Water Research

Modelling nitrification, heterotrophic growth and predation in activated sludge

Authors: Moustaf Samir Moussa, Christine M. Hooijmans, H J Lubberding, Huub J. Gijzen, M C M van Loosdrecht

Abstract

A mathematical model describing the interaction between nitrifiers, heterotrophs and predators in wastewater treatment has been developed. The inclusion of a predation mechanism is a new addition to the existing activated sludge models. The developed model considered multi-substrate consumption and multi-species growth, maintenance and decay in a culture where nitrifiers, heterotrophs and predators (protozoa and metazoa) are coexisting. Two laboratory-scale sequenced batch reactors (SBRs) operated at different sludge retention time (SRT) of 30 and 100 days for a period of 4 years were used to calibrate and validate the model. Moreover, to assess the predator activity, a simple procedure was developed, based on measuring the respiration rate with and without the presence of the predators. The model successfully described the performance of two SBRs systems. The fraction of active biomass (ammonia oxidisers, nitrite oxidisers and heterotrophs) predicted by the proposed model was only 33% and 14% at SRT of 30 and 100 days, respectively. The high fraction of inert biomass predicted by the model was in accordance with the microscopic investigations of biomass viability in both reactors. The presented model was used to investigate the effect of increasing sludge age and the role of predators on the biomass composition of the tested SBR system.

Objective

Q: What problem does this paper attempt to solve?

Kimi: The paper "Modelling nitrification, heterotrophic growth and predation in activated sludge" primarily focuses on the interactions between nitrification, heterotrophic growth, and predation within activated sludge systems, and attempts to describe these interactions through the development of a mathematical model. Specifically, the paper aims to address the following issues:

1. Interaction between nitrification and heterotrophic microorganisms: Nitrifying bacteria and heterotrophic microorganisms play crucial roles in the treatment of nitrogen and organic matter in wastewater. The paper describes the interactions between these microbial communities and how they collectively affect the efficiency of the system through a model.

2. Introduction of predation mechanisms: Traditional activated sludge models often do not consider the impact of predators. This paper attempts to provide a more comprehensive understanding of how predation behavior affects the activity and biomass composition of nitrifying and heterotrophic bacteria by introducing predation mechanisms into the model.

3. Multi-substrate consumption and multi-species growth, maintenance, and decay: The model in the paper considers the consumption of multiple substrates and the growth, maintenance, and decay of multiple microbial populations. This multi-substrate and multi-species approach helps to more accurately simulate actual activated sludge systems.

4. The role of sludge age (SRT) on biomass composition: The paper explores how different sludge ages (i.e., the residence time of sludge in the system) affect the composition of active biomass (including nitrifying bacteria, heterotrophic bacteria, and predators) and the accumulation of inert biomass through the model.

5. Impact of predators on nitrification and heterotrophic activity: The paper also investigates the influence of predators on the presence and activity of nitrifying and heterotrophic bacteria, which is crucial for understanding the role of predators in activated sludge systems.

6. Application of laboratory-scale sequencing batch reactors (SBRs): To calibrate and validate the model, the researchers used two laboratory-scale sequencing batch reactors operating under different sludge age conditions. The model was able to successfully describe the performance of the SBR systems and predict the proportion of active biomass.

In summary, this paper attempts to enhance our understanding of microbial interactions within activated sludge systems, especially considering the role of predators, and how to optimize sludge age and improve the overall treatment efficiency of the system. This has significant practical implications for the design and optimization of wastewater treatment processes.

Related

Q: What are the relevant studies?

Kimi: The study of interactions between nitrification, heterotrophic growth, and predation in activated sludge is a complex field with numerous related research efforts. Here are some areas and specific studies that are relevant to the topic:

1. Activated Sludge Modeling:

   • ASM models (Henze et al., 2000) are a series of mathematical models that describe the biological processes in activated sludge systems. These models have been continuously developed and improved upon by various researchers to better represent the complex interactions within the system.

2. Microbial Ecology in Wastewater Treatment:

   • Curds and Cockburn (1970) conducted early studies on the role of protozoa in activated sludge systems, highlighting their importance in effluent quality and solids removal.
   • Martin-Cereceda et al. (1996) and Salvadó et al. (1995) have used ciliated protozoa as bio-indicators of process performance in wastewater treatment.

3. Nitrification and Denitrification:

   • Hellinga et al. (1998, 1999) developed the SHARON process, an innovative method for nitrogen removal from ammonia-rich wastewater, focusing on the control of nitrification.
   • Brouwer et al. (1998) studied the feed-forward control of nitrification by manipulating the aerobic volume in activated sludge plants.

4. Predation and Its Impact on Microbial Communities:

   • Curds (1971a, b, 1973) conducted theoretical and simulation studies on predator-prey relationships in activated sludge systems.
   • Lee and Welander (1994) investigated the influence of predators on nitrification in aerobic biofilm processes.

