Landfill leachate as an additional substance in the Johannesburg-Sulfur autotrophic denitrification system in the treatment of municipal wastewater with low strength and low COD/TN ratio

Enhancing nitrogen removal in low C/N wastewater with recycled sludge-derived biochar: A sustainable solution

Denitrification is an important biological process in wastewater treatment plants (WWTPs). However, a low carbon-to-nitrogen (C/N) ratio limits the availability of organic carbon, potentially reducing denitrification efficiency. This study investigates the impact of sludge-derived biochar on the nitrogen removal of activated sludge for low C/N ratio municipal wastewater. Sludge-based biochar was characterized by its physicochemical properties, revealing that biochar prepared at 400 °C exhibited the highest specific surface area and the most favorable surface functional groups for electron transfer. The results from batch tests showed that adding 4 g/L of biochar dosage enhanced denitrification rates and total nitrogen (TN) removal efficiency the most. Sequencing batch reactors (SBRs) experiments further confirmed that biochar dosgae improved the removal efficiencies of COD, NH4+-N, and TN, achieving stable values of 97.2 ± 1.2 %, 99.2 ± 0.6 %, and 83.8 ± 2.4 %, respectively. Metabolic and electrochemical analyses revealed that biochar addition enhanced the activity of denitrification enzymes, increasing the ammonia oxidation rate by 12.9 ± 0.7 %, nitrite oxidation rate by 14.7 ± 1.2 %, nitrate reduction rate by 36.9 ± 1.5 %, and nitrite reduction rate by 16.4 ± 0.8 %. The relative abundance of denitrification functional genes (amoA, nirS, nirK, narG, nosZ) increased, and the activities of the corresponding enzymes (AMO, NXR, NAP, NIR) rose by 23±6 %, 53±5 %, 260±15 %, and 55±7 %, respectively. This increase in enzyme activity suggested enhanced denitrification processes, which was further supported by the 60.1 ± 3.7 % increase in electron transfer system activity (ETSA), indicating that biochar acted as an electron shuttle. This study proposes a potential sustainable approach for sludge recycling and enhanced wastewater nitrogen removal under low C/N conditions.

Inhibitory mechanism of Cr(VI) on sulfur-based denitrification: Bio-toxicity, bio-electron characteristics, and microbial evolution

Sulfur-based denitrification is a promising technology for efficient nitrogen removal in low-carbon wastewater, while it is easily affected by toxic substances. This study revealed the inhibitory mechanism of Cr(VI) on thiosulfate-based denitrification, including bio-toxicity and bio-electron characteristics response. The activity of nitrite reductase (NIR) was more sensitive to Cr(VI) than that of nitrate reductase (NAR), and NIR was inhibited by 21.32 % and 19.86 % under 5 and 10 mg/L Cr(VI), resulting in 10.12 and 15.62 mg/L of NO2--N accumulation. The biofilm intercepted 36.57 % of chromium extracellularly by increasing 25.78 % of extracellular polymeric substances, thereby protecting microbes from bio-toxicity under 5 mg/L Cr(VI). However, it was unable to resist 20-30 mg/L of Cr(VI) bio-toxicity as 19.95 and 14.29 mg Cr/(g volatile suspended solids) invaded intracellularly, inducing the accumulation of reactive oxygen species by 165.98 % and 169.12 %, which triggered microbial oxidative-stress and damaged the cells. In terms of electron transfer, S2O32- oxidation was inhibited, and parts of electrons were redirected intracellularly to maintain microbial activity, resulting in insufficient electron donors. Meanwhile, the contents of flavin adenine dinucleotide and cytochrome c decreased under 5-30 mg/L Cr(VI), reducing the electron acquisition rate of denitrification. Thermomonas (the dominant genus) possessed denitrification and Cr(VI) resistance abilities, playing an important role in antioxidant stress and biofilm formation. ENVIRONMENTAL IMPLICATION: Sulfur-based denitrification (SBD) is a promising method for nitrate removal in low-carbon wastewater, while toxic heavy metals such as Cr(VI) negatively impair denitrification. This study elucidated Cr(VI) inhibitory mechanisms on SBD, including bio-toxicity response, bio-electron characteristics, and microbial community structure. Higher concentrations Cr(VI) led to intracellular invasion and oxidative stress, evidenced by ROS accumulation. Moreover, Cr(VI) disrupted electron flow by inhibiting thiosulfate oxidation and affecting electron acquisition by denitrifying enzymes. This study provided valuable insights into Cr(VI) toxicity, which is of great significance for improving wastewater treatment technologies and maintaining efficient and stable operation of SBD in the face of complex environmental challenges.

