation of sulfur autotrophic-heterotrophic denitrification biofilters at low temperatures.

Uncovering interactions among ternary electron donors of organic carbon source, thiosulfate and Fe0 in mixotrophic advanced denitrification: Proof of concept from simulated to authentic secondary effluent

To offset the imperfections of higher cost and emission of CO2 greenhouse gas in heterotrophic denitrification (HDN) as well as longer start-up time in autotrophic denitrification (ADN), we synergized the potential ternary electron donors of organic carbon source, thiosulfate and zero-valent iron (Fe0) to achieve efficient mixotrophic denitrification (MDN) of oligotrophic secondary effluent. When the influent chemical oxygen demand to nitrogen (COD/N) ratio ascended gradually in the batch operation with sufficient sulfur to nitrogen (S/N) ratio, the MDN with thiosulfate and Fe0 added achieved the highest TN removal for treating simulated and authentic secondary effluents. The external carbon is imperative for initiating MDN, while thiosulfate is indispensable for promoting TN removal efficiency. Although Fe0 hardly donated electrons for denitrification, the suitable circumneutral environment for denitrification was implemented by OH- released from Fe0 corrosion, which neutralized H+generated during thiosulfate-driven ADN. Meanwhile, Fe0 corrosion consumed the dissolved oxygen (DO) and created the low DO environment suitable for anoxic denitrification. This process was further confirmed by the continuous flow operation for treating authentic secondary effluent. The TN removal efficiency achieved its maximum under the combination condition of influent COD/N ratio of 3.1-3.5 and S/N ratio of 2.0-2.1. Whether in batch or continuous flow operation, the coordination of thiosulfate and Fe0 maintained the dominance of Thiobacillus for ADN, with the dominant heterotrophic denitrifiers (e.g., Plasticicumulans, Terrimonas, Rhodanobacter and KD4-96) coexisting in MDN system. The interaction insights of ternary electron donors in MDN established a pathway for realizing high-efficiency nitrogen removal of secondary effluent.

Promoting heterotrophic denitrification of Pseudomonas hunanensis strain PAD-1 using pyrite: A mechanistic study

Denitrification is critical for removing nitrate from wastewater, but it typically requires large amounts of organic carbon, which can lead to high operating costs and secondary environmental pollution. To address this issue, this study proposes a novel method to reduce the demand for organic carbon in denitrification. In this study, a new denitrifier, Pseudomonas hunanensis strain PAD-1, was obtained with properties for high efficiency nitrogen removal and trace N2O emission. It was also used to explore the feasibility of pyrite-enhanced denitrification to reduce organic carbon demand. The results showed that pyrite significantly improved the heterotrophic denitrification of strain PAD-1, and optimal addition amount was 0.8-1.6 g/L. The strengthening effect of pyrite was positively correlated with carbon to nitrogen ratio, and it could effectively reduce demand for organic carbon sources and enhance carbon metabolism of strain PAD-1. Meanwhile, the pyrite significantly up-regulated electron transport system activity (ETSA) of strain PAD-1 by 80%, nitrate reductase activity by 16%, Complex III activity by 28%, and napA expression by 5.21 times. Overall, the addition of pyrite presents a new avenue for reducing carbon source demand and improving the nitrate harmless rate in the nitrogen removal process.

Carbon source shaped microbial ecology, metabolism and performance in denitrification systems

The limited information on microbial interactions and metabolic patterns in denitrification systems, especially those fed with different carbon sources, has hindered the establishment of ecological linkages between microscale connections and macroscopic reactor performance. In this work, denitrification performance, metabolic patterns, and ecological structure were investigated in parallel well-controlled bioreactors with four representative carbon sources, i.e., methanol, glycerol, acetate, and glucose. After long-term acclimation, significant differences were observed among the four bioreactors in terms of denitrification rates, organic utilization, and heterotrophic bacterial yields. Different carbon sources induced the succession of denitrifying microbiota toward different ecological structures and exhibited distinct metabolic patterns. Methanol-fed reactors showed distinctive microbial carbon utilization pathways and a more intricate microbial interaction network, leading to significant variations in organic utilization and metabolite production compared to other carbon sources. Three keystone taxa belonging to the Verrucomicrobiota phylum, SJA-15 order and the Kineosphaera genus appeared as network hubs in the methanol, glycerol, and acetate-fed systems, playing essential roles in their ecological functions. Several highly connected species were also identified within the glucose-fed system. The close relationship between microbial metabolites, ecological structures, and system performances suggests that this complex network relationship may greatly contribute to the efficient operation of bioreactors.

Organic carbon source excites extracellular polymeric substances to boost Fe0-mediated autotrophic denitrification in mixotrophic system

Fe⁰-mediated autotrophic denitrification (ADN) can be suppressed by iron oxide coverage resulting from Fe⁰ corrosion. The mixotrophic denitrification (MDN) coupling Fe⁰-mediated ADN with heterotrophic denitrification (HDN) can circumvent the weakening of Fe⁰-mediated ADN over operation time. But the interaction between HDN and Fe⁰-mediated ADN for nitrogen removal of secondary effluent with deficient bioavailable organics remains unclear. When the influent COD/NO₃^--N ratio increased from 0.0 to 1.8-2.1, the TN removal efficiency was promoted significantly. The increased carbon source did not inhibit ADN, but promoted ADN and HDN synchronously. The formation of extracellular polymeric substances (EPS) was also facilitated concomitantly. Protein (PN) and humic acid (HA) in EPS increased significantly, which capable of accelerating electron transfer of denitrification. Due to that the electron transfer of HDN occurs intracellularly, the EPS with the capacity of accelerating electron transfer had a negligible influence on HDN. But for Fe⁰-mediated ADN, the increased EPS as well as corresponding PN and HA facilitated TN and NO₃^--N removal significantly, while accelerated the electron release originating from Fe⁰ corrosion. The bioorganic-Fe complexes were generated on Fe⁰ surface after used, meaning that the soluble EPS and soluble microbial products (SMP) participated in the electron transfer of Fe⁰-mediated ADN. The coexistence of HDN and ADN denitrifiers demonstrated the synchronous enhancement of HDN and ADN by the external carbon source. From the perspective of EPS and related SMP, the insight of enhancing Fe⁰-mediated ADN by external carbon source is beneficial to implement high-efficiency MDN for organics-deficient secondary wastewater.

Microbial mixotrophic denitrification using iron(II) as an assisted electron donor

Mixotrophic denitrification processes have a great potential in nitrogen removal in biological wastewater treatment processes. However, so far, few studies have focused on the mixotrophic denitrification system using Fe(II) as an exclusively assisted electron donors and the underlying mechanisms in such a process remain unclear. Furthermore, the mechanisms by which microorganisms cover carbon, nitrogen, phosphorus and iron in an iron-assisted mixotrophic system remain unrevealed. In this work, we explore the feasibility of using Fe(II) as an assisted electron donor for enhancing simultaneous nitrogen and phosphorus removal via long-term reactor operation and batch tests. The results show that Fe(II) could provide electrons for efficient nitrate reduction and that biological reactions played a predominant role in these systems. In these systems Thermomonas, a strain of nitrate-reduction Fe(II)-oxidation bacterium, was enriched and accounted for a maximum abundance of 60.2%. These findings indicate a great potential of the Fe(II)-assisted mixotrophic denitrification system for practical use as an efficient simultaneous nitrogen and phosphorus removal process.

