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

Iron sulfide mineral/polylactic acid mixotrophic biofilter for simultaneous nitrate and phosphate removal

Heterotrophic denitrification based on polylactic acid (PLAHD) can remove nitrate effectively, but it is expensive and can't remove phosphate. Autotrophic denitrification based on iron sulfide (ISAD) can simultaneously remove nitrate and phosphate cost-effectively, but its nitrate rate is slow. So, iron sulfide mineral/polylactic acid mixotrophic biofilter (ISPLAB) was constructed to combine advantages of ISAD and PLAHD. ISPLAB achieved nitrogen and phosphorus removal rates of 98.04 % and 94.12 %, respectively, at a hydraulic retention time (HRT) of 24 h. The study also revealed that controlling molecular weight (MW) of PLA improved the release of soluble organic matter; adding iron sulfide enhanced the hydrolysis of PLA and precipitated PO43- of Fe2+/Fe3+, thereby facilitated simultaneous nitrogen and phosphorus removal. Microbial community analysis resulted that denitrifying bacterias (Phaeodactylibacter and Methylotenera), sulfur-reducing bacterias (Hyphomicrobium), sulfur-oxidizing bacteria (Denitratisoma), iron-reducing bacteria (Romboutsia) and hydrolyzed bacterias (norank_f_norank_o_1-20 and norank_f_Caldilineaceae) coexisted in the ISPLAB system. Organics and iron sulfide drived the denitrification process in ISPLAB.

Minimizing byproduct formation in bioelectrochemical denitrification with anammox bacteria

Autotrophic bioelectrochemical denitrification (BED) holds promise for nitrate remediation. However, the accumulation of byproducts such as NO2-, N2O, and NH4+, poses a significant challenge to effluent quality and climate adaptation. This study hypothesized that introducing anaerobic ammonium oxidation bacteria (anammox) to BED could alleviate this issue through synergy: a) anammox can utilize NH4+ and NO2- from BED without producing N2O, as seen in canonical denitrification, and b) BED can recycle NO3- from the anammox anabolic pathway. Results showed that Anammox_BED reduced NO2- accumulation by two-thirds, lowered the relative abundance of N2O by 80 %, and eliminated NO. Metagenomic analysis revealed that the anammox species Ca. Brocadia sapporoensis tripled in abundance in the bulk sludge. Meanwhile, Pseudomonas stutzeri and Bosea robiniae, species capable of reducing nitrate via extracellular electron transfer (EET) and supplying NO2- to anammox, halved in relative abundance, while the abundance of Stenotrophomonas acidaminiphila, a non-EET, ammonia assimilation species, doubled following anammox introduction. Metatranscriptomic analysis found upregulation of denitrification-related functional genes in Anammox_BED biofilm and survival- and motility- related genes in bulk sludge, possibly due to insufficient substrate. Overall, BED-Anammox successfully diverted the rate-limiting EET nitrite reduction towards anammox-driven nitrite utilization thereby mitigating the generation of unwanted intermediates.

Enhanced and sustainable advanced nitrogen removal in mixotrophic systems using pyrite and solid carbon source

Utilizing widespread minerals/solid wastes as electron donors for denitrification is conducive to sustainable wastewater treatment. The current denitrification technologies based on single pyrite/solid carbon sources have problems of limited removal efficiency or unstable carbon release. In this study, two continuous biofilters, pyrite-corncob mixotrophic system (RPCM) and pyrite-polybutylene succinate mixotrophic system (RPPM), were conducted and operated steadily for a long period (>326 d). The mixotrophic systems achieved advanced removal of NO3--N (18 mg L-1) and a small amount of NH4+-N (2.5 mg L-1), with stabilized effluent TIN less than 2 mg L-1 at HRT of 4 h. Additionally, the systems demonstrated several distinct advantages, including no additional alkalinity requirement and a low risk of secondary contamination. RPCM could achieve advanced nitrogen removal at a higher nitrogen loading rate (93.6 mg L-1 d-1) but demanded periodic replenishment of corncob. In contrast, the organic matter release and nitrogen removal performance of RPPM exhibited stability throughout the operation. The increased abundance of functional microorganisms related to C, N, S, and Fe metabolism was essential for advanced nitrogen removal through synergistic effects. This study will provide implications for developing novel wastewater treatment processes emphasizing both nitrogen removal and waste valorization.

