Potential causes of partial-denitrification (PD) granular sludge breakdown under high nitrate loading rates

Rapid formation of partial denitrification biofilm using gas-liquid separation membrane as carrier: Performance and mechanism

Partial denitrification (PD) can ensure stable supply of electron acceptors for anaerobic ammonia oxidation, and biofilm is an effective method to prevent biomass loss, which are crucial for stable operation of PD. In this study, hydrophobic hollow-fiber gas-liquid separation membranes were placed in a denitrification sequencing batch reactor, and dense biofilms were formed within just 3 days. Confocal laser microscopy showed the preferential attachment of the protein (PN) content in extracellular polymeric substances (EPS) to the membrane surface, followed by exopolysaccharides. Further analyses showed the decrease in the types of signal molecules from six to two (i.e., C4-HSL, C6-HSL) due to negative pressure operation. Importantly, the concentration of C4-HSL increased dramatically with the increase in PN concentration, suggesting that negative pressure promoted the synthesis of C4-HSL signal molecules, which further mediated the secretion of PN for biofilm formation. In addition, biofilm formation was accompanied by nitrite accumulation, leading to successful achievement of PD. Furthermore, 60 % of nitrate-to-nitrite transformation ratio was obtained even when COD/N was increased from 4.5 to 5.0 and influent nitrate concentration was reduced to 25 mg/L. This confirmed the stability of PD, which was mainly attributed to a change in the microbial community and a decrease in nitrite reductase (Nir) activity, with microorganisms enriched through the gas-liquid separation operation exhibiting low Nir activity. This study provides a new method for rapid formation of biofilm for wastewater treatment and stable operation of PD.

From acetate-driven partial denitrification (PD) to N-Methylpyrrolidone-driven PD: Microbial community, metabolic pathway and functional genes

With the rapid development of the new energy industry, the wastewater discharged from battery production industries has increased significantly, in which N-Methylpyrrolidone (NMP) have aroused widespread attention. It is valuable to develop low-cost biological methods for NMP containing wastewater treatment. In this study, an NMP-driven PD system was acclimated in an sequencing batch reactor (SBR) by gradually replacing the sodium acetate with NMP. The nitrate-to-nitrite transformation ratio (NTR) of the acclimated PD sludge was stabilized around 60 % and reached up to 68.1 %.The TOC removal was maintained above 90 %, and the NO-3-N reduction reached up to 96.7 %. The relative abundance of denitrifying bacterium Paracoccus increased during domestication (from 0.11 % to 9.38 %), and the abundance of NMP metabolism-related genes (hyuA, hyuB, mao, and gabD) also increased significantly. Moreover, the contribution of Paracoccus to NMP metabolism-related genes gradually increased. It suggests that Paracoccus may play a major role in this system. In conclusion, this study verified the feasibility of NMP as a carbon source to drive PD process to achieve NO-2-N accumulation, and provided a novel strategy for nitrogen removal of battery wastewater.

Fluoride-induced stress shapes partial denitrification granules to sustain microbial metabolism

The presence of fluoride ions (F-) in nitrogen-rich wastewater from photovoltaic and semiconductor industries introduces a significant challenge to biological treatment processes, particularly for the innovative partial denitrification (PD) process, which supplies nitrite for anaerobic ammonium oxidation (Anammox). This study provides the first comprehensive and systematic investigation of the effects of F- stress on the granule-based PD process through batch tests and long-term operation. Results indicate that PD activity remains resilient to F- shock up to 1.5 g/L but is markedly impaired at concentrations of 2.0-3.0 g/L, despite maintaining a nitrate-to-nitrite transformation ratio (NTR) of approximately 80 %. Under long-term F- stress at 0.5 g/L, NTR gradually reduces to 50 %, but subsequently recovers to and maintains at 70 %. The increased secretion of loosely bound extracellular polymeric substances and proteins likely enhances the resistance of PD granules to F- stress, though excessive amounts degrade their settling properties. F--induced microbial community succession shapes a predominance of medium granules (1.0 < d < 2.0 mm of 60.2 %) by enhancing aggregation of smaller granules and disintegration of larger ones. This enhances the mechanical strength and microbial activity of PD granules, aiding in resistance to F- stress to sustain microbial metabolism. Thauera is selectively enriched under long-term F- stress, with upregulated nirBDS genes contributing to the reduced NTR. Additionally, increased electron metabolism activity and a robust antioxidative response help to maintain higher microbial metabolic activity, mitigating F--induced oxidative stress. These findings advance our understanding of the resilience and adaptability of the PD process under F- stress, providing critical insights for optimizing biological wastewater treatment systems in challenging environments.

