Modelling nitrification, heterotrophic growth and predation in activated sludge

Nitrification characteristics and microbial community changes during conversion of freshwater to seawater in down-flow hanging sponge reactor

In recirculating aquaculture systems (RAS), maintaining water quality in aquaculture tanks is a paramount factor for effective fish production. A down-flow hanging sponge (DHS) reactor, a trickling filter system used for water treatment of RAS that employs sponges to retain biomass, has high nitrification activity. However, nitrification in seawater RAS requires a long start-up time owing to the high salinity stress. Therefore, this study aimed to evaluate the nitrification characteristics and changes in the microbial community during the conversion of freshwater to seawater in a DHSreactor fed with ammonia-based artificial seawater. The total ammonia nitrogen concentration reached 1.0 mg-N·L-1 (initial concentration 10 mg-N·L-1) within 11 days of operation, and nitrate production was observed. The 16 S rRNA gene sequence of the DHS-retained sludge indicated that the detection rate of the ammonia-oxidizing archaeon Candidatus Nitrosocosmicus decreased from 23.9 % to 14.0 % and 25.8-17.6 % in the upper and lower parts of the DHS reactor, respectively, after the introduction of seawater. In contrast, the nitrite-oxidizing bacteria Nitrospira spp. increased from 0.1 % to 9.5 % and from 0.5 % to 10.5 %, respectively. The ammonia oxidation rates of 0.12 ± 0.064 and 0.051 ± 0.0043 mg-N·g-MLVSS-1·h-1 on the 37th day in the upper and bottom layers, respectively. Thus, nitrification in the DHS reactor performed well, even under high-salinity conditions with short operational days. This finding makes the transition from freshwater to saltwater fish in the RAS system simple and economical, and has the potential for early start-up of the RAS.

Nitrite Oxidation in Wastewater Treatment: Microbial Adaptation and Suppression Challenges

Microbial nitrite oxidation is the primary pathway that generates nitrate in wastewater treatment systems and can be performed by a variety of microbes: namely, nitrite-oxidizing bacteria (NOB). Since NOB were first isolated 130 years ago, the understanding of the phylogenetical and physiological diversities of NOB has been gradually deepened. In recent endeavors of advanced biological nitrogen removal, NOB have been more considered as a troublesome disruptor, and strategies on NOB suppression often fail in practice after long-term operation due to the growth of specific NOB that are able to adapt to even harsh conditions. In line with a review of the history of currently known NOB genera, a phylogenetic tree is constructed to exhibit a wide range of NOB in different phyla. In addition, the growth behavior and metabolic performance of different NOB strains are summarized. These specific features of various NOB (e.g., high oxygen affinity of Nitrospira, tolerance to chemical inhibitors of Nitrobacter and Candidatus Nitrotoga, and preference to high temperature of Nitrolancea) highlight the differentiation of the NOB ecological niche in biological nitrogen processes and potentially support their adaptation to different suppression strategies (e.g., low dissolved oxygen, chemical treatment, and high temperature). This review implicates the acquired physiological characteristics of NOB to their emergence from a genomic and ecological perspective and emphasizes the importance of understanding physiological characterization and genomic information in future wastewater treatment studies.

Expanding mechanistic models to represent purple phototrophic bacteria enriched cultures growing outdoors

The economic feasibility of purple phototrophic bacteria (PPB) for resource recovery relies on using enriched-mixed cultures and sunlight. This work presents an extended Photo-Anaerobic Model (ePAnM), considering: (i) the diverse metabolic capabilities of PPB, (ii) microbial clades interacting with PPB, and (iii) varying environmental conditions. Key kinetic and stoichiometric parameters were either determined experimentally (with dedicated tests), calculated, or gathered from literature. The model was calibrated and validated using different datasets from an outdoors demonstration-scale reactor, as well as results from aerobic and anaerobic batch tests. The ePAnM was able to predict the concentrations of key compounds/components (e.g., COD, volatile fatty acids, and nutrients), as well as microbial communities (with anaerobic systems dominated by fermenters and PPB). The results underlined the importance of considering other microbial clades and varying environmental conditions. The model predicted a minimum hydraulic retention time of 0.5 d^-1. A maximum width of 10 cm in flat plate reactors should not be exceeded. Simulations showed the potential of a combined day-anaerobic/night-aerobic operational strategy to allow efficient continuous operation.

A quantified nitrogen metabolic network by reaction kinetics and mathematical model in a single-stage microaerobic system treating low COD/TN wastewater

A single-stage intermittent aeration microaerobic reactor (IAMR) has been developed for the cost-effective nitrogen removal from piggery wastewater with a low ratio of chemical oxygen demand (COD) to total nitrogen (TN). In this study, a quantified nitrogen metabolic network was constructed based on the metagenomics, reaction kinetics and mathematical model to provide a revealing insight into the nitrogen removal mechanism in the IAMR. Metagenomics revealed that a complex nitrogen metabolic network, including aerobic ammonia and nitrite oxidation, anammox, denitrification via nitrate and nitrite, and nitrate respiration, existed in the IAMR. A novel method for solving kinetic parameters with high stability was developed based on a genetic algorithm. Use this method to calculate the kinetics of various reactions involved in nitrogen metabolism. Kinetics revealed that simultaneous partial nitritation-anammox (PN/A) and partial denitrification-anammox (PDN/A) were the dominant approaches to nitrogen removal in the IAMR. Finally, a kinetics-based model was proposed for quantitatively describing the nitrogen metabolic network under the limitation of COD. 58% ∼ 67% of nitrogen was removed via the anammox-based processes (PN/A and PDN/A), but only 7% ∼ 12% and 1% ∼ 2% of nitrogen were removed via heterotrophic denitrification of nitrite and nitrate, respectively. The half-inhibition constant of dissolved oxygen (DO) on anammox was simulated as 0.37 ∼ 0.60 mg L^-1, filling the gap in quantifying DO inhibition on anammox. High-frequency intermittent aeration was identified as the crucial measure to suppress nitrite-oxidizing bacteria, although it has a high affinity for DO and NO₂^--N. In continuous aeration mode, the simulated NO₃^--N in the IAMR would rise by 39.6%. The research provides a novel insight into the nitrogen removal mechanism in single-stage microaerobic systems and provides a reliable approach to practicing PN/A and PDN/A for cost-effective nitrogen removal.