Integrated evaluation for advanced removal of nitrate using novel solid carbon biochar/corncob/PHBV composite: Insight into electron transfer and metabolic pathways

This study developed a novel Biochar/Corncob/PHBV (BCP) composite material, integrating the electron transfer capability of biochar, the cost-effectiveness of corncob, and the sustained carbon release performance of PHBV. The BCP system achieved a maximum nitrate removal efficiency of 97.3 %, significantly outperforming the single PHBV system (91.05 %), while effectively reducing nitrous oxide and other greenhouse gas emissions. It also demonstrated stable carbon release and enhanced electron transfer capabilities, contributing to a more sustainable denitrification process. The physical and chemical characterization of BCP confirmed that its superior performance is attributed to the uniformly distributed functional groups (e.g., CO and -COOH) on the surface and its porous structure, which facilitated electron transfer and microbial adhesion. Metagenomic and microbial analyses further revealed that BCP enriched functional genera such as Cellulomonas and Chryseobacterium and significantly increased the abundance of key functional genes related to nitrate reduction (e.g., NaR and NiR), enhancing organic matter decomposition and microbial nitrogen transformation. Beyond improving nitrate removal efficiency compared to PHBV, the BCP material offers practical engineering value by addressing carbon source limitations in long-term wastewater treatment applications. Its enhanced electron transfer and microbial enrichment suggest strong potential for application in constructed wetlands, biofilters, and other decentralized wastewater treatment systems. The study demonstrates that the BCP composite is not only a viable alternative to traditional PHBV but also a cost-effective and environmentally friendly material with broad applicability in nitrogen pollution control.

Carbon sources derived from the invasive plant Spartina alterniflora improved the nitrogen removal in seawater constructed wetland

Invasive alien plants pose a great threat to local plants and ecosystems. How to effectively alleviate this hazard is an unresolved issue. This study explored the carbon release characteristics of an invasive plant Spartina alterniflora and evaluated the ability of nitrogen removal from shrimp culture wastewater through constructing seawater wetland. The results showed that fresh S. alterniflora had a significantly higher carbon release potential and bioavailability than that of withered S. alterniflora, and alkali-heat treatment could increase the carbon release with an average COD release rate of 33.39 mg/g from fresh S. alterniflora. The removal rate of total nitrogen was improved by about 22% in seawater constructed wetlands by adding fresh S. alterniflora biomass. Moreover, the addition of fresh S. alterniflora biomass was beneficial to the increase in the abundance of denitrification-related genera Vibrio, which might be the key to the improvement of nitrate removal efficiency in seawater constructed wetland systems. These findings indicated that invasive plants S. alterniflora as carbon sources of seawater wetland was a feasible and effective resource utilization strategy.

Carbon slow-release and enhanced nitrogen removal performance of plant residue-based composite filler and ecological mechanisms in constructed wetland application

Stable carbon release and coupled microbial efficacy of external carbon source solid fillers are the keys to enhanced nitrogen removal in constructed wetlands. The constructed wetland plant residue Acorus calamus was cross-linked with poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) to create composite solid carbon source fillers (Ac-BDPs). The study demonstrated the slow release of carbon sources from Ac-BDPs with 35.27 mg/g under an average release rate of 0.88 mg/(g·d). Excellent denitrification was also observed in constructed wetlands with Ac-BDPs. Moreover, the average removal rate of nitrate nitrogen (NO3--N) was increased by 1.94 and 3.85 times of the blank groups under initial NO3--N inputs of 5 and 15 mg/L, respectively. Furthermore, the relatively high abundances of nap, narG, nirKS, norB, qnorZ and nosZ guaranteed efficient denitrification performance in constructed wetlands with Ac-BDPs. The study introduced a reliable technique for biological nitrogen removal by using composite carbon source fillers in constructed wetlands.

