Highly efficient transformation of slowly-biodegradable organic matter into endogenous polymers during hydrolytic fermentation for achieving effective nitrite production by endogenous partial denitrification

Engineered stable partial nitrification/endogenous partial denitrification-anammox process for enhanced nitrogen removal from low carbon-to-nitrogen ratio wastewater

Addressing the intractable challenges of nitrite instability and slow start-up in anammox for low carbon-to-nitrogen (C/N) ratio wastewater treatment, a one-stage partial nitrification/endogenous partial denitrification-anammox (PN/EPD-A) process in a sequencing batch biofilm reactor was proposed. By synergistically coupling PN and EPD, self-sustained nitrite supply for anammox was achieved. Concurrently, a layered biofilm structure, engineered through tailored aeration and carrier addition, facilitated the rapid enrichment of anammox bacteria. The results demonstrated exceptional performance, achieving a total nitrogen removal efficiency of 83.3 %, with anammox consistently contributing 75.8 % of the nitrogen removed. Microbial community analysis further indicated the stable coexistence of anammox bacteria, ammonia-oxidizing bacteria, and glycogen-accumulating organisms, with their relative abundance reaching 1.36 %, 2.19 % and 9.80 %, respectively. These findings unveiled a robust and efficient strategy to overcome the limitations of anammox technology in low C/N wastewater treatment, paving the way for its broader application in nitrogen removal.

Regulation of microcurrent on carbon and nitrogen metabolism in denitrification under low carbon-to-nitrogen ratio: Optimizing carbon flux distribution

Synergy of autotrophic and heterotrophic denitrification can achieve low-carbon and high-efficient nitrogen removal. However, it remains unclear how microcurrent-driven hydrogen autotrophic denitrification regulates carbon flux distribution (nitrogen reduction, poly-β-hydroxyalkanoate (PHA) storage, and cell growth) in heterotrophic denitrification. This work compared biofilm reactor with biofilm electrode reactor under different carbon-to-nitrogen (C/N) ratios (10 - 3). At C/N ratio of 3, microcurrent accelerated nitrate reduction rate by 0.35 mg/(L·min) and reduced nitrite accumulation by 10.29 mg/L, thus decreasing nitrogen reduction proportion by 11.21%. Meanwhile, PHA storage and cell growth proportions increased by 0.03% and 11.18%, respectively. PHA was initially synthesized and subsequently utilized for nicotinamide adenine dinucleotide and energy production, while cell growth preferentially utilized limited carbon sources to maintain system stability. Increased abundance of hydrogen autotrophic denitrifiers, heterotrophic denitrifiers, and PHA storage bacteria confirmed optimization of microcurrent on carbon flux distribution. These findings advanced the understanding of microcurrent regulation on carbon flux.

Genome-resolved metagenomic analysis reveals a novel denitrifier with truncated nitrite reduction pathway from the genus SC-I-84

Understanding the genomic and ecological traits of partial denitrification (PD) bacteria is of high importance for developing wastewater treatment technologies. In this study, a PD-based bioreactor was operated, resulting in a mixed culture dominated by a potentially novel PD functional bacterium (SC-I-84). Progressively increased activity in both nitrate reduction and nitrite production were observed in the SC-I-84 enrichment system, whereas the nitrite reduction activity was always negligible. The phylogenetic analysis indicated that SC-I-84 was closely related to an uncultured beta-proteobacterium (99 %), whereas its denitrification functional genes (napA, napB, narV, and narY) exhibited evidence of co-evolution with chromosomal genes from the genus Cupriavidus, order Burkholderiales. In the genetic sketch of SC-I-84, only nitrate-reduction genes (nar and nap) were identified, whereas nitrite-reduction genes (nir) were absent. Notably, nitrate reduction genes were adjacent to carbon metabolism genes (sucB/C, mdh, idh) and a high abundance of tricarboxylic acid (TCA) cycling genes were found. This can promote the utilization efficiency of electron donors by nitrate reduction genes in SC-I-84, thus enhancing the denitrification activity. Furthermore, SC-I-84 positively cooperated with some bacteria that participate in nitrogen and carbon metabolism and other PD bacteria, but negatively interacted with full-denitrification bacteria. These results indicate that the enrichment of SC-I-84 restricted the growth of full-denitrification bacteria, aiding in the maintenance of a stable PD process. Taken together, the meta-genomic analysis of the novel PD functional bacterium is expected to enhance our understanding of PD processes and aid in the development of PD-based wastewater treatment processes.

