Synergistic Integration of Anammox and Endogenous Denitrification Processes for the Simultaneous Carbon, Nitrogen, and Phosphorus Removal.

The feasibility of a synergistic endogenous partial denitrification-phosphorus removal coupled anammox (SEPD-PR/A) system was investigated in a modified anaerobic baffled reactor (mABR) for synchronous carbon, nitrogen, and phosphorus removal. The mABR comprising four identical compartments (i.e., C1-C4) was inoculated with precultured denitrifying glycogen-accumulating organisms (DGAOs), denitrifying polyphosphate-accumulating organisms, and anammox bacteria. After 136 days of operation, the chemical oxygen demand (COD), total nitrogen, and phosphorus removal efficiencies reached 88.6 ± 1.0, 97.2 ± 1.5, and 89.1 ± 4.2%, respectively. Network-based analysis revealed that the biofilmed community demonstrated stable nutrient removal performance under oligotrophic conditions in C4. The metagenome-assembled genomes (MAGs) such as MAG106, MAG127, MAG52, and MAG37 annotated as denitrifying phosphorus-accumulating organisms (DPAOs) and MAG146 as a DGAO were dominated in C1 and C2 and contributed to 89.2% of COD consumption. MAG54 and MAG16 annotated as Candidatus_Brocadia (total relative abundance of 16.5% in C3 and 4.3% in C4) were responsible for 74.4% of the total nitrogen removal through the anammox-mediated pathway. Functional gene analysis based on metagenomic sequencing confirmed that different compartments of the mABR were capable of performing distinct functions with specific advantageous microbial groups, facilitating targeted nutrient removal. Additionally, under oligotrophic conditions, the activity of the anammox bacteria-related genes of hzs was higher compared to that of hdh. Thus, an innovative method for the treatment of low-strength municipal and nitrate-containing wastewaters without aeration was presented, mediated by an anammox process with less land area and excellent quality effluent.

Bidirectional Enhancement of Nitrogen Removal by Indigenous Synergetic Microalgal-Bacterial Consortia in Harsh Low-C/N Wastewater.

Conventional microalgal-bacterial consortia have limited capacity to treat low-C/N wastewater due to carbon limitation and single nitrogen (N) removal mode. In this work, indigenous synergetic microalgal-bacterial consortia with high N removal performance and bidirectional interaction were successful in treating rare earth tailing wastewaters with low-C/N. Ammonia removal reached 0.89 mg N L-1 h-1, 1.84-fold more efficient than a common microalgal-bacterial system. Metagenomics-based metabolic reconstruction revealed bidirectional microalgal-bacterial interactions. The presence of microalgae increased the abundance of bacterial N-related genes by 1.5- to 57-fold. Similarly, the presence of bacteria increased the abundance of microalgal N assimilation by 2.5- to 15.8-fold. Furthermore, nine bacterial species were isolated, and the bidirectional promotion of N removal by the microalgal-bacterial system was verified. The mechanism of microalgal N assimilation enhanced by indole-3-acetic acid was revealed. In addition, the bidirectional mode of the system ensured the scavenging of toxic byproducts from nitrate metabolism to maintain the stability of the system. Collectively, the bidirectional enhancement system of synergetic microalgae-bacteria was established as an effective N removal strategy to broaden the stable application of this system for the effective treatment of low C/N ratio wastewater.

Resourceful application and mechanism of oyster shell-microalgae synergistic system：Sustainable treatment of harsh low carbon nitrogen ratio actual wastewater.

Microalgal technology holds great promise for both low C/N wastewater treatment and resource recovery simultaneously. Nevertheless, the advancement of microalgal technology is hindered by its reduced nitrogen removal efficiency in low C/N ratio wastewater. In this work, microalgae and waste oyster shells were combined to achieve a total inorganic nitrogen removal efficiency of 93.85% at a rate of 2.05 mg L-1 h-1 in low C/N wastewater. Notably, over four cycles of oyster shell reuse, the reactor achieved an average 85% ammonia nitrogen removal extent, with a wastewater treatment cost of only $0.092/ton. Moreover, microbial community analysis during the reuse of oyster shells revealed the critical importance of timely replacement in inhibiting the growth of non-functional bacteria (Poterioochromonas_malhamensi). The work demonstrated that the oyster shell - microalgae system provides a time- and cost-saving, environmental approach for the resourceful treatment of harsh low C/N wastewater.