Model study on real-time aeration based on nitrite for effective operation of single-stage anammox

Anaerobic ammonia oxidation (Anammox) is an innovative technology for cost-efficient nitrogen removal without intensive aeration. However, effective control of the competition between nitrite oxidizing bacteria (X_NOB) and Anammox bacteria (X_ANA) for nitrite is a key challenge for broad applications of single-stage Anammox processes in real wastewater treatment. Therefore, a real-time aeration scheme was proposed to determine dissolved oxygen (DO) based on nitrite concentration for effective control of X_NOB growth while maintaining the X_ANA activity in a single-stage Anammox process. In this study, a non-steady state mathematical model was developed and calibrated using previously reported lab-scale Anammox results to investigate the efficiency of the proposed real-time aeration scheme in enhancing the Anammox process. Based on the calibrated model simulation results, DO of about 0.10 mg-O₂/L was found to be ideal for maintaining effective nitrite creation by ammonia oxidizing bacteria (X_AOB) while slowing down the growth of X_NOB. If DO is too low (e.g., 0.01 mg-O₂/L or lower), the overall rate of the ammonia removal is limited due to slow growth of X_AOB. On the other hand, high DO (e.g., 1.0 mg-O₂/L or higher) inhibits the growth of X_ANA, resulting in dominancy of X_AOB and X_NOB. According to the simulation results, nitrite concentration was found to be a rate-limiting parameter on effective nitrogen removal in single-stage Anammox processes. We also found that nitrite concentration can be used as a real-time switch for aeration in a single-stage Anammox process. A schematic aeration method based on real-time nitrite concentration was proposed and examined to control the competition between X_ANA and X_NOB. In the model simulation, the X_ANA activity was successfully maintained because the schematic aeration prevented an outgrowth of X_NOB, allowing energy-efficient nitrogen removal using single-stage Anammox processes.

Minimizing Foaming and Bulking in Activated Sludge with Bacteriophage Treatment: A Review of Mathematical Modeling

The interest in the ability of phages to control bacterial populations has extended from medical applications into the fields of agriculture, aquaculture, and the food industry. In particular, several authors have proposed using bacteriophages as an alternative method to control foaming and bulking in wastewater treatment. This strategy has shown successful results at the laboratory scale. However, this technology is still in development, and there are several challenges to overcome before bacteriophages can be widely used to control foaming and bulking in pilot or larger-scale treatment plants. Several models of the infection mechanisms in individual bacteria–phage pairs have been reported, i.e., for controlled systems with only one bacterium species in the presence of one phage species. However, activated sludge treatment systems largely differ from this situation, which opens a large horizon for future research. Mathematical models will play a key role in this development process, and this review offers an overview of the proposed models: their applications, potential, and challenges. A particular focus is placed on the model properties, such as parameter identifiability and states’ observability, which are essential for process prediction, monitoring, or dynamic optimization.

Immobilization of Ochrobactrum sp. on Biochar/Clay Composite Particle: Optimization of Preparation and Performance for Nitrogen Removal

The objective of this study was to prepare biochar/clay composite particle (BCCP) as carrier to immobilize Ochrobactrum sp. to degrade ammonia nitrogen (NH4 +-N), and the effects of calcined program and immobilizing material were investigated. Results reflected that the parameters were as follows: calcined temperature 400°C, heating rate 20°C min-1, and holding time 2 h, and the adsorption capacity could reach 0.492 mg g-1. Sodium alginate/polyvinyl alcohol, as embedding material, jointed with NH4 +-N adsorption process and then degraded by Ochrobactrum sp. with 79.39% degradation efficiency at 168 h. Immobilizing Ochrobactrum sp. could protect strain from high salt concentration to achieve the exceeding degradation efficiency than free bacteria, but could not block the impact of low temperature.

Dynamic modelling of N2 O emissions from a full-scale granular sludge partial nitritation-anammox reactor

Partial nitration-anammox is a resource-efficient pathway for nitrogen removal from wastewater. However, the advantages of this nitrogen removal technology may be counter-acted by the emission of N₂ O, a potent greenhouse gas. In this study, mathematical modelling was applied to analyse N₂ O formation and emission dynamics and to develop N₂ O mitigation strategies for a one-stage partial nitritation-anammox granular sludge reactor. Dynamic model calibration for such a full-scale reactor was performed, applying a 1-dimensional biofilm model and including several N₂ O formation pathways. Simultaneous calibration of liquid phase concentrations and N₂ O emissions leads to improved model fit compared to their consecutive calibration. The model could quantitatively predict the average N₂ O emissions and qualitatively characterize the N₂ O dynamics, adjusting only seven parameter values. The model was validated with N₂ O data from an independent data set at different aeration conditions. Nitrifier nitrification was identified as the dominating N₂ O formation pathway. Off-gas recirculation as a potential N₂ O emission reduction strategy was tested by simulation and showed indeed some improvement, be it at the cost of higher aeration energy consumption. This article is protected by copyright. All rights reserved.

