Good servant, bad master: sulfide influence on partial nitritation of sewage

Development of an integrated fixed-film activated sludge (IFAS) reactor treating landfill leachate for the biological nitrogen removal through nitritation-denitritation

The current work investigated the performance of an Integrated Fixed-Film Activated Sludge Sequencing Batch Reactor (IFAS-SBR) for Biological Nitrogen Removal (BNR) from mature landfill leachate through the nitritation-denitritation process. During the experimental period two IFAS-SBR configurations were examined using two different biocarrier types with the same filling ratio (50%). The dissolved oxygen (DO) concentration ranged between 2 and 3 mg/L and 4-6 mg/L in the first (baseline-IFAS) and the second (S8-IFAS) setup, respectively. Baseline-IFAS operated for 542 days and demonstrated a high and stable BNR performance maintaining a removal efficiency above 90% under a Nitrogen Loading Rate (NLR) up to 0.45 kg N/m3-d, while S8-IFAS, which operated for 230 days, was characterized by a limited and unstable BNR performance being unable to operate sufficiently under an NLR higher than 0.20 kg N/m3-d. It also experienced a severe inhibition period, when the BNR process was fully deteriorated. Moreover, S8-IFAS suffered from extensive biocarrier stagnant zones and a particularly poor sludge settleability. The attached biomass cultivated in both IFAS configurations had a negligible content of nitrifying bacteria, probably attributed to the insufficient DO diffusion through the biofilm, caused by the low DO concentration in the liquid in the baseline case and the extensive stagnant zones in the S8-IFAS case. As a result of the high biocarrier filling ratio, the S8-IFAS was unstable and low. This was probably attributed to the mass transfer limitations caused by the biocarrier stagnant zones, which hinder substrate and oxygen diffusion, thus reducing the biomass activity and increasing its vulnerability to inhibitory and toxic factors. Hence, the biocarrier filling fraction is a crucial parameter for the efficient operation of the IFAS-SBR and should be carefully selected taking into consideration both the media type and the overall reactor configuration.

Achieving Long-Term Stability of Partial Nitrification and Autotrophic Denitrification in an MABR via Sulfide Dosing

While partial nitrification (PN) has the potential to reduce energy for aeration, it has proven to be unstable when treating low-strength wastewater. This study introduces an innovative combined strategy incorporating a low rate of oxygen supply, pH control, and sulfide addition to selectively inhibit nitrite-oxidizing bacteria (NOB). This strategy led to a stable PN in a laboratory-scale membrane aerated biofilm reactor (MABR). Over a period of 260 days, the nitrite accumulation ratio exceeded 60% when treating synthetic sewage containing 50 mg NH4+-N/L. Through in situ activity testing and high-throughput sequencing, the combined strategy led to low levels of nitrite-oxidation activity (<5.5 mg N/m2 h), Nitrospira species (relative abundance <1%), and transcription of nitrite-oxidation genes (undetectable). The addition of sulfide led to simultaneous PN and autotrophic denitrification in the single-stage MABR, resulting in over 60% total inorganic nitrogen removal. Sulfur-based autotrophic denitrification consumed nitrite and inhibited NOB conversion of nitrite to nitrate. The combined strategy has potential to be applied in large-scale sewage treatment and deserves further exploration.

Advances in nitrogen removal and recovery technologies from reject water: Economic and environmental perspectives

This review critically assesses nitrogen removal technologies applied in the reject water treatment, across different stages of technological development, with a focus on their economic and environmental impacts. The prevalent use of biological processes raises concerns due to potential environmental impacts caused by N2O emissions. However, partial nitritation-anaerobic ammonium oxidation demonstrated economic benefits and the potential for positive environmental outcomes when properly operated and controlled. Furthermore, reject water, in many cases, provides sufficient nitrogen concentrations for nitrogen recovery processes, such as ammonia stripping, substituting production of industrial fertilizers and contributing to a circular economy. Nonetheless, their financial competitiveness is subject to various conditions, including the nitrogen concentration or reject water flow. As the environmental benefits of bioprocesses and economic benefits of nitrogen recovery processes may vary, it is crucial to further optimize both and investigate novel promising technologies such as electrochemical systems, denitrifying anaerobic methane oxidation or direct ammonia oxidation.