Towards efficient heterotrophic recovery of N2O via Fe(II)EDTA-NO: A modeling study

Efficient recovery of nitrous oxide (N₂O) through heterotrophic denitrification with the help of Fe(II)EDTA-NO as a chelating agent has been regarded as an ideal technology to treat nitric oxide (NO)-rich flue gas. In this study, an integrated NO-based biological denitrification model was developed to describe the sequential reduction of the NO fixed in Fe(II)EDTA-NO with organic carbon as the electron donor. With the inclusion of only the key pathways contributing to nitrogen transformation, the model was firstly developed and then calibrated/validated and evaluated using the data of batch tests mediated by the identified functional heterotrophic bacteria at various substrates concentrations and then used to explore the possibility of enhancing N₂O recovery by altering the substrates condition and reactor setup. The results demonstrated that the optimal COD/N ratio decreased consistently from 1.5 g-COD/g-N at the initial NO concentration of 40 g-N/m³ to 1.0 g-COD/g-N at the initial NO concentration of 420 g-N/m³. Furthermore, sufficiently increasing the headspace volume of the reactor was considered an ideal strategy to obtain ideal N₂O production of 86.6 % under the studied conditions. The production of high-purity N₂O (98 %) confirmed the practical application potential of this integrated treatment technology to recover a valuable energy resource from NO-rich flue gas.

A hormesis-like effect of FeS on heterotrophic denitrification and its mechanisms

To alleviate the insufficiency of carbon source in sewage, many sulfur-containing inorganic electron donors were added into traditional heterotrophic denitrification process. However, the effects of extraneous inorganic electron donors on heterotrophic denitrification were still largely unknown. In this study, a hormesis-like effect of ferrous sulfide (FeS, a representative inorganic electron donors) on Paracoccus denitrificans was observed. Total nitrogen (TN) removal efficiency of P. denitrificans rose by 15% with the increase of FeS dosage from 0 to 0.3 g L^-1 (low level), whereas the TN removal significantly decreased to 53% as the dosage of FeS mounted up to 5.0 g L^-1 (high level). Furthermore, the impacts of FeS on glucose utilization and bacterial growth exhibited hormesis-like effects. A subsequent mechanistic study revealed that above influences were caused by its released ions (Fe²⁺, Fe³⁺, and S^2-) rather than particle size. Further study illustrated that low dosage of FeS released a small amount of Fe²⁺ and Fe³⁺, which provided sufficient electrons via promoting glucose utilization, then improved denitrification. Conversely, FeS with high dosage inhibited denitrification via its released S^2-, which suppressed the activity of key denitrifying enzymes rather than influenced glucose metabolism and electron provision. Our results provide an insight into improving denitrification efficiency of the mixotrophic process coexisting with autotrophic and heterotrophic denitrifiers.

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.

Influences of carbon sources on N2O production during denitrification in freshwaters: Activity, isotopes and functional microbes

Denitrification is one of the major sources of N₂O in freshwaters. Diverse forms of organic compounds act as the electron donors for microbial denitrification. However, the influences of carbon sources on N₂O production, N₂O reduction, isotope fractionation and functional microbes during denitrification were largely unknown. In this study, five forms of carbon sources (i.e. acetate, citrate, glucose, cellobiose and leucine) were used to enrich denitrifiers in freshwater sediments. N₂O conversion in the enrichments was investigated by a combination of inhibition technique, natural stable isotope method and metagenomics. Acetylene was effective in inhibiting N₂O reduction without influencing the isotopic characteristics during N₂O production. Glucose led to the least N₂O production and reduction, in accordance with the lowest abundance of both NO and N₂O reductases in this enrichment. δ¹⁸O and site preference value (SP, =δ¹⁵N^α-δ¹⁵N^β) of N₂O were sensitive to discriminate the five carbon sources, except when comparing acetate and leucine. Isotopic values of N₂O were not significantly different in these two enrichments due to the similarity of NO reductases - Pseudomonas-type cNorB. Specifically, the enrichment with cellobiose produced N₂O with the lowest δ¹⁸O values (39.4‰±1.1‰), due to Alicycliphilus with both cNorB and qNorB. The enrichment with glucose led to the highest SP values (8.9‰±8.6‰), caused by Thiobacillus-type cNorB. Our results demonstrated the link between carbon sources, N₂O production and reduction, isotopic signatures, microbial populations and enzymes during denitrification in freshwaters.

Cysteine reduced the inhibition of CO2 on heterotrophic denitrification: Restoring redox balance, facilitating iron acquisition and carbon metabolism

The direct effect of CO₂ on denitrification has attracted great attention currently. Our previous studies have confirmed that CO₂ inhibited heterotrophic denitrification and caused high nitrite accumulation and nitrous oxide emission. Cysteine is a widely reported bio-accelerator; however, its effect on denitrification under CO₂ exposure remains unknown. In this paper, the effect of cysteine on heterotrophic denitrification and its mechanisms under CO₂ exposure were explored with the model denitrifier, Paracoccus denitrificans. We observed that total nitrogen removal increased from 17.9% to 90.4% as cysteine concentration increased from 0 to 50 μM, probably due to restoration of cell growth and viability. Further study showed that cysteine reduced the inhibition of CO₂ on denitrification due to multiple positive influences: (1) regulating glutathione metabolism to eliminate intracellular reactive nitrogen species (RNS), while reducing extracellular polymeric substances (EPS) levels and altering its composition, ultimately restoring cell membrane integrity (2) facilitating the transport and metabolism of carbon sources to increase NADH production, and (3) increasing intracellular iron and up-regulating the expression of key iron transporters genes (AfuA, AfuB, ExbB and TonB) to restore the transport and consumption of electron. This study suggests that cysteine can be added to recover heterotrophic denitrification performance after inhibition by elevated CO₂.

Reduced fertilization mitigates N2O emission and drip irrigation has no impact on N2O and NO emissions in plastic-shed vegetable production in northern China

Plastic-shed vegetable production in China creates hotspots for emission of the potent greenhouse gas nitrous oxide (N₂O) and the atmospheric pollutant nitric oxide (NO). To mitigate N₂O and NO emissions, determination of the predominant processes of N₂O and NO generation in plastic-shed vegetable production is important. Here, we reported the findings of a 2-year experimental study on the effects of reduced fertilization and/or drip irrigation on N₂O and NO emissions during plastic-shed tomato production in northern China. Five treatments were applied: 1) over fertilization and flood irrigation (conventional practice); 2) fertilization reduced by 20% and flood irrigation; 3) fertilization reduced by 20% and drip irrigation; 4) fertilization reduced by 30% and drip irrigation, and 5) control (no fertilizer input and flood irrigation). Reduced both basal and top-dressed fertilization maintained tomato yields. Compared with conventional practices (mean annual N₂O and NO emissions: 18.1 ± 1.3 and 0.79 ± 0.02 kg N ha^-1 yr ^-1, respectively), fertilization reduction by 20%-30% decreased the annual N₂O emission by 21.2%-27.0% owing to lower soil inorganic nitrogen (SIN) contents under the reduced fertilization practices. Switching from flood to drip irrigation might weaken denitrification due to lower soil moisture and less wet soil area, but increased SIN contents, and thus had no significant impact on annual N₂O and NO emissions. Peak N₂O fluxes occurred at soil temperature 28 °C and water-filled pore space (WFPS) > 60%, were higher than those for NO, and peak NO fluxes appeared 4-6 days later than N₂O fluxes, consistent with the decline in WFPS. These observations indicated that N₂O and NO from alkaline plastic-shed soil may be mainly generated via heterotrophic denitrification and nitrification, respectively. Reduced fertilization and drip irrigation in plastic-shed tomato production maintained crop productivity and mitigated N₂O emission. These results could be integrated into the decision-making in sustainable plastic-shed production.