Mixotrophic denitrification of waste activated sludge fermentation liquid as an alternative carbon source for nitrogen removal: Reducing N2O emissions and costs

Heterotrophic-sulfur autotrophic denitrification (HAD) has been proposed to be a prospective nitrogen removal process. In this work, the potential of fermentation liquid (FL) from waste-activated sludge (WAS) as the electron donor for denitrification in the HAD system was explored and compared with other conventional carbon sources. Results showed that when FL was used as a carbon source, over 99% of NO3--N was removed and its removal rate exceeded 14.00 mg N/g MLSS/h, which was significantly higher than that of methanol and propionic acid. The produced sulfate was below the limit value and the emission of N2O was low (1.38% of the NO3--N). Microbial community analysis showed that autotrophic denitrifiers were predominated in the HAD system, in which Thiobacillus (16.4%) was the dominant genus. The economic analysis showed the cost of the FL was 0.062 €/m3, which was 30% lower than that in the group dosed with methanol. Our results demonstrated the FL was a promising carbon source for the HAD system, which could reduce carbon emission and cost, and offer a creative approach for waste-activated sludge resource reuse.

Preparation and Performance Verification of a Solid Slow-Release Carbon Source Material for Deep Nitrogen Removal in Urban Tailwater

Urban tailwater typically has a low carbon-to-nitrogen ratio and adding external carbon sources can effectively improve the denitrification performance of wastewater. However, it is difficult to determine the dosage of additional carbon sources, leading to insufficient or excessive addition. Therefore, it is necessary to prepare solid slow-release carbon source (SRC) materials to solve the difficulty in determining the dosage of carbon sources. This study selected two SRCs of slow-release carbon source 1 (SRC1) and slow-release carbon source 2 (SRC2), with good slow-release performance after static carbon release and batch experiments. The composition of SRC1 was: hydroxypropyl methylcellulose/disodium fumarate/polyhydroxy alkanoate (HPMC/DF/PHA) at a ratio of 3:2:4, with an Fe3O4 mass fraction of 3%. The composition of SRC2 was: HPMC/DF/PHA with a ratio of 1:1:1 and an Fe3O4 mass fraction of 3%. The fitted equations of carbon release curves of SRC1 and SRC2 were y = 61.91 + 7190.24e-0.37t and y = 47.92 + 8770.42e-0.43t, respectively. The surfaces of SRC1 and SRC2 had a loose and porous morphological structure, which could increase the specific surface area of materials and be more conducive to the adhesion and metabolism of microorganisms. The experimental nitrogen removal by denitrification with SRCs showed that when the initial total nitrogen concentration was 40.00 mg/L, the nitrate nitrogen (NO3--N) concentrations of the SRC1 and SRC2 groups on the 10th day were 2.57 and 2.66 mg/L, respectively. On the 20th day, the NO3--N concentrations of the SRC1 and SRC2 groups were 1.67 and 2.16 mg/L, respectively, corresponding to removal efficiencies of 95.83% and 94.60%, respectively. The experimental results indicated that SRCs had a good nitrogen removal effect. Developing these kinds of materials can provide a feasible way to overcome the difficulty in determining the dosage of carbon sources in the process of heterotrophic denitrification.

Enhanced denitrification using high-molecular-weight poly(lactic acid) blended with lactide as an effective carbon source

The utilization of wasted Poly(lactic acid) (PLA) as low-cost carbon sources in solid-phase denitrification is hindered by its low biodegradability, which can be attributed to its high molecular weight. This study presents a new approach by blending high-molecular-weight PLA with a small amount of ʟ-lactide (PLA/LAx) to treat nitrate-contaminated wastewater. The addition of ʟ-lactide enhanced the release of carbon from high-molecular-weight PLA. An impressive denitrification efficiency of 96.7% was achieved, accompanied by extremely low levels of accumulated NO2--N (0.1 mg/L) and NH4+-N (0.4 mg/L). The quantity of ʟ-lactide used significantly impacted the bacterial community structure. A high abundance of the phyla Bacteroidota and Chloroflexi associated with polymer degradation was observed. The most dominant denitrifier was the genus unclassified_f__Rhodocyclaceae belonged to the phylum Proteobacteria. This study demonstrates that blending PLA with just 5 wt% lactide can transform it into a highly effective solid-phase carbon source to eliminate nitrates.