The effect of reflux ratio on sulfur disproportionation tendency in anaerobic baffled reactor with the heterotrophic combining sulfur autotrophic processes under high concentration perchlorate stress

In a laboratory scale, an anaerobic baffled reactor (ABR) consisting of eight compartments, the heterotrophic combining sulfur autotrophic processes under different reflux ratios were constructed to achieve effective perchlorate removal and alleviate sulfur disproportionation reaction. Perchlorate was efficiently removed with effluent perchlorate concentration below 0.5 μg/L when the influent perchlorate concentration was 1030 mg/L during stages I ~ V, indicating that heterotrophic combining sulfur autotrophic perchlorate reduction processes can effectively achieve high concentration perchlorate removal. Furthermore, the 100% reflux ratio could reduce the contact time between sulfur particles and water; thus, the sulfur disproportionation reaction was inhibited. However, the inhibition effect of reflux on sulfur disproportionation was attenuated due to dilute perchlorate concentration when a reflux ratio of 150% and 200% was implemented. Meanwhile, the content of extracellular polymeric substances (EPS) in the heterotrophic unit (36.79 ~ 45.71 mg/g VSS) was higher than that in the sulfur autotrophic unit (22.19 ~ 25.77 mg/g VSS), indicating that high concentration perchlorate stress in the heterotrophic unit promoted EPS secretion. Thereinto, the PN content of sulfur autotrophic unit decreased in stage III and stage V due to decreasing perchlorate concentration in the autotrophic unit. Meanwhile, the PS content increased with increasing reflux in the autotrophic unit, which was conducive to the formation of biofilm. Furthermore, the high-throughput sequencing result showed that Proteobacteria, Chloroflexi, Firmicutes, and Bacteroidetes were the dominant phyla and Longilinea, Diaphorobacter, Acinetobacter, and Nitrobacter were the dominant genus in ABR, which were associated with heterotrophic or autotrophic perchlorate reduction and beneficial for effective perchlorate removal. The study indicated that reflux was a reasonable strategy for alleviating sulfur disproportionation in heterotrophic combining sulfur autotrophic perchlorate removal processes.

Long-term effects of thiosulfate on the competition between sulfur-mediated bacteria and glycogen accumulating organisms in sulfate-rich carbon-deficient wastewater