Analyzing the mechanism of nitrous oxide production in aerobic phase of anoxic/aerobic sequential batch reactor from the perspective of key enzymes

How the vast majority of nitrous oxide (N₂O) in the aerobic zone of nitrogen bio-removal process is produced is still a controversial issue. To solve this issue, this study measured the activities of two key denitrifying enzymes (nitric oxide reductase (Nor) and nitrous oxide reductase (N₂OR)) in an A/O SBR with different chemical nitrogen demand (COD)/total nitrogen (TN) ratios. By analyzing the Spearman's correlations between the N₂O production, the enzyme activities, and the factors, the main N₂O production process was identified. By comparing the activities of these enzymes, this study analyzed the reasons for the N₂O production. Results show that Nor activities had a linear relationship with total N₂O concentrations (y = 0.34749 + 31.31365x, R² = 0.83362) and were not affected by COD (r = 0.299, N = 15, P = 0.279 > 0.05), which showed that most of the N₂O released and produced came from the autotrophic denitrification. N₂OR activities had a positive correlation with COD (r = 0.692, N = 15, P = 0.004 < 0.01), which showed that heterotrophic denitrification played a role as an N₂O consumer. Nor activities were much higher than N₂OR activities and the gap between them increased when the total N₂O concentration increased, showing that the heterotrophic denitrification was difficult to consume all the N₂O produced by the autotrophic denitrification. Reducing autotrophic denitrification is the best way to reduce N₂O production in aerobic phase.

Application of external carbon source in heterotrophic denitrification of domestic sewage: A review

The carbon source is essential as an electron donor in the heterotrophic denitrification process. When there is a lack of organic carbon sources in the system, an external carbon source is needed to improve denitrification efficiency. This review compiles the effects of liquid, solid and gaseous carbon sources on denitrification. Sodium acetate has better denitrification efficiency and is usually the first choice for external carbon sources. Fermentation by-products have been demonstrated to have the same denitrification efficiency as sodium acetate. Compared with cellulose-rich materials, biodegradable polymers have better and more stable denitrification performance in solid-phase nitrification, but their price is higher than the former. Methane as a gaseous carbon source is studied mainly by aerobic methane oxidation coupled with denitrification, which is feasible using methane as a carbon source. Liquid carbon sources are better controlled and utilized than solid carbon sources and gaseous carbon sources. In addition, high carbon to nitrogen ratio and hydraulic retention time can promote denitrification, while high dissolved oxygen (DO>2.0 mg L^-1) will inhibit the denitrification process. At the same time, high temperature is conducive to the decomposition of carbon sources by microorganisms. This review also considers the advantages and disadvantages of different carbon sources and cost analysis to provide a reference for looking for more economical and effective external carbon sources in the future.

A novel anammox aggregate nourished sustainably internal heterotrophic nitrate removal pathway with endogenous carbon source

Anaerobic ammonium oxidation (anammox) is a cost-effective nitrogen removal pathway but instinctively generated nitrate limits its application. A novel anammox aggregate reduced the production of nitrate significantly with efficient removal of ammonia and nitrite in this work. The results demonstrated that the internal heterotrophic nitrate removal (IHNAR) pathway exists stably at inner of anammox aggregates, which eliminated 42.31 ± 3.85 % nitrate generated in anammox at without consuming external carbon source. The observed volatile fatty acids (VFAs) and adequate protein, polysaccharide and humic acids in the aggregates verified that the in-situ fermentation supplied sustainably endogenous carbon sources for the IHNAR. The efficient interspecies cooperation between anammox bacteria, heterotrophic denitrifiers and fermentative bacteria was identified, as the intrinsic justification for the obtained sustainability of IHNAR pathway. The findings were expected to provide theoretical guidance for promotions and applications of the anammox process with high-efficiency total nitrogen removal capabilities.

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

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

Simultaneous reduction of perchlorate and nitrate using fast-settling anoxic sludge

Fast-settling, anoxic sludge (FAS) was cultivated and utilized in this study to simultaneously reduce elevated levels of perchlorate and nitrate in an anaerobic sequencing batch reactor (AnSBR). Average perchlorate and nitrate removal efficiencies of 96.5 ± 8.44 % and 99.8 ± 0.32 %, respectively, were achieved from an average perchlorate and nitrate loading rate of 159 ± 101 g ClO₄^-/m³·d and 10.8 ± 7.25 g NO₃^--N/m³·d, respectively, throughout long-term operation (>500-d). Batch activity tests revealed a preferential utilization of nitrate over perchlorate, where significant perchlorate reduction inhibition occurred when nitrate was present as a competing electron acceptor under carbon-limiting conditions. Specific perchlorate and nitrate reduction rates were shown to increase as the hydraulic retention time (HRT) of the AnSBR was step-wise decreased and subsequently the perchlorate and nitrate loading rates were step-wise increased. Functional, mRNA-based expression of the nitrite reductase (nirS and nirK), nitrous oxide reductase (nosZ), perchlorate reductase subunit A (pcrA), and the chlorite dismutase (cld) genes illustrated the simultaneous activity of heterotrophic denitrification and perchlorate reduction occurring throughout a complete standard reactor operational cycle, and allowed for expression trends to be documented as the HRT of the AnSBR was reduced from 5-d to 1.25-d. Nitrous oxide (N₂O) production was detected as a result of incomplete denitrification, where the largest N₂O production occurred at the highest nitrate loading rates investigated in this study. Thauera species were heavily enriched at a longer HRT of 5-d, but were out-competed by Dechloromonas species as the HRT of the AnSBR was step-wise reduced to 1.25-d.

Mixotrophic denitrification processes based on composite filler for low carbon/nitrogen wastewater treatment

Removal of nitrogen from wastewater with low carbon/nitrogen ratio was treated by using a denitrification packed bed reactor. Composite fillers with both autotrophic and heterotrophic denitrification capacity were prepared by mixing melted polycaprolactone and elemental sulfur at various alkalinity ratios (heterotrophic to autotrophic ratios of 1:2, 1:1, 3:2, and 2:1). Optimum denitrification was achieved at a ratio of 2:1. The diversity of the microbial community in the biofilm on the surface of the composite fillers showed that the increase of the elemental sulfur in the composite fillers has led to the increase of the microbial abundance. Furthermore, biofilm composition developed from a single dominant species to multiple species, and genes related to sulfur metabolism increased while those related to denitrification decreased slightly.

Simultaneous carbon and nitrogen removal by anaerobic ammonium oxidation and denitrification under different operating strategies

Real domestic wastewater was treated initially in a sequencing batch reactor (SBR), with partial nitrification achieved before the effluent was used as the influent for an anaerobic ammonium oxidation (anammox) reactor (ASBR) system. The effects of three factors, hydraulic retention time (HRT), substrate (NO₂^-/NH₄⁺) ratio, and the ratio of COD to NH₄⁺ (C/N), on the removal of carbon and nitrogen by an anammox and denitrification process were investigated in the ASBR reactor at 24°C. The response surface methodology was used to explore the interactions of the three factors. The results indicated that the nitrogen and carbon removal efficiency was optimum when HRT, substrate ratio, and C/N ratio were 33 h, 1.4-1.6, and 3-5, respectively. The optimal removal rates of NH₄⁺, NO₂^-, and COD were 96.30%, 97.79%, and 72.91%, respectively. The ΔNO₂^-/ΔNH₄⁺ and ΔNO₃^-/ΔNH₄⁺ ratios of the first two conditions were less than the theoretical anammox values of 1.32 and 0.26 due to heterotrophic denitrification. The optimum nitrogen and carbon removal efficiencies of the third condition could be realized by the synergistic effect of denitrification and the anammox process. Analysis of variance (ANOVA) results showed that when the HRT was 33.48 h, the substrate ratio was 1.46, and the C/N ratio was 4.28, the total nitrogen removal rate (TNR) was optimum (90.12 ± 0.1%), verified by parallel experiments.

