Insight into the mechanism of nitritation establishment through external fermented sludge addition

Feasibility of double nitrite supply through partial nitrification and partial denitrification driven by sludge fermentation

Challenges in obtaining stable nitrite have impeded the use of anammox in municipal wastewater treatment. This study explored the feasibility of using sludge fermentation products as carbon source and selective nitrification inhibitor to supply nitrite via partial nitrification (PN) and partial denitrification (PD). PD was initiated within 15 days, achieving nitrite transformation rate of over 90 % with a carbon/nitrogen ratio of 3 and a reaction time of 0.75 h. The dominant genus, Romboutsia, increased in relative abundance from 4.1 to 35 %. Organic acids in sludge fermentation products, like acetate (200 mg/L) and propionate (400 mg/L), selectively suppressed nitrite-oxidizing bacteria (NOB) more than ammonia-oxidizing bacteria (AOB), leading to PN. Combining anaerobic exposure with sludge fermentation products addition achieved PN with over 80.0 % nitrite accumulation. AOB increased tenfold in the long term, significantly outpacing NOB growth. This strategy simplifies difficulty of anammox application and shows broad application potential in municipal wastewater treatment.

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.

Nitrite accumulation in activated sludge through cyclic anaerobic exposure with acetate

Achieving nitrite accumulation still remains challenging for efficient short-cut biological nitrogen removal in municipal wastewater treatment. To tackle the problem of insufficient carbon in incoming wastewater for biological nutrient removal, a return activated sludge (RAS) fermentation method has been proposed and demonstrated to enable producing supplemental volatile fatty acids (VFAs) and enhance biological phosphorus removal via sludge cycling between mainstream and a sidestream anaerobic reactor. However, the impacts of long anaerobic exposure with acetate on nitrifying bacteria, known as the aerobic chemoautotrophic microorganisms, remains unexplored. In this study, the activated sludge underwent a cyclic anaerobic treatment with the addition of acetate (Ac), the effects on nitrification rate, abundance and microdiversity of nitrifying communities were comprehensively assessed. Firstly, batch activity tests proved the direct addition of high acetate (above 1000 mg/L) could cause inhibition on the nitrification rate, moreover, the inhibitory effect was stronger on nitrite-oxidizing bacteria (NOB) activity than that of ammonia-oxidizing bacteria (AOB). Then, a sequencing batch reactor (SBR) was applied to test the nitrogen conversion performance for low-strength ammonium wastewater. Nitrite accumulation could be achieved via the cyclic anaerobic exposure with 1000-5000 mg Ac/L. The maximum effluent concentration of nitrite was 40.8 ± 3.5 mg N/L with nitrite accumulation ratio (NAR) of 67.6 ± 3.5%. The decrease in NOB activity (72.7%) was greater than AOB of 42.4%, promoting nitrite accumulation via nitritation process. Furthermore, the cyclic anaerobic exposure with acetate can largely reshape the nitrifying communities. As the dominant AOB and NOB, the abundance of Nitrosomonas and Nitrospira were both decreased with species-level microdiversity in the nitrifying communities. However, the heterotrophic microorganism, Thauera, were found to be highly enriched (from 0 to 17.3%), which may act as the potential nitrite producer as proved by the increased nitrate reduction gene abundance. This study can provide new insights into achieving mainstream nitrite accumulation by involving sidestream RAS fermentation towards efficient wastewater treatment management.

Magnetite enhancing sludge anaerobic fermentation to improve wastewater biological nitrogen removal: Pilot-scale verification

Alkaline anaerobic fermentation for acids production has been considered as an effective method to recover resources from waste activated sludge, and magnetite could improve the quality of fermentation liquid. Here we have constructed a pilot-scale sludge alkaline anaerobic fermentation process enhanced by magnetite to produce short chain fatty acids (SCFAs), and used them as external carbon sources to improve the biological nitrogen removal of municipal sewage. Results showed that the addition of magnetite could significantly increase the production of SCFAs. The average concentration of SCFAs in fermentation liquid reached 3718.6 ± 101.5 mg COD/L and the average concentration of acetic acid reached 2368.8 ± 132.1 mg COD/L. The fermentation liquid enhanced by magnetite were used in the mainstream A²O process, and the TN removal efficiency increased from 48.0% ± 5.4%-62.2% ± 6.6%. The main reason is that the fermentation liquid is conducive to the succession of microbial community in the denitrification process, increasing the abundance of denitrification functional bacteria and realizing the enhancement of denitrification process. Besides, magnetite can promote the activity of enzyme to enhance biological nitrogen removal. Finally, the economic analysis showed that magnetite enhancing sludge anaerobic fermentation was economically and technically feasible to promote biological nitrogen removal of municipal sewage.