Sewage nutrient (e.g., nitrogen and phosphorus) biological removal performance is often limited by the deficient carbon source and undesirable glycogen accumulating organisms (GAOs), even in sulfate-containing wastewater. Thiosulfate (S2O32-) as a bioavailable, environmentally-benign, metastable and cost-effective agent has been regarded as electron carriers that induces high sulfur-mediated bacterial activity for nutrient removal from wastewater. In this study, the long-term effects of thiosulfate on the competition between sulfur-mediated bacteria (SMB, including sulfur-reducing bacteria (SRB) and sulfur-oxidizing bacteria (SOB)) and GAOs were explored to further close the gap of our knowledge on the control of GAOs under carbon deficient wastewater. Three reactors were continuously operated for over 100 days and were fed with 200 mg acetate-COD/L and 20 (R1), 50 (R2) and 80 (R3) mg S/L thiosulfate respectively. The results revealed that adding thiosulfate at the beginning of the anoxic phase promoted sulfur metabolism and increased the proliferation of SRB (mainly Desulfobacter) and SOB (mainly Chromatiaceae). Correspondingly, the relative abundance of GAOs (mainly Candidatus_Competibacter) decreased. After the carbon source was reduced, the abundance of GAOs increased and the competitive activity of SRB was weakened, resulting in the reduced sulfate reduction, which could be attributed to the fact that GAOs had a higher carbon source competitiveness than SRB under low carbon source conditions. While SOB maintained a high abundance due to the addition of thiosulfate as an additional electron donor, which enhanced the denitrification efficiency. Additionally, the dominant SOB shifted from Thiobacillus to Chromatiaceae during the long-term operation, indicating that Chromatiaceae had a higher competitive advantage for reduced sulfur (e.g., S2O32-, Polysulfide (Poly-S)) and nitrate compared to Thiobacillus. Furthermore, microbial functional genes revealed that S metabolism was enhanced during long-term operation. The potential mechanism and optimization strategy regarding the competition between sulfur-mediated bacteria and GAOs were revealed.

A novel sulfur autotrophic denitrification in-situ coupled sequencing batch reactor system to treat low carbon to nitrogen ratio municipal wastewater: Performance, niche equilibrium and pollutant removal mechanisms

A novel integrated sulfur fixed-film activated sludge in SBR system (IS0FAS-SBR) was proposed to treat the low C/N ratio municipal wastewater. The effluent total inorganic nitrogen (TIN) and PO43--P decreased from 17 mg/L and 3.5 mg/L to 8.5 mg/L and 0.5 mg/L, and higher nitrogen removal efficiency was contributed by the autotrophic denitrification. Microbial response characteristics showed that catalase (CAT), reduced nicotinamide adenine dinucleotide (NADH) and extracellular polymeric substance (EPS) alleviated the oxidative stress of sulfur carrier to maintain cell activity, while metabolic activity analysis indicated that the electron transfer rate was enhanced to improve mixotrophic denitrification efficiency. Meanwhile, the increased key enzyme activities further facilitated nitrogen removal and sulfur oxidation process. Additionally, the microbial community, functional proteins and genes revealed a niche equilibrium of C, N, S metabolic bacteria. Sulfur autotrophic in-situ coupled SBR system enlarged a promising strategy for treatment of low C/N ratio municipal wastewater.

Sulfur autotrophic denitrification as an efficient nitrogen removals method for wastewater treatment towards lower organic requirement: A review

The sulfur autotrophic denitrification (SADN) process is an organic-free denitrification process that utilizes reduced inorganic sulfur compounds (RISCs) as the electron donor for nitrate reduction. It has been proven to be a cost-effective and environment-friendly approach to achieving carbon neutrality in wastewater treatment plants. However, there is no consensus on whether SADN can become a dominant denitrification process to treat domestic wastewater or industrial wastewater if organic carbon is desired to be saved. Through a comprehensive summary of the SADN process and extensive discussion of state-of-the-art SADN-based technologies, this review provides a systematic overview of the potential of the SADN process as a sustainable alternative for the heterotrophic denitrification (HD) process (organic carbons as electron donor). First, we introduce the mechanism of the SADN process that is different from the HD process, including its transformation pathways based on different RISCs as well as functional bacteria and key enzymes. The SADN process has unique theoretical advantages (e.g., economy and carbon-free, less greenhouse gas emissions, and a great potential for coupling with novel autotrophic processes), even if there are still some potential issues (e.g., S intermediates undesired production, and relatively slow growth rate of sulfur-oxidizing bacteria [SOB]) for wastewater treatment. Then we present the current representative SADN-based technologies, and propose the outlooks for future research in regards to SADN process, including implement of coupling of SADN with other nitrogen removal processes (e.g., HD, and sulfate-dependent anaerobic ammonium oxidation), and formation of SOB-enriched biofilm. This review will provide guidance for the future applications of the SADN process to ensure a robust-performance and chemical-saving denitrification for wastewater treatment.