Various electron donors for biological nitrate removal: A review

Nitrate (NO₃^-) pollution in water and wastewater has become a serious global issue. Biological denitrification, which reduces NO₃^- to N₂ (nitrogen gas) by denitrifying microorganisms, is an efficient and economical process for the removal of NO₃^- from water and wastewater. During the denitrification process, electron donor is required to provide electrons for reduction of NO₃^-. A variety of electron donors, including organic and inorganic compounds, can be used for denitrification. This paper reviews the state of the art of various electron donors used for biological denitrification. Depending on the types of electron donors, denitrification can be classified into heterotrophic and autotrophic denitrification. Heterotrophic denitrification utilizes organic compounds as electron donors, including low-molecular-weight organics (e.g. acetate, methanol, glucose, benzene, methane, etc.) and high-molecular-weight organics (e.g. cellulose, polylactic acid, polycaprolactone, etc.); while autotrophic denitrification utilizes inorganic compounds as electron donors, including hydrogen (H₂), reduced sulfur compounds (e.g. sulfide, element sulfur and thiosulfate), ferrous iron (Fe²⁺), iron sulfides (e.g. FeS, Fe_1-xS and FeS₂), arsenite (As(Ш)) and manganese (Mn(II)). The biological denitrification processes and the representative denitrifying microorganisms are summarized based on different electron donors, and their denitrification performance, operating costs and environmental impacts are compared and discussed. The pilot- or full-scale applications were summarized. The concluding remarks and future prospects were provided. The biodegradable polymers mediated heterotrophic denitrification, as well as H₂ and sulfur mediated autotrophic denitrification are promising denitrification processes for NO₃^- removal from various types of water and wastewater.

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

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

Fe(II) enhances simultaneous phosphorus removal and denitrification in heterotrophic denitrification by chemical precipitation and stimulating denitrifiers activity

Using Fe(II) salt as the precipitant in heterotrophic denitrification achieves improved TP removal, and enhancement in denitrification was often observed. This study aimed to obtain a better understanding of Fe(II)-enhanced denitrification with sufficient carbon source supply. Laboratory-scale experiments were conducted in SBRs with or without Fe(II) addition. Remarkably improved TP removal was experienced. TP removal efficiency in Fe(II) adding reactor was 85.8 ± 3.4%; whereas, that in the reactor without Fe(II) addition was 31.1 ± 2.8%. Besides improved TP removal, better TN removal efficiency (94.1 ± 1.1%) were recorded when Fe(II) was added, and that in the reactor without Fe(II) addition was 89 ± 0.8%. The specific denitrification rate were observed increase by 12.6% when Fe(II) was added. Further microbial analyses revealed increases in the abundances of typical denitrifiers (i.e. Niastella, Opitutus, Dechloromonas, Ignavibacterium, Anaeromyxobacter, Pedosphaera, and Myxococcus). Their associated denitrifying genes, narG, nirS, norB, and nosZ, were observed had 14.2%, 19.4%, 21.6%, and 9.9% elevation, respectively. Such enhancement in denitrification shall not be due to nitrate-dependent ferrous oxidation, which prevails in organic-deficient environments. In an environment with a continuous supply of Fe(II) and plenty of carbon sources, a cycle of denitrifying enzyme activity enhancement in the presence of Fe(II) facilitating nitrogen substrate utilization, stimulating denitrifier metabolism and growth, elevating denitrifying genes abundance, and increasing denitrifying enzymes expression were thought to be responsible for the Fe(II)-enhanced heterotrophic denitrification. Fe(II) salt is often a less expensive precipitant and has recently become attractive for TP removal in wastewater. The findings of this study solidify previous observation of enhancement of both TP and TN removal by adding Fe(II) in denitrification, and would be helpful for developing cost-effective pollutant removal processes.

Research progress in solid carbon source-based denitrification technologies for different target water bodies

Nitrogen pollution in water bodies is a serious environmental issue which is commonly treated by various methods such as heterotrophic denitrification. In particular, solid carbon source (SCS)-based denitrification has attracted widespread research interest due to its gradual carbon release, ease of management, and long-term operation. This paper reviews the types and properties of SCSs for different target water bodies. While both natural (wheat straw, wood chips, and fruit shells) and synthetic (polybutylene succinate, polycaprolactone, polylactic acid, and polyhydroxyalkanoates) SCSs are commonly used, it is observed that the denitrification performance of the synthetic sources is generally superior. SCSs have been used in the treatment of wastewater (including aquaculture wastewater), agricultural subsurface drainage, surface water, and groundwater; however, the key research aspects related to SCSs differ markedly based on the target waterbody. These key research aspects include nitrogen pollutant removal rate and byproduct accumulation (ordinary wastewater); water quality parameters and aquatic product yield (recirculating aquaculture systems); temperature and hydraulic retention time (agricultural subsurface drainage); the influence of dissolved oxygen (surface waters); and nitrate-nitrogen load, HRT, and carbon source dosage on denitrification rate (groundwater). It is concluded that SCS-based denitrification is a promising technique for the effective elimination of nitrate-nitrogen pollution in water bodies.

Insights into heterotrophic denitrification diversity in wastewater treatment systems: Progress and future prospects based on different carbon sources

Nitrate, as the most stable form of nitrogen pollution, widely exists in aquatic environment, which has great potential threat to ecological environment and human health. Heterotrophic denitrification, as the most economical and effective method to treat nitrate wastewater, has been widely and deeply studied. From the perspective of heterotrophic denitrification, this review discusses nitrate removal in the aquatic environment, and the behaviors of different carbon source types were classified and summarized to explain the cyclical evolution of carbon and nitrogen in global biochemical processes. In addition, the denitrification process, electron transfer as well as denitrifying and hydrolyzing microorganisms among different carbon sources were analyzed and compared, and the commonness and characteristics of the denitrification process with various carbon sources were revealed. This study provides theoretical support and technical guidance for further improvement of denitrification technologies.

Coupling transformation of carbon, nitrogen and sulfur in a long-term operated full-scale constructed wetland

The coupling transformation of carbon, nitrogen and sulfur compounds has been studied in lab-scale and pilot-scale constructed wetlands (CWs), but few studies investigated full-scale CW. In this study, we used batch experiments to investigate the potentials of carbon, nitrogen and sulfur transformation in a long-term operated, full-scale horizontal subsurface flow wetland. The sediments collected from the HSFW were incubated for 48 h in the laboratory with supplying various dosages of carbon, nitrogen and sulfur compounds. The results showed that heterotrophic denitrification was the main pathway. At the same time, the sulfide (S^2-)-based autotrophic denitrification was also present. Increasing TOC concentration or NO₃^- concentration could promote heterotrophic denitrification but did not inhibit the sulfide-based autotrophic denitrification. In our experiment, the highest NO₃^- removal via autotrophic denitrification was 25.23% while that via heterotrophic denitrification was 73.66%, leading to the total NO₃^- removal of 98.89%. The results also demonstrated that NO₃^- rather than NO₂^- was the preferable electron acceptor for both heterotrophic and sulfide-based autotrophic denitrification in the CW. Increasing S^2- concentrations promote NO₃^- removal from 12.99% to 25.23% without organic carbon, but varying NO₃^- or NO₂^- has no effects. These results indicated that concentrations of S^2-, instead of NO₃^- or NO₂^-, was the limiting factor for sulfide-based autotrophic denitrification in the studied CW. The microbial community analysis and correlation analysis between the transformation of carbon, nitrogen and sulfur compounds and relative abundance of bacteria further confirmed that in the CW, the key pathways coupling transformation were heterotrophic denitrification and sulfide-based autotrophic denitrification. Overall, the current study will enhance understanding of carbon, nitrogen, and sulfur transformation in CW and support better design and treatment efficiency.