Coupling Thiosulfate-Driven denitrification and anammox to remove nitrogen from actual wastewater

A coupled thiosulfate-driven denitrification and anammox (TDDA) process was established to remove nitrogen from wastewater. It was optimized in an up-flow anaerobic sludge blanket reactor using synthetic wastewater, and its reliability was then verified with actual wastewater. The results demonstrated that nitrate, nitrite, and ammonium could be synergistically removed, and the highest total nitrogen removal efficiency reached 97.8% at a loading of 1.39 kgN/(m3·d). Anammox bacteria, primarily Candidatus_Brocadia, were the main contributors to nitrogen removal, while sulfur-oxidizing bacteria such as Thiobacillus and Rhodanobacter played a supportive role. By optimizing substrate conditions to enhance the anammox process, the coupled system attained higher abundances of functional genes such as napA, nirS, hzs, soxXA, and soxYZ, along with the corresponding microbial species. The data suggested that microbial cross-feeding and self-adaptation strategies were key to efficient nitrogen removal by TDDA.

Increasing biomass concentration facilitates simultaneous nitrogen removal and sludge reduction under low C/N conditions

To overcome the issues of limited carbon source and high sludge production in partial denitrification/anammox (PD/A) process, the effects of mixed liquor suspended solids (MLSS) and carbon/nitrogen ratio (C/N) on PD/A were investigated through parallel experiments. Nitrogen removal efficiencies decreased significantly when C/N was reduced (1.5 → 0.75). When MLSS was doubled, the nitrogen removal efficiencies in the two parallel reactors increased from 75.3 %, 72.9 % to 86.9 %, 89.7 %, respectively, and sludge yields decreased obviously. Combining with in-situ test, it was speculated when MLSS increased, fermentation was enhanced, providing substrate for partial denitrification. Thauera, involved in partial denitrification, decreased obviously with reduced C/N, but increased from 9.93 % to 38.16 % when MLSS doubled, which could promote the PD/A process. Terrimonas and Ignavibacterium (fermentative bacteria) increased from 1.26 %, 5.22 % to 6.62 %, 6.30 %, respectively. These results proved that increasing MLSS under low C/N ratios promoted fermentation in PD/A system, facilitating efficient nitrogen removal and sludge reduction.

Achieving advanced nitrogen removal with anammox and endogenous partial denitrification driven by efficient hydrolytic fermentation of slowly-biodegradable organic matter

Anammox-based processes are pivotal for elevating nitrogen removal efficiency in municipal wastewater treatment. This study established a novel HF-EPDA system combined in-situ hydrolytic fermentation (HF) with endogenous partial denitrification (EPD) and anammox. Slowly-biodegradable organic matter (SBOM) was degraded and transformed into endogenous polymers for driving production of sufficient nitrite by EPD, further promoted the nitrogen removal via anammox process. Processes above formed positive feedback, guaranteeing the robustness and recoverability of system. After a 92-day suspension during operation, advanced nitrogen removal was still achieved with excellent nitrogen removal efficiency of 95.84 ± 1.73 %, treating with actual domestic wastewater and synthetic nitrate wastewater. Candidatus Brocadia and Candidatus Competibacter were dominant bacteria on biofilms responsible for the anammox and EPD process respectively, while the main hydrolytic fermentation organisms norank_o SBR1031 was enriched in floc sludge. This study highlights the reliable potential for expanding anammox application with simultaneous improvement of SBOM utilization.

Efficient nitrite accumulation in partial sulfide autotrophic denitrification (PSAD) system: insights of S/N ratio, pH and temperature

