Increasing carbon to nitrogen ratio promoted anaerobic ammonia-oxidizing bacterial enrichment and advanced nitrogen removal in mainstream anammox system

Enhanced nitrogen removal in constructed wetlands filled with iron-carbon substrates: Reexploring unique roles of iron-cycling and electroactive microorganisms

Bacteria involved in iron cycle and electroactivity are commonly found in intensified constructed wetlands (CWs) with iron-carbon substrates. However, their roles in nitrogen (N) removal remain unclear. Here, two types of CWs with different filling modes (separate and mixed) of sponge iron and biochar substrates under micro-oxygen regulation (2 h/d, 60 mL min-1) were constructed for nitrogen removal for 240 days. The results revealed that CWs amended with iron-biochar substrates separately (CW-D) achieved a higher total nitrogen removal performance (81 %) and lower greenhouse gas emission (global warming potential reduced by 3.07 × 105 μg CO2-eq m-2h-1) compared with Fe-C micro-electrolysis CWs (CW-E). In this process, although 41 genera of nitrogen-transforming bacteria (NTB) were detected in CWs, no NTB members had a significant difference (P < 0.05) in relative abundance between CW-D and CW-E. However, 11 genera of iron-cycling bacteria (ICB, e.g. Pseudomonas) with electroactive and 5 genera of electroactive bacteria (EAB, e.g. Tetrasphaera) were significantly enriched in CW-D and CW-E, respectively, both showing significant negative correlations (P < 0.05) with NO2--N content. It indicated that ICB and EAB rather than specific NTB members were decisive in N removal in iron and carbon CWs in low C/N ratio wastewater treatment and regulated by filling modes. Our findings expand the knowledge of the application of iron and carbon substrates in CWs and provide an initial assessment of the effect of different filling modes of iron and carbon on nitrogen removal in CWs.

Enhanced nitrogen removal from low C/N municipal wastewater in a step-feed integrated fixed-film activated sludge system: Synergizing anammox and partial denitrification with sludge fermentation liquid supplementation

The scarcity of rapidly biodegradable organics, which serve as essential electron donors for the partial denitrification (PD) process, significantly hinders the combined application of PD coupled with anammox (PDA) in municipal wastewater treatment plants. This study innovatively applied, for the first time, a step-feed strategy combined with the use of sludge fermentation liquid (SFL) as an external carbon source in an integrated fixed-film activated sludge (IFAS) system, successfully driving full nitrification and PDA to achieve advanced nitrogen removal from low C/N real municipal wastewater. Moreover, the associated nitrogen removal mechanism of this system was systematically analyzed. By employing second-step SFL feed as a supplementary carbon source, the nitrogen removal efficiency reached 92.26 ± 2.77 % and the effluent total inorganic nitrogen was 6.43 ± 2.23 mg/L, with anammox contributing approximately 70 % to total inorganic nitrogen removal. 16S rRNA gene sequencing and fluorescence in situ hybridization analysis unveiled the extensive cooperation and synergistic interactions among anammox bacteria, denitrifying bacteria, and nitrifying bacteria, with Candidatus Brocadia being highly enriched in biofilms with a relative abundance of 2.21 %. Metagenomic sequencing confirmed that the relative abundance of the narGHI gene was greater than that of the nirS gene, providing stable nitrite accumulation conditions for the anammox process. Overall, this study proposes an innovative synergistic treatment scheme that utilizes a step-feed full nitrification-PDA process driven by SFL to achieve advanced nitrogen removal in municipal wastewater treatment plants. This approach is characterized by low energy consumption, low operational costs and a high nitrogen removal efficiency.

Application of sponge iron-carbon to enrich anaerobic ammonia-oxidizing bacteria from sludge mixture and coupled denitrification for degradation of industrial wastewater