Sulfur autotrophic denitrification filter and heterotrophic denitrification filter: Comparison on denitrification performance, hydrodynamic characteristics and operating cost

Sulfur autotrophic denitrification (SAD) process, as an alternative to heterotrophic denitrification (HD) filter, receives growing interest in polishing the effluent from secondary sewage treatment. Although individual studies have indicated several advantages of SAD over HD, rare study has compared these two systems under identical condition and by using real secondary effluent. In this study, two small pilot scale filters (SAD and HD) were designed with identical configuration and operated parallelly by feeding the real secondary effluent from a WWTP. The results showed SAD filter can be started up without the addition of soluble electron donor, although the time (14 days) was about 3 times longer than that of HD filter. The nitrate removal rate of SAD filter at HRT of 1.4 h was measured as 0.268 ± 0.047 kg N/(m³∙d). Similar value was observed in HD filter with supplementing 90 mg/L COD. The COD concentration of effluent always kept lower than that of influent in SAD filter but not in HD filter. In addition, SAD filter could maintain a stable denitrification performance without backwash for 15 days, while decline of nitrate removal rate was observed in HD filter just 2 days after stopping the backwash. This different behavior was further confirmed as the SAD filter had a better hydraulic flow pattern. Analysis according to high-throughput 16S rRNA gene-based Illumina MiSeq sequencing clearly showed the microbial community evolution and differentiation among the samples of seed sludge, SAD and HD filters. Finally, the economic assessment was carried out, showing the operation cost of SAD filter was over 50% lower than that of HD filter.

Is the ammonia stripping pre-treatment suitable for the nitrogen removal via partial nitritation-anammox of OFMSW digestate?

Treating the organic fraction of municipal solid waste (OFMSW) can be performed by coupling the anaerobic digestion (AD) and partial nitritation-anammox (PN-AMX) processes for organic matter and nitrogen removal, respectively. Besides, an ammonia stripping (AS) step before the AD benefit the removal of organic matter. In the present study, the operation of two PN-AMX sequencing batch reactors with and without AS pre-treated OFMSW digestate (AS-SBR and nAS-SBR, respectively) was assessed. The specific anammox activity decreased by 90 % for increasing proportions of fed OFMSW in both cases, indicating no differences over the anammox activity whether the AS pre-treatment is implemented or not. For 100 % OFMSW proportion, the AS-SBR achieved better effluent quality than the nAS-SBR (127 ± 88 vs. 1050 ± 23 mg N/L) but with lower nitrogen removal rates (58 ± 8 vs. 687 ± 32 g N/(L·d)). Still, the latter required successive re-inoculations to obtain higher removal rates. Changes in the microbial communities were mainly correlated to sCOD/N ratios in the OFMSW, being Candidatus Brocadia the dominant anamnmox species. The results proved the AS to be a suitable pre-treatment, despite the higher sCOD/N ratios in the OFMSW digestate, achieving good synergy between the PN-AMX and heterotrophic denitrification processes.

Tertiary denitrification and organic matter variations of secondary effluent from wastewater treatment plant by the 3D-BER system

A three-dimensional biofilm-electrode reactor (3D-BER) was constructed to facilitate the tertiary denitrification of the secondary effluent of wastewater treatment plants (SEWTP) under 12 mA and in the absence of a carbon source. The TN removal efficiency was 63.8%. The path of the formation and transformation of nitrogen, the relationship between the TN and COD removal rate and the relative concentration and composition of organic matter in the influent and effluent were analyzed to clarify the possible pathways of N and C transformation in the 3D-BER system. Under the action of an electric current, 4.4 mg NH₄⁺-N·L^-1 and 17.7 mg COD·L^-1 accumulated in the 3D-BER system, and the removal rates of TN and COD were strongly and positively correlated (R² = 0.9353). The microorganisms in the 3D-BER system under the action of electric current secreted organic matter, some of which (humic acid and microbial metabolites) could be further electrolyzed by microorganisms into bioavailable organic matter for heterotrophic denitrification. Partially dissolved organic matter (DOM, tryptophan aromatic protein, humic acid and microbial metabolites) in the SEWTP could be hydrolyzed under the action of the electric current in the 3D-BER system and consisted of bioavailable organic matter for heterotrophic denitrification. The contribution of heterotrophic denitrification to TN removal was greater than 11.7%. Therefore, the 3D-BER system removed a portion of DOM through microbial electrohydrolysis and promoted the coupling of hydrogen autotrophic denitrification and heterotrophic denitrification to enhance the effectiveness of nitrogen removal in SEWTP. Overall, this technique is effective for enhancing tertiary denitrification in SEWTP.

New insights into the cultivation of N, N-dimethylformamide-degrading methanogenic consortium: A long-term investigation on the variation of prokaryotic community inoculated with activated sludge

The cultivation of the N, N-dimethylformamide (DMF)-degrading methanogenic consortium is considered difficult. In this study, an up-flow anaerobic sludge blanket (UASB) was inoculated with activated sludge in order to culture the DMF-degrading anaerobic sludge under a constant DMF concentration of approximately 2000 mg L^-1. While the UASB realized a nearly 100% degradation of DMF and a high methane production of 1.03 L d^-1 for the first two months, both the removal efficiency and methane production continued to decrease until the end. The characterization of the prokaryotic community reveals that those DMF-hydrolyzing bacteria (DHB) originating from the activated sludge were responsible for the effective degradation of DMF. However, even when fed with a constant concentration of DMF, the DHB kept decreasing all the time while methane-producing archaea were rapidly cultivated. The variation of prokaryotic community suggests that the DHB could not proliferate anaerobically without utilizing the intermediate products from the hydrolysis of DMF, resulting in an unstable DMF-degrading consortium. The cultivation of DHB under the anaerobic condition of the UASB was therefore difficult. The reason it was not possible to culture a DMF-degrading methanogenic consortium in this study is that the DHB are denitrifying bacteria which require nitrate for their cell growth under the anaerobic condition. The solution to maintain the abundance of these DHB is to add doses of nitrate into the system. Nitrate is likely to help these DHB recapture intermediates from methanogens, enabling them to perform a heterotrophic denitrification by using a small proportion of DMF as the carbon source while simultaneously maintaining the cell growth of DHB.

Overlooked pathways of denitrification in a sulfur-based denitrification system with organic supplementation

Elemental sulfur-driven autotrophic denitrification (SADN) is a cost-effective approach for treating secondary effluent from wastewater treatment plants (WWTPs). Additional organics are generally supplemented to promote total nitrogen (TN) removal, reduce nitrite accumulation and sulfate production, and balance the pH decrease induced by SADN. However, understanding of the impacts of organic supplementation on microbial communities, nitrogen metabolism, denitrifier activity, and SADN rates in sulfur-based denitrification reactors is still limited. Here, a sulfur-based denitrification reactor was continuously operated for 272 days during which six different C/N ratios were tested successively (2.7, 1.5, 0.7, 0.5, 0.25, and 0). Organic supplementation improved TN removal and decreased NO₂^- accumulation, but reduced the relative abundance of denitrifiers and the contribution of autotrophic nitrate-reducing bacteria (aNRB) to TN removal during the long-term operation of reactor. Predictive functional profiling showed that nitrogen metabolism potential increased with decreasing C/N ratios. SADN was the predominant removal process when the C/N ratio was ≤0.7 (achieving 60% contribution when C/N = 0.7). Although organic supplementation weakened the dominant role of aNRB in denitrification, batch tests for the first time demonstrated that it could accelerate the SADN rate, attributed to the improvement of sulfur bioavailability, likely via the formation of polysulfide. A possible nitrogen removal pathway with multiple electron donors (i.e., sulfur, organics, sulfide, and polysulfide) in a sulfur-based denitrification reactor with organic supplementation was therefore proposed. However, supplementation with a high level of organics could increase the operational cost and effluent concentrations of sulfide and organics as well as enrich heterotrophic denitrifiers. Moreover, microbial community had substantial changes at C/N ratios of >0.5. Accordingly, an optimal C/N ratio of 0.25-0.5 was suggested, which could simultaneously minimize the additional operating cost associated with organic supplementation and maximize TN removal and SADN rates.