The hybrid MABR process achieves intensified nitrogen removal while N2O emissions remain low

The hybrid membrane aerated biofilm reactor (MABR) process represents a full-scale solution for sustainable municipal wastewater treatment. However, most of the existing hybrid MABR processes retain large aerobic bioreactor volumes for nitrification, which is undesirable for energy and carbon savings. In this study, we used the plant-wide modeling approach with dynamic simulations to examine a novel hybrid MABR configuration with aeration controls that change the anoxic and aerobic fractions of the bioreactor volume. Result showed that the novel hybrid MABR showed "swinging" nitrification and denitrification capacities in response to diurnal loadings, achieving intensified nitrogen removal performance under both warm and cold temperature scenarios. N2O emissions from the hybrid MABR were only 1/5 of the emissions from the conventional activated sludge. The model predicted higher CH4 emissions from the hybrid MABR than the activated sludge process due to the methanogen growth in the oxygen-depleted MABR biofilm layer. Future measurements for CH4 emission are needed to obtain a holistic picture of the carbon footprint of the hybrid MABR process.

A critical review on microbial ecology in the novel biological nitrogen removal process: Dynamic balance of complex functional microbes for nitrogen removal

The novel biological nitrogen removal process has been extensively studied for its high nitrogen removal efficiency, energy efficiency, and greenness. A successful novel biological nitrogen removal process has a stable microecological equilibrium and benign interactions between the various functional bacteria. However, changes in the external environment can easily disrupt the dynamic balance of the microecology and affect the activity of functional bacteria in the novel biological nitrogen removal process. Therefore, this review focuses on the microecology in existing the novel biological nitrogen removal process, including the growth characteristics of functional microorganisms and their interactions, together with the effects of different influencing factors on the evolution of microbial communities. This provides ideas for achieving a stable dynamic balance of the microecology in a novel biological nitrogen removal process. Furthermore, to investigate deeply the mechanisms of microbial interactions in novel biological nitrogen removal process, this review also focuses on the influence of quorum sensing (QS) systems on nitrogen removal microbes, regulated by which bacteria secrete acyl homoserine lactones (AHLs) as signaling molecules to regulate microbial ecology in the novel biological nitrogen removal process. However, the mechanisms of action of AHLs on the regulation of functional bacteria have not been fully determined and the composition of QS system circuits requires further investigation. Meanwhile, it is necessary to further apply molecular analysis techniques and the theory of systems ecology in the future to enhance the exploration of microbial species and ecological niches, providing a deeper scientific basis for the development of a novel biological nitrogen removal process.

Response of nitritation granules to anaerobically pre-treated municipal wastewater at low temperatures in a continuous-flow reactor

Achieving mainstream nitritation with aerobic granules is attractive based on increasing evidence but generally treating artificial low-ammonium wastewater. Real municipal wastewater is much more complex in composition, the behavior of the nitritation granules would be different when treating real municipal wastewater. Herein, the response of nitritation granules to influent shift from artificial low-ammonium (35-40 mg/L) wastewater to anaerobically pre-treated municipal wastewater (MWW_pre-treated) was investigated at low temperatures. Results showed that MWW_pre-treated caused the outgrowth of filamentous bacteria on the granule surface and developed into finger-like structures, which in turn resulted in the decrease of the overall granular sludge settleability. Batch-tests and microbial analysis indicated the functional and microbial differentiation between the newly formed fluffy exterior and the original compact granule. The fluffy exterior was dominated by genus Flavobacterium (66.6%) and primarily functioned as COD removal, whereas the nitrifiers (mainly Nitrosomonas) were still located in the compact core and performed nitritation. Moreover, the heterotrophs-dominated fluffy exterior hindered the oxygen transfer towards nitrifiers located in the compact granule and thereby facilitated the stable NOB repression in the granule particularly at low temperatures (<10 °C). Finally, gradual recovery of the granular sludge morphology and settleability occurred after the influent reverted to synthetic low-ammonium wastewater. Overall, this work demonstrated that the feeding of MWW_pre-treated only caused morphological changes of the nitritation granules, but its structural and functional stability could be maintained stably.