Impacts of O2:CH4 ratios and CH4 concentrations on the denitrification and CH4 oxidations of a novel AME-AD system.

Aerobic methane (CH4) oxidation coupled to denitrification (AME-D) is a promising process for the denitrification of low C/N wastewater. Compared with anaerobic denitrifying bacteria, aerobic denitrifying bacteria may enable AME-D have high denitrification ability under aerobic conditions. This study constructed a novel aerobic methane oxidation coupled to aerobic denitrification (AME-AD) system using the typical aerobic denitrifying bacteria Paracoccus pantotrophus ATCC35512 and the typical aerobic methane oxidizing bacteria Methylosinus trichosporium OB3b. The denitrification and CH4 oxidations of AME-AD with different O2:CH4 ratios (0:1, 0.25:1, 0.5:1, 0.75:1, 1:1 and 1.25:1) and CH4 concentrations (0, 14000, 28000, 42000, 56000 and 70000 mg m-3) were investigated in batch experiments. Higher O2:CH4 ratios can significantly improve the denitrification and CH4 oxidations of the AME-AD (P < 0.05). The treatment with an O2:CH4 ratio of 1.25:1 had the highest denitrification rate (0.036 mg h-1) and highest CH4 oxidation rate (0.20 mg h-1). The CH4 concentration in the headspace was positively correlated with the AME-AD denitrification rate. The calculated CH4/NO3-(mol/mol) in most treatments ranged from 5.76 to 6.84. In addition, excessively high O2 and CH4 concentrations can lead to increased nitrous oxide (N2O) production in AME-AD. The N2O production rate was up to 1.00 µg h-1 when the O2:CH4 was 1.25:1. These results can provide data support for the application of AME-AD for low-C/N wastewater treatment and greenhouse gas emission reduction.

Start-up of glycerol-driven denitrifying phosphorus removal from wastewater: The effects of the microaerobic environment.

Glycerol, an abundant by-product of biodiesel production, represented a promising carbon source for enhancing nutrient removal from low C/N ratio wastewater. This study discovered a novel approach to initiate glycerol-driven denitrifying phosphorus removal (DPR) in situ by creating a short-term microaerobic environment within the aerobic zone. This approach facilitated the in-situ conversion of glycerol, which was subsequently utilized by denitrifying phosphate accumulating organisms (DPAOs) for DPR. The feasibility and stability of glycerol-driven DPR were validated in a continuous-flow pilot-scale reactor. Anaerobic phosphorus release increased from 1.0 mg/L/h to 2.5 mg/L/h, with fermentation bacteria and related functional genes showing significant increases. The stable stage exhibited 92.8% phosphorus removal efficiency and 55.5% DPR percentage. The microaerobic environment enhanced fermentation bacteria enrichment, crucial for glycerol-driven DPR stability. The collaborative interaction between fermentation bacteria and phosphate accumulating organisms (PAOs) played a key role in sustaining glycerol-driven DPR stability. These findings provide a robust theoretical foundation for applying glycerol-driven DPR in established wastewater treatment plants.

Ammonia oxidizing bacteria (AOB) denitrification and bacterial denitrification as the main culprit of high N2O emission in SBR with low C/N ratio wastewater.

A large amount of greenhouse gas nitrous oxide (N2O) will be produced during the biological nitrogen removal process for organic wastewater of low C/N ratio. One of the effective methods to solve this problem is to incorporate inexpensive carbon source. In this study, raw wastewater (RW) from pig farm, that was not anaerobically digested, was utilized as exogenous carbon in both A/O and SBR aerobic reactor to treat liquid digestate with high ammonia nitrogen and low C/N ratio. The results showed that N2O emission in SBR was higher than that of A/O process under the same nitrogen load. The N2O conversion in the biological nitrogen removal process was investigated by the strategy of integrating stable isotope method and metagenomics. The δO18-N2O, δN15-N2O, and SP values of the SBR were closer to the denitrification values of Ammonia-Oxidizing Bacteria (AOB) than those of A/O. The abundance of AOB in the SBR reactor was higher than that in the A/O reactor, while the abundance of denitrifying bacteria was lower. The amoA/B/C gene abundance in the SBR was greater than that in the A/O, and the NOS gene abundance was the opposite. The results indicated that both AOB denitrification and bacterial denitrification led to the increase of N2O emissions of the SBR.