Assessing the feasibility of sequential aerobic respiration and heterotrophic denitrification of a high-strength mixture of phenol and its derivatives in the field single-well-drift test

Biological degradation of high strength phenol and its derivatives in groundwater is problematic because these compounds are toxic to human and microbes. To evaluate the feasibility of in situ bioremediation using sequential aerobic respiration and heterotrophic denitrification, a field single-well-drift test (SWDT) was conducted in groundwater contaminated with coal tar distillates. To stimulate indigenous phenol degrading microorganisms, a 1400 L of oxygen-saturated test solution containing bromide (3.96 ± 0.179 mmol-Br/L) and nitrate (5.34 ± 0.187 mmol NO₃^--N/L) was injected into an aquifer. After injection of the test solution, significant consumption of dissolved oxygen (DO) was immediately observed; then, degradation of the methyl derivatives o-cresol and m,p-cresol was observed with average zero-order rate coefficients of 0.047 mmol/L/d and 0.23 mmol/L^/d, respectively. After 73% of the injected DO was consumed, significant NO₃^- consumption was observed along with degradation of phenol and the dimethyl derivatives 2,4-xylenol and 3,5-xylenol, which had average zero-order rate coefficients of 0.17 mmol/L/d, 0.060 mmol/L/d, and 0.018 mmol/L/d, respectively. The production of CO₂, NO₂^-, and N₂O along with significant consumption of DO and NO₃^- suggest that phenolic compounds were biologically degraded by sequential aerobic respiration and heterotrophic denitrification. The results of 16s RNA analysis revealed that, after injection of the test solution, a bacterium that shared a 99% 16s rRNA sequence similarity with an uncultured bacterium revealed to be Pseudomonas stutzeri, a facultative heterotrophic denitrifier, was found in the aquifer. Thus, these results suggest that simultaneous injection of DO and NO₃^- is an appropriate in situ bioremediation strategy for degrading mixtures of high-strength phenolic compounds in an aquifer.

Evaluation of denitrification performance and bacterial community of a sequencing batch reactor under intermittent aeration

Effects of operational parameters (initial nitrite concentration, initial nitrate concentration, carbon source, and COD/N ratio) on denitrification performance was evaluated using a sequencing batch reactor (SBR) under intermittent aeration. Complete denitrification was observed without N₂O accumulation when the initial nitrite concentration was 100-500 mg-N·L^-1. When the initial nitrate concentration was 75-300 mg-N·L^-1, 95-96% of NO₃^--N was completely reduced to N₂ gas. Acetate was the most effective sole carbon source for the complete denitrification of the SBR under intermittent aeration, and 99% of NO₃^--N was reduced to N₂ gas. The optimum COD/N ratio was 8-12 for the complete denitrification, while NO₂^- accumulation was observed at low COD/N ratios of 1 and 2. In this study, N₂O accumulation was not observed during the denitrification process regardless of operational condition. Paracoccus (15-68%), a representative aerobic denitrifying bacterium, was dominant in the SBR during the denitrification process, and the intermittent aeration condition could affect the abundance of Paracoccus in this study.

Microbial community modulates electrochemical performance and denitrification rate in a biocathodic autotrophic and heterotrophic denitrifying microbial fuel cell

A comparison of autotrophic (AD) and heterotrophic (HD) cathodic denitrification in a Microbial Fuel Cell (MFC) was made in this study. Denitrifying microbial consortia were developed from cow manure and soil and acclimatized under AD and HD conditions. The AD MFC supported the power output of 4.45 W m^-3 while removing nitrate nitrogen (NO₃^--N) at the rate of 0.118 kg NO₃^--N m^-3 d^-1. Significant power output (3.02 W m^-3) and nitrate removal rate (2.06 kg NO₃^--N m^-3 d^-1) were achieved in HD MFC. Further, 16S rDNA based community analysis revealed higher diversity in HDMFC. The genus Thauera and Pseudomonas were predominant in ADMFC while genus Klebsiella and Alkaliphilus were abundant in HDMFC. The abundance of the denitrifying genes namely narG, nirS, and nosZ were assessed with the help of quantitative PCR and presence of all the genes in both the conditions ensured the necessary molecular requirements for complete denitrification.

Effects of carbon source, C/N ratio, nitrate, temperature, and pH on N2O emission and functional denitrifying genes during heterotrophic denitrification

The effects of operational parameters such as carbon source, C/N ratio, initial nitrate concentration, temperature, and pH value on heterotrophic denitrification and functional denitrifying genes were evaluated. When methanol was used as the sole carbon source, complete denitrification was performed in a short time without nitrous oxide (N₂O) emission. Complete denitrification was performed at high C/N ratios (5.14 and 12.85) and low initial nitrate concentrations (75.9 and 151.6 mg N L^-1). The denitrification rate was not temperature-sensitive in the range of 25-35 °C, but tended to decrease at a low pH of 5-6. The relationships between N₂O emission and functional genes under various operational conditions were investigated by Pearson correlation and association network analyses. The C/N ratio was a key factor for N₂O emission during the heterotrophic denitrification process. This information on the denitrification performance and its association with functional gene dynamics under various operational conditions is useful for N₂O mitigation strategies for wastewater treatment processes.

Comparison of heterotrophic and autotrophic denitrification processes for nitrate removal from phosphorus-limited surface water

Phosphorus (P) limitation has been demonstrated for micro-polluted surface water denitrification treatment in previous study. In this paper, a lab-scale comparative study of autotrophic denitrification (ADN) and heterotrophic denitrification (HDN) in phosphorus-limited surface water was investigated, aiming to find out the optimal nitrogen/phosphorus (N/P) ratio and the mechanism of the effect of P limitation on ADN and HDN. Furthermore, the optimal denitrification process was applied to the West Lake denitrification project, aiming to improve the water quality of the West Lake from worse than grade V to grade IV (GB3838-2006). The lab-scale study showed that the lack of P indeed inhibited HDN more greatly than ADN. The optimal N/P ratio for ADN and HDN was 25 and a 0.15 mg PO₄^3--P L^-1 of microbial available phosphorus (MAP) was observed. P additions could greatly enhance the resistance of ADN and HDN to hydraulic loading shock. Besides, The P addition could effectively stimulate the HDN performance via enriching the heterotrophic denitrifiers and the denitrifying phosphate-accumulating organisms (DNPAOs). Additionally, HDN was more effective and cost-effective than ADN for treating P-limited surface water. The study of the full-scale HDBF (heterotrophic denitrification biofilter) indicated that the denitrification performance was periodically impacted by P limitation, particularly at low water temperatures.

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

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

Effects of salinity, C/S ratio, S/N ratio on the BESI process, and treatment of nanofiltration concentrate

A laboratory-scale biodegradation and electron transfer based on the sulfur metabolism in the integrated (BESI^®) process was used to treat a saline petrochemical nanofiltration concentrate (NFC). The integrated process consisted of activated sludge sulfate reduction (SR), and sulfide oxidation (SO) reactors, and a biofilm nitrification reactor. During the process, the total removal efficiencies of chemical oxygen demand (COD), ammonia nitrogen, and total nitrogen (TN) were 76.2, 83.8, and 73.1%, respectively. In the SR reactor, most of the organic degradation occurred and approximately 70% COD were removed by the sulfate-reducing bacteria (SRB). In the SO reactor, both the autotrophic and heterotrophic denitrifications were observed to take place. In parallel, batch experiments were conducted to detect the effects of different C/S and S/N ratios on COD removal and denitrification efficiency. The batch experiments were also conducted to detect the effects of salinity on COD and sulfate reduction. The composition of pollutants in the wastewater was complex, and some existing organics were not degraded by the SRB. The non-SRB groups also played important roles in the reactor. Under salinity-induced stress, the metabolisms of the SRBs and non-SRB groups were both inhibited. However, 6 g/L NaCl did not have much effect on the final COD removal efficiency. In the batch experiments, the added sulfide served as the electron donor for autotrophic denitrification. The added organics provided substance for heterotrophic denitrification.