Superior nitrogen removal and efficient sludge reduction via partial nitrification-anammox driven by addition of sludge fermentation products for real sewage treatment

Efficient retention and enrichment of anammox bacteria (AnAOB) are essential for the application of municipal wastewater anammox. Herein, an innovative process for highly enriching AnAOB within suspended carrier was developed in a single-stage anaerobic/oxic/anoxic reactor with 5.5 % carrier filling ratio for real sewage. Addition of sludge fermentation products promoted stable maintenance of partial nitrification (nitrite accumulation rate > 90.0 %) and achieved efficient external sludge reduction (27.6-37.9 %). Continuous nitrite supply and carrier addition promoted AnAOB enrichment (2.4 × 1011 gene copies/g dry sludge). Candidatus Brocadia was the predominant bacteria in carriers (18.6 %). The average effluents of total inorganic nitrogen (TIN) and NH4+-N were 1.9 and 0.8 mg/L with removal rates of 97.0 % and 98.7 %. In the anoxic stage, TIN removal rate reached 71.5 %, and the proportion of anammox to nitrogen removal accounted for 82.7 %. This study broadens the application of mainstream sewage anammox and the resource utilization of waste activated sludge.

Effect of free ammonia shock on Chlorella sp. in wastewater: Concentration-dependent activity response and enhanced settleability

The unstable microbial activity and unsatisfactory settling performance impede the development and implementation of microalgal wastewater treatment, especially in high-ammonium wastewater in the presence of free ammonia (FA). The shock of FA due to the nutrient fluctuation in wastewater was demonstrated as the primary stress factor suppressing microalgal activities. Recent study has clearly revealed the inhibition mechanism of FA at a specific high level (110.97 mg/L) by inhibiting the genetic information processing, photosynthesis, and nutrient metabolism. However, the effects of various FA shock concentrations on microalgal activities and settling performance remain unknown, limiting the wastewater bioremediation efficiencies improvement and the process development. Herein, a concentration-dependent shock FA (that was employed on microalgae during their exponential growth stages) effect on microalgal growth and photosynthesis was observed. Results showed that the studied five FA shock concentrations ranging from 25 to 125 mg/L significantly inhibited biomass production by 14.7-57.0%, but sharp reductions in photosynthesis with the 36.0-49.0% decreased Fv/Fm values were only observed when FA concentration was above 75.0 mg/L. On the other hand, FA shock enhanced microalgal settling efficiency by 12.8-fold, which was believed to be due to the stimulated intra- and extracellular protein contents and thereby the enhanced extracellular polymer substances (EPS) secretion. Specifically, FA shock induced 40.2 ± 2.3% higher cellular protein content at the cost of the decreased carbohydrates (22.6 ± 1.3%) and fatty acid (39.0 ± 0.8%) contents, further improving the protein secretion by 1.21-fold and the EPS production by 40.2 ± 2.3%. These FA shock-induced variations in intra- and extracellular biomolecules were supported by the up-regulated protein processing and export at the assistance of excessive energy generated from fatty acid degradation and carbohydrates consumption. In addition, FA shock significantly decreased the biomass nutritional value as indicated by the 1.86-fold lower essential amino acid score and nearly 50% reduced essential to non-essential amino acids ratio, while slightly decreased the biodiesel quality. This study is expected to enrich the knowledge of microalgal activities and settling performance in response to fluctuant ammonium concentrations in wastewater and to promote the development of microalgal wastewater treatment.