Effects of eukaryotic predation on nitrifying MABR biofilms

This research explored the effects of eukaryotic predation on nitrifying membrane-aerated biofilm reactor (MABR) biofilms. Past research on heterotrophic MABR biofilms showed that predation could create internal voids that promoted sloughing. However, the no past research addressed the effects of predation on nitrifying MABRs, even though nitrification is the most common MABR application. Nitrifying biofilms are typically denser, and ammonia oxidizing bacteria (AOB) form large, dense clusters within the biofilm. This could affect their susceptibility to predation. Nitrifying biofilms were grown in flat-sheet MABRs. Images of the biofilm were captured using optical coherence tomography (OCT). For detachment tests, an increased shear flow (Re≅140) was used, and a shear rheometer was used to measure the biofilm mechanical properties. The nitrifying community was analyzed with fluorescence in situ hybridization (FISH) and quantitative PCR (qPCR). Predation increased internal void ratios from 54 ± 5% to 69 ± 6%. Biofilms were weakened by predation, with a storage modulus (G') and loss modulus (G'') of 242 ± 135 and 1,649 ± 853 Pa with predation and 3,644 ± 1,857 and 23,334 ± 11,481 Pa for the control with suppressed predation. Predation increased the relative biofilm detachment from 4 ± 5 to 18 ± 12%, decreased the amount of biomass, i.e., the average biofilm thickness, from 502 ± 150 to 266 ± 54 µm, and decreased the nitrification flux from 1.00 to 0.61 g NH4+-N/m2day. Also, predation decreased the abundance of nitrite oxidizing bacteria (NOB) relative to AOB, consistent with the observed nitritation. These results show that predation can significantly impact the structural stability, bacterial community and removal rates of nitrifying MABR biofilms. Lumping the effects of predation into the detachment or decay coefficients of biofilm models may not accurately reflect the behavior of nitrifying MABR biofilms.

Elucidating salinity adaptation and shock loading on denitrification performance: Focusing on microbial community shift and carbon source evaluation

To understand the denitrification efficiency and microbial community shift with increasing salinity in salinity adaptation and shock loading process, nitrate (NO3--N), nitrite (NO2--N) and chemical oxygen demand (COD) removal efficiencies were monitored feeding acetate and primary sludge fermentation liquid. During adaptation process, salinity had little effect on NO3--N removal efficiency (>99.0%) with acetate-fed, while for fermentation liquid-fed, it decreased to around 97% at high salinity (>2.5%). Effluent NO2--N was lower than 0.1 mg/L, though obvious fluctuation of NO2--N was observed with fermentation liquid-fed when salinity change. During shock loading process, denitrification process all had slight decrease when the salinity abruptly increased to 5.0%. Traditional denitrifier of Thauera was the dominant genus, and a specialized microbial community of Azoarcus in salinity adaptation and Paracoccus in shock loading for denitrification showed high salinity tolerant. Meanwhile, microbial diversity was enriched with fermentation liquid-fed at high salinity condition.

Modeling the formation of microorganism-derived dissolved organic nitrogen (mDON) in the activated sludge system

Microorganism-derived dissolved organic nitrogen (mDON) represents a significant and inevitable portion of dissolved organic nitrogen (DON) in the wastewater biotreatment processes. In the existing method, mDON concentrations are indirectly measured by the values of DON concentrations from the reactors with DON-free influent. However, this becomes problematic when influent contains DON. Especially when the real wastewater is involved, the paucity of the direct methods to quantitatively measure mDON is a major barrier to further research. This limitation is due to the difficulty of segregating mDON from the other nitrogenous organics, e.g., influent DON. In this study, we propose the ASM-mDON model based on ASM #1, which incorporates the production and consumption of mDON in the activated sludge processes to predict the mDON concentrations. In four independent lab-scale tests, our model was established and calibrated to obtain the accurate values of mDON (R² = 0.929, p < 0.05), and the validity and applicability of the model were successfully examined by comparing the simulated and measured data. Moreover, the universality of the ASM-mDON model was further confirmed by simulating mDON production in a full-scale wastewater treatment plant. A reasonable prediction of mDON formation was shown in a full-scale test (1.98 ± 0.71 mg/L in June and 1.51 ± 0.54 mg/L in July) and is indirectly supported by an algal bioassay (p < 0.05, t-test). This study provides a useful approach to the efficient and accurate evaluation of mDON formation, which will improve current strategies designed to minimize the effluent mDON in wastewater bioprocesses.

Role of bacterivorous organisms on fungal-based systems for natural tannin degradation

Tannery wastewater presents high concentrations of organic load and pollutant recalcitrant molecules (e.g. tannins), which reduce the efficiency of biological treatment processes. Recent studies showed that several fungal species and strains are effective in the degradation of tannins. However, high bacterial load can negatively affect fungal growth, reducing system stability and degradation performances.

The aim of the present study was to evaluate the effects of the introduction of bacterivorous grazers (ciliates and/or rotifers) in batch scale experiments using fungi to remove Tara tannin, i.e. to check the potential synergistic effect between fungi and bacterivorous grazers in the degradation of recalcitrant compounds. In this context, the ciliated grazers Paramecium calkinsi, Tetrahymena sp., Pseudovorticella sp., and the rotifer Lecane inermis, preliminary selected according to their ability to grow in a solution prepared with Tara tannin, were separately tested. Activated sludge, including a complex mixture of native grazers, was used as experimental control. The following parameters were monitored: bacterial load, number of grazers/mL and Soluble Chemical Oxygen Demand (SCOD). Colony Forming Unit (CFU)/grazers ratio was also calculated. Particular attention was paid to: i) bacterial load reduction and ii) enhancement of recalcitrant compounds degradation, and we observed that in all experimental conditions where grazers occurred bacterial load was significantly reduced and the system achieved a higher SCOD removal in a shorter time. Our findings provide useful insights for the stabilization of fungal-based systems in non-sterile conditions.

Effect of aquaculture salinity on nitrification and microbial community in moving bed bioreactors with immobilized microbial granules

The novel immobilized microbial granules (IMG) shows a significant effect of nitrification for freshwater aquaculture. However, there is lack of evaluation study on the performance of nitrification at high salinity due to the concentration of recycled water or seawater utilization. A laboratory scale moving bed bioreactor (MBBR) with IMG was tested on recycled synthetic aquaculture wastewater for the nitrification at 2.5 mg/L NH₃-N daily. The results indicated that IMG showed a high salinity tolerance and effectively converted ammonia to nitrate up to 92% at high salinity of 35.0 g/L NaCl. As salinity increased from near zero to 35.0 g/L, the microbial activity of nitrite oxidation bacteria (NOB) in the IMG decreased by 86.32%. The microbial community analysis indicated that salinity significantly influenced the community structure. It was found that Nitrosomonas sp. and Nitrospira sp. were the dominant genera for ammonia oxidation bacteria (AOB) and NOB respectively at different salinity levels.

