Kinetic parameters and inhibition response of ammonia- and nitrite-oxidizing bacteria in membrane bioreactors and conventional activated sludge processes

Kinetic and elemental characterization of HAP-based high-rate partial nitritation/anammox system orienting stability and inorganic elemental requirements

Anammox-based processes are attractive for biological nitrogen removal, and the combination of anammox and hydroxyapatite (HAP) is promising for the simultaneous removal of nitrogen and phosphorus from wastewater. However, the kinetics of one-stage partial nitritation/anammox (PNA) in which ammonia-oxidizing bacteria (AOB) and anammox bacteria (AnAOB) exist in a reactor are poorly understood. Moreover, inorganic elements are required to promote microbial cell synthesis and growth; therefore, monitoring of elements to prevent the limitation and inhibition of the process is critical. The minimum amounts of inorganic elements required for a one-stage PNA process and the elemental flow remain unknown. Therefore, in this study, kinetics, stoichiometry, and element flow in the long-term, high-rate, continuous, one-stage HAP-PNA process with microaerobic granular sludge at 25 °C were determined using process modeling, parameter estimation, and mass balance. The biomass elemental composition was determined to be CH2.2O0.89N0.18S0.0091, and the biomass yield (Yobs) was calculated to be 0.0805 g/g NH4+-N. Therefore, a stoichiometric reaction equation for the one-stage HAP-PNA system was also proposed. The maximum specific growth rate (μm) of AnAOB and AOB were 0.0360 and 0.0982 d-1 with doubling times of 19 and 7.1 d, respectively. Finally, the elemental requirements for stable and high-rate performance were determined using element flow analysis. These findings are essential for developing the anammox-based process in a stable and resource-efficient manner and determining engineering applicability.

Thermodynamic Analysis of Intermediary Metabolic Steps and Nitrous Oxide Production by Ammonium-Oxidizing Bacteria

Nitrous oxide (N₂O) is a greenhouse gas emitted from wastewater treatment, soils, and agriculture largely by ammonium-oxidizing bacteria (AOB). While AOB are characterized by being aerobes that oxidize ammonium (NH₄⁺) to nitrite (NO₂^-), fundamental studies in microbiology are revealing the importance of metabolic intermediates and reactions that can lead to the production of N₂O. These findings about the metabolic pathways for AOB were integrated with thermodynamic electron-equivalents modeling (TEEM) to estimate kinetic and stoichiometric parameters for each of the AOB's nitrogen (N)-oxidation and -reduction reactions. The TEEM analysis shows that hydroxylamine (NH₂OH) oxidation to nitroxyl (HNO) is the most energetically efficient means for the AOB to provide electrons for ammonium monooxygenation, while oxidations of HNO to nitric oxide (NO) and NO to NO₂^- are energetically favorable for respiration and biomass synthesis. The respiratory electron acceptor can be O₂ or NO, and both have similar energetics. The TEEM-predicted value for biomass yield, maximum-specific rate of NH₄⁺ utilization, and maximum specific growth rate are consistent with empirical observations. NO reduction to N₂O is thermodynamically favorable for respiration and biomass synthesis, but the need for O₂ as a reactant in ammonium monooxygenation likely precludes NO reduction to N₂O from becoming the major pathway for respiration.

Comparing three mathematical models using different substrates for prediction of partial nitrification

Modelling of partial nitrification process is affected by several factors such as selection of true substrates, FA and FNA inhibition, and pH effect on growth rate. Among these factors, the selection of true substrates is very critical as it affects the structure of the model. In the present work, a new model adopting free ammonia (FA) and free nitrous acids (FNA) as the true substrate for ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) was proposed. Then the proposed model was compared with two reported models which adopted ammonium and nitrite, and FA and nitrite as the true substrate for AOB and NOB, respectively. The three mathematical models were compared in terms of predicted minimum dissolved oxygen (DO) in response to varied solids retention time (SRT) (10-30 d), pH (7-8.5), and temperature (10-35 °C). The input kinetic values were justified and updated based on statistical analysis of literature data. Adopting FA as the true substrate increased the minimum DO for AOB. Further, experimental data from different literature studies were taken for model simulation and comparison. Inconsistency was observed between the model prediction and literature data for all three models. The model that adopted ammonium and nitrite as the true substrate for AOB and NOB had better consistency with literature data than other two models. The affecting factors for the model prediction was classified into three levels and discussed in detail. Future work was proposed. The results of this study provide valuable information for the design and modelling of partial nitrification process.