Humic acid recovery from stabilized leachate: Characterization and interference with chemical oxygen demand-colour removal

Heterogeneous combinations of organic compounds (humic acid (HA) and fulvic acid) are the prime factor for the high concentration of colour and chemical oxygen demand (COD) in semi-aerobic stabilized landfill leachate. These organics are less biodegradable and cause a severe threat to environmental elements. Microfiltration and centrifugation processes were applied in this study to investigate the HA removal from stabilized leachate samples and its corresponding interference with COD and colour. The three-stage extraction process recovered a maximum of 1412 ± 2.5 mg/L (Pulau Burung landfill site (PBLS) leachate), 1510 ± 1.5 mg/L (Alor Pongsu landfill site (APLS leachate) at pH 1.5 and 1371 ± 2.5 mg/L (PBLS) and 1451 ± 1.5 mg/L (APLS) of HA (about 42% of the total COD concentration) at pH 2.5, which eventually indicates the process efficiency. Comparative characteristics analysis of recovered HA by scanning electron microscopy, energy-dispersive X-ray, X-ray photoelectron spectroscopy, and Fourier transform infrared significantly indicate the existence of identical elements in the recovered HA compared with the previous studies. The higher reduction (around 37%) in ultraviolet (UV) absorbance values (UV254 and UV280) in the final effluent indicates the elimination of aromaticity and conjugated double-bond compounds from leachate. Moreover, 36 and 39% COD and 39 and 44% colour removal exhibit substantial interference.

Discovering dominant ammonia assimilation: Implication for high-strength nitrogen removal in full scale biological treatment of landfill leachate

Landfill leachate containing high-strength nitrogen is generated in domestic waste landfilling. The integration of anoxic and aerobic process (AO) based on nitrification and denitrification, has been a mainstream process of biological nitrogen removal (BNR). But the high-strength organics as well as aerobic effluent reflux might change the biochemical environment designed and operated as AO. In view of the nitrogen balance in a full scale landfill leachate treatment plant with two-stage AO, we found that approximately 90% removal of total nitrogen (TN) and ammonia (NH₄⁺-N) focused on primary anoxic and aerobic stage. Meanwhile, the less nitrate and nitrite in the aerobic effluent were incapable of sustaining denitrification or anaerobic ammonia oxidation (anammox). The high reflux flow from aerobic to anoxic process enabled the similar microbial community and functional genes in anoxic and aerobic process units. However, the functional genes involving ammonia assimilation in all process units showcased the highest abundance compared to those correlated with other BNR pathways, including nitrification and denitrification, assimilatory and dissimilatory nitrate reduction, nitrogen fixation and anammox. The ammonia assimilation dominated the removals of TN and NH₄⁺-N, rather than other BNR mechanism. The insight of dominant ammonia assimilation is favorable for illustrating the authentic BNR mechanism of landfill leachate in AO, thereby guiding the optimization of engineering design and operation.

Rapid start-up and stable operation of an aerobic/oxic/anoxic simultaneous nitrification, denitrification, and phosphorus removal reactor with no sludge discharge

An anaerobic/aerobic/anoxic mode simultaneous nitrification, denitrification, and phosphorus removal system was visited for enhanced low-strength wastewater treatment and dramatic in situ sludge reduction. Results showed that rapid start-up was achieved with conventional activated sludge after 15 days, with effluent ammonia nitrogen, total nitrogen, total phosphorus, and chemical oxygen demand being 0.25, 7.89, 0.12, 24.37 mg/L, respectively. A two-stage biomass growth rate was observed with the sludge yield of 0.285 (day 1-50) and 0.017 g MLSS/g COD (day 51-110) without sludge discharge. Dynamics of bacterial community has been identified with outstanding accumulation of Candidatus_Competibacter up to 29.06 %, which contributed to both simultaneous nutrients removal and sludge reduction. Further analysis via PICRUSt2 revealed the main pathway of nitrogen metabolism, while proposed mechanism for phosphorus removal with no sludge discharge was analyzed from the intracellular and extracellular perspectives. Overall, this study provided guidance and reference for the development and application of A/O/A-SNDPR technology.