High-efficient nutrient removal in a single-stage electrolysis-integrated sequencing batch biofilm reactor (E-SBBR) for low C/N sanitary sewage treatment.

To efficiently remove nutrients from low C/N sanitary sewage by conventional biological process is challenging due to the lack of sufficient electron donors. A novel electrolysis-integrated sequencing batch biofilm reactor (E-SBBR) was established to promote nitrogen and phosphorus removal for sanitary sewage with low C/N ratios (3.5-1.5). Highly efficient removal of nitrogen (>79%) and phosphorus (>97%) was achieved in the E-SBBR operating under alternating anoxic/electrolysis-anoxic/aerobic conditions. The coexistence of autotrophic nitrifiers, electron transfer-related bacteria, and heterotrophic and autohydrogenotrophic denitrifiers indicated synergistic nitrogen removal via multiple nitrogen-removing pathways. Electrolysis application induced microbial anoxic ammonia oxidation, autohydrogenotrophic denitrification and electrocoagulation processes. Deinococcus enriched on the electrodes were likely to mediate the electricity-driven ammonia oxidation which promoted ammonia removal. PICRUSt2 indicated that the relative abundances of key genes (hyaA and hyaB) associated with hydrogen oxidation significantly increased with the decreasing C/N ratios. The high autohydrogenotrophic denitrification rates during the electrolysis-anoxic period could compensate for the decreased heterotrophic rates resulting from insufficient carbon sources and nitrate removal was dramatically enhanced. Electrocoagulation with iron anode was responsible for phosphorus removal. This study provides insights into mechanisms by which electrochemically assisted biological systems enhance nutrient removal for low C/N sanitary sewage.

Partial nitrification-denitrification and enrichment of paracoccus induced by iron-chitosan beads addition in an intermittently-aerated activated sludge system.

Insufficient carbon source has become the main limiting factor for efficient nitrogen removal in wastewater treatment. In this study, an intermittently-aerated activated sludge system with iron-chitosan (Fe-CS) beads addition was proposed for nitrogen removal from low C/N wastewater. By adding Fe-CS beads, partial nitrification-denitrification (PND) process and significant enrichment of Paracoccus (with ability of iron reduction/ammonium oxidation/aerobic denitrification) were observed in the reactor. The accumulation rate of NO2--N reached 81.9 %, and the total nitrogen removal efficiency was improved to 93.9 % by shortening the aeration time. The higher activity of ammonium oxidizing bacteria and inhibited activity of nitrite-oxidizing bacteria in Fe-CS assisted system mediated the occurrence of PND. In contrast, the traditional nitrification and denitrification process occurred in the control group. The high-throughput sequencing analysis and metagenomic results confirmed that the addition of Fe-CS induced 77.8 % and 54.9 % enrichment of Paracoccus in sludge and Fe-CS beads, respectively, while almost no enrichment was observed in control group. Furthermore, with the addition of Fe-CS beads, the expression of genes related to outer membrane porin, cytochrome c, and TCA was strengthened, thereby enhancing the electron transport of Fe(Ⅱ) (electron donor) and Fe(Ⅲ) (electron acceptor) with pollutants in the periplasm. This study provides new insights into the direct enrichment of iron-reducing bacteria and its PND performance induced by the Fe-CS bead addition. It therefore offers an appealing strategy for low C/N wastewater treatment.

How ß-Cyclodextrin-Functionalized Biochar Enhanced Biodenitrification in Low C/N Conditions via Regulating Substrate Metabolism and Electron Utilization.