Functional Response of MBR Microbial Consortia to Substrate Stress as Revealed by Metaproteomics

Bacterial consortia have a primary role in the biological degradations occurring in activated sludge for wastewater treatment, for their capacities to metabolize the polluting matter. Therefore, the knowledge of the main metabolic pathways for the degradation of pollutants becomes critical for a correct design and operation of wastewater treatment plants. The metabolic activity of the different bacterial groups in activated sludge is commonly investigated through respirometry. Furthermore, in the last years, the development of "omic" approaches has offered more opportunities to integrate or substitute the conventional microbiological assays and to deeply understand the taxonomy and dynamics of complex microbial consortia. In the present work, an experimental membrane bioreactor (MBR) was set up and operated for the treatment of municipal wastewater, and the effects of a sudden decrease of the organic supply on the activated sludge were investigated. Both respirometric and metaproteomic approaches revealed a resistance of autotrophic bacteria to the substrate stress, and particularly of nitrifying bacteria. Furthermore, metaproteomics allowed the identification of the taxonomy of the microbial consortium based on its protein expression, unveiling the prevalence of Sorangium and Nitrosomonas genera both before and after the organic load decrease. Moreover, it confirmed the results obtained through respirometry and revealed a general expression of proteins involved in metabolism and transport of nitrogen, or belonging to nitrifying species like Nitrosomonas europeae, Nitrosomonas sp. AL212, or Nitrospira defluvii.

Long-term operation of oxygen-limiting membrane bioreactor (MBR) for the development of simultaneous partial nitrification, anammox and denitrification (SNAD) process

In this study, an oxygen-limiting membrane bioreactor (MBR) with recirculation of biogas for relieving membrane fouling was successfully operated to realize the simultaneous partial nitrification, anammox and denitrification (SNAD) process. The MBR operation was considered effective in the long-term test with total nitrogen (TN) and chemical oxygen demand (COD) removal efficiencies of 94.86% and 98.91%, respectively. Membrane fouling was significantly alleviated due to the recirculation of biogas and the membrane had been cleaned four times with a normal filtration period of 52 days. The co-existence of ammonia-oxidizing bacteria (AOB), anammox and denitrifying bacteria in MBR was confirmed by scanning electron microscopy (SEM) and fluorescence in situ hybridizations (FISH) analysis. Furthermore, AOB were found close to the granule surface, while denitrifying bacteria and anammox were in the deeper layer of granules. Potential in excellent TN and COD removal, operational stability and sustainability, as well as in alleviating membrane fouling is expected by using this oxygen-limiting MBR.

Removal of pharmaceuticals and personal care products by ammonia oxidizing bacteria acclimated in a membrane bioreactor: Contributions of cometabolism and endogenous respiration

We carried out batch experiments using biomass from a membrane bioreactor (MBR) to study the influence of ammonia oxidizing bacteria (AOB) on the removal of 45 pharmaceuticals and personal care products (PPCPs). Kinetic parameters such as biodegradation constants and adsorption coefficients with and without AOB inhibition were estimated. No significant differences in adsorption tendency were found, but the biodegradability of most compounds was enhanced when ammonia was completely oxidized, indicating that AOB present in MBR played a critical role in eliminating the PPCPs. Moreover, target PPCPs were degraded in 2 stages, first by cometabolic degradation related to AOB growth, and then by endogenous respiration by microorganisms in the absence of other growth substrate. The compounds were classified into 3 groups according to removal performance and cometabolic degradation. Our approach provides new insight into the removal of PPCPs via cometabolism and endogenous respiration under AOB enrichment cultures developed in MBR.