Predation creates unique void layer in membrane-aerated biofilms

The membrane-aerated biofilm reactor (MABR) is a novel wastewater treatment technology based on oxygen-supplying membranes. The counter diffusion of oxygen and electron donors in MABRs leads to unique behavior, and we hypothesized it also could impact predation. We used optical coherence tomography (OCT), microsensor analyses, and mathematical modeling to investigate predation in membrane-aerated biofilms (MABs). When protozoa were excluded from the inoculum, the MAB's OCT-observable void fraction was around 5%. When protozoa were included, the void fraction grew to nearly 50%, with large, continuous voids at the base of the biofilm. Real-time OCT imaging showed highly motile protozoa in the voids. MABs with protozoa and a high bulk COD (270 mg/L) only had 4% void fraction. DNA sequencing revealed a high relative abundance of amoeba in both high and low-COD MABs. Flagellates were only abundant in the low-COD MAB. Modeling also suggested a relationship between substrate concentrations, diffusion mode (co- or counter-diffusion), and biofilm void fraction. Results suggest that amoeba proliferate in the biofilm interior, especially in the aerobic zones. Voids form once COD limitation at the base of MABs allows predation rates to exceed microbial growth rates. Once formed, the voids provide a niche for motile protozoa, which expand the voids into a large, continuous gap. This increases the potential for biofilm sloughing, and may have detrimental effects on slow-growing, aerobic microorganisms such as nitrifying bacteria.

Understanding of aerobic sludge granulation enhanced by sludge retention time in the aspect of quorum sensing

Aerobic granular sludge (AGS) reactors with different sludge retention times (SRTs) were established for enhanced functional microorganism enrichment and granular formation. Results showed that higher total nitrogen (TN) removal efficiency and compact granules were achieved in the 6-day-SRT reactor. Also, Xanthomonadaceae, Rhodobacteraceae and Hyphomonadaceae with AHL-producing and EPS-secreting functions also enriched under 6-day SRT. For investigating the enhanced mechanism of sludge granulation, typical quorum sensing signals of acylated-homoserine-lactones (AHLs) and extracellular polymeric substances (EPS) were analyzed. Tryptophan-and-protein-like substances were major EPS components in granules formed at 6-day SRT. Meanwhile, most detected AHLs, i.e. C8-HSL and 3OHC8-HSL, were correlated positively with contents of tryptophan-and-protein-like substances. According to AHLs add-back test, AHLs especially those with 8-carbon sidechains, played important roles in aerobic sludge granulation via secreting special extracellular proteins by functional microbes enrichment.

Incorporating the influent cellulose fraction in activated sludge modelling

Cellulose represents a significant fraction of domestic wastewater, but it is not considered as a separate state variable in the conventional Activated Sludge Models (ASM). Cellulose is a very slowly degradable substrate that in traditional wastewater characterisation methods would be characterised partly as slowly biodegradable and partly as inert material. This can be problematic when the same model is used under different temperature conditions or different solid retention times. Also with the emerging attention for cellulose recovery, inclusion of this compound in models helps to assess the impact of cellulose recovery on operations and on operational costs. But also in membrane bioreactors, sieves are used in order to remove fibrous material, mainly cellulose. In this study a modification of ASM1 is proposed, where cellulose is introduced as a separate state variable and it is supposed to degrade with a first order hydrolysis rate. With the aid of this model, the effect of fine sieves is simulated using two alternatives, by either sieving the influent or the activated sludge. The model proved to be useful for operational purposes, and illustrates the need for including cellulose as separate state variable.

Preliminary investigation of quorum quenching effects on sludge quantity and quality of activated sludge process

The Quorum Sensing (QS) system has attracted the interest of researchers as a cell-cell communication system. In activated sludge processes, the production of extracellular polymeric substances (EPS), biofilms and floc formation are regulated by the QS system. Hence, disruption of the QS system, called Quorum Quenching (QQ), could have a significant effect on the quality and quantity of excess sludge. In the present research, the quorum quenching bacteria, Rhodococcus sp. BH4 was used as a quorum quencher and was entrapped in an alginate structure (QQ beads). Three separate sequential batch reactors (SBR) were constructed and operated as a control reactor, a Low-QQ reactor (containing 150 QQ beads), and a High-QQ reactor (containing 600 QQ beads). Results indicated that the presence of QQ beads in the aeration reactor leads to a decrease in EPS content and mean floc particle size in the both Low-QQ and High-QQ reactors. The eukaryotic community was changed significantly so that the QS disruption caused an enhancement in microbial predation. The presence of QQ beads also led to a 16 and a 26% decrease in the Y_obs coefficient within the Low-QQ and High-QQ reactors, respectively. Findings of this research revealed a new application of the QQ system in the activated sludge process, but additional studies are needed.

Deterioration of the anammox process at decreasing temperatures and long SRTs

The implementation of autotrophic nitrogen removal in the mainstream of a municipal wastewater treatment plant is currently pursued. Among the crucial unknown factors are the kinetic properties of anaerobic ammonium oxidising (anammox) bacteria at low temperatures. In this study we investigated the adaptation of a fast-growing anammox culture to a lower temperature. In a membrane bioreactor a highly enriched anammox community was obtained at 30°C, 25°C and 20°C. This culture was exposed to long- and short-term temperature changes. In short-term experiments the decrease in biomass-specific activity due to decrease in temperature can be described by an activation energy of 64 ± 28 kJ mol^-1. Prolonged cultivation (months) implies that cultivation at low temperatures resulted in deterioration of biomass-specific activity (Ea^LT 239 kJ mol^-1). The growth rate and specific anammox activity in the system decreased from 0.33 d^-1 and 4.47 g NO₂-N g VSS^-1 d^-1 at 30°C to 0.0011 d^-1 and 0.037 g NO₂-N g VSS^-1 d^-1 at 20°C. The reason for the deterioration of the system was related to the required long SRT in the system. The long SRT leads to an increase of non-active and non-anammox cells in the reactor, thereby decreasing the biomass-specific activity.