Biodenitrification plays a vital role in the remediation of nitrogen-contaminated water. However, influent with a low C/N ratio limits the efficiency of denitrification and causes the accumulation/emission of noxious intermediates. In this study, ß-cyclodextrin-functionalized biochar (BC@ß-CD) was synthesized and applied to promote the denitrification performance of Paracoccus denitrificans when the C/N was only 4, accompanied by increased nitrate reduction efficiency and lower nitrite accumulation and nitrous oxide emission. Transcriptomic and enzymatic activity analyses showed BC@ß-CD enhanced glucose degradation by promoting the activities of glycolysis (EMP), the pentose phosphate pathway (PPP), and the tricarboxylic acid (TCA) cycle. Notably, BC@ß-CD drove a great generation of electron donors by stimulating the TCA cycle, causing a greater supply of substrate metabolism to denitrification. Meanwhile, the promotional effect of BC@ß-CD on oxidative phosphorylation accelerates electron transfer and ATP synthesis. Moreover, the presence of BC@ß-CD increased the intracellular iron level, causing further improved electron utilization in denitrification. BC@ß-CD helped to remove metabolites and induced positive feedback on the metabolism of P. denitrificans. Collectively, these effects elevated the glucose utilization for supporting denitrification from 36.37% to 51.19%. This study reveals the great potential of BC@ß-CD for enhancing denitrification under low C/N conditions and illustrates a potential application approach for ß-CD in wastewater bioremediation.

Aerobic Denitrification Enhanced by Immobilized Slow-Released Iron/Activated Carbon Aquagel Treatment of Low C/N Micropolluted Water: Denitrification Performance, Denitrifying Bacterial Community Co-occurrence, and Implications.

The key limiting factors in the treatment of low C/N micropolluted water bodies are deficient essential electron donors for nitrogen removal processes. An iron/activated carbon aquagel (IACA) was synthesized as a slowly released inorganic electron donor to enhance aerobic denitrification performance in low C/N micropolluted water treatment. The denitrification efficiency in IACA reactors was enhanced by more than 56.72% and the highest of 94.12% was accomplished compared with those of the control reactors. Moreover, the CODMn removal efficiency improved by more than 34.32% in IACA reactors. The Illumina MiSeq sequencing consequence explained that the denitrifying bacteria with facultative denitrification, iron oxidation, and iron reduction function were located in the dominant species niches in the IACA reactors (e.g., Pseudomonas, Leptothrix, and Comamonas). The diversity and richness of the denitrifying bacterial communities were enhanced in the IACA reactors. Network analysis indicated that aerobic denitrifying bacterial consortia in IACA reactors presented a more complicated co-occurrence structure. The IACA reactors presented the potential for long-term denitrification operation. This study affords a pathway to utilize IACA, promoting aerobic denitrification during low C/N micropolluted water body treatment.

Hybrid bioreactor built-in with fixed bio-carriers for denitrification with low C/N ratio: Hydrodynamic optimization and microbial divergence.

Hydrodynamics played an important role in the design and operation of bioreactors for wastewater treatment. In this work, an up-flow anaerobic hybrid bioreactor built-in with fixed bio-carriers was designed and optimized using computational fluid dynamics (CFD) simulation. The results indicated that the flow regime involving with vortex and dead zone was greatly affected by the positions of water inlet and bio-carrier modules. The ideal hydraulic features were obtained when the water inlet and bio-carrier modules located 9 cm and 60 cm above the bottom of reactor. Using the optimum hybrid system for nitrogen removal from wastewater with low carbon-to-nitrogen ratio (C/N = 3), the denitrification efficiency could reach 80.9 ± 0.4%. Illumina sequencing of 16S rRNA gene amplicons revealed that the microbial community divergence occurred among the biofilm on bio-carrier, the suspended sludge phase and the inoculum. Especially, the relative abundance of denitrifying genera Denitratisoma in the biofilm of bio-carrier reaches 5.73%, 6.2 times higher than that in the suspended sludge, implying the imbedded bio-carrier was conductive to enrich the specific denitrifiers to polish the denitrification performance with low carbon source. This work provided an effective method for the design optimization of bioreactor based on CFD simulation, and developed a hybrid reactor with fixed bio-carrier for nitrogen removal from wastewater with low C/N ratio.