For the treatment of industrial wastewater, coupled iron‑carbon micro-electrolysis (ICME) with anaerobic ammonia oxidation (anammox) and denitrification was optimized under the following conditions: Fe/C = 2, C/N ≤ 2, and the temperature was 30 °C. The coupled ICME enriched ammonia-oxidizing bacteria (AnAOB) and denitrifying bacteria (DB) in the mixed sludge on the 76th day of the present experiment. Stable operation was achieved on the 78th day. The COD and TN removal rates during the operation were 86.20 % and 87.12 %, respectively, while the control group (without iron and carbon) had removal rates of 74.30 % and 60.31 % which were 11.9 % and 26.81 % higher, respectively. Notably, the abundance of AnAOB in the system increased from 0.44 % to 1.43 % during the operation from day 76 to day 100. High-throughput sequencing demonstrated that Candidatus_Kuenenia was a key anaerobic ammonia-oxidizing bacterium. Based on the experimental results, the ICME process could rapidly enrich anaerobic ammonia-oxidizing bacteria to change the microbial community structure of the sludge under the water quality conditions of industrial wastewater and increasing the tolerance of certain DB and Candidatus_Kuenenia to water quality. By combining with iron‑carbon, the rapid modification of mixed sludge was achieved, and the iron‑carbon micro-electrolysis coupled denitrification anaerobic ammonia oxidation process was established, which provides a certain reference value for treating industrial wastewater.

Interpretable causal machine learning optimization tool for improving efficiency of internal carbon source-biological denitrification

Interpretable causal machine learning (ICML) was used to predict the performance of denitrification and clarify the relationships between influencing factors and denitrification. Multiple models were examined, and XG-Boost model provided the best prediction (R2 = 0.8743). Based on the ICML framework, hydraulic retention time (HRT), mixture chemical oxygen demand/total nitrogen (COD/TN = C/N), mixture COD concentration, and pretreatment technology were identified as important features affecting the denitrification performance. Further, tapping point and partial dependence analyses provided the range of key factors that precisely regulate denitrification. In the application analysis, HRT (6-10.5 h), mixture C/N (6-12), and mixture COD concentration (300-600 mg L-1) were the appropriate operating ranges, achieving TN removal of approximately 73 %-77 %. The effluent TN and COD concentrations met the discharge standards for wastewater in China (class 1A) and EU. These findings provide support for regulating excess sludge as internal carbon source to promote denitrification.

Responses of suspended sludge and biofilm in a SNAD system under C/N elevation: Microbial activity, nitrogen conversion flux and molecular ecological network

The simultaneous partial nitrification, anammox and denitrification (SNAD) process had received widespread attention as an advanced wastewater treatment process. In this study, the SNAD mainstream nitrogen removal process with the incorporation of polyurethane sponge packing under different C/N conditions was investigated. Results showed that the highest nitrogen removal efficiency of the system was achieved at the C/N of 2.0, while the high C/N (3.5) significantly deteriorate the nitrogen removal efficiency. Meanwhile, high C/N (3.5) significantly inhibited the activity and abundance of anammox bacteria (mainly Candidatus_Kuenenia), resulting in the decreased contribution of anammox (from 63.14 % to 48.09 %). The significant divergence of microbial interactions in the suspended sludge and biofilm was observed with increasing C/N. Compared with suspended sludge, biofilm facilitated higher abundance and activity of anammox bacteria, and the molecular ecological network of biofilm displayed better stability and more efficient mass transfer efficiency between microorganisms. The C/N of 3.5 simplified the subnetworks of Chloroflexi and Proteobacteria but increased the positive interactions between Planctomycetota and other microbes. Anammox bacteria were found as keystone species only in biofilm system. This study provided a theoretical basis and technical guidance for the application of SNAD process in municipal wastewater treatment.

Enhancing collaboration of anammox with heterotrophic microbes mediated selectively by iron of different valences: Activities balance, metabolic mechanism, and functional genes regulation

The partial denitrification/anammox (PD/A) process is receiving increasing attention due to its cost-effectiveness advantages. However, effective strategies to alleviate organic matter inhibition and promote anammox activity have been proven to be a big challenge. This study investigated the effects of three types of iron (nano zero-valent iron (nZVI), Fe(II), and Fe(III)) on the PD/A process. It is worth noting that nZVI of 5-50 mg/L and Fe(III) of 5-120 mg/L promoted both PD and anammox activity. Long-term intermittent addition of nZVI (50 mg/L) resulted in a nitrogen removal efficiency of 98.2% in the mixotrophic PD/A system driven by iron and organic matter. The contribution of anammox for nitrogen removal reached as high as 93.8%. The organic carbon demand decreased due to the external electron donor provided by nZVI for PD. Multiple Fe-N metabolic pathways, primarily involving ammonia oxidation by Fe(III) and nitrate reduction by nZVI, play a crucial role in facilitating nitrogen transformation. Conversely, the direct addition of 30-120 mg/L Fe (II) resulted in a significant decrease in pH to below 5.0 and severe inhibition of PD and anammox activity. Following prolonged operation in the presence of nZVI, it was demonstrated that there is an enhancing effect on robust nitrite production for anammox. This was accompanied by a remarkable up-regulation of genes encoding nitrate reductase and iron-transporting proteins dominated by Thauera. Overall, this study has provided an efficient approach for advanced nitrogen removal through organic- and iron-driven anammox processes.