To provide the necessary nitrite for the Anaerobic Ammonium Oxidation (ANAMMOX) process, the effect of nitrite accumulation in the partial sulfide autotrophic denitrification (PSAD) process was investigated using an SBR reactor. The results revealed that the effectiveness of nitrate removal was unsatisfactory when the S/N ratio (mol/mol) fell below 0.6. The optimal conditions for nitrate removal and nitrite accumulation were achieved within the S/N ratio range of 0.7-0.8, resulting in an average Nitrate Removal Efficiency (NRE) of 95.84%±4.89% and a Nitrite Accumulation Rate (NAR) of 75.31%±6.61%, respectively. It was observed that the nitrate reduction rate was three times faster than that of nitrite reduction during a typical cycle test. Furthermore, batch tests were conducted to assess the influence of pH and temperature conditions. In the pH tests, it became evident that the PSAD process performed more effectively in alkaline environment. The highest levels of nitrate removal and nitrite accumulation were achieved at an initial pH of 8.5, resulting in a NRE of 98.30%±1.93% and a NAR of 85.83%±0.47%, respectively. In the temperature tests, the most favourable outcomes for nitrate removal and nitrite accumulation were observed at 22±1 ℃, with a NRE of 100.00% and a NAR of 81.03%±1.64%, respectively. Moreover, a comparative analysis of 16S rRNA sequencing results between the raw sludge and the sulfide-enriched culture sludge sample showed that Proteobacteria (49.51%) remained the dominant phylum, with Thiobacillus (24.72%), Prosthecobacter (2.55%), Brevundimonas (2.31%) and Ignavibacterium (2.04%) emerging as the dominant genera, assuming the good nitrogen performance of the system.

Refractory dissolved organic matters in sludge leachate trigger the combination of anammox and denitratation for advanced nitrogen removal

The cost-effective treatment of sludge leachate (SL) with high nitrogen content and refractory dissolved organic matter (rDOM) has drawn increasing attention. This study employed, for the first time, a rDOM triggered denitratation-anammox continuous-flow process to treat landfill SL. Moreover, the mechanisms of exploiting rDOM from SL as an inner carbon source for denitratation were systematically analyzed. The results demonstrated outstanding nitrogen and rDOM removal performance without any external carbon source supplement. In this study, effluent concentrations of 4.27 ± 0.45 mgTIN/L and 5.58 ± 1.64 mgTN/L were achieved, coupled with an impressive COD removal rate of 65.17 % ± 1.71 %. The abundance of bacteria belonging to the Anaerolineaceae genus, which were identified as rDOM degradation bacteria, increased from 18.23 % to 35.62 %. As a result, various types of rDOM were utilized to different extents, with proteins being the most notable, except for lignins. Metagenomic analysis revealed a preference for directing electrons towards NO3--N reductase rather than NO2--N reductase, indicating the coupling of denitratation bacteria and anammox bacteria (Candidatus Brocadia). Overall, this study introduced a novel synergy platform for advanced nitrogen removal in treating SL using its inner carbon source. This approach is characterized by low energy consumption and operational costs, coupled with commendable efficiency.

Feasibility of in-situ sludge fermentation coupled with partial denitrification: Key roles of initial organic matters and alkaline pH

Achieving partial denitrification (PD) by using fermentation products extracted from waste activated sludge (WAS) rather than commercial organic matters is a promising approach for providing nitrite for anammox, while sludge reduction could also be realized by WAS reutilization. This study proposed an In-situ Sludge Fermentation coupled with Partial Denitrification (ISFPD) system and explored its performance under different conditions, including initial pH, nitrate concentrations, and organic matters. Results showed that nitrite production increased with the elevation of initial pH (from 6 to 9), and the highest nitrate-to-nitrite transformation ratio (NTR) reached 77% at initial pH 9. The PD rates and NTR were observed to be minimally influenced by initial nitrate concentrations. Acetate was preferred by denitrifying bacteria, while macromolecules such as proteins necessitated be hydrolyzed to be suitable for further utilization. The insights gained through this study paved the way for efficient nitrite production and sustainable WAS reutilization in harmony.

Mechanisms of endogenous and exogenous partial denitrification in response to different carbon/nitrogen ratios: Transcript levels, nitrous oxide production, electron transport

Nitrite as an important substrate for Anammox can be provided by partial denitrification (PD). In this study, endogenous partial denitrification (EdPD) and exogenous partial denitrification (ExPD) sludge were domesticated and their nitrite transformation rate reached 74.4% and 83.4%, respectively. The impact of four carbon/nitrogen (C/N) ratios (1.5, 3.0, 5.0 and 6.0) on nitrous oxide (N2O) emission and denitrification functional genes expression in both PD systems were investigated. Results showed that elevated C/N ratios enhanced most denitrification genes expression, but in EdPD, high nitrite levels suppressed nosZ genes expression (from 9.4% to 1.4%), leading to increased N2O emission (0 to 3.4%). EdPD also exhibited lower electron transfer system activity, resulting in slower nitrogen oxide conversion efficiency and more stable nitrite accumulation compared to ExPD. These findings offer insights for optimizing PD systems under varying water quality conditions.

