4.1 Nitrogen removal processes

Nitrogen removal in wastewater treatment involves two key processes: nitrification and denitrification. These processes convert harmful ammonia to harmless nitrogen gas, improving water quality and protecting aquatic ecosystems.

Efficient nitrogen removal depends on factors like dissolved oxygen, temperature, pH, and carbon-to-nitrogen ratio. Various treatment methods exist, from conventional systems to innovative approaches like simultaneous nitrification-denitrification and partial nitritation-anammox, each with unique advantages and applications.

Nitrogen Removal Principles

Principles of nitrification and denitrification

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• Nitrification
  • Aerobic process conducted by autotrophic bacteria converts ammonia to nitrate
  • Ammonia ($NH_4^+$) oxidized to nitrite ($NO_2^-$) by ammonia-oxidizing bacteria (AOB) (Nitrosomonas)
  • Nitrite further oxidized to nitrate ($NO_3^-$) by nitrite-oxidizing bacteria (NOB) (Nitrobacter)
  • Requires sufficient dissolved oxygen (DO) (>2 mg/L) and alkalinity (7.14 mg CaCO₃ per mg NH₄⁺-N oxidized)
• Denitrification
  • Anoxic process conducted by heterotrophic bacteria reduces nitrate to nitrogen gas
  • Nitrate ($NO_3^-$) reduced to nitrogen gas ($N_2$) using organic carbon as electron donor (methanol, acetate)
  • Occurs in absence of DO (<0.5 mg/L) and requires carbon source (C/N ratio of 4-6 g COD/g N)
  • Produces alkalinity (3.57 mg CaCO₃ per mg NO₃⁻-N reduced) and reduces overall nitrogen content in wastewater

Factors in nitrogen removal efficiency

• Dissolved oxygen (DO)
  • Nitrification requires minimum DO of 2 mg/L, higher levels improve rates but increase energy consumption
  • Denitrification requires anoxic conditions with DO <0.5 mg/L to prevent inhibition
• Temperature
  • Optimal temperature range for nitrification is 25-35°C, rates decrease significantly below 15°C
  • Denitrification less sensitive to temperature but performs best at 20-30°C
• pH
  • Optimal pH range for nitrification is 7.5-8.5, rates decrease significantly at pH <6.5 or >9.0
  • Denitrification less sensitive to pH but performs best at pH 7.0-8.0
• Carbon-to-nitrogen ratio (C/N)
  • Denitrification requires sufficient carbon source, typically expressed as C/N ratio
  • Optimal C/N ratio for denitrification is 4-6 g COD/g N, lower ratios may limit nitrate reduction

Nitrogen Removal Processes and Design

Comparison of nitrogen removal processes

• Conventional nitrification-denitrification
  • Two-stage process with separate aerobic and anoxic zones
  • Requires internal recirculation of nitrate from aerobic to anoxic zone
  • Can achieve high nitrogen removal efficiencies (>90%) but requires larger footprint and higher energy consumption
• Simultaneous nitrification-denitrification (SND)
  • Occurs within same reactor under low DO conditions (0.5-1.5 mg/L)
  • Utilizes oxygen gradient within microbial flocs or biofilms
  • Requires less space and energy compared to conventional process but may have lower removal efficiencies (70-80%)
• Partial nitritation-anammox (PN/A)
  • Two-stage autotrophic nitrogen removal process
  • Partial nitritation converts ammonia to nitrite under aerobic conditions
  • Anaerobic ammonium oxidation (anammox) converts nitrite and remaining ammonia directly to nitrogen gas
  • Requires less oxygen and no external carbon source compared to conventional processes
  • Suitable for high-strength, low-carbon wastewater (sludge digester effluent, landfill leachate)

Design of nitrogen removal systems

• Influent characterization
  • Determine influent nitrogen concentrations (ammonia, nitrite, nitrate) and C/N ratio
  • Assess readily biodegradable COD (rbCOD) content and temperature/pH variations
• Process selection
  • Choose suitable nitrogen removal process based on wastewater characteristics and treatment goals
  • Consider factors such as available space, energy consumption, and effluent quality requirements
• Design parameters
  1. Determine required solids retention time (SRT) for nitrification based on temperature and target effluent ammonia
  2. Calculate aerobic and anoxic volume fractions based on selected process and influent C/N ratio
  3. Size reactors and clarifiers based on design flow rate and selected SRT
• Aeration and mixing
  • Design aeration system to maintain required DO levels in aerobic zones (fine bubble diffusers, mechanical aerators)
  • Ensure adequate mixing in anoxic zones to promote denitrification and prevent settling (submersible mixers, recirculation pumps)
• Recirculation and carbon dosing
  • Determine internal recirculation rate based on influent nitrogen load and target effluent nitrate
  • Consider external carbon dosing if influent rbCOD is insufficient for complete denitrification (methanol, glycerol)
• Process control and optimization
  • Implement online monitoring and control strategies for key parameters (DO, pH, ORP)
  • Adjust operational parameters (SRT, recirculation rates, carbon dosing) based on performance data to optimize nitrogen removal efficiency and minimize energy consumption