Relevance of Candidatus Nitrotoga for nitrite oxidation in technical nitrogen removal systems

Abstract

Many biotechnological applications deal with nitrification, one of the main steps of the global nitrogen cycle. The biological oxidation of ammonia to nitrite and further to nitrate is critical to avoid environmental damage and its functioning has to be retained even under adverse conditions. Bacteria performing the second reaction, oxidation of nitrite to nitrate, are fastidious microorganisms that are highly sensitive against disturbances. One important finding with relevance for nitrogen removal systems was the discovery of the mainly cold-adapted Cand. Nitrotoga, whose activity seems to be essential for the recovery of nitrite oxidation in wastewater treatment plants at low temperatures, e.g., during cold seasons. Several new strains of this genus have been recently described and ecophysiologically characterized including genome analyses. With increasing diversity, also mesophilic Cand. Nitrotoga representatives have been detected in activated sludge. This review summarizes the natural distribution and driving forces defining niche separation in artificial nitrification systems. Further critical aspects for the competition with Nitrospira and Nitrobacter are discussed. Knowledge about the physiological capacities and limits of Cand. Nitrotoga can help to define physico-chemical parameters for example in reactor systems that need to be run at low temperatures.

Key points

• Characterization of the psychrotolerant nitrite oxidizer Cand. Nitrotoga

• Comparison of the physiological features of Cand. Nitrotoga with those of other NOB

• Identification of beneficial environmental/operational parameters for proliferation

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11487-5.

Simultaneous nitrification/denitrification and desulfurization of wastewater polluted with ammonium, COD and sulfide: effectiveness of a new up-flow vertical hybrid reactor

Industrial wastewater discharges pose an environmental risk. Here, the effectiveness of an up-flow vertical hybrid system, operating with synthetic and industrial wastewater was investigated, as a new approach to perform nitrification/denitrification and desulfurization within a single reactor. The hybrid reactor is divided in two reaction zones, the oxic and anoxic. The removal of chemical oxygen demand (COD), ammonium, and sulfide was investigated, highlighting changes in microbial diversity. The reactor was evaluated at hydraulic residence time (HRT) of 1.6 days, and its performance throughout 180 days is presented in four stages. In stages I-II, high COD and ammonium removal was obtained with synthetic wastewater. In stage-III, sulfide-rich synthetic wastewater did not alter the system, attaining COD, ammonium, and sulfide removal efficiencies of 81, 99.5, and 99.7%, respectively. In the last stage, a mixture of effluents was fed into the reactor at loading rates of 277 mg COD/L-d, 46.5 mg NH₄ ⁺-N /L-d, and 15 mg HS^--S /L-d. Sulfide and ammonium removals were 100% and 99.9%, respectively. However, low COD removal was observed, being of 51%, and the system removed 97% in terms of BOD₅. The structure and microbial diversity also changed. Sulfide feeding, induced the proliferation of sulfur oxidizers like Thiomiscropira and Thiobacillus. Industrial wastewater enhanced the abundance of Pseudomonas (15.53%) and favored the proliferation of new bacteria of the genus Truepera (2.98%) and Alicyclipilus (7.56%). This is the first study reporting simultaneous nitrification/denitrification and desulfurization to remove ammonium, COD and sulfide from complex industrial wastewater using an up-flow vertical hybrid reactor.

Modeling of the effect of zeolite concentration on the biological nitrification process in the presence of sulfide and organic matter

The evaluation of the nitrification kinetics in the simultaneous presence of sulfide and organic matter using zeolite as improver was the main goal of this work. According to the sensitivity and collinearity analyses, five parameters were the most sensitive in the model, whose calibrated values were: μ _{max, AOB} = 0.02642 ± 0.002 h^-1; μ _{max, NOB} = 0.3307 ± 0.416 h^-1; K _S,NOB = 1.65·10^-6 ± 2.85·10^-6 mgHNO₂-N/L; k _S2 = 0.8213 ± 0.076 and n = 0.6537 ± 0.030. A good fit between the experimental data and the model's results including the effect of zeolite on the kinetic parameters was obtained, with Theil inequality coefficient values between 0.109 and 0.007 for all the variables studied, with all of these values lower than 0.3. Thus, the model proposed is robust and can simulate the nitrification process in the presence of sulfide and organic matter when zeolite was used as improver.