Mixed culture biotechnology and its versatility in dark fermentative hydrogen production

Over the years, extensive research has gone into fermentative hydrogen production using pure and mixed cultures from waste biomass with promising results. However, for up-scaling of hydrogen production mixed cultures are more appropriate to overcome the operational difficulties such as a metabolic shift in response to environmental stress, and the need for a sterile environment. Mixed culture biotechnology (MCB) is a robust and stable alternative with efficient waste and wastewater treatment capacity along with co-generation of biohydrogen and platform chemicals. Mixed culture being a diverse group of bacteria with complex metabolic functions would offer a better response to the environmental variations encountered during biohydrogen production. The development of defined mixed cultures with desired functions would help to understand the microbial community dynamics and the keystone species for improved hydrogen production. This review aims to offer an overview of the application of MCB for biohydrogen production.

How to Provide Nitrite Robustly for Anaerobic Ammonium Oxidation in Mainstream Nitrogen Removal

Innovation in decarbonizing wastewater treatment is urgent in response to global climate change. The practical implementation of anaerobic ammonium oxidation (anammox) treating domestic wastewater is the key to reconciling carbon-neutral management of wastewater treatment with sustainable development. Nitrite availability is the prerequisite of the anammox reaction, but how to achieve robust nitrite supply and accumulation for mainstream systems remains elusive. This work presents a state-of-the-art review on the recent advances in nitrite supply for mainstream anammox, paying special attention to available pathways (forward-going (from ammonium to nitrite) and backward-going (from nitrate to nitrite)), key controlling strategies, and physiological and ecological characteristics of functional microorganisms involved in nitrite supply. First, we comprehensively assessed the mainstream nitrite-oxidizing bacteria control methods, outlining that these technologies are transitioning to technologies possessing multiple selective pressures (such as intermittent aeration and membrane-aerated biological reactor), integrating side stream treatment (such as free ammonia/free nitrous acid suppression in recirculated sludge treatment), and maintaining high activity of ammonia-oxidizing bacteria and anammox bacteria for competing oxygen and nitrite with nitrite-oxidizing bacteria. We then highlight emerging strategies of nitrite supply, including the nitrite production driven by novel ammonia-oxidizing microbes (ammonia-oxidizing archaea and complete ammonia oxidation bacteria) and nitrate reduction pathways (partial denitrification and nitrate-dependent anaerobic methane oxidation). The resources requirement of different mainstream nitrite supply pathways is analyzed, and a hybrid nitrite supply pathway by combining partial nitrification and nitrate reduction is encouraged. Moreover, data-driven modeling of a mainstream nitrite supply process as well as proactive microbiome management is proposed in the hope of achieving mainstream nitrite supply in practical application. Finally, the existing challenges and further perspectives are highlighted, i.e., investigation of nitrite-supplying bacteria, the scaling-up of hybrid nitrite supply technologies from laboratory to practical implementation under real conditions, and the data-driven management for the stable performance of mainstream nitrite supply. The fundamental insights in this review aim to inspire and advance our understanding about how to provide nitrite robustly for mainstream anammox and shed light on important obstacles warranting further settlement.

Enhancement strategies of aerobic denitrification for efficient nitrogen removal from low carbon-to-nitrogen ratio shale oil wastewater

The strategy of high reflux ratio and long solids retention time was adopted to realize efficient nitrogen removal from real shale oil wastewater. This was undertaken with a low chemical oxygen demand to total nitrogen (COD/TN) ratio by strengthening aerobic denitrification in an anoxic/aerobic membrane bioreactor (A/O-MBR). The TN removal load climbed from 22 to 25 g N/(kg MLSS·d) as the COD/TN ratio declined from 8 to 3. The abundance of heterotrophic nitrifying and aerobic denitrifying (HNAD) bacteria increased by 13.8 times to 42.5%, displacing anoxic denitrifying bacteria as the predominant bacteria. The abundance of genes involved in denitrification (napAB, narGHI, norBC, nosZ) increased, however the genes related to assimilatory nitrate reduction (nirA, narB, nasC) decreased. The capacity of the dominant HNAD bacteria in an A/O-MBR to efficiently utilize a carbon source is the key to efficient nitrogen removal from shale oil wastewater with a low COD/TN ratio.