Effects of salinity and COD/N on denitrification and bacterial community in dicyclic-type electrode based biofilm reactor

A dicyclic-type electrode based biofilm electrode reactor (BER) was developed for advanced nitrate removal from saline municipal wastewater. The denitrification efficiency was evaluated with a synthetic feed (NO₃^--N, 20 mg L^-1) under different salinity and COD to nitrogen ratios (COD/N). As the salinity increased from 0% to 1.0%, the denitrification performance of both the traditional biofilm reactor (BR) and BER was inhibited; however, the BER showed better adaptation and ability to recover. The BER achieved a high nitrate removal efficiency (≥90%) at a salinity of 1.0% and a low COD/N of 2.5 (theoretical stoichiometric 2.86 ignoring microbial growth). The abundance of Methylotenera mobilis in BR and Clostridium sticklandii in BER was higher than in the initial sludge sample used as inoculum. Likewise, the abundance of napA, nirS and nosZ genes increased as the COD/N further decreased. Under high salinity stress, the BER had a higher denitrification efficiency and the consumption of the organic carbon source (i.e., methanol) was reduced compared to BR. The cooperation between heterotrophic and autotrophic denitrifiers in the BER system provides a more efficient and feasible solution for nitrate removal from saline municipal wastewater.

Nitrification in a soil-aquifer treatment system: comparison of potential nitrification and concentration profiles in the vadose zone

The oxidation of ammonium in the vadose zone of soil aquifer systems is discussed and examined by detailed analysis of the depth profiles of dissolved oxygen, nitrate and ammonium concentrations in the vadose zone of a soil-aquifer treatment (SAT) system of a municipal wastewater treatment system of the Tel Aviv metropolitan area. Nitrification kinetics and ammonium adsorption capacity studies show that neither the nitrification rate nor the ammonium adsorption capacity controls the capacity of the Shafdan SAT system for ammonium removal. Evaluation of the ammonium adsorption capacity of the soil reveals that under ideal conditions, a depth of less than 50 cm is sufficient to adsorb all the ammonium supplied in a flooding cycle. In-field studies show that all the ammonium is concentrated within the first 80 cm of the vadose zone. A depth profile of the Potential Nitrification (P.N), a measure of the local amount and activity of nitrifiers, is presented for the first time in the vadose zone of a SAT system showing that there are sufficient nitrifiers to oxidize all the ammonia that is supplied in a flooding cycle within less than 2 h, under optimal microbiological conditions based on the existing nitrifiers and their spatial distribution. The biodegradation rate in the field corresponds to first order ammonium conversion with a kinetic coefficient of 8.0 ± 0.2 d^-1. Accordingly, the average measured rate was 8.6 ± 5.8 mg NH₄⁺-N per kg per d for in-field tests, which can be compared to the average P.N, with a value of 34.5 ± 16.8 mg NH₄⁺-N per kg per d. The results suggest that a SAT design, taking into account full ammonium removal capacity, is feasible and can rely on the evaluation of the ammonium adsorption capacity in the SAT soil, the ammonium input and the P.N of the equilibrated target soil under conditions simulating the operation of the infiltrating basins.

Long-term dynamic and pseudo-state modeling of complete partial nitrification process at high nitrogen loading rates in a sequential batch reactor (SBR)

Recently, partial nitrification has been adopted widely either for the nitrite shunt process or intermediate nitrite generation step for the Anammox process. However, partial nitrification has been hindered by the complexity of maintaining stable nitrite accumulation at high nitrogen loading rates (NLR) which affect the feasibility of the process for high nitrogen content wastewater. Thus, the operational data of a lab scale SBR performing complete partial nitrification as a first step of nitrite shunt process at NLRs of 0.3-1.2kg/(m³d) have been used to calibrate and validate a process model developed using BioWin® in order to describe the long-term dynamic behavior of the SBR. Moreover, an identifiability analysis step has been introduced to the calibration protocol to eliminate the needs of the respirometric analysis for SBR models. The calibrated model was able to predict accurately the daily effluent ammonia, nitrate, nitrite, alkalinity concentrations and pH during all different operational conditions.

Assessment of the endogenous respiration rate and the observed biomass yield for methanol-fed denitrifying bacteria under anoxic and aerobic conditions

In this study, the endogenous respiration rate and the observed biomass yield of denitrifying methylotrophic biomass were estimated through measuring changes in denitrification rates (DNR) as a result of maintaining the biomass under methanol deprived conditions. For this purpose, activated sludge biomass from a full-scale wastewater treatment plant was kept in 10-L batch reactors for 8 days under fully aerobic and anoxic conditions at 20 °C without methanol addition. To investigate temperature effects, another biomass sample was placed under starvation conditions over a period of 10 days under aerobic conditions at 25 °C. A series of secondary batch tests were conducted to measure DNR and observed biomass yields. The decline in DNR over the starvation period was used as a surrogate to biomass decay rate in order to infer the endogenous respiration rates of the methylotrophs. The regression analysis on the declining DNR data shows 95% confidence intervals of 0.130 ± 0.017 day^-1 for endogenous respiration rate under aerobic conditions at 20 °C, 0.102 ± 0.013 day^-1 under anoxic conditions at 20 °C, and 0.214 ± 0.044 day^-1 under aerobic conditions at 25 °C. Results indicated that the endogenous respiration rate of methylotrophs is 20% slower under anoxic conditions than under aerobic conditions, and there is a significant temperature dependency, with an Arrhenius coefficient of 1.10. The observed biomass yield value showed an increasing trend from approximately 0.2 to 0.6 when the starvation time increased from 0 to 10 days.