Autotrophic nitrogen removal characteristics of PN-anammox process enhanced by sulfur autotrophic denitrification under mainstream conditions

Stabilization of nitrification process and reduction of NO₃^--N concentration in effluent are the keys to realize mainstream application of partial nitrification-anaerobic ammonia oxidation (PN-anammox) process. The sulfur-based autotrophic denitrification (SADN) process was coupled with the PN-anammox in a single reactor to enhance and stabilize the nitrogen removal performance, and the feasibility and reaction characteristics of the coupling system under mainstream conditions were investigated. The results showed that the NO₃^- of PN-anammox effluent dropped from 22 to 24 mg/L to 5 mg/L after the SADN process coupled, and the total nitrogen removal efficiency and total nitrogen removal rate reached 83.5% and 0.15 kg/(m³·d), respectively. This coupling system doesn't need to over-strengthen PN control. Batch experiments showed that sulfur autotrophic oxidizing bacteria used O₂ to oxidize S^2- in the coupling system, which competed with SADN to remove NO₃^-. Moreover, Nitrosomonas, Candidatus Brocadia and Thiobacillus were the main genera for nitrogen and sulfur conversion.

Biological ammonium and sulfide oxidation in a nitrifying sequencing batch reactor: Kinetic and microbial population dynamics assessments

A kinetic study was carried out in a sequencing batch reactor (SBR) (125 mg NH₄⁺-N/L) inoculated with a physiologically stable nitrifying sludge not previously acclimated to sulfur compounds and fed at different initial sulfide concentrations (2.5-20.0 mg HS^--S/L). Up to 10.0 mg HS^--S/L, the nitrifying process kept stable and complete, reaching an ammonium consumption efficiency (E_NH4+) of 100% and a nitrate yield (Y_NO3-) of 0.95 ± 0.03 mg NO₃^--N/mg NH₄⁺-N consumed. At 15.0 and 20.0 mg HS^--S/L, after an initial alteration in the nitrite oxidizing process, the Y_NO2- was decreasing throughout the cycles and the Y_NO3- increasing, obtaining in the last cycle at 20.0 mg HS^--S/L, an E_NH4+ of 100%, a Y_NO2- of zero, and a Y_NO3- of 0.80 mg NO₃^--N/mg NH₄⁺-N consumed. At the end of the period at 20.0 mg HS^--S/L, the specific rates of ammonium consumption and nitrate formation were 15 and 55% lower than their respective values in the control period without sulfide addition, showing that the sludge had a better metabolic adaptation for ammonium oxidizing activity than for nitrite oxidizing activity. The sludge acquired a higher sulfide oxidation capacity along the cycles. Bacterial population dynamics assessment indicated that the ammonium oxidizing bacteria (AOB) community was more diverse and stable than the nitrite oxidizing bacteria (NOB) community. The use of consortia with a previously stabilized nitrifying activity in SBR may constitute an alternative for eliminating simultaneously ammonium by nitrification and sulfide by sulfide oxidation and be implemented for the treatment of wastewater with ammonium and sulfide.

Bioaugmented methanol production using ammonia oxidizing bacteria in a continuous flow process

Organic compounds such as methanol are widely used for enhancing denitrification at wastewater treatment plants (WWTPs) to meet effluent water quality permits. On the other hand, methane, which is the main feedstock for industrial methanol production, is also generated during anaerobic digestion in WWTPs, but is often flared to mitigate its greenhouse impacts. The overarching goal herein was to develop a novel continuous process for methanol production from methane using nitrifying activated sludge. The maximum AOB specific methanol production rate using hydroxylamine as electron donor was 1.61 ± 0.15 and 1.27 ± 0.15 mg-COD-CH3OH/(mg-COD-AOB*d), for hydraulic retention times (HRTs) of 7.5 h and 2 h, respectively. The corresponding production rate using ammonia as electron donor was 0.31 ± 0.08 mg-COD-CH3OH/(mg-COD-AOB*d) at a HRT of 2 h. These results show that nitrifier-mediated methanol production in a continuous-flow system can enhance the efficiency of WWTPs through internal production of biomethanol for denitrification, while simultaneously minimizing wasteful biogas flaring.