Elemental sulfur-driven autotrophic denitrification process for effective removal of nitrate in mariculture wastewater: Performance, kinetics and microbial community

To date, there is a lack of systematic investigation on the elemental sulfur-driven autotrophic denitrification (SDAD) process for removing nitrate (NO3--N) from mariculture wastewater deficient in organic carbon sources. Therefore, a packed-bed reactor was established and continuously operated for 230 days to investigate the operation performance, kinetic characteristics and microbial community of SDAD biofilm process. Results indicate that the NO3--N removal efficiencies and rates varied with the operational conditions including HRT (1-4 h), influent concentrations of NO3--N (25-100 mg L-1) and DO (0.2-7.0 mg L-1), and temperature (10oC-30 °C), in the ranges of 51.4%-98.6% and 0.054-0.546 g L-1 d-1, respectively. Limestone could partially neutralize the produced acidity. Small portions of NO3--N were converted to nitrite (<4.5%) and ammonia (<2.8%) in the reactor. Operational conditions also influenced the production of acidity, nitrite and ammonia as well as sulfate. Shortening HRT and increasing influent NO3--N concentration turned the optimal fitting model depicting the NO3--N removal along the reactor from half-order to zero-order. Furthermore, the NO3--N removal was accelerated by a higher temperature and influent NO3--N concentration and a lower HRT and influent DO concentration. Microbial richness, evenness and diversity gradually decreased during the autotrophic denitrifier enrichment cultivation and the reactor start-up and operation. Sulfurimonas constituted the predominate genus and the primary functional bacteria in the reactor. This study highlights the SDAD as a promising way to control the coastal eutrophication associated with mariculture wastewater discharge.

Systematic review: External carbon source for biological denitrification for wastewater

Nitrogen mitigation is serious environmental issue around the globe. Several methods for wastewater treatment have been introduced, but biological denitrification has been recommended, particularly with addition of the best external carbon source. The key sites of denitrification are wetlands; it can be carried out with different methods. To highlight the aforementioned technology, this paper deals to review the literature to evaluate biological denitrification and to demonstrate cost effective external carbon sources. The results of systematic review disclose the denitrification process and addition of different external carbon sources. The online literature exploration was accomplished using the most well-known databases, that is, science direct and the web of science database, resulting 625 review articles and 3084 research articles, published in peer-reviewed journals between 2015 and 2021 were identified in first process. After doing an in-depth literature survey and exclusion criteria, we started to shape the review from selected review and research articles. A number of studies confirmed that both nitrification and denitrification are significant for biological treatment of wastewater. The studies proved that the carbon source is the main contributor and is a booster for the denitrification. Based on the literature reviewed it is concluded that biological denitrification with addition of external carbon source is cost effective and best option in nitrogen mitigation in a changing world. Our study recommends textile waste for recovery of carbon source.

The research progress, hotspots, challenges and outlooks of solid-phase denitrification process

Nitrogen pollution is one of the main reasons for water eutrophication. The difficulty of nitrogen removal in low-carbon wastewater poses a huge potential threat to the ecological environment and human health. As a clean biological nitrogen removal process, solid-phase denitrification (SPD) was proposed for long-term operation of low-carbon wastewater. In this paper, the progress, hotspots, and challenges of the SPD process based on different solid carbon sources (SCSs) are reviewed. Compared with synthetic SCS and natural SCS, blended SCSs have more application potential and have achieved pilot-scale application. Differences in SCSs will lead to changes in the enrichment of hydrolytic microorganisms and hydrolytic genes, which indirectly affect denitrification performance. Moreover, the denitrification performance of the SPD process is also affected by the physical and chemical properties of SCSs, pH of wastewater, hydraulic retention time, filling ratio, and temperature. In addition, the strengthening of the SPD process is an inevitable trend. The strengthening measures including SCSs modification and coupled electrochemical technology are regarded as the current research hotspots. It is worth noting that the outbreak of the COVID-19 epidemic has led to the increase of disinfection by-products and antibiotics in wastewater, which makes the SPD process face challenges. Finally, this review proposes prospects to provide a theoretical basis for promoting the efficient application of the SPD process and coping with the challenge of the COVID-19 epidemic.