Iron mediated autotrophic denitrification for low C/N ratio wastewater: A review.

In recent years, iron mediated autotrophic denitrification has been a concern because it overcomes the absence of organic carbon and has been successfully used in denitrification for low C/N ratio wastewater. However, there is currently a lack of a more systematic summary of iron-based materials that can be used for denitrification, and no detailed overview about the mechanism of iron mediated autotrophic denitrification has been reported. In this study, the iron materials with different valence states that can be used for denitrification were summarized, and emphasized, as well as the mechanism in different interaction systems were emphasize. In addition, the contribution of various microorganisms in nitrate reduction were analyzed and the effects of operating conditions and water quality were evaluated. Finally, the challenges and shortcomings of the denitrification process were discussed aiming to find better practical engineering applications of iron-based denitrification.

Enhanced denitrification of the AO-MBBR system used for expressway service area sewage treatment: A new perspective on decentralized wastewater treatment.

Decentralized wastewater treatment warrants considerable development in numerous countries and regions. Owing to the unique characteristics of high ammonia nitrogen concentrations and low carbon/nitrogen ratio, nitrogen removal is a key challenge in treating expressway service area sewage. In this study, an anoxic/oxic-moving bed biofilm reactor (A/O-MBBR) and a traditional A/O bioreactor were continuously operated for 115 days and their outcomes were compared to investigate the enhancement effect of carriers on the total nitrogen removal (TN) for expressway service area sewage. Results revealed that A/O-MBBR required lower dissolved oxygen, exhibited higher tolerance toward harsh conditions, and demonstrated better shock load resistance than traditional A/O bioreactor. The TN removal load of A/O-MBBR reached 181.5 g‧N/(m3‧d), which was 15.24% higher than that of the A/O bioreactor. Furthermore, under load shock resistance, the TN removal load of A/O-MBBR still reached 327.0 g‧N/(m3‧d), with a TN removal efficiency of above 80%. Moreover, kinetics demonstrated that the denitrification rate of the A/O-MBBR was 121.9% higher than that of the A/O bioreactor, with the anoxic tank biofilm contributing 60.9% of the total denitrification rate. Community analysis results revealed that the genera OLB8, uncultured_f_Saprospiraceae and OLB12 were the dominant in biofilm loaded on carriers, and OLB8 was the key for enhanced denitrification. FAPROTAX and PICRUSt2 analyses confirmed that more bacteria associated with nitrogen metabolism were enriched by the A/O-MBBR carriers through full denitrification metabolic pathway and dissimilatory nitrate reduction pathway. This study offers a perspective into the development of cost-effective and high-efficiency treatment solutions for expressway service area sewage.

Recent advances in co-digestion conjugates for anaerobic digestion of food waste.

Anaerobic digestion (AD) is a biological process that breaks down organic waste materials, such as food waste (FW) that produces biogas and digestate. The biogas can be utilized as biofuel, and digestate could be applied as fertilizer. However, AD of FW alone has limitations on optimal degradation, digester stability and biogas yield. Co-digestion of FW along with other organic wastes such as animal manure, agricultural residue, sewage sludge and industrial organic waste, has shown substantial improvement in degradation process with increased biogas yield. The inadequacies in FW for optimum AD, like low carbon-to-nitrogen ratio (C/N ratio), lack of trace elements and irregular particle sizes, can be nullified by adding appropriate co-digestion conjugates. This review aims to describe the characteristic inadequacies of FW and examines the effect on mesophilic co-digestion of FW with animal manure, waste sludge and agricultural wastes for biogas production optimization. A critical review on the impact of pretreatment and co-digestion to enrich the methane (CH4) content in biogas has been performed. The review also examines the microbial community shift due to co-digestion, which is critical for the stability of an anaerobic digester. Finally, it discusses the prospects and challenges for the widespread application of the co-digestion technique as an effective organic waste management practice.

Insights on microbial fuel cells for sustainable biological nitrogen removal from wastewater: A review.