Effects of free nitrous acid treatment conditions on the nitrite pathway performance in mainstream wastewater treatment

Inline sludge treatment using free nitrous acid (FNA) was recently shown to be effective in establishing the nitrite pathway in a biological nitrogen removal system. However, the effects of FNA treatment conditions on the nitrite pathway performance remained to be investigated. In this study, three different FNA treatment frequencies (daily sludge treatment ratios of 0.22, 0.31 and 0.38, respectively), two FNA concentrations (1.35 mgN/L and 4.23 mgN/L, respectively) and two influent feeding regimes (one- and two-step feeding) were investigated in four laboratory-scale sequencing batch reactors. The nitrite accumulation ratio was positively correlated to the FNA treatment frequency. However, when a high treatment frequency was used e.g., daily sludge treatment ratio of 0.38, a significant reduction in ammonia oxidizing bacteria (AOB) activity occurred, leading to poor ammonium oxidation. AOB were able to acclimatise to FNA concentrations up to of 4.23 mgN/L, whereas nitrite oxidizing bacteria (NOB) were limited by an FNA concentration of 1.35 mgN/L over the duration of the study (up to 120 days). This difference in sensitivity to FNA could be used to further enhance nitrite accumulation, with 90% accumulation achieved at an FNA concentration of 4.23 mgN/L and a daily sludge treatment ratio of 0.31 in this study. However, this high level of nitrite accumulation led to increased N₂O emission, with emission factors of up to 3.9% observed. The N₂O emission was mitigated (reduced to 1.3%) by applying two-step feeding resulting in a nitrite accumulation ratio of 45.1%. Economic analysis showed that choosing the optimal FNA treatment conditions depends on a combination of the wastewater characteristics, the nitrogen discharge standards, and the operational costs. This study provides important information for the optimisation and practical application of FNA-based sludge treatment technology for achieving the mainstream stable nitrite pathway.

Synergistic Exposure of Return-Sludge to Anaerobic Starvation, Sulfide, and Free Ammonia to Suppress Nitrite Oxidizing Bacteria

A key step toward energy-positive sewage treatment is the development of mainstream partial nitritation/anammox, a nitrogen removal technology where aerobic ammonium-oxidizing bacteria (AerAOB) are desired, while nitrite-oxidizing bacteria (NOB) are not. To suppress NOB, a novel return-sludge treatment was investigated. Single and combined effects of sulfide (0-600 mg S L^-1), anaerobic starvation (0-8 days), and a free ammonia (FA) shock (30 mg FA-N L^-1 for 1 h) were tested for immediate effects and long-term recovery. AerAOB and NOB were inhibited immediately and proportionally by sulfide, with AerAOB better coping with the inhibition, while the short FA shock and anaerobic starvation had minor effects. Combinatory effects inhibited AerAOB and NOB more strongly. A combined treatment of sulfide (150 mg S L^-1), 2 days of anaerobic starvation, and FA shock (30 mg FA-N L^-1) inhibited AerAOB 14% more strongly compared to sulfide addition alone, while the AerAOB/NOB activity ratio remained constant. Despite no positive change being observed in the immediate-stress response, AerAOB recovered much faster than NOB, with a nitrite accumulation ratio (effluent nitrite on nitrite + nitrate) peak of 50% after 12 days. Studying long-term recovery is therefore crucial for design of an optimal NOB-suppression treatment, while applying combined stressors regularly may lead toward practical implementation.

Sulfide inhibition of nitrite oxidation in activated sludge depends on microbial community composition

Increasingly, technologies that use sulfide as an electron donor are being considered for nitrogen removal; however, our understanding of how sulfide affects microbial communities in nitrifying treatment processes is limited. In this study, we used batch experiments to quantify sulfide inhibition of both ammonium oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) using activated sludge from two full-scale treatment plants with distinct treatment processes. The batch experiments showed that NOB were more vulnerable to sulfide inhibition than AOB, and that inhibition constants (K_I) for NOB were distinct between the two treatment plants, which also had distinct nitrite oxidizing microbial communities. A Nitrospira-rich, less diverse NOB community was inhibited more by sulfide than a more diverse community rich in Nitrotoga and Nitrobacter. Therefore, sulfide-induced nitritation may be more successful in less diverse, Nitrospira-rich communities. Additionally, sulfide significantly influenced the activity of non-nitrifying microbial community members, as measured by 16S rRNA cDNA sequencing. Overall, these results indicate that sulfide has a strong impact on both nitrification and the activity of the underlying microbial communities, and that the response is community-specific.