Pyrite and PHBV combined as substrates for groundwater denitrification

Nitrate pollution in groundwater has steadily increased globally, posing a potential threat to human health. Introduction of exogenous electron donors can significantly enhance nitrogen removal from nitrate-contaminated groundwater. Yet, conventional individual autotrophic or heterotrophic denitrification approaches have the disadvantage of low efficiency or high cost. This study investigated the performance of a laboratory-scale solid-phase denitrification (SPD) permeable reactive barrier (PRB) using a polyhydroxybutyrate-co-valerate (PHBV)/pyrite mixture as an electron donor for groundwater denitrification. Two different mass ratios (1:1 and 1:2) were established for the experimental setup. The results showed that under influent levels between 20 and 37 mg·L-1, the PHBV/pyrite system at a ratio of 1:1 achieved a maximum nitrate removal efficiency of 97.03%, with a nitrate removal rate of 99.13 mg NO 3 - - N NO 3 - - N ·L-1·d-1. Moreover, the PHBV/pyrite system at 1:2 reached 97.65% and 111.04 mg NO 3 - - N ·L-1·d-1 in terms of the optimum nitrate removal efficiency and rate. Dissolved organic carbon was undetectable in the effluent in both systems. The nitrate removal performance of the PHBV/pyrite system at 1:2 was superior to the one at 1:1, implying appropriate addition of pyrite in mixtrophic systems could enhance denitrification in groundwater. Additionally, the dominant genera identified were respectively Cloacibacterium and Acinetobacter in two systems, indicating that varying PHBV/pyrite ratios can modulate the succession of dominant nitrogenremoving microorganisms. Specifically, the system at 1:2 favoured aerobic microbial growth, thereby enhancing the efficiency of biological nitrogen removal. The findings have provided a valuable alternative for mixtrophic denitrification in in-situ remediation of nitrate-polluted groundwater.

Reason and control strategy for denitrification and anammox sludge flotation in nitrogen removal process: Mechanisms, strategies and perspectives

Anaerobic biological treatment technology, especially denitrification and anaerobic ammonia oxidation (anammox) technology as mainstream process, played dominant role in the field of biological wastewater treatment. However, the above process was prone to sludge floating during high load operation and thereby affecting the efficient and stable operation of the system. Excessive production of extracellular polymeric substance (EPS) was considered to be the main reason for anaerobic granular sludge flotation, but the summaries in this area were not comprehensive enough. In this review, the potential mechanisms of denitrification and anammox sludge floatation were discussed from the perspective of granular sludge structural characteristics, nutrient transfer, and microbial flora change respectively, and the corresponding control strategies were also summarized. Finally, this paper indicated that future research on sludge flotation should focus on reducing the negative effects of EPS in sludge particles.

Selective genes expression and metabolites transformation drive a robust nitrite accumulation during nitrate reduction under alternating feast-famine condition

Nitrite production via denitrification has been regarded as a key approach for survival of anaerobic ammonium oxidation (anammox) bacteria. Despite the important carbon substrate, little is known about the role of differential genes expression and extracellular metabolite regulation among diverse microbial communities. In this study, a novel alternating feast-famine strategy was proposed and demonstrated to efficiently accumulate nitrite in a low-nitrogen loading rate (NLR) (0.2∼0.8 kg N/m3/d) denitrification system. Highly selective expression of denitrifying genes was revealed as key regulators. Interestingly, in absence of carbon source (ACS) condition, the expression of narG and narI/V genes responsible for reduction of nitrate to nitrite jumped to 2.5 and 5.1 times higher than that in presence of carbon source (PCS) condition with carbon to nitrate ratio of 3.0. This fortunately facilitated a rapid nitrite accumulation once acetate was added, despite a significantly down-regulated narG and narI/narV and up-regulated nirS/nirK. This strategy selected Thauera as the most dominant denitrifier (50.2 %) with the highest contribution to narG and narI/narV genes, responsible for the high nitrite accumulation. Additionally, extracellular xylose, pyruvate, and glucose jointly promoted carbon-central metabolic pathway of key denitrifiers in ACS stage, playing an important role in the process of self-growth and selective enrichment of functional bacteria. The relatively rapid establishment and robust performance obtained in this study shows an engineering-feasible and economically-favorable solution for the regulation of partial denitrification in practical application.