Microbial fuel cells (MFCs) have emerged as a promising technology for energy-efficient wastewater treatment. The feasibility of integrating biological nitrogen removal into MFC systems has been reported. However, better pollutant removal efficiency and power production need to be achieved at a lower cost for a sustainable wastewater treatment system. The objective of this paper is to critically review the nitrogen removal process in various MFC configurations, factors that influence this process, and challenges that should be overcome in future studies. Based on the results of the review, shortcut nitrification-autotrophic denitrification in an MFC is an option as it minimizes the aeration energy and C/N ratio requirement; however, it is necessary to evaluate the N2O emission further. Another attractive option is the heterotrophic anodic denitrification process as it demonstrates the potential for free-buffer MFCs, but the nitrogen removal efficiency at low C/N ratios needs improvement. Bacteria population in MFC system also plays an essential role in both contaminant removal and electricity generation. It can be concluded that MFCs can be a low cost, sustainable solution for the treatment of wastewater and removal of nitrogen. Moreover, selection of MFC configuration will depend on the nature of the wastewater.

Rapid start-up and advanced nutrient removal of simultaneous nitrification, endogenous denitrification and phosphorus removal aerobic granular sequence batch reactor for treating low C/N domestic wastewater.

The rapid start-up and advanced nutrient removal of simultaneous nitrification, endogenous denitrification, and phosphorus (P) removal aerobic granular sequence batch reactor (SNEDPR-AGSBR) is a challenge in the treatment of low carbon/nitrogen (C/N) domestic sewage. In this study, the feasibility of the SNEDPR-AGSBR process was examined in an exceedingly single-stage anaerobic/aerobic/anoxic sequencing batch reactor for treating low C/N ratio (3.3-5.0) domestic sewage. The initial results showed that accompanied by the rapid formation of the mature aerobic granular sludge based on the selection for slow-growing organisms, the rapid start-up (38 d) of the SNEDPR-AGSBR process was successfully realized. The formed mature aerobic granules had a dense structure with an average diameter of 667.7 µm and SVI30 of 30.0 mL/g. Two conditions for achieving the competitive balance between phosphorus-accumulating organisms/denitrifying phosphorus-accumulating organisms (PAOs/DPAOs) and glycogen accumulating organisms/denitrifying glycogen accumulating organisms (GAOs/DGAOs) were revealed by the long-term operation results. First, the dissolved oxygen (DO) concentration needed to be decreased to 3.0 mg/L in the aerobic phase, and then, the aerobic and anoxic phase hydraulic retention time (HRT) should be increased to 3.0 h. Notably, high removal efficiencies for NH4+-N (100%), total nitrogen (84.3%), and P (91.8%) of the SNEDPR-AGSBR process were stably obtained with a low C/N ratio of 3.9 domestic sewage. Simultaneous nitrification and endogenous denitrification (SNED) efficiency of 61.6% was achieved during a long-term operation of 142 days. Finally, microbial community analysis confirmed that GAOs (Defluviicoccus)/DGAOs (Candidatus_Competibacter) were responsible for the removal N, and PAOs (Acinetobacter, Candidatus_Accumulibacter, Hypomicrobinm)/DPAOs (Pseudomonas and Dechloromonas) ensured P removal.

Simultaneous denitrification and antibiotic degradation of low-C/N-ratio wastewater by a three-dimensional biofilm-electrode reactor: Performance and microbial response.