5. Biomass and Sludge Age Studies:

   • Pollice et al. (2002) studied the influence of aeration and sludge retention time on ammonia oxidation to nitrite and nitrate.
   • Ratsak et al. (1996) researched the effects of protozoa on carbon mineralization in activated sludge.

6. Mathematical and Computational Modeling:

   • Smolders et al. (1994) developed stoichiometric models for the aerobic metabolism in biological phosphorus removal processes.
   • Meijer et al. (2001) worked on metabolic modeling of full-scale biological nitrogen and phosphorus removing WWTPs.

7. Laboratory and Full-Scale Studies:

   • Tijhuis and van Loosdrecht (1994) studied the solid retention time in spherical biofilms in a biofilm airlift suspended reactor.
   • van Dongen et al. (2001) explored the Combined Sharon/Anammox Process as a sustainable method for nitrogen removal from sludge water.

These studies, among many others, contribute to the broader understanding of biological processes in wastewater treatment, with a particular focus on the interactions between different microbial groups and the environmental conditions that influence their activities. The research presented in the paper "Modelling nitrification, heterotrophic growth and predation in activated sludge" builds upon this foundation, integrating the role of predation into the modeling framework to provide a more comprehensive view of activated sludge system dynamics.

Solution

Q: How the paper solves this problem?

Kimi: This paper addresses the issue of interactions between nitrification, heterotrophic growth, and predation in activated sludge systems through the following key steps:

1. Development of a Mathematical Model: The researchers developed a mathematical model to describe the interactions between nitrifying bacteria, heterotrophic microorganisms, and predators (such as protozoa and metazoa). This model considers multi-substrate consumption and the growth, maintenance, and decay of multiple species.

2. Experimental Validation: To calibrate and validate this model, the researchers used two laboratory-scale Sequencing Batch Reactors (SBRs) operated under different Sludge Retention Times (SRTs). By running these reactors over a long period and collecting data, the researchers were able to adjust the model parameters to ensure that the model accurately predicts the performance of actual systems.

3. Analysis of Microbial Community Structure: Through the model, the researchers analyzed the impact of different SRTs on the composition of active biomass (nitrifying bacteria, heterotrophic bacteria, and predators) and the accumulation of inert biomass. This helps to understand how SRT affects the microbial ecology and treatment efficiency of the system.

4. Assessment of Predators' Role: The researchers assessed the role of predators in the system through the model, especially the changes in the fraction of active biomass in the absence of predators. This helps to understand the impact of predators on nitrification and heterotrophic processes and explores the possibility of optimizing system performance by controlling predators.

5. Model Application and Optimization: The researchers used the model to simulate and predict the performance of the system under different operational conditions, including the performance of reactors at different SRTs. This provides a theoretical basis for designing and optimizing actual wastewater treatment systems.

Through these methods, the paper not only provides a theoretical framework for understanding the complex microbial interactions in activated sludge systems but also offers a tool that can help environmental engineers and researchers optimize wastewater treatment processes to improve system efficiency and stability.

Experiment

Q: What experiments were done for the paper?

Kimi: The paper "Modelling nitrification, heterotrophic growth and predation in activated sludge" conducted the following experiments to support the development and validation of the mathematical model:

1. Laboratory-Scale Sequencing Batch Reactors (SBRs): The researchers operated two SBRs at different Sludge Retention Times (SRTs) of 30 and 100 days for a period of 4 years. These reactors were used to simulate real-world conditions and to collect data for model calibration and validation.

2. Continuous Operation and Monitoring: The SBRs were automatically operated in cycles that included a fill period, reaction period, settling time, and effluent discharge. The researchers continuously monitored parameters such as dissolved oxygen (DO), pH, and temperature, and controlled the pH using automated systems.

3. Substrate Addition and Biomass Analysis: A synthetic medium containing ammonia and nutrients was used to feed the SBRs. The mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) were measured, along with ammonia (NH4+), nitrite (NO2-), and nitrate (NO3-) concentrations.

4. Respiration Activity Assay: The oxygen uptake rate (OUR) was measured using a biological oxygen monitor (BOM) to estimate the activity and viability of the biomass. This involved injecting different substrates (NO2, NH4, COD) into the reaction chamber and monitoring the decrease in oxygen concentration.

5. Determination of Predators' Activity: The activity of predators was determined by measuring the respiration under starvation conditions with and without the presence of predators. A salt shock (NaCl) was used to eliminate predators and measure the impact on the remaining biomass.

6. Analytical Procedures: Various analytical methods were employed to measure the concentrations of nitrogen species and organic matter. This included spectrophotometry, ion chromatography, and filtration techniques for MLSS and MLVSS determination.

7. Staining Techniques (Live-Dead Stain): Molecular Probes' LIVE/DEAD BacLightTM Bacterial Viability Kits were used to discriminate between viable cells and dead cells in the microbial community.