Efficient use of electrons in a double-anode microbial fuel cell-biofilm electrode reactor self-powered coupled system for degradation of azo dyes

A coupled system consisting of a double-anode microbial fuel cell (MFC) unit and a biofilm electrode reactor (BER) has been applied to degrade the azo dye reactive brilliant red X-3B. In this system, the MFC effluent was used as the input of the BER. The MFC preliminarily degraded X-3B while generating electricity, and the BER obtained electrons from the MFC through the external circuit to continue degrading pollutants without the need for an external power supply. The X-3B removal efficiency was 41.93% higher in the coupled system than the control when the X-3B concentration was 3000 mg/L. The analysis of intermediate products showed that the azo bond of X-3B broke in the MFC, generating a large number of complex intermediates such as anthraquinones, which were further degraded into simple organic compounds in the BER. Meanwhile, the abundance of microbial taxa related to the degradation of refractory organics in the MFC was high, as was that of microbial taxa related to the degradation of simple organics in the BER. Furthermore, the abundance of microorganisms related to power generation in the MFC increased. These results provided an efficient strategy for improving electron utilization efficiency in the coupling system of bioelectrochemical system.

Formation of anaerobic granular sludge (AnGS) to treat high-strength perchlorate wastewater via anaerobic baffled reactor (ABR) system: Electron transfer characteristic, bacterial community and positive feedback mechanism

Anaerobic granular sludge (AnGS) was cultured to treat high-strength perchlorate (reaching to 4800 mg/L) wastewater by an anaerobic baffled reactor (ABR) system with five equal-volume compartments (C1-C5 compartments). Inoculated sludge completely granulated on day 104 with granule size of 0.50-0.75 mm and perchlorate removal efficiency reaching to 97% (influent perchlorate of 2000-4800 mg/L). The Cyclic voltammetry (CV) capacitance increased from 487.5, 465.8 and 407.8 μF to 576.5, 552.4, 549.6 μF in C1, C3 and C5 compartments of ABR system, respectively, suggesting the electron transfer capacity was enhanced under high-strength perchlorate stress. Meanwhile, adenosine triphosphate (ATP) value and electron transport system activity (ETSA) increased to 25.05, 22.87, 20.43 and 6.22, 4.87, 3.95 of C1, C3 and C5 compartments, respectively. The results suggested that high-strength perchlorate stress improved the microbial metabolic activity, which promoted secretion of extracellular polymeric substances (EPS). The more EPS could facilitate the formation and stability of AnGS under high-strength perchlorate stress. In addition, more reasonable metabolic division of labor in functional bacterial (Thauera and Comamonas) was beneficial to AnGS formation, which achieved high-strength perchlorate efficient removal. Finally, a positive feedback mechanism between AnGS formation and high-strength perchlorate removal was established through EPS, microbial metabolic activity and electron transfer characteristic in ABR system. However, excessive perchlorate (5800 mg/L) would exceed the treatment capacity of AnGS, which resulted in the deterioration of removal performance. This work provided an effective information for AnGS application to treat high-strength perchlorate wastewater.

Enhanced simultaneous partial nitrification and denitrification performance of aerobic granular sludge via tapered aeration in sequencing batch reactor for treating low strength and low COD/TN ratio municipal wastewater

