🚰Advanced Wastewater Treatment Unit 4 – Biological Nutrient Removal

Key Concepts and Principles

• Biological nutrient removal (BNR) aims to remove nitrogen and phosphorus from wastewater through microbial processes
• BNR relies on creating specific environmental conditions that promote the growth of desired microorganisms
  • Alternating aerobic and anoxic/anaerobic zones in the treatment process
  • Providing suitable carbon sources for microbial growth
• Nitrification converts ammonia to nitrate under aerobic conditions by nitrifying bacteria (Nitrosomonas and Nitrobacter)
• Denitrification reduces nitrate to nitrogen gas under anoxic conditions by denitrifying bacteria (Pseudomonas and Paracoccus)
• Enhanced biological phosphorus removal (EBPR) encourages phosphorus accumulating organisms (PAOs) to uptake and store excess phosphorus
  • PAOs release phosphorus under anaerobic conditions and uptake it under aerobic conditions
• Carbon to nitrogen (C/N) ratio plays a crucial role in BNR efficiency
  • Sufficient readily biodegradable carbon sources are required for effective denitrification and EBPR
• Sludge age or solids retention time (SRT) affects the growth and selection of desired microorganisms in BNR systems

Biological Processes Involved

• Nitrification is a two-step aerobic process carried out by autotrophic bacteria
  • Ammonia-oxidizing bacteria (AOB) convert ammonia to nitrite ($NH_4^+ + 1.5O_2 \rightarrow NO_2^- + 2H^+ + H_2O$)
  • Nitrite-oxidizing bacteria (NOB) convert nitrite to nitrate ($NO_2^- + 0.5O_2 \rightarrow NO_3^-$)
• Denitrification is an anoxic process performed by heterotrophic bacteria that reduces nitrate to nitrogen gas
  • Occurs in the absence of dissolved oxygen and presence of organic carbon source
  • Nitrate serves as an electron acceptor for microbial respiration ($NO_3^- \rightarrow NO_2^- \rightarrow NO \rightarrow N_2O \rightarrow N_2$)
• Enhanced biological phosphorus removal (EBPR) is carried out by phosphorus accumulating organisms (PAOs)
  • PAOs release phosphorus under anaerobic conditions and uptake excess phosphorus under aerobic conditions
  • Phosphorus is removed from the system through waste activated sludge (WAS)
• Anaerobic ammonia oxidation (Anammox) is an autotrophic process that converts ammonia and nitrite directly to nitrogen gas
  • Anammox bacteria (Candidatus Brocadia and Kuenenia) perform this process under anoxic conditions ($NH_4^+ + NO_2^- \rightarrow N_2 + 2H_2O$)
• Simultaneous nitrification-denitrification (SND) can occur in systems with low dissolved oxygen concentrations
  • Nitrification and denitrification happen concurrently in the same reactor due to oxygen concentration gradients

Nutrient Removal Mechanisms

• Assimilation is the uptake of nutrients by microorganisms for cell growth and reproduction
  • Nitrogen and phosphorus are incorporated into the biomass
  • Nutrients are removed from the system through waste activated sludge (WAS) disposal
• Nitrification converts ammonia to nitrate, which can be subsequently removed through denitrification
  • Nitrification requires aerobic conditions and sufficient alkalinity
  • Nitrifying bacteria have slower growth rates compared to heterotrophic bacteria
• Denitrification reduces nitrate to nitrogen gas, effectively removing nitrogen from the wastewater
  • Denitrification requires anoxic conditions and a readily biodegradable carbon source
  • Internal carbon sources (e.g., primary effluent, fermented sludge) or external carbon sources (e.g., methanol, acetate) can be used
• Enhanced biological phosphorus removal (EBPR) relies on the ability of PAOs to store excess phosphorus as polyphosphate
  • PAOs release phosphorus under anaerobic conditions and uptake it under aerobic conditions
  • Phosphorus is removed from the system through the disposal of phosphorus-rich waste activated sludge
• Precipitation is a chemical process that can complement biological nutrient removal
  • Chemical additives (e.g., ferric chloride, alum) can precipitate phosphorus as insoluble compounds
  • Precipitation is often used as a polishing step after biological treatment

System Design and Configuration

• Plug-flow configuration is commonly used in BNR systems
  • Wastewater flows through a series of reactors or zones with different environmental conditions
  • Typical sequence: anaerobic, anoxic, aerobic, and clarification zones
• Modified Ludzack-Ettinger (MLE) process is a popular BNR configuration for nitrogen removal
  • Consists of an anoxic zone followed by an aerobic zone with internal mixed liquor recirculation
  • Denitrification occurs in the anoxic zone using nitrate from the aerobic zone
• A2O (Anaerobic-Anoxic-Oxic) process is designed for both nitrogen and phosphorus removal
  • Includes an anaerobic zone for phosphorus release, followed by anoxic and aerobic zones for denitrification and nitrification
  • Nitrate recirculation from the aerobic to the anoxic zone enhances denitrification
• Bardenpho process is an advanced BNR configuration with a 4-stage or 5-stage layout
  • 4-stage: anaerobic, anoxic, aerobic, and second anoxic zones
  • 5-stage: anaerobic, anoxic, aerobic, second anoxic, and re-aeration zones
  • Provides higher nitrogen and phosphorus removal efficiencies
• Sequencing batch reactors (SBRs) can be used for BNR in a single reactor with alternating phases
  • Fill, react (anaerobic, anoxic, aerobic), settle, decant, and idle phases
  • Flexibility in operation and control of environmental conditions