Challenges in using allylthiourea and chlorate as specific nitrification inhibitors

Allylthiourea (ATU) and chlorate (ClO₃^-) are often used to selectively inhibit nitritation and nitratation. In this work we identified challenges with use of these compounds in inhibitory assays with filter material from a biological rapid sand filter for groundwater treatment. Inhibition was investigated in continuous-flow lab-scale columns, packed with filter material from a full-scale filter and supplied with NH₄⁺ or NO₂^-. ATU concentrations of 0.1-0.5 mM interfered with the indophenol blue method for NH₄⁺ quantification leading to underestimation of the measured NH₄⁺ concentration. Interference was stronger at higher ATU levels and resulted in no NH₄⁺ detection at 0.5 mM ATU. ClO₃^- at typical concentrations for inhibition assays (1-10 mM) inhibited nitratation by less than 6%, while nitritation was instead inhibited by 91% when NH₄⁺ was supplied. On the other hand, nitratation was inhibited by 67-71% at 10-20 mM ClO₃^- when NO₂^- was supplied, suggesting significant nitratation inhibition at higher NO₂^- concentrations. No chlorite (ClO₂^-) was detected in the effluent, and thus we could not confirm that nitritation inhibition was caused by ClO₃^- reduction to ClO₂^-. In conclusion, ATU and ClO₃^- should be used with caution in inhibition assays, because analytical interference and poor selectivity for the targeted process may affect the experimental outcome and compromise result interpretation.

Comparing the Resistance, Resilience, and Stability of Replicate Moving Bed Biofilm and Suspended Growth Combined Nitritation-Anammox Reactors

Combined partial nitritation-anammox (PN/A) systems are increasingly being employed for sustainable removal of nitrogen from wastewater, but process instabilities present ongoing challenges for practitioners. The goal of this study was to elucidate differences in process stability between PN/A process variations employing two distinct aggregate types: biofilm [in moving bed biofilm reactors (MBBRs)] and suspended growth biomass. Triplicate reactors for each process variation were studied under baseline conditions and in response to a series of transient perturbations. MBBRs displayed elevated NH₄⁺ removal rates relative to those of suspended growth counterparts over six months of unperturbed baseline operation but also exhibited significantly greater variability in performance. Transient perturbations led to strikingly divergent yet reproducible behavior in biofilm versus suspended growth systems. A temperature perturbation prompted a sharp reduction in NH₄⁺ removal rates with no accumulation of NO₂^- and rapid recovery in MBBRs, compared to a similar reduction in NH₄⁺ removal rates but a high level of accumulation of NO₂^- in suspended growth reactors. Pulse additions of a nitrification inhibitor (allylthiourea) prompted only moderate declines in performance in suspended growth reactors compared to sharp decreases in NH₄⁺ removal rates in MBBRs. Quantitative fluorescence in situ hybridization demonstrated a significant enrichment of anammox in MBBRs compared to suspended growth reactors, and conversely a proportionally higher AOB abundance in suspended growth reactors. Overall, MBBRs displayed significantly increased susceptibility to transient perturbations employed in this study compared to that of suspended growth counterparts (stability parameter), including significantly longer recovery times (resilience). No significant difference in the maximal impact of perturbations (resistance) was apparent. Taken together, our results suggest that aggregate architecture (biofilm vs suspended growth) in PN/A processes exerts an unexpectedly strong influence on process stability.

Impacts of variable pH on stability and nutrient removal efficiency of aerobic granular sludge

The impact of pH variation on aerobic granular sludge stability and performance was investigated. A 9-day alkaline (pH=9) and acidic (pH=6) pH shocks were imposed on mature granules with simultaneous chemical oxygen demand (COD), nitrogen and phosphorus removal. The imposed alkaline pH shock (pH 9) reduced nitrogen and phosphorus removal efficiency from 88% and 98% to 66% and 50%, respectively, with no further recovery. However, acidic pH shock (pH 6) did not have a major impact on nutrient removal and the removal efficiencies recovered to their initial values after 3 days of operation under the new pH condition. Operating the reactors under alkaline pH induced granules breakage and resulted in an increased solids concentration in the effluent and a significant decrease in the size of the bio-particles, while acidic pH did not have significant impacts on granules stability. Changes in chemical structure and composition of extracellular polymeric substances (EPS) matrix were suggested as the main factors inducing granules instability under high pH.