The aerobic granular sludge simultaneous partial nitrification, denitrification and phosphorus removal (AGS-SPNDPR) process was carried out via tapered aeration in sequencing batch reactor (SBR) for treating low strength and low COD/TN ratio municipal wastewater. The results showed that aerobic granular sludge was successfully cultivated with good sedimentation performance when treating the municipal wastewater. Meanwhile, the median granule size increased to 270 (R1) and 257 (R2) μm on day 80. The excellent removal performance of COD (92%) and NH₄⁺-N (95%) were achieved under different aeration modes, while the higher TN removal efficiency (76%) was achieved by tapered aeration. The accumulation of NO₂^--N in R2 indicated that the tapered aeration was beneficial to achieve simultaneously partial nitrification and denitrification. Meanwhile, the high-efficiency phosphorus (95%) removal was realized via additional carbon source, and SPNDPR process was formed under tapered aeration. The bacterial community analysis indicated denitrifying glycogen-accumulating organisms (DGAOs) Candidatus_Competibacter and ammonia-oxidizing bacteria (AOB) Nitrosomonas were more effectively enriched via tapered aeration, while phosphorus-accumulating organisms (PAOs) Candidatus_Accumulibacter were effectively enriched under additional organic carbon. AOB, denitrifying bacteria and PAOs were simultaneously enriched by tapered aeration and additional carbon source, which was beneficial to nutrients removal. This study might be conducive to the application of AGS-SPNDPR system for treating low strength and low COD/TN ratio municipal wastewater under tapered aeration.

Study and evaluation of the characteristics of saline wastewater (brine) produced by desalination and industrial plants

Desalination and industrial plants all around the world generate large amounts of saline wastewater (brine). The discharge of brine from facilities poses a severe environmental threat, while at the same time, the opportunity to recover resources is being lost as discharged brine is rich in valuable metals that could be recovered as salts/minerals. To this aim, this study presents and analyzes for the first time the characteristics of different brine effluents (from industries such as desalination, oil and gas production, petrochemical, aquaculture, pharmaceutical, textile) to prevent environmental pollution and to recover valuable resources (i.e., salts, minerals, metals, chemicals) enabling the concept of waste-to-resource (circular water economy model). The results revealed that the common salinity values in brine effluents range from 0.5 to 150 g/L, while the only exception is the produced water from the oil and gas industry (up to 400 g/L). Brine effluents from all sectors contain sodium, chloride, calcium, and potassium ions in high concentrations, while the production of common salts such as NaCl, CaCl₂, and MgCl₂ from brine can be economically profitable. Besides common ions, precious metals such as lithium, rubidium, and cesium are present in low concentrations (<25 mg/L); however, their extraction from brine effluents can be significantly profitable due to their very high sale price. The treatment and valorization of brine can be implemented by the hybridization of membrane-based, chemical, biological, and thermal-based technologies/processes in minimal and zero liquid discharge (MLD/ZLD) systems.

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.

Evaluating the impacts of leachate co-treatment on a full-scale municipal wastewater treatment plant in Canada

The objective of this study was to evaluate the impacts of leachate co-treatment on a full-scale municipal WWTPby comparing plant performance at varying levels of leachate contributions and hydraulic loadings.Leachate BOD:COD ratio was 0.08 ± 0.07 and indicated a stabilized, old matrix and concentrations of zinc, iron, aluminum, chloride and sulfate were 0.174, 38, 1.47, 1803 and 119.1 mg/L, respectively. The average volumetric leachate ratio (VLR%) was approximately 0.01% corresponding to a daily volume of 30 m³ but reaching a maximum of 270 m³(VLR% = 0.1%) and fluctuating on a daily-basis. A cluster analysis revealed 5 VLR% groupings that were used for subsequent analyses:no leachate, 0 < Low ≤ 0.001, 0.001 < Medium ≤ 0.02, 0.02 < High ≤ 0.05, 0.05 < Very high ≤ 0.2. Treated effluent concentrations of TKN, ammonia, fecal coliforms (FC),E. coli(EC), TSS and TP experienced atrend where effluent quality was improved at low and medium VLR%compared to no leachate addition, but deteriorated in high and very high VLR%.Treated effluent UVT% and EC were not statistically significantly different at varying VLR%, but FC was.Plant hydraulic had a significant impact on removal rates.Ammonia removals and nitrite concentrations improved inhigh flow conditions, whileTP, BOD and cBODremovals deteriorated. Finally,VLR%, leachate COD, TKN ammonia, chloride and arsenic had significant relationships with plant performance. Thus,for leachate with comparable age and strength, VLR% should not exceedlow to medium contributions(0 and 0.02%)during co-treatment at this WWTP.