Operational Parameters and Control

• Dissolved oxygen (DO) concentration is a critical parameter in BNR systems
  • Aerobic zones require sufficient DO (>2 mg/L) for nitrification and phosphorus uptake
  • Anoxic zones should maintain low DO (<0.5 mg/L) to promote denitrification
• pH affects the growth and activity of nitrifying and phosphorus accumulating organisms
  • Optimal pH range for nitrification is between 7.5 and 8.5
  • EBPR performs best at a pH range of 7.0 to 8.0
• Temperature influences microbial growth rates and reaction kinetics
  • Nitrification and denitrification rates decrease at lower temperatures (<10°C)
  • EBPR performance can deteriorate at temperatures below 15°C
• Sludge age or solids retention time (SRT) should be controlled to maintain desired microbial populations
  • Longer SRTs (>8 days) favor the growth of slow-growing nitrifying bacteria
  • SRT of 3-5 days is suitable for EBPR to prevent excessive growth of glycogen accumulating organisms (GAOs)
• Recirculation rates (e.g., internal mixed liquor, nitrate, RAS) affect the distribution of substrates and microorganisms
  • Internal mixed liquor recirculation (IMLR) from the aerobic to the anoxic zone enhances denitrification
  • Nitrate recirculation (NARCY) from the aerobic to the anoxic zone provides nitrate for denitrification
• Carbon to nitrogen (C/N) ratio should be optimized for effective BNR
  • Sufficient readily biodegradable carbon source is required for denitrification and EBPR
  • External carbon sources (e.g., methanol, acetate) can be added if internal carbon is limited

Performance Monitoring and Optimization

• Influent and effluent characterization is essential for assessing BNR performance
  • Monitor parameters such as COD, BOD, TKN, ammonia, nitrate, nitrite, total phosphorus, and orthophosphate
  • Evaluate removal efficiencies and compliance with discharge standards
• Online sensors and real-time monitoring systems can provide continuous data for process control
  • DO, pH, ORP (oxidation-reduction potential), and nutrient sensors
  • Feedback and feedforward control strategies can be implemented based on real-time data
• Microscopic examination of activated sludge can reveal the presence and abundance of key microbial populations
  • Nitrifying bacteria, PAOs, GAOs, and filamentous organisms
  • Regular monitoring helps in identifying process issues and optimizing operating conditions
• Batch tests and respirometric assays can be conducted to assess the activity of specific microbial groups
  • Nitrification inhibition tests, denitrification potential tests, and EBPR batch tests
  • Results can be used to fine-tune process parameters and troubleshoot performance issues
• Mass balances and modeling tools can assist in process optimization and scenario analysis
  • Steady-state and dynamic models (e.g., activated sludge models, ASM1, ASM2d, ASM3)
  • Predict the impact of operational changes and evaluate alternative control strategies

Challenges and Troubleshooting

• Insufficient carbon source can limit denitrification and EBPR performance
  • Evaluate the C/N ratio and consider adding external carbon sources if necessary
  • Implement carbon management strategies (e.g., primary sludge fermentation, high-rate anaerobic digestion)
• Nitrite accumulation can occur due to incomplete nitrification or denitrification
  • Adjust DO levels, pH, and SRT to promote complete nitrification
  • Ensure sufficient anoxic volume and carbon source for complete denitrification
• Competing microbial populations, such as glycogen accumulating organisms (GAOs), can hinder EBPR performance
  • Control the anaerobic contact time, pH, and temperature to favor PAO growth over GAOs
  • Implement a longer aerobic phase to promote PAO growth and phosphorus uptake
• Filamentous bulking can cause poor sludge settleability and clarifier performance
  • Identify the dominant filamentous species and their growth conditions
  • Adjust DO levels, nutrient ratios, and SRT to control filamentous growth
• Struvite (magnesium ammonium phosphate) precipitation can cause operational issues, such as pipe clogging
  • Control the pH and magnesium concentrations in the system
  • Implement struvite recovery processes to remove precipitates and recover nutrients
• Toxic compounds and inhibitory substances can affect the activity of nitrifying and phosphorus accumulating organisms
  • Identify potential sources of toxicity (e.g., industrial discharges, pesticides)
  • Implement pretreatment strategies or source control measures to minimize the impact on BNR processes

Emerging Technologies and Future Trends

• Mainstream partial nitritation-anammox (PN/A) processes aim to reduce energy and carbon requirements for nitrogen removal
  • Combine partial nitritation and anammox in the main wastewater treatment line
  • Challenges include maintaining stable partial nitritation and anammox activity under variable conditions
• Granular sludge systems, such as aerobic granular sludge (AGS), offer compact and efficient BNR
  • Granules contain layered microbial communities that perform simultaneous nitrification, denitrification, and phosphorus removal
  • Advantages include smaller footprint, reduced energy consumption, and improved settleability
• Microalgae-based systems can integrate nutrient removal with bioenergy production and CO2 sequestration
  • Microalgae assimilate nitrogen and phosphorus from wastewater while producing biomass for biofuels or high-value products
  • Challenges include harvesting, downstream processing, and maintaining stable algal growth
• Resource recovery from BNR systems is gaining attention for sustainable wastewater management
  • Struvite precipitation for phosphorus recovery as a slow-release fertilizer
  • Biogas production from anaerobic digestion of waste activated sludge
  • Integrated systems for simultaneous nutrient recovery and energy generation
• Advanced control strategies and data-driven approaches are being developed for optimizing BNR performance
  • Model predictive control (MPC) for real-time optimization of process parameters
  • Machine learning and artificial intelligence techniques for fault detection, diagnosis, and process control
  • Integration of sensors, online monitoring, and advanced analytics for improved decision-making