8. Model Calibration and Validation: The collected data from the SBRs were used to calibrate the model parameters, such as the predation rate, decay rate of predators, and the fraction of inert COD generated in biomass lysis. The calibrated model was then validated against the observed performance of the SBRs.

These experiments provided a comprehensive dataset that allowed the researchers to develop a detailed understanding of the interactions between different microbial groups in the activated sludge system and to validate the mathematical model's predictions against real-world observations.

Exploration

Q: What are the points that can be explored further?

Kimi: Although the paper "Modelling nitrification, heterotrophic growth and predation in activated sludge" provides an in-depth understanding of the interactions between nitrification, heterotrophic growth, and predation in activated sludge systems, there are several areas that can be further explored:

1. Regional Adaptability of Model Parameters: Researchers may need to validate and adjust model parameters in different geographical regions and types of wastewater treatment plants to ensure the broad applicability and accuracy of the model.

2. In-depth Study of Predator Behavior: Further research could investigate how predators behave under different environmental conditions, such as varying temperatures, pH values, and pollutant concentrations.

3. Microbial Community Dynamics: High-throughput sequencing and other molecular biology techniques could be used to study microbial diversity and community dynamics in activated sludge in more detail, as well as how they respond to the presence of predators and changes in operating conditions.

4. System Stability and Robustness: The impact of different operational strategies (such as cycle times, substrate concentrations, aeration volumes, etc.) on system stability and robustness could be studied, along with how to optimize these strategies to improve overall system performance.

5. Economic and Environmental Sustainability: Assess the long-term impact of process improvements guided by the model on economic costs and environmental sustainability, including energy consumption, greenhouse gas emissions, and resource recovery.

6. Integration with Other Biological Processes: Explore the possibility of integrating the nitrification, heterotrophic growth, and predation model with other biological processes (such as anaerobic digestion, biological phosphorus removal, etc.) to simulate more complex wastewater treatment systems.

7. Practical Application of the Model: Investigate how to apply the model to full-scale wastewater treatment plants and how to combine model predictions with actual operational data for more precise process control and optimization.

8. Design of Novel Bioreactors: Use insights from the model to guide the design of novel bioreactors that can more effectively promote the interactions between nitrification, heterotrophic growth, and predation, thereby improving wastewater treatment efficiency.

Through these further explorations, researchers can continue to enhance their understanding and control of the complexity of activated sludge systems, contributing more significantly to the development of wastewater treatment technologies and environmental protection.

Summary

Q: Summarize the main points of the paper.

Kimi: The paper "Modelling nitrification, heterotrophic growth and predation in activated sludge" presents a comprehensive study on the interactions between different microbial groups within activated sludge systems, focusing on nitrifiers, heterotrophs, and predators such as protozoa and metazoa. Here is a summary of the main content of the paper:

1. Objective: The paper aims to develop a mathematical model that describes the complex interactions between nitrifiers, heterotrophs, and predators in wastewater treatment. The model is intended to improve the understanding of how these organisms coexist and influence the performance of activated sludge systems.

2. Model Development: A dynamic mathematical model is developed, which includes six soluble compounds and five types of biomass as particulate compounds. The model considers multi-substrate consumption, multi-species growth, maintenance, and decay, incorporating a predation mechanism as a new aspect to the existing activated sludge models.

3. Experimental Setup: Two laboratory-scale Sequencing Batch Reactors (SBRs) with different Sludge Retention Times (SRTs) of 30 and 100 days were used to calibrate and validate the model. The SBRs were operated for four years, and various parameters such as dissolved oxygen, pH, and concentrations of nitrogen species were monitored.

4. Model Calibration and Validation: The model was calibrated using data from the SBRs and validated against the performance of a second reactor operated at a different SRT. The model successfully described the performance of the SBR systems and predicted the fraction of active biomass.

5. Predator Activity Assessment: A simple procedure was developed to assess predator activity based on measuring the respiration rate with and without the presence of predators. The use of salt (NaCl) as a selective agent to eliminate predators and observe the immediate effects on the system was also explored.

6. Key Findings: The model predicted a high fraction of inert biomass in both reactors, in line with microscopic investigations. It also showed the impact of increasing sludge age on the biomass composition of the SBR system and the role of predators in controlling the active biomass fraction.

7. Conclusions: The paper concludes that the developed model is a valuable tool for understanding the biological processes in activated sludge systems and can help optimize the operation of such systems. The model demonstrates the importance of considering predators in the design and management of wastewater treatment plants.

Overall, the paper contributes to the field of environmental engineering by providing a detailed model that can be used to better understand and optimize the biological treatment processes in wastewater treatment facilities.

More

These are just a few basic FAQs for understanding a paper with Kimi. if you would like to discuss the paper further with it, click here to jump to the Kimi web version and start a new session related to the paper.