Survival strategy of comammox bacteria in a wastewater nutrient removal system with sludge fermentation liquid as additional carbon source

Complete ammonia oxidizing (comammox) bacteria are frequently detected in wastewater biological nutrient removal (BNR) systems. This study identified "Candidatus Nitrospira nitrosa"-like comammox bacteria as the predominant ammonia oxidizers (97.5-99.4%) in a lab-scale BNR system with acetate and sludge fermentation liquid as external carbon sources. The total nitrogen and phosphorus removals of the system were 75.9% and 86.9% with minimal N₂O emission (0.27%). Low ammonia concentration, mixotrophic growth potentials and metabolic interactions with diverse heterotrophs collectively contributed to the survival of comammox bacteria in the system. The recovered draft genomes of comammox bacteria indicated their potentials in using acetate and propionate but not butyrate. Acetate and propionate indeed stimulated the transcription of comammox amoA genes (up-regulated by 4.1 folds compared with no organic addition), which was positively correlated with the ammonia oxidation rate of the community (r = 0.75, p < 0.05). Comammox bacteria could provide vitamins/cofactors (e.g., cobalamin and biotin) to heterotrophs (e.g., Burkholderiaceae), and in return receive amino acids (e.g., phenylalanine and tyrosine) from heterotrophs, which they cannot synthesize. Compared with comammox bacteria, ammonia oxidizing bacteria (AOB) exhibited lower metabolic versatility, and lacked more pathways for the synthesis of amino acids and vitamin/cofactors, leading to their washout in the studied system. BNRs with comammox bacteria as the major nitrifiers hold great potentials in achieving superior performance at low aeration cost and low N₂O emission and at full-scale plants.

Sulfur transformation and bacterial community dynamics in both desulfurization-denitrification biofilm and suspended activated sludge

Types of microbial aggregates have essential effects on bacterial communities' characteristics, thus affecting the pollutants removal. An up-flow biofilm reactor was used to study the different performances of S^2-/NO₂^- removal and functional genes in suspended sludge and biofilms. The metabolic pathways of sulfurous and nitrogenous pollutants in the desulfurization-denitrification process were proposed. The results showed that S⁰ formation dominated the reactor with a high S^2- concentration. Autotrophic Sulfurovum responsible for S^2-/S⁰ oxidation was the only dominant bacteria in suspended sludge. Heterotrophic Desulfocapsa responsible for SO₄^2- reduction coexisted with Sulfurovum and dominated in biofilms. S^2- oxidation to S⁰ was catalyzed via fccA/B and sqr genes in suspended sludge. S₃^2-/S⁰ oxidation to SO₄^2- was catalyzed via dsrA/B gene in biofilms. SO₄^2- and NO₂^- were removed via the dissimilatory sulfate reduction and denitrification pathway, respectively. This work provides a fundamental and practical basis for optimizing suspended sludge/biofilm systems for S^2-/NO₂^- removal.

Novel tapered variable diameter biological fluidized bed for treating pesticide wastewater with high nitrogen removal efficiency and a small footprint

In this study, the removal efficiency of nitrogen, specific nitrification rate (SNR), specific denitrification rate (SDNR) and compliance rate of the novel tapered variable diameter biological fluidized bed (TVDBFB) and anoxic/oxic (AO) process were compared at different temperatures. The results showed that the optimal TN, NH₄⁺-N, and TKN removal efficiencies of the TVDBFB were 76%, 89% and 88%, respectively, and those of AO were 65%, 67% and 69%, respectively. The SNR and SDNR of the TVDBFB were significantly higher than those of AO. The TVDBFB had a smaller footprint than AO. The alkalinity/NH₄⁺-N, BOD₅/TN and temperature play important roles in the compliance rate. Increasing the carrier packing rate has emerged as a new strategy for enhancing the compliance rate. Mathematical models were developed and determined to be well-fitted with the experimental values, which can be employed to predict the SNR and SDNR of the TVDBFB.