Denitrification of nitrate and nitrite by 'Candidatus Accumulibacter phosphatis' clade IC

Phosphate accumulating organisms (PAO) are assumed to use nitrate as external electron acceptor, allowing an efficient integration of simultaneous nitrogen and phosphate removal with minimal organic carbon (COD) requirements. However, contradicting findings appear in literature regarding the denitrification capacities of PAO due to the lack of clade specific highly enriched PAO cultures. Whereas some studies suggest that only PAO clade I may be capable of using nitrate as external electron acceptor for anoxic P-uptake, other studies indicate that PAO clade II may be responsible for anoxic P-removal. In the present study, a highly enriched PAO clade IC culture (>99% according to FISH) was cultivated in an SBR operated under Anaerobic/Oxic conditions and subsequently exposed to Anaerobic/Anoxic/Oxic conditions using nitrate as electron acceptor. Before and after acclimatization to the presence of nitrate, the aerobic and anoxic (nitrate and nitrite) activities of the PAO I culture were assessed through the execution of batch tests using either acetate or propionate as electron donor. In the presence of nitrate, significant P-uptake by PAO I was not observed before or after acclimatization. Using nitrite as electron acceptor, limited nitrite removal rates were observed before acclimatization with lower rates in the acetate fed reactor without P-uptake and slightly higher in the propionate fed reactor with a marginal anoxic P-uptake. Only after acclimatization to nitrate, simultaneous P and nitrite removal was observed. This study suggests that PAO clade IC is not capable of using nitrate as external electron acceptor for anoxic P-removal. The elucidation of the metabolic capacities for individual PAO clades helps in better understanding and optimization of the relation between microbial ecology and process performance in enhanced biological phosphate removal processes.

Analysis and modelling of predation on biofilm activated sludge process: Influence on microbial distribution, sludge production and nutrient dosage

The influence of predation on the biofilm activated sludge (BAS) process is studied using a unified model that incorporates hydrolysis and predation phenomena into the two stages of the BAS system: moving bed biofilm reactor pre-treatment (bacterial-predator stage) and activated sludge (predator stage). The unified model adequately describes the experimental results obtained in a cellulose and viscose full-scale wastewater plant and has been used to evaluate the role and contribution of predator microorganisms towards removal of COD, nutrient requirements, sludge production and microbial distribution. The results indicate that predation is the main factor responsible for the reduction of both nutrient requirements and sludge production. Furthermore, increasing the sludge retention time (SRT) does not influence the total biomass content in the AS reactor of a BAS process in two different industrial wastewater treatments.

An integrated mathematical model for chemical oxygen demand (COD) removal in moving bed biofilm reactors (MBBR) including predation and hydrolysis

An integrated mathematical model is proposed for modelling a moving bed biofilm reactor (MBBR) for removal of chemical oxygen demand (COD) under aerobic conditions. The composite model combines the following: (i) a one-dimensional biofilm model, (ii) a bulk liquid model, and (iii) biological processes in the bulk liquid and biofilm considering the interactions among autotrophic, heterotrophic and predator microorganisms. Depending on the values for the soluble biodegradable COD loading rate (SCLR), the model takes into account a) the hydrolysis of slowly biodegradable compounds in the bulk liquid, and b) the growth of predator microorganisms in the bulk liquid and in the biofilm. The integration of the model and the SCLR allows a general description of the behaviour of COD removal by the MBBR under various conditions. The model is applied for two in-series MBBR wastewater plant from an integrated cellulose and viscose production and accurately describes the experimental concentrations of COD, total suspended solids (TSS), nitrogen and phosphorous obtained during 14 months working at different SCLRs and nutrient dosages. The representation of the microorganism group distribution in the biofilm and in the bulk liquid allow for verification of the presence of predator microorganisms in the second reactor under some operational conditions.

Modeling of Nitrous Oxide Production from Nitritation Reactors Treating Real Anaerobic Digestion Liquor

In this work, a mathematical model including both ammonium oxidizing bacteria (AOB) and heterotrophic bacteria (HB) is constructed to predict N2O production from the nitritation systems receiving the real anaerobic digestion liquor. This is for the first time that N2O production from such systems was modeled considering both AOB and HB. The model was calibrated and validated using experimental data from both lab- and pilot-scale nitritation reactors. The model predictions matched the dynamic N2O, ammonium, nitrite and chemical oxygen demand data well, supporting the capability of the model. Modeling results indicated that HB are the dominant contributor to N2O production in the above systems with the dissolved oxygen (DO) concentration of 0.5-1.0 mg O2/L, accounting for approximately 75% of N2O production. The modeling results also suggested that the contribution of HB to N2O production decreased with the increasing DO concentrations, from 75% at DO = 0.5 mg O2/L to 25% at DO = 7.0 mg O2/L, with a corresponding increase of the AOB contribution (from 25% to 75%). Similar to HB, the total N2O production rate also decreased dramatically from 0.65 to 0.25 mg N/L/h when DO concentration increased from 0.5 to 7.0 mg O2/L.

Physiological adaptation of growth kinetics in activated sludge

Physiological adaptation as it occurs in bacterial cells at variable environmental conditions influences characteristic properties of growth kinetics significantly. However, physiological adaptation to growth related parameters in activated sludge modelling is not yet recognised. Consequently these parameters are regarded to be constant. To investigate physiological adaptation in activated sludge the endogenous respiration in an aerobic degradation batch experiment and simultaneous to that the maximum possible respiration in an aerobic growth batch experiment was measured. The activated sludge samples were taken from full scale wastewater treatment plants with different sludge retention times (SRTs). It could be shown that the low SRT sludge adapts by growth optimisation (high maximum growth rate and high decay rate) to its particular environment where a high SRT sludge adapts by survival optimization (low maximum growth rate and low decay rate). Thereby, both the maximum specific growth rate and the decay rate vary in the same pattern and are strongly correlated to each other. To describe the physiological state of mixed cultures like activated sludge quantitatively a physiological state factor (PSF) is proposed as the ratio of the maximum specific growth rate and the decay rate. The PSF can be expressed as an exponential function with respect to the SRT.

Tracking the dynamics of heterotrophs and nitrifiers in moving-bed biofilm reactors operated at different COD/N ratios

In this study, the impact of COD/N ratio and feeding regime on the dynamics of heterotrophs and nitrifiers in moving-bed biofilm reactors was addressed. Based on DGGE analysis of 16S rRNA genes, the influent COD was found to be the main factor determining the overall bacterial diversity. The amoA-gene-based analysis suggested that the dynamic behavior of the substrate in continuous and pulse-feeding reactors influenced the selection of specific ammonium-oxidizing bacteria (AOB) strains. Furthermore, AOB diversity was directly related to the applied COD/N ratio and ammonium-nitrogen load. Maximum specific ammonium oxidation rates observed under non-substrate-limiting conditions were observed to be proportional to the fraction of nitrifiers within the bacterial community. FISH analysis revealed that Nitrosomonas genus dominated the AOB community in all reactors. Moreover, Nitrospira was found to be the only nitrite-oxidizing bacteria (NOB) in the fully autotrophic system, whereas Nitrobacter represented the dominant NOB genus in the organic carbon-fed reactors.