Machine learning estimation of biodegradable organic matter concentrations in municipal wastewater

This study investigates whether a novel estimation method based on machine learning can feasibly predict the readily biodegradable chemical oxygen demand (RB-COD) and slowly biodegradable COD (SB-COD) in municipal wastewater from the oxidation-reduction potential (ORP) data of anoxic batch experiments. Anoxic batch experiments were conducted with highly mixed liquor volatile suspended solids under different RB-COD and SB-COD conditions. As the RB-COD increased, the ORP breakpoint appeared earlier, and fermentation occurred in the interior of the activated sludge, even under anoxic conditions. Therefore, the ORP decline rates before and after the breakpoint were significantly correlated with the RB-COD and SB-COD, respectively (p < 0.05). The two biodegradable CODs were estimated separately using six machine learning models: an artificial neural network (ANN), support vector regression (SVR), an ANN-based AdaBoost, a SVR-based AdaBoost, decision tree, and random forest. Against the ORP dataset, the RB-COD and SB-COD estimation correlation coefficients of SVR-based AdaBoost were 0.96 and 0.88, respectively. To identify which ORP data are useful for estimations, the ORP decline rates before and after the breakpoint were separately input as datasets to the estimation methods. All six machine learning models successfully estimated the two biodegradable CODs simultaneously with accuracies of ≥0.80 from only ORP time-series data. Sensitivity analysis using the Shapley additive explanation method demonstrated that the ORP decline rates before and after the breakpoint obviously contributed to the estimation of RB-COD and SB-COD, respectively, indicating that acquiring the ORP data with various decline rates before and after the breakpoint improved the estimations of RB-COD and SB-COD, respectively. This novel estimation method for RB-COD and SB-COD can assist the rapid control of biological wastewater treatment when the biodegradable organic matter concentration dynamically changes in influent wastewater.

Corncob-pyrite bioretention system for enhanced dissolved nutrient treatment: Carbon source release and mixotrophic denitrification

Solid biomass waste amendment and substrates modification in bioretention systems have been increasingly used to achieve effective dissolved nutrients pollution control in stormwater runoff. However, the risk of excess chemical oxygen demand (COD) leaching from organic carbon sources is often overlooked on most occasions. Pyrite is an efficient electron donor for autotrophic denitrification, but little is known about the efficacy of autotrophic-heterotrophic synergistic effect between additional carbon source and pyrite in bioretention. Here, four bioretention columns (i.e., corncob column (C), pyrite column (P), the corncob-pyrite layered column (L-CP), and the corncob-pyrite mixed column (M-CP)) were designed and filled with soil, quartz sand, and modified media to reveal the synergistic effects. The results showed that the corncob-pyrite layered bioretention could maintain low COD effluent concentration with high stability and efficiency in treating dissolved nutrients. When the influent nitrogen and phosphorus concentrations were 8.46 mg/L and 0.94 mg/L, the average removal rates of ammonia nitrogen, total inorganic nitrogen, and phosphate were 83.6%, 70.52%, and 76.35%, respectively. The scouring experiment showed that placing the corncob in the mulch layer was beneficial to the sustained release of dissolved organic carbon (DOC). Erosion pits were found in the SEM images of used pyrite, indicating that autotrophic denitrifying bacteria in the bioretention could react with pyrite as an electron donor. The relative abundance of Thiobacillus in the submerged zone of the corncob-pyrite layered bioretention reached 38.39%, indicating that the carbon source in the mulch layer increased the relative abundance of Thiobacillus. Coexisting heterotrophic and autotrophic denitrification in this bioretention created a more abundant microbial community structure in the submerged zone. Overall, the corncob-pyrite layered bioretention is highly promising for stormwater runoff treatment, with effective pollution removal and minimal COD emission.