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.

Removal of pharmaceuticals in pre-denitrifying MBBR - Influence of organic substrate availability in single- and three-stage configurations

Due to the limited efficiency of conventional biological treatment, innovative solutions are being explored to improve the removal of trace organic chemicals in wastewater. Controlling biomass exposure to growth substrate represents an appealing option for process optimization, as substrate availability likely impacts microbial activity, hence organic trace chemical removal. This study investigated the elimination of pharmaceuticals in pre-denitrifying moving bed biofilm reactors (MBBRs), where biofilm exposure to different organic substrate loading and composition was controlled by reactor staging. A three-stage MBBR and a single-stage reference MBBR (with the same operating volume and filling ratio) were operated under continuous-flow conditions (18 months). Two sets of batch experiments (day 100 and 471) were performed to quantify and compare pharmaceutical removal and denitrification kinetics in the different MBBRs. Experimental results revealed the possible influence of retransformation (e.g., from conjugated metabolites) and enantioselectivity on the removal of selected pharmaceuticals. In the second set of experiments, specific trends in denitrification and biotransformation kinetics were observed, with highest and lowest rates/rate constants in the first (S1) and the last (S3) staged sub-reactors, respectively. These observations were confirmed by removal efficiency data obtained during continuous-flow operation, with limited removal (<10%) of recalcitrant pharmaceuticals and highest removal in S1 within the three-stage MBBR. Notably, biotransformation rate constants obtained for non-recalcitrant pharmaceuticals correlated with mean specific denitrification rates, maximum specific growth rates and observed growth yield values. Overall, these findings suggest that: (i) the long-term exposure to tiered substrate accessibility in the three-stage configuration shaped the denitrification and biotransformation capacity of biofilms, with significant reduction under substrate limitation; (ii) biotransformation of pharmaceuticals may have occurred as a result of cometabolism by heterotrophic denitrifying bacteria.

Effect of dissolved oxygen on nitrate removal using polycaprolactone as an organic carbon source and biofilm carrier in fixed-film denitrifying reactors

Nitrate-nitrogen (NO3(-)-N) always accumulates in commercial recirculating aquaculture systems (RASs) with aerobic nitrification units. The ability to reduce NO3(-)-N consistently and confidently could help RASs to become more sustainable. The rich dissolved oxygen (DO) content and sensitive organisms stocked in RASs increase the difficulty of denitrifying technology. A denitrifying process using biologically degradable polymers as an organic carbon source and biofilm carrier was proposed because of its space-efficient nature and strong ability to remove NO3(-)-N from RASs. The effect of dissolved oxygen (DO) levels on heterotrophic denitrification in fixed-film reactors filled with polycaprolactone (PCL) was explored in the current experiment. DO conditions in the influent of the denitrifying reactors were set up as follows: the anoxic treatment group (Group A, average DO concentration of 0.28±0.05mg/L), the low-oxygen treatment DO group (Group B, average DO concentration of 2.50±0.24mg/L) and the aerated treatment group (Group C, average DO concentration of 5.63±0.57mg/L). Feeding with 200mg/L of NO3(-)-N, the NO3(-)-N removal rates were 1.53, 1.60 and 1.42kg/m(3) PCL/day in Groups A, B and C, respectively. No significant difference in NO3(-)-N removal rates was observed among the three treatments. It was concluded that the inhibitory effects of DO concentrations lower than 6mg/L on heterotrophic denitrification in the fixed-film reactors filled with PCL can be mitigated.

Microbial fuel cell assisted nitrate nitrogen removal using cow manure and soil

Microbial fuel cells (MFCs) are emerging wastewater treatment systems with a proven potential for denitrification. In this study, we have developed a high-rate denitrifying MFC. The anode consisted of cow manure and fruit waste and the cathode consisted of cow manure and soil. The initial chemical oxygen demand (COD)/nitrate nitrogen (NO3 (-)-N) was varied from 2 to 40 at the cathode while keeping the anode ratio fixed at 100. NO3 (-)-N removal rate of 7.1 ± 0.9 kg NO3 (-)-N/m(3) net cathodic compartment (NCC)/day was achieved at cathode COD/NO3 (-)-N ratio 7.31 with the current density of 190 ± 9.1 mA/m(2) and power density of 31.92 ± 4 mW/m(2) of electrode surface area. We achieved an open-circuit voltage (OCV) of 410 ± 20 mV at initial cathodic NO3 (-)-N of 0.345 g/l. The cathode COD/NO3 (-)-N ratio had a significant influence on MFC's OCV and nitrate removal rate. Lower OCV (<150 mV) and NO3 (-)-N removal rates were observed at COD/NO3 (-)-N ratio >12 and <7. Experiments done at different cathode pH values indicated that the optimum pH for denitrification was 7. Under optimized biochemical conditions, nitrate removal rate of 6.5 kg NO3 (-)-N/m(3) net cathodic compartment (NCC)/day and power density of 210 mW/m(2) were achieved in a low resistance MFC. The present study thus demonstrates the utility of MFCs for the treatment of high nitrate wastes.

Evaluating simultaneous chromate and nitrate reduction during microbial denitrification processes

Sulfur-based autotrophic denitrification and heterotrophic denitrification have been demonstrated to be promising technological processes for simultaneous removal of nitrate NO3(-) and chromate (Cr (VI)), two common contaminants in surface and ground waters. In this work, a mathematical model was developed to describe and evaluate the microbial and substrate interactions among sulfur oxidizing denitrifying organism, methanol-based heterotrophic denitrifiers and chromate reducing bacteria in the biofilm systems for simultaneous nitrate and chromate removal. The concomitant multiple chromate reduction pathways by these microbes were taken into account in this model. The validity of the model was tested using experimental data from three independent biofilm reactors under autotrophic, heterotrophic and mixotrophic conditions. The model sufficiently described the nitrate, chromate, methanol, and sulfate dynamics under varying conditions. The modeling results demonstrated the coexistence of sulfur-oxidizing denitrifying bacteria and heterotrophic denitrifying bacteria in the biofilm under mixotrophic conditions, with chromate reducing bacteria being outcompeted. The sulfur-oxidizing denitrifying bacteria substantially contributed to both nitrate and chromate reductions although heterotrophic denitrifying bacteria dominated in the biofilm. The mixotrophic denitrification could improve the tolerance of autotrophic denitrifying bacteria to Cr (VI) toxicity. Furthermore, HRT would play an important role in affecting the microbial distribution and system performance, with HRT of higher than 0.15 day being critical for a high level removal of nitrate and chromate (over 90%).

Recent advances in mathematical modeling of nitrous oxides emissions from wastewater treatment processes

Nitrous oxide (N2O) can be emitted from wastewater treatment contributing to its greenhouse gas footprint significantly. Mathematical modeling of N2O emissions is of great importance toward the understanding and reduction of the environmental impact of wastewater treatment systems. This article reviews the current status of the modeling of N2O emissions from wastewater treatment. The existing mathematical models describing all the known microbial pathways for N2O production are reviewed and discussed. These included N2O production by ammonia-oxidizing bacteria (AOB) through the hydroxylamine oxidation pathway and the AOB denitrification pathway, N2O production by heterotrophic denitrifiers through the denitrification pathway, and the integration of these pathways in single N2O models. The calibration and validation of these models using lab-scale and full-scale experimental data is also reviewed. We conclude that the mathematical modeling of N2O production, while is still being enhanced supported by new knowledge development, has reached a maturity that facilitates the estimation of site-specific N2O emissions and the development of mitigation strategies for a wastewater treatment plant taking into the specific design and operational conditions of the plant.