Modeling effects of DO and SRT on activated sludge decay and production

The effect of dissolved oxygen (DO) on the endogenous decay of active heterotrophic biomass and the hydrolysis of cell debris were studied. With the inclusion of a hydrolysis process for the cell debris, mathematical models that are capable of quantifying the effects of DO and sludge retention time (SRT) on concentrations of active biomass and cell debris in activated sludge are presented. By modeling the biomass cultivated with unlimited DO, the values of endogenous decay coefficient for heterotrophic biomass, the hydrolysis constant of cell debris, and the fraction of decayed biomass that became cell debris were determined to be 0.38 d(-1), 0.013 d(-1), and 0.28, respectively. Results from modeling the biomass cultivated under different DO conditions suggested that the experimental low DO (∼0.2 mg/L) did not inhibit the endogenous decay of heterotrophic biomass, but significantly inhibited the hydrolysis of cell debris with a half-velocity constant value of 2.1 mg/L. Therefore, the increase in sludge production with low DO was mainly contributed by cell debris rather than the active heterotrophic biomass. Maximizing sludge production during aerobic wastewater treatment could reduce aeration energy consumption and improve biogas energy recovery potential.

Simultaneous removal of nanosilver and fullerene in sequencing batch reactors for biological wastewater treatment

Increasing use of engineered nanomaterials (ENMs) inevitably leads to their potential release to the sewer system. The co-removal of nano fullerenes (nC60) and nanosilver as well as their impact on COD removal were studied in biological sequencing batch reactors (SBR) for a year. When dosing nC60 at 0.07-2mgL(-1), the SBR removed greater than 95% of nC60 except for short-term interruptions occurred (i.e., dysfunction of bioreactor by nanosilver addition) when nC60 and nanosilver were dosed simultaneously. During repeated 30-d periods of adding both 2 mg L(-1) nC60 and 2 mg L(-1) nanosilver, short-term interruption of SBRs for 4d was observed and accompanied by (1) reduced total suspended solids in the reactor, (2) poor COD removal rate as low as 22%, and (3) decreased nC60 removal to 0%. After the short-term interruption, COD removal gradually returned to normal within one solids retention time. Except for during these "short-term interruptions", the silver removal rate was above 90%. A series of bottle-point batch experiments was conducted to determine the distribution coefficients of nC60 between liquid and biomass phases. A linear distribution model on nC60 combined with a mass balance equation simulated well its removal rate at a range of 0.07-0.76 mg L(-1) in SBRs. This paper illustrates the effect of "pulse" inputs (i.e., addition for a short period of time) of ENMs into biological reactors, demonstrates long-term capability of SBRs to remove ENMs and COD, and provides an example to predict the removal of ENMs in SBRs upon batch experiments.

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.

A new interpretation of endogenous respiration profiles for the evaluation of the endogenous decay rate of heterotrophic biomass in activated sludge

In current activated sludge models aerobic degradation, resulting in loss of activity and mass of activated sludge is expressed with only one process called decay. The kinetics of this process is regarded to be first order and constant with respect to the loading conditions. In this work twelve aerobic digestion batch experiments were conducted for the activated sludge of seven different water resource recovery facilities (WRRFs). An analysis of the obtained respirograms shows three clearly distinguishable phases. The first phase is assumed to be due to the degradation of stored material (X(STOR)) and active biomass simultaneously. The second phase is exclusively due to the degradation of active biomass that is regarded to consist mainly of ordinary heterotrophic biomass (X(OHO)). The first order decay rate is slower than the degradation rate in phase 1 and varies between samples. The decay rate correlates with the activity of the activated sludge expressed as the ratio of initial heterotrophic OUR and the initial organic fraction X(ORG) of the activated sludge. This second phase was detectable until day 5 of most of the experiments. After that time within phase 3 the OUR decrease slows down and the OUR even increased for short intervals. This behaviour is thought to be due to the activity of higher organisms and the adaptation of microorganisms to starvation.

Simulation and optimization of ammonia removal at low temperature for a double channel oxidation ditch based on fully coupled activated sludge model (FCASM): a full-scale study

An optimal operating condition for ammonia removal at low temperature, based on fully coupled activated sludge model (FCASM), was determined in a full-scale oxidation ditch process wastewater treatment plant (WWTP). The FCASM-based mechanisms model was calibrated and validated with the data measured on site. Several important kinetic parameters of the modified model were tested through respirometry experiment. Validated model was used to evaluate the relationship between ammonia removal and operating parameters, such as temperature (T), dissolved oxygen (DO), solid retention time (SRT) and hydraulic retention time of oxidation ditch (HRT). The simulated results showed that low temperature have a negative effect on the ammonia removal. Through orthogonal simulation tests of the last three factors and combination with the analysis of variance, the optimal operating mode acquired of DO, SRT, HRT for the WWTP at low temperature were 3.5 mg L(-1), 15 d and 14 h, respectively.

Stabilization of unstable steady states of a continuous stirred tank bioreactor with predator-prey kinetics

Nonlinear properties of a bioreactor with a developed microbiological predator-prey food chain are discussed. The presence of the predator microorganism completely changes the position and stability of the stationary states. A wide range of unstable steady states appears, associated with high amplitude oscillations of the state variables. Without automatic control such a system can only operate in transient states, with the yield undergoing periodic changes following the dynamics of the stable limit cycle. Technologically, this is undesirable. It has been shown that the oscillations can be removed by employing continuous P or PI controllers. Moreover, with a PI-controller, the predator can be eliminated from the system.