Exploring recycled agricultural wastes for high-rate removal of nitrogen in wastewater: Emphasizing on the investigation of the inner driving force and comparison with conventional liquid carbon sources

In this study, four typical recycled agricultural wastes (AWs), corn cob, wheat straw, sawdust and walnut shells (named AW1, AW2, AW3 and AW4, respectively), were selected as external solid carbon sources to enhance the removal of nitrogen in wastewater, and specifically, the driving mechanism was thoroughly investigated. The leaching experiments showed that the dissolved organic carbon (DOC) release capacity followed the order of AW1>AW2>AW3>AW4, ranging from 6.21 to 31.92 mg/g. DOC released from AWs mainly consisted of protein-like substances, fulvic acid-like substances and humic-like substances. AW1 and AW2 achieved comparable NO_x^--N removal performance with a liquid carbon source of sodium acetate (SA) during the long-term denitrification experiments (>94.2%) but not for the other two AWs (only 16.8%-38.1%). Denitrification performance relied on DOC released from AWs at the beginning, while the enrichment of the functional CAZymes (including glycoside hydrolase and carbohydrate esterase) involved in cellulose and hemicellulose decomposition of AWs and functional genes (GAPDH, gap 2, PK, etc.) related to glycolysis were the inner driving force, which guaranteed the continuous supply of electron donors for denitrification. The relatively high abundances of napAB, narGHI, nirKS, norBC and nosZ, which encode nitrate reductase, nitrite reductase, NO reductase and N₂O reductase, assured the better denitrification performance in the SA, AW1 and AW2 groups. In addition to denitrification-related functional genes, the relative abundances of nirBD and nrfAH associated with dissimilatory nitrate reduction were much higher in AW1 and AW2 groups than in SA group, implying that the nitrogen removal mechanism should be different in liquid carbon source and AW-based solid carbon source systems. In addition, GLU, gltBD and glnA, which participate in ammonia assimilation were the highest in the AW2 group, resulting in a large amount of organic nitrogen accumulation (peak concentration of approximately 24.5 mg/L), and this finally ruled it out as an alternative external carbon source. The abovementioned microbial mechanism was verified based on the correlation analysis of nutrient removal and functional genes combined with host bacterial analysis. Our study can provide valuable information for understanding the mechanism of using AWs as alternative external carbon sources to promote the removal of nitrogen in wastewater.

New insights on simultaneous nitrate and phosphorus removal in pyrite-involved mixotrophic denitrification biofilter for a long-term operation: Performance change and its underlying mechanism

Simultaneous nitrate and phosphorus removal can be completed by pyrite- and influent organics-involved mixotrophic denitrification and chemical phosphorus removal via iron precipitation. However, so far, how their removal performances change with iron precipitation accumulation remains unclear. In this study, the differences in nitrate and phosphorus removal from municipal tailwater between volcanic and pyrite supported biofilters (V-BF, P-BF) for a long-term operation were investigated, as well as the underlying mechanism for these differences. The nitrate removal efficiencies (NREs) in P-BF were greater than those in V-BF due to the synergistic effect of influent organic and pyrite, as evidenced by comparable TOC consumption and Fe²⁺/SO₄^2- production. The NREs in P-BF were gradually lower than in V-BF as a result of bacterial cell-iron encrustation observed in TEM images, which would deteriorate microbial activity. However, the phosphorus removal efficiencies (PREs) in P-BF remained consistently higher than in V-BF, resulting from chemical phosphorus removal which was confirmed that P, Fe and O elements dominated on the pyrite surface after use by SEM-EDS. The dominant denitrifying bacteria differed significantly, autotrophic and heterotrophic denitrifying microorganisms coexisted in P-BF. The relative abundances of the narG coding gene in P-BF were higher than that in V-BF, which was consistent with the total relative abundances of identified denitrifying bacteria. Besides, the mechanism of simultaneous nitrogen and phosphorus removal in the pyrite-involved mixotrophic denitrification process has been deduced. This work has significant implications for the practical application of a pyrite-involved mixotrophic denitrification process for low C/N wastewater treatment.