Exploring transformation of dissolved organic matters and dissolved organic nitrogen in full-scale anammox wastewater treatment: Temperature and microbial roles

Variation of dissolved organic matters (DOM) in mainstream anammox process has received limited attention. This study systematically characterized DOM and dissolved organic nitrogen (DON) in a full-scale mainstream anammox wastewater treatment plant (WWTP) using spectroscopy and Fourier transform-ion cyclotron resonance mass spectrometry. Roles of bacterial community structures related with temperatures on DOM and DON transformations were analyzed. Results indicated that the WWTP removed highly bioavailable, S-containing DOM while producing more unsaturated, aromatic, and N-containing DOM. Higher relative abundances of Proteobacteria and Chloroflexi at low temperature resulted in greater removal rates of proteins, SMP-like and humic acid-like substances. At high temperature, higher relative abundance of Actinobacteriota increased lignin production. Principal component analysis revealed that temperature significantly impacted DOM characteristics compared to DON. These findings are crucial for understanding DOM and DON transformation during mainstream anammox WWTP.

Wastewater treatment from a science faculty during the COVID-19 pandemic by using ammonium-oxidising and heterotrophic bacteria

During and after the pandemic caused by the SARS-CoV-2 virus, the use of personal care products and disinfectants increased in universities worldwide. Among these, quaternary ammonium-based products stand out; these compounds and their intermediates caused substantial changes in the chemical composition of the wastewater produced by these institutions. For this reason, improvements and environmentally sustainable biological alternatives were introduced in the existing treatment systems so that these institutions could continue their research and teaching activities. For this reason, the objective of this study was to develop an improved culture medium to cultivate ammonium oxidising bacteria (AOB) to increase the biomass and use them in the treatment of wastewater produced in a faculty of sciences in Bogotá, D.C., Colombia. A Plackett Burman Experimental Design (PBED) and growth curves served for oligotrophic culture medium, and production conditions improved for the AOB. Finally, these bacteria were used with total heterotrophic bacteria (THB) for wastewater treatment in a pilot plant. Modification of base ammonium broth and culture conditions (6607 mg L-1 of (NH4)2SO4, 84 mg L-1 CaCO3, 40 mg L-1 MgSO4·7H2O, 40 mg L-1 CaCl2·2H2O and 200 mg L-1 KH2PO4, 10% (w/v) inoculum, no copper addition, pH 7.0 ± 0.2, 200 r.p.m., 30 days) favoured the growth of Nitrosomonas europea, Nitrosococcus oceani, and Nitrosospira multiformis with values of 8.23 ± 1.9, 7.56 ± 0.7 and 4.2 ± 0.4 Log10 CFU mL-1, respectively. NO2- production was 0.396 ± 0.0264, 0.247 ± 0.013 and 0.185 ± 0.003 mg L-1 for Nitrosomonas europea, Nitrosococcus oceani and Nitrosospira multiformis. After the 5-day wastewater treatment (WW) by co-inoculating the three studied bacteria in the wastewater (with their self-microorganisms), the concentrations of AOB and THB were 5.92 and 9.3 Log10 CFU mL-1, respectively. These values were related to the oxidative decrease of Chemical Oxygen Demand (COD), (39.5 mg L-1), Ammonium ion (NH4+), (6.5 mg L-1) Nitrite (NO2-), (2.0 mg L-1) and Nitrate (NO3-), (1.5 mg L-1), respectively in the five days of treatment. It was concluded, with the improvement of a culture medium and production conditions for three AOB through biotechnological strategies at the laboratory scale, being a promising alternative to bio-augment of the biomass of the studied bacteria under controlled conditions that allow the aerobic removal of COD and nitrogen cycle intermediates present in the studied wastewater.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-024-03961-4.