Interactions of nitrifying bacteria and heterotrophs: identification of a Micavibrio-like putative predator of Nitrospira spp

Chemolithoautotrophic nitrifying bacteria release soluble organic compounds, which can be substrates for heterotrophic microorganisms. The identities of these heterotrophs and the specificities of their interactions with nitrifiers are largely unknown. In this study, we incubated nitrifying activated sludge with (13)C-labeled bicarbonate and used stable isotope probing of 16S rRNA to monitor the flow of carbon from uncultured nitrifiers to heterotrophs. To facilitate the identification of heterotrophs, the abundant 16S rRNA molecules from nitrifiers were depleted by catalytic oligonucleotides containing locked nucleic acids (LNAzymes), which specifically cut the 16S rRNA of defined target organisms. Among the (13)C-labeled heterotrophs were organisms remotely related to Micavibrio, a microbial predator of Gram-negative bacteria. Fluorescence in situ hybridization revealed a close spatial association of these organisms with microcolonies of nitrite-oxidizing sublineage I Nitrospira in sludge flocs. The high specificity of this interaction was confirmed by confocal microscopy and a novel image analysis method to quantify the localization patterns of biofilm microorganisms in three-dimensional (3-D) space. Other isotope-labeled bacteria, which were affiliated with Thermomonas, colocalized less frequently with nitrifiers and thus were commensals or saprophytes rather than specific symbionts or predators. These results suggest that Nitrospira spp. are subject to bacterial predation, which may influence the abundance and diversity of these nitrite oxidizers and the stability of nitrification in engineered and natural ecosystems. In silico screening of published next-generation sequencing data sets revealed a broad environmental distribution of the uncultured Micavibrio-like lineage.

Simulation of the performance of aerobic granular sludge SBR using modified ASM3 model

The activated sludge model No. 3 (ASM3) was modified to describe the biological reactions in aerobic granular sludge SBR. The simultaneous storage and growth, nitrification and denitrification were all accounted for in modified model. The sensitivities of effluent COD, NH(4)(+) -N, and TN toward the stoichiometric and kinetic coefficients were analyzed. A standard set of parameters obtained from a combination of literature data was chosen for the model. The experimental results for the time profile of COD, NH(4)(+) -N, and TN in a typical cycle were used to verify the ASM3 model. The verification results show the model established is applicable for simulating the perfo rmance of an aerobic granule-based SBR. A comparison of the measured and predicted values of substrate removal for both the modified ASM3 and the original ASM3 was also performed. The verification and comparison results show the modified ASM3 model describes the aerobic granule-based SBR better and more mechanistically.

Effect of pH, substrate and free nitrous acid concentrations on ammonium oxidation rate

Respirometric techniques have been used to determine the effect of pH, free nitrous acid (FNA) and substrate concentration on the activity of the ammonium oxidizing bacteria (AOB) present in an activated sludge reactor. With this aim, bacterial activity has been measured at different pH values (ranging from 6.2 to 9.7), total ammonium nitrogen concentrations (ranging from 0.1 to 10 mg TAN L(-1)) and total nitrite concentrations (ranging from 3 to 43 mg NO(2)-NL(-1)). According to the results obtained, the most appropriate kinetic expression for the growth of AOB in activated sludge reactors has been established. Substrate half saturation constant and FNA and pH inhibition constants have been obtained by adjusting model predictions to experimental results. Different kinetic parameter values and different Monod terms should be used to model the growth of AOB in activated sludge processes and SHARON reactors due to the different AOB species that predominate in both systems.

Evaluating the solid retention time of bacteria in flocculent and granular sludge

The specific solid retention time for different bacteria within flocculent and granular sludge was determined. Samples were collected from reactor and effluent sludge and the number of a specific bacterial group was evaluated in respect to the total bacterial community with quantitative polymerase chain reaction (qPCR). The ratio of the relative presence of a specific bacterial group in the reactor sludge and wasted sludge was established to observe if preferential wash-out occurred. From the data also the solid retention time for different microbial groups can be estimated. Using this tool, we were able to show that the SRT of populations found on the exterior of granules is slightly lower than the SRT for population in the interior. Archaea were not found in the flocculent system but were present in small amounts within the granular system. It was further observed that protozoa were grazing on the bacterial community within the system indicating that they have the potential to shorten the specific SRT of bacteria.

Bioreactor function under perturbation scenarios is affected by interactions between bacteria and protozoa

This study investigated the impact of transient cadmium perturbations on the structure and function of the microbial community in an activated sludge system. The impact of cadmium perturbation on the bioreactor performance, bacterial activity, bacterial community structure, and bacteria-protozoa interactions was examined. The bacterial community exhibited a short-term inhibition following a pulse perturbation of cadmium. Process recovery was associated with an increase in bacterial abundance above the unperturbed control reactor, followed by high biomass activity after the washout of cadmium. This trend was seen for multiple experiments at both laboratory- and pilot-scale. The increase in biomass activity could not be explained by changes in bacterial community structure. Independent experiments showed that the increase in bacterial abundance, and by association biomass activity, was caused by the decrease in the protozoal grazing due to the higher inhibition of ciliated protozoa as compared to bacteria when exposed to cadmium. This paper highlights the importance of expanding the investigative boundaries of the microbial ecology of bioengineered systems to include protozoal grazing, especially under perturbation scenarios.

Effect of different operational conditions on biofilm development, nitrification, and nitrifying microbial population in moving-bed biofilm reactors

In this study, the effect of different operational conditions on biofilm development and nitrification in three moving-bed biofilm reactors (MBBRs) was investigated: two reactors were operated in a continuously fed regime and one in sequencing-batch mode. The presence of organic carbon reduced the time required to form stable nitrifying biofilms. Subsequent stepwise reduction of influent COD caused a decrease in total polysaccharide and protein content, which was accompanied by a fragmentation of the biofilm, as shown by scanning electron microscopy, and by an enrichment of the biofilm for nitrifiers, as observed by fluorescent in situ hybridization (FISH) analysis. Polysaccharide and protein concentrations proved to be good indicators of biomass development and detachment in MBBR systems. Ammonium- and nitrite-oxidizing bacteria activities were affected when a pulse feeding of 4 g of NH(4)-N/(m(2)·day) was applied. Free nitrous acid and free ammonia were likely the inhibitors for ammonium- and nitrite-oxidizing bacteria.