Three-dimensional biofilm-electrode reactors (3D-BERs) were fabricated and used to simultaneously remove nitrate and metronidazole (MNZ) from low-C/N-ratio wastewater. The results showed that 1 mg/L MNZ significantly promoted nitrate removal. After MNZ was added to the reactor, the removal efficiencies of total nitrogen (TN) and NO3--N increased significantly from 18.97% and 52.09% to 71.63% and 99.98% within 6 h, respectively. The MNZ-removal kinetics conformed to a pseudo-first-order model, and the removal rate constant reached a maximum value of 0.853 h-1, which was 4.1 and 2.8 times higher than that of pure microorganisms and pure electrochemical reactors, respectively. This indicated that the 3D-BERs constructed in this study were capable of simultaneous MNZ degradation and denitrification. In the presence of nitrate, six MNZ-degradation intermediates were identified, and four MNZ transformation pathways were proposed, including cleavage of hydroxyethyl groups, reduction of nitro groups, N-denitration, and deprotonation of side-chain hydroxyl groups. High-throughput sequencing revealed that the reactor was rich in various MNZ-degraders and denitrifiers, such as Hydrogenophaga, Methylomonas, Crenohrix, Dechloromonas, and Methylophilus. A function prediction analysis of nitrogen metabolism showed that the 3D-BER reactor with MNZ had higher denitrification activity than the other reactors tested. It was speculated that the intermediates produced by MNZ could act as carbon sources allowing denitrifying bacteria to perform denitrification, which made a nonnegligible contribution to the removal of nitrogen.

Bioaugmentation for low C/N ratio wastewater treatment by combining endogenous partial denitrification (EPD) and denitrifying phosphorous removal (DPR) in the continuous A2/O - MBBR system.

Endogenous partial denitrification (EPD) and denitrifying phosphorous removal (DPR) were combined in a novel A2/O - MBBR (Anaerobic Anoxic Oxic - Moving Bed Biofilm Reactor) system for low carbon/nitrogen (C/N) ratio wastewater treatment. The DPR performance was compared and the nutrient metabolism was elucidated based on the optimization of hydraulic retention time (HRT, 4-12 h) and nitrate recycling (R, 200%-600%). In the continuous-flow, the nitrate (NO3-) denitrification accompanied by nitrite (NO2-, via EPD) accumulation with the nitrate-to-nitrite transformation ratio (NTR) of 35.87%-43.31% in the anoxic zones. At HRT of 12 h with R of 500%, batch test initially revealed the DPR mechanism using both NO3- and NO2- as electron acceptor, where denitrifying phosphorus accumulation organisms (DPAOs) and denitrifying glycogen accumulation organisms (DGAOs) were the main contributors for EPD with incomplete denitrification (NO3- → NO2-). Furthermore, stoichiometry-based functional bacteria analysis displayed that higher bioactivity of DPAOs (NO2-→N2, 57.30%; NO3-→N2, 35.85%) over DGAOs (NO3-→N2, 6.85%) facilitated the anoxic NO3- reduction. Microbial community analysis suggested that Cluster I of Defluviicoccus-GAO group (∼4%) was responsible for stable NO2- accumulation performance via EPD, while increased Accumulibacter-PAO group (by ∼15%) contributed to the advanced nutrient removal. Based on the achievement of NO2- accumulation, the application feasibility of integrated EPD - DPR - Anammox for deep-level nutrient removal was discussed.

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

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

Biological nitrogen removal capability and pathways analysis of a novel low C/N ratio heterotrophic nitrifying and aerobic denitrifying bacterium (Bacillus thuringiensis strain WXN-23).

A novel heterotrophic nitrification and aerobic denitrification (HNAD) bacteria, identified as Bacillus thuringiensis strain WXN-23, was isolated from husk feed filtrate of a pig farm. It was the first report of Bacillus thuringiensis with the capability for HNAD and could adapt to the condition of low Carbon/Nitrogen (C/N) ratio. Nitrogen could be efficiently removed by the strain WXN-23 in simulated wastewater, be it in single or mixed form nitrogen sources. The nitrogen balance revealed that 63.5% of the initial nitrogen (5.32 mg) was lost in the form of N2. The conditions for maximum total nitrogen (TN) removal efficiency (95.996%) were shaking speed of 126.89 r/min, a carbon C/N ratio of 5.91, the temperature of 32.81 °C, and a pH value of 8.17. The nitrification-denitrification metabolic pathway (NH4+-N→NH2OH→NO2--N→NO3--N→NO2--N→NO→N2O→N2) under aerobic conditions was determined on the basic of characteristic of N removal, N balance analysis, enzyme assay and functional genes amplification results. Strain WXN-23 was effective at wastewater treatment, with TN, NH4+-N, NO3--N and NO2--N removal efficiencies of 82.12%, 86.74%, 90.74% and 100%, respectively.