An overlooked effect of hydroxylamine on anammox granular sludge: Promoting granulation and boosting activity

The exogenous hydroxylamine dosing has been proven to enhance nitrite supply for anammox bacteria. In this study, exogenous hydroxylamine was fed into a sequencing batch reactor to investigate its long-term effect on anammox granular sludge. The results showed that hydroxylamine enhanced the reactor's performance with an increase in total nitrogen removal rate from 0.23 to 0.52 kg N/m3/d and an increase in bacterial activity from 11.65 to 78.24 mg N/g VSS/h. Meanwhile, hydroxylamine promoted granulation by eluting flocs. And higher anammox activity and granulation were supported by extracellular polymeric substances (EPS) characteristics. Moreover, Candidatus Brocadia's abundance increased from 1.10 % to 3.03 %, and its symbiosis with heterotrophic bacteria was intensified. Additionally, molecular docking detailed the mechanism of the hydroxylamine effect. Overall, this study would provide new insights into the hydroxylamine dosing strategy application.

Granulation of partial denitrification sludge: Advances in mechanism understanding, technologies development and perspectives

The high-rate and stably efficient nitrite generation is vital and still challenges the wide application of partial denitrification (PD) and anammox technology. Increasing attention has been drawn to the granulation of PD biomass. However, the knowledge of PD granular sludge is still limited in terms of granules characterization and mechanisms of biomass aggregation for high nitrite accumulation. This work reviewed the performance and granulation of PD biomass for high nitrite accumulation via nitrate reduction, including the system start-up, influential factors, granular characteristics, hypothetical mechanism, challenges and perspectives in future application. The physiochemical characterization and key influential factors were summarized in view of nitrite production, morphology analysis, extracellular polymer substance structure, as well as microbial mechanisms. The PD granules exhibit potential advantages of a high biomass density, good settleability, high hydraulic loading rates, and strong shock resistance. A novel granular sludge-based PD combined with anammox process was proposed to enhance the capability of nitrogen removal. In the future, PD granules utilizing different electron donors is a promising way to broaden the application of anammox technology in both municipal and industrial wastewater treatment.

Hydroxyapatite (HAP) formation in acetate-driven partial denitrification process: Enhancing sludge granulation and phosphorus removal

Partial denitrification/anammox (PD/A) processes have emerged as a promising technology for efficient nitrogen removal from wastewater. However, these processes fail to remove phosphorus (P), a key pollutant that contributes to water eutrophication. To address this issue, the potential of inducing hydroxyapatite (HAP) precipitation in PD processes to achieve simultaneous P removal was investigated for the first time. Specifically, three SBRs (R1-R3) for PD were operated with adding varying concentrations of external Ca (30, 60, and 120 mg/L, respectively). Results demonstrated significant P reduction in all three SBRs, particularly in R3 with high Ca, which achieved an 80 % removal efficiency. Notably, sludge granulation was observed during operation, with the granule size in R3 with high Ca reaching 906.1 μm during the stable period, exceeding those in R2 (788.7 μm) and R1 (707.1 μm). This led to good settle ability of the PD sludge, as demonstrated by the lowest SVI5 (20 mL/g MLSS). Moreover, the decrease in the MLVSS/MLSS ratio suggested that the inorganic content accumulated, as observed by confocal laser scanning microscopy in the interior of the granules. Elemental composition analysis suggested that PD granules contained high P and Ca, while the X-ray diffraction (XRD) results confirmed the formation of HAP. Overall, this study demonstrated that PD-HAP coupled granular sludge process has potential as a robust and efficient method for nitrite production, as well as effective P removal and recovery, thereby advancing the application of anammox processes in wastewater treatment.