Simultaneous heterotrophic and FeS2-based ferrous autotrophic denitrification process for low-C/N ratio wastewater treatment: Nitrate removal performance and microbial community analysis

Heterotrophic-autotrophic denitrification reduces the cost of wastewater treatment and the risk of excess chemical oxygen demanded (COD) in the effluent. A mixotrophic denitrification system involving mixed heterotrophic and ferrous autotrophic bacteria was investigated to treat low-C/N ratio (C/N, defined as chemical oxygen demand (COD)/total nitrogen (TN)) wastewater with pyrite and organic carbon as electron donors. The system yielded effluent total nitrogen (TN) of 0.38 mg/L in 48 h due to a synergistic effect when the C/N ratio was 0.5 and influent nitrate nitrogen (NO₃^--N) was 20 mg/L; this TN value was significantly lower than those of the heterotrophic system (14.08 mg/L) and ferrous autotrophic system (12.00 mg/L). The highest abundance of the narG gene was observed in the mixotrophic denitrification system, along with more abundant microbial species. The dominant denitrification bacteria in each system included Thaurea, Ferritrophicum, Pseudomonas, and Thiobacillus, which varied with the initial inoculum source and the environment. Nevertheless, the abundance of the heterotrophic bacteria Thaurea decreased with prolonged operation of the systems. Together, these results implied that the simultaneous heterotrophic and FeS₂-based ferrous autotrophic denitrification process can be an alternative approach for the treatment of low-C/N ratio wastewater.

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.

Nitrate removal under low carbon to nitrogen ratio by modified corn straw bioreactor: Optimization and possible mechanism

ABSTRACTThe removal of nitrate (NO₃^--N) from water bodies under the conditions of poor nutrition and low carbon to nitrogen (C/N) ratio is a widespread problem. In this study, modified corn stalk (CS) was used to immobilize Burkholderia sp. CF6 with cellulose-degrading and denitrifying abilities. The optimal operating parameters of the bioreactor were explored. The results showed that under the hydraulic retention time (HRT) of 3 h and the C/N ratio of 2.0, the maximum nitrate removal efficiency was 96.75%. In addition, the organic substances in the bioreactor under different C/N ratios and HRT were analyzed by three-dimensional fluorescence excitation-emission mass spectrometry (3D-EEM), and it was found that the microorganisms have high metabolic activity. Scanning electron microscope (SEM) showed that the new material has excellent immobilization effects. Fourier transform infrared spectrometer (FTIR) showed that it has potential as a solid carbon source. Through high-throughput sequencing analysis, Burkholderia sp. CF6 was observed as the main bacteria present in the bioreactor. These research results showed that the use of waste corn stalks waste provides a theoretical basis for the advanced treatment of low C/N ratio wastewater.

Denitrification performance and microbial community of bioreactor packed with PHBV/PLA/rice hulls composite

In this study, a novel biodegradable PHBV/PLA/rice hulls (PPRH) composite was applied and tested as biofilm attachment carrier and carbon source in two bioreactors for biological denitrification process. The denitrification performance, effect of operational conditions and microbial community structure of PPRH biofilm were evaluated. The batch experiment results showed that PPRH-packed bioreactor could completely remove 50 mg L^-1 of NO₃^--N at natural pH (ca. 7.5) and room temperature. The continuous flow experiments indicated that high NO₃^--N removal efficiency (77%-99%) was achieved with low nitrite (<0.48 mg L^-1) and ammonia (<0.81 mg L^-1) accumulation, when influent NO₃^--N concentration was 30 mg L^-1 and hydraulic retention time was 2-6 h. Furthermore, the microbial community analysis indicated that bacteria belonging to genus Diaphorobacter in phylum Proteobacteria were the most dominant and major denitrifiers in denitrification. In summary, PPRH composite was a promising carbon source for biological nitrate removal from water and wastewater.