From the affinity constant to the half-saturation index: understanding conventional modeling concepts in novel wastewater treatment processes

The "affinity constant" (KS) concept is applied in wastewater treatment models to incorporate the effect of substrate limitation on process performance. As an increasing number of wastewater treatment processes rely on low substrate concentrations, a proper understanding of these so-called constants is critical in order to soundly model and evaluate emerging treatment systems. In this paper, an in-depth analysis of the KS concept has been carried out, focusing on the different physical and biological phenomena that affect its observed value. By structuring the factors influencing half-saturation indices (newly proposed nomenclature) into advectional, diffusional and biological, light has been shed onto some of the apparent inconsistencies present in the literature. Particularly, the importance of non-ideal mixing as a source of variability between observed KS values in different systems has been illustrated. Additionally, discussion on the differences existent between substrates that affect half-saturation indices has been carried out; it has been shown that the observed KS for some substrates will reflect transport or biological limitations more than others. Finally, potential modeling strategies that could alleviate the shortcomings of the KS concept have been provided. These could be of special importance when considering the evaluation and design of emerging wastewater treatment processes.

Biodegradation and cometabolic modeling of selected beta blockers during ammonia oxidation

Accurate prediction of pharmaceutical concentrations in wastewater effluents requires that the specific biochemical processes responsible for pharmaceutical biodegradation be elucidated and integrated within any modeling framework. The fate of three selected beta blockers-atenolol, metoprolol, and sotalol-was examined during nitrification using batch experiments to develop and evaluate a new cometabolic process-based (CPB) model. CPB model parameters describe biotransformation during and after ammonia oxidation for specific biomass populations and are designed to be integrated within the Activated Sludge Models framework. Metoprolol and sotalol were not biodegraded by the nitrification enrichment culture employed herein. Biodegradation of atenolol was observed and linked to the activity of ammonia-oxidizing bacteria (AOB) and heterotrophs but not nitrite-oxidizing bacteria. Results suggest that the role of AOB in atenolol degradation may be disproportionately more significant than is otherwise suggested by their lower relative abundance in typical biological treatment processes. Atenolol was observed to competitively inhibit AOB growth in our experiments, though model simulations suggest inhibition is most relevant at atenolol concentrations greater than approximately 200 ng·L(-1). CPB model parameters were found to be relatively insensitive to biokinetic parameter selection suggesting the model approach may hold utility for describing pharmaceutical biodegradation during biological wastewater treatment.

Treatment of wastewater using a sequencing batch reactor

The aim of this study was to evaluate the efficiency of two sequencing batch reactors (R1 and R2) at removing nutrient (N and P) and chemical oxygen demand (COD). The two reactors (R1 and R2) were of the same design, operating under identical cycles and had a sludge retention time of 5 d. In R1, the substrate was sewage enriched with cooked and triturated cereals. In R2, the substrate was raw sewage mixed with triturated discarded excess sludge. Respirometry tests were performed to compare the biodegradability of the substrates used during the experimental period. The efficiency of R1 in removing soluble P and N-ammonia was considerably higher (90.4 and 97.2%, respectively) than reactor R2 (60 and 39.2%, respectively). While the effluent generated by R1 contained only minor amounts of N-nitrite and N-nitrate (0.5 +/- 0.4 and 1.7 +/- 0.8 mg L(-1), respectively). The concentrations of nitrite and nitrate in the effluent from R2 were 2 and 7 times higher. The lack of biodegradable COD available for denitrification was responsible for the high concentrations of nitrite and nitrate in the effluent of R2.