A novel partial denitrification, anammox-biological phosphorus removal, fermentation and partial nitrification (PDA-PFPN) process for real domestic wastewater and waste activated sludge treatment

A novel process was developed for real domestic wastewater and waste activated sludge (WAS) treatment based on partial denitrification, anammox-biological phosphorus removal, fermentation and partial nitrification (PDA-PFPN). After 246 days of operation, the effluent concentrations of NH₄⁺-N, NO₂^--N and NO₃^--N were below detection limits (0.1 mg/L), and the effluent concentration of PO₄^3--P was 0.1 mg/L without the addition of external carbon source in PDA-PFPN system. Moreover, the sludge reduction efficiency reached 48.1% due to fermentation. The nitrite accumulation ratios by ammonia oxidation and nitrate reduction pathway were 60.6% and 87%, respectively. Intracellular metabolites measured by liquid chromatography mass spectrometer (LC-MS/MS) suggested that different intracellular amino acids were stored and consumed at different duration, and intracellular Valine, Glycine and Lysine were not utilized in oxic stage. Results of flow cytometry showed that the proportion of intact cells decreased from 94.7% to 82.9%, and necrotic cells increased from 5.3% to 17.1% with the increase of DNA content in sludge supernatant and cell decay rate, indicating the occurrence of cell death and lysis and leading to WAS reduction. Analysis of transcriptional community composition revealed that partial denitrification bacteria (Thauera), anammox bacteria (Candidatus Brocadia and Candidatus Kuenenia), simultaneous phosphorus removal and fermentation bacteria (Tetrasphaera) and partial nitrification bacteria (Nitrosomonas) coexisted and actually worked in PDA-PFPN system. The novel PDA-PFPN process simultaneously achieved highly efficient nitrogen and phosphorus removal and WAS reduction without the addition of external carbon source, which greatly reduced the operation cost of carbon source dosing and WAS treatment in wastewater treatment.

Successful mainstream nitritation through NOB inactivation

The development of new wastewater treatment processes can assist in reducing the impact of wastewater treatment on the environment. The recently developed partial nitritation anammox (PNA) process, for example, consumes less energy for aeration and reduces nitrate in the effluent without requiring additional organic carbon. However, achieving stable nitritation (ammonium oxidation; NH₄⁺ → NO₂^-) at mainstream conditions (T = 10-25 °C, C:N > 10, influent ammonium < 50 mgNH₄-N/L and effluent < 1 mgNH₄-N/L) remains challenging. This study explores the potential and mechanism of nitrite-oxidizing bacteria (NOB) suppression in a bottom-fed sequencing batch reactor (SBR). Two bench-scale (11 L) reactors and a pilot-scale reactor (8 m³) were operated for over a year and were fed with organic substrate depleted municipal wastewater. Initially, nitratation (nitrite oxidation; NO₂^- → NO₃^-) occurred occasionally until an anaerobic phase was integrated into the operating cycle. The introduction of the anaerobic phase effectively suppressed the regrowth of NOB while nitritation was stable over 300 days, down to 8 °C and at ammonium influent concentrations < 25 mgNH₄-N/L. Batch experiments and process data revealed that parameters typically affecting NOB growth (e.g., dissolved oxygen, alkalinity, trace elements, lag-phase after anoxia, free nitrous acid (FNA), free ammonia (FA), pH, sulfide, or solids retention time (SRT)) could not fully explain the suppression of nitratation. Experiments in which fresh nitrifying microbial biomass was added to the nitritation system indicated that NOB inactivation explained NOB suppression better than NOB washout at high SRT. This study concludes that bottom-fed SBRs with anaerobic phases allow for stable nitritation over a broad range of operational parameters. Coupling this type of SBR to an anammox reactor can enable efficient mainstream anammox-based wastewater treatment.

Realization of partial nitrification and in-situ anammox in continuous-flow anaerobic/aerobic/anoxic process with side-stream sludge fermentation for real sewage

A continuous-flow anaerobic/aerobic/anoxic reactor with complete suspended activated sludge using sludge alkaline fermentation products as carbon source was utilized to strengthen nitrogen removal performance for low C/N ratio (<4) wastewater. Long-time performance indicated that the nitrite accumulation rate reached 60.40%, which strengthened the contribution of anammox. The average total inorganic nitrogen removal efficiency improved 19.40%. The abundance of ammonia oxidizing bacteria has not changed, but the abundance of nitrite oxidizing bacteria reduced from 5.79% to 0.69%. Quantitative PCR results demonstrated that the abundance of anammox bacteria has raised by 80.5 times. These results indicated that side-stream sludge alkaline fermentation promoted the mainstream partial nitrification, consequently accelerating the in-situ enrichment of anammox bacteria. No external carbon source dosing and short oxic hydraulic retention time (5.3 h) save energy and reduce consumption significantly in this system.