Nitrogen, carbon, and sulfur isotopic change during heterotrophic (Pseudomonas aureofaciens) and autotrophic (Thiobacillus denitrificans) denitrification reactions

In batch culture experiments, we examined the isotopic change of nitrogen in nitrate (δ(15)NNO3), carbon in dissolved inorganic carbon (δ(13)CDIC), and sulfur in sulfate (δ(34)SSO4) during heterotrophic and autotrophic denitrification of two bacterial strains (Pseudomonas aureofaciens and Thiobacillus denitrificans). Heterotrophic denitrification (HD) experiments were conducted with trisodium citrate as electron donor, and autotrophic denitrification (AD) experiments were carried out with iron disulfide (FeS2) as electron donor. For heterotrophic denitrification experiments, a complete nitrate reduction was accomplished, however bacterial denitrification with T. denitrificans is a slow process in which, after seventy days nitrate was reduced to 40% of the initial concentration by denitrification. In the HD experiment, systematic change of δ(13)CDIC (from -7.7‰ to -12.2‰) with increase of DIC was observed during denitrification (enrichment factor εN was -4.7‰), suggesting the contribution of C of trisodium citrate (δ(13)C=-12.4‰). No SO4(2-) and δ(34)SSO4 changes were observed. In the AD experiment, clear fractionation of δ(13)CDIC during DIC consumption (εC=-7.8‰) and δ(34)SSO4 during sulfur use of FeS2-S (around 2‰), were confirmed through denitrification (εN=-12.5‰). Different pattern in isotopic change between HD and AD obtained on laboratory-scale are useful to recognize the type of denitrification occurring in the field.

Nitrogen removal from wastewater by anaerobic methane-driven denitrification in a lab-scale reactor: heterotrophic denitrifiers associated with denitrifying methanotrophs

Nitrite-dependent anaerobic methane oxidation (n-damo) is a newly discovered bioprocess that reduces nitrite to dinitrogen with methane as electron donor, which has promising potential to remove nitrogen from wastewater. In this work, a lab-scale sequencing batch reactor (SBR) was operated for 609 days with methane as the sole external electron donor. In the SBR, nitrite in synthetic wastewater was removed continuously; the final volumetric nitrogen removal rate was 12.22±0.02 mg N L(-1) day(-1) and the percentage of nitrogen removal was 98.5 ± 0.2 %. Microbial community analysis indicated that denitrifying methanotrophs dominated (60-70 %) the population of the final sludge. Notably, activity testing and microbial analysis both suggested that heterotrophic denitrifiers existed in the reactor throughout the operation period. After 609 days, the activity testing indicated the nitrogen removal percentage of heterotrophic denitrification was 17 ± 2 % and that of n-damo was 83 ± 2 %. A possible mutualism may be developed between the dominated denitrifying methanotrophs and the associated heterotrophs through cross-feed. Heterotrophs may live on the microbial products excreted by denitrifying methanotrophs and provide growth factors that are required by denitrifying methanotrophs.

Effect of dissolved oxygen on heterotrophic denitrification using poly(butylene succinate) as the carbon source and biofilm carrier

The effect of dissolved oxygen (DO) on heterotrophic denitrification using poly(butylene succinate) as the carbon source and biofilm carrier was evaluated in a lab-scale experiment. Aerated, low-oxygen, and anoxic treatment groups were set up, which had average DO concentrations of 5.2±1.0, 1.4±1.2, and 0.5±0.3 mg L(-1), respectively. The NO3(-)-N and total nitrogen (TN) removal rates in the aerated group (37.44±0.24 and 36.24±0.48 g m(-3) d(-1), respectively) were higher than those in the other two groups. There was no significant difference between the low-oxygen and anoxic groups for the NO3(-)-N or TN removal rate. Accumulation of NO2(-)-N reached 5.0 mg L(-1) in the aerated group; no nitrite accumulation was found in the other two treatment groups. Bacterial communities of the low-oxygen and anoxic groups showed high similarity and were significantly different from those of the aerated group.

Heterotrophic denitrification plays an important role in N₂O production from nitritation reactors treating anaerobic sludge digestion liquor

Nitrous oxide (N2O) emissions from nitritation reactors receiving real anaerobic sludge digestion liquor have been reported to be substantially higher than those from reactors receiving synthetic digestion liquor. This study aims to identify the causes for the difference, and to develop strategies to reduce N2O emissions from reactors treating real digestion liquor. Two sequencing batch reactors (SBRs) performing nitritation, fed with real (SBR-R) and synthetic (SBR-S) digestion liquors, respectively, were employed. The N2O emission factors for SBR-R and SBR-S were determined to be 3.12% and 0.80% of the NH4(+)-N oxidized, respectively. Heterotrophic denitrification supported by the organic carbon present in the real digestion liquor was found to be the key contributor to the higher N2O emission from SBR-R. Heterotrophic nitrite reduction likely stopped at N2O (rather than N2), with a hypothesised cause being free nitrous acid inhibition. This implies that all nitrite reduced by heterotrophic bacteria was converted to and emitted as N2O. Increasing dissolved oxygen (DO) concentration from 0.5 to 1.0 mg/L, or above, decreased aerobic N2O production from 2.0% to 0.5% in SBR-R, whereas aerobic N2O production in SBR-S remained almost unchanged (at approximately 0.5%). We hypothesised that DO at 1 mg/L or above suppressed heterotrophic nitrite reduction thus reduced aerobic heterotrophic N2O production. We recommend that DO in a nitritation system receiving anaerobic sludge digestion liquor should be maintained at approximately 1 mg/L to minimise N2O emission.

Heterotrophic and elemental-sulfur-based autotrophic denitrification processes for simultaneous nitrate and Cr(VI) reduction

Nitrate and chromate can be present together in water resources as nitrate is a common co-contaminant in surface and ground waters. This study aims at comparatively evaluating simultaneous chromate and nitrate reduction in heterotrophic and sulfur-based autotrophic denitrifying column bioreactors. In sulfur-based autotrophic denitrification process, elemental sulfur and nitrate act as an electron donor and an acceptor, respectively, without requirement of organic supplementation. Autotrophic denitrification was complete and not adversely affected by chromate up to 0.5 mg/L. Effluent chromate concentration was <50 μg/L provided that influent chromate concentration was ≤0.5 mg/L. Heterotrophic denitrification performance was not adversely affected even at 20 mg/L chromate and complete chromate reduction was attained up to 10 mg/L. Although autotrophic denitrification rate was much lower compared with heterotrophic one, it may be preferred in drinking water treatment due to the elimination of organic supplementation and the risk of treated effluent contamination.

Slowly released molasses barrier system for controlling nitrate plumes in groundwater: a pilot-scale tank study

A well-type barrier system containing solidified molasses as a reactive medium was developed to promote the indigenous denitrifying activity and to treat nitrate plumes in groundwater. Three slowly released molasses (SRM) barrier systems harboring 60, 120, and 120 SRM rods, which were named System A, B, and C, respectively, were operated to examine nitrate removal efficiency in a pilot-scale sandy tank. These SRM systems induced a consistent removal of nitrate without pore clogging and hydraulic disturbance during the test period. The initial nitrate concentration was 142mgL(-1), and the concentrations decreased by 80%, 84%, and 79% in System A, B, and C, respectively. In particular, System C was inoculated with heterotrophic denitrifiers, but the nitrate removal efficiency was not enhanced compared to System B, probably due to the prior existence of indigenous denitrifiers in the sandy tank. The presence of nitrite reductase-encoding gene (i.e. nirK) at the site was confirmed by denatured gradient gel electrophoresis analysis.