Comparison of vertical-flow constructed wetlands with and without supplementary aeration treating decentralized domestic wastewater

Constructed wetlands (CWs) are efficient in reducing excessive contamination from wastewaters. However, oxygen inside CW beds is frequently low especially when substrate clogging problems appear after long-term operation, and this may become a limited factor for the treatment of wastewaters. Aimed at dealing with the issue of a low oxygen content in CW systems, two laboratory-scale vertical-flow constructed wetlands (VFCWs) with and without an aeration device (called VFCW-a and VFCW-c, respectively) were designed in this study to test the contribution of supplementary aeration to the treatment of decentralized domestic wastewater. Results showed that under the intermittent operation of about 45 days, two VFCW units were successfully started up by using activated sludge as seed sludge. Compared to VFCW-c, VFCW-a had a better resistance ability to organic shock loads and its removal function could be effectively recovered within a short period after the introduction of organic shock loads. Under intermittent operation with a 12 h idling time, the ideal hydraulic retention time (HRT) of VFCW-a was 42 h, about 6 h shorter than that of VFCW-c. Likewise, under intermittent operation with 42 h HRT, the ideal idling time of VFCW-a was 12 h, still about 6 h shorter than that of VFCW-c. Under intermittent operation with HRT-42 h and an idling time of 12 h, SS, COD, TN and TP removal efficiencies in VFCW-a could reach 81.2%, 85%, 89.9% and 77.9%, respectively. The VFCW unit with supplementary aeration is an efficient innovation for the treatment of decentralized domestic wastewater.

Fast start-up and controlled operation during a long-term period of a high-rate partial nitrification activated sludge system

Partial nitrification of a high-strength ammonium wastewater (1150 +/- 150 mg N-NH4(+) L(-1)), mimicking reject water, was achieved in an activated sludge pilot plant with a configuration of three continuous reactors in series plus a settler. Stable and robust partial nitrification was maintained during 800 days of operation at 30 degrees C with a sludge retention time (SRT) of 8 +/- 3 days. A high volumetric ammonium oxidation rate (2.0 g N L(-1) d(-1)) was obtained with a [N-NO2-]/[N-NO(x)-] ratio of 1, i.e. full nitritation. The start-up of the partial nitrification system was quickly and successfully performed with an on-line control system using municipal wastewater treatment plant (WWTP) sludge as inoculum. An ammonia-oxidizing bacteria (AOB) fraction of 72 +/- 10% was obtained after only 30 days of start-up. The applied SRT of 7-10 days with the combination of free ammonia inhibition and dissolved oxygen limitation provided the selective washout of nitrite-oxidizing bacteria (NOB) and an active nitrifying population with high ammonium oxidizing rates.

Factors affecting the growth rates of ammonium and nitrite oxidizing bacteria

The maximum specific growth rates of both ammonium oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) were investigated under varying aerobic solids retention time (SRT(a)) and in the presence/absence of anoxic (alternating) conditions. Two bench SBRs, reactor R1 and R2, were run in parallel for 150d. Reactor R1 was operated in aerobic conditions while R2 operated in alternating anoxic/aerobic conditions. The feed (synthetic wastewater), temperature, hydraulic retention time and mixing were identical in both reactors. The SRT(a) in both reactors was, sequentially, set at four values: 5, 4, 3 and 2d. Kinetic tests with the biomasses from both reactors were carried out to estimate the maximum specific growth rates (μ(max)) at each tested SRT(a) and decay rates, in both aerobic and anoxic conditions. The kinetic parameters of nitrifier were estimated through the calibration of a two step nitrification-denitrification activated sludge model. The results point to a slightly higher μ(max,AOB) and μ(max,NOB) in alternating conditions, while both μ(max,AOB) and μ(max,NOB) were shown not to vary in the tested range of SRT(a) (from 2 to 5d) at 20°C. They were relatively high when compared to literature data: 1.05d(-1)<μ(max,AOB)<1.4d(-1) and 0.91d(-1)<μ(max,NOB)<1.31d(-1). The decay coefficients of both AOB and NOB were much higher in aerobic (from 0.22d(-1) to 0.28d(-1)) than in anoxic (0.04d(-1) to 0.16d(-1)) conditions both in R1 and R2, which explained the higher nitrification rates observed in the alternating reactor.