Sulfur transformation in sulfur autotrophic denitrification using thiosulfate as electron donor

Thiosulfate is frequently used as an energy source and electron donor in autotrophic denitrification (AD) for removing nitrate from wastewater. However, transforming pathways of S₂O₃^2- in this process is unclear. Herein, the aim of this study is to explore possible transforming pathways of sulfur compounds in thiosulfate-based AD process. After measuring the variation of NO₃^-, NO₂^-, and various sulfur compounds such as S⁰, SO₄^2-, S₂O₃^2-, acid volatile sulfide (AVS), and S^2- in the presence and absence of S₂O₃^2-, the variation process of S₂O₃^2- and the contribution of various sulfur compounds were analyzed. The results indicated that S⁰, AVS, and S^2- were the intermediate products when S₂O₃^2- was applied as an electron donor. All S₂O₃^2-, S⁰, AVS, and S^2- could act as electron donors in the nitrate removal process with the final products of SO₄^2-. The utilization priority of these four sulfur sources was presumed in the following order: S^2- > S₂O₃^2- > AVS ≈ S⁰. Furthermore, sulfur transformation and balance in nitrate removal process was also investigated. This suggests the transforming pathways of sulfur compounds in denitrification process. Nitrogen removal and sulfur conversion process are dependent on the presence of microorganisms in the sludge.

Molecular investigation into the transformation of dissolved organic matter in mature landfill leachate during treatment in a combined membrane bioreactor-reverse osmosis process

This study investigated the effectiveness of a combined membrane bioreactor (MBR) and reverse osmosis (RO) process for treating leachate produced by a large-scale anaerobic landfill. The MBR process had limited treatment efficiency for removing organic pollutants, but when combined with RO, the integrated system completely removed macromolecular compounds (i.e., humic- and fulvic-like substances) and produced effluent that satisfied the applicable discharge standard. The landfill leachate contained many types of DOM that had high molecular weight and were highly unsaturated. Although the MBR process removed some DOM that had a relatively low saturated degree (mainly aliphatic compounds (2.0 ≥ H/C ≥ 1.5) with relatively high bioavailability), many bio-refractory compounds were not removed. The RO system greatly reduced the content of residual DOM in MBR effluent and was effective for removing heteroatom DOM, especially polycyclic aromatics (AI > 0.66) and polyphenols (0.66 ≥ AI > 0.50). The effluent from the combined process of MBR and RO treatment mainly contained a small number of aliphatic compounds and phenolic compounds (AI ≤ 0.50 and H/C < 1.5) that had higher bioavailability than DOM in the raw leachate and posed little environmental risk.

Sulphur-based autotrophic denitrification of wastewater obtained following graphite production: Long-term performance, microbial communities involved, and functional gene analysis

Sulphur-based autotrophic denitrification is an energy-efficient NO3--N removal process; it does not require carbon and may potentially replace traditional denitrification processes. This process was used to treat graphite production-derived wastewater and achieved almost complete removal of NO3--N (concentration in effluent: 5.2 mg/L; concentration in influent: 606 mg/L) at a salinity of 15 g/L with a 30 h hydraulic retention time. A unique microbial community was established, in which the abundance of Thiobacillus increased with the increase of the NO3--N concentration and salinity. Metagenomic analysis revealed that the denitrification metabolic pathway in the bioreactor was active. It also revealed the increased activation of nhaH, a gene encoding Na+/H+ antiporters; proA, proB, and proC, genes encoding proline; and Trk and Kdp systems during the treatment of graphite production-derived wastewater to maintain cell function, providing valuable information about utilizing the sulphur-based autotrophic denitrification process to treat graphite production-derived wastewater.