5.1 Anammox process principles and microbiology

The Anammox process is a game-changer in wastewater treatment. It converts ammonium to nitrogen gas using nitrite, all without oxygen. This anaerobic method involves unique bacteria and specialized biochemical reactions, making it more efficient than traditional nitrification-denitrification.

Anammox bacteria are slow-growing and oxygen-sensitive, thriving in specific conditions. They require anoxic environments, optimal temperatures, and pH levels. Compared to conventional processes, Anammox needs less space and resources, producing less sludge and cutting operational costs.

Anammox Process Principles

Biochemical reactions of Anammox

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• Anammox converts $NH_4^+$ directly to $N_2$ using $NO_2^-$ as an electron acceptor under anaerobic conditions
  • Overall reaction: $NH_4^+ + NO_2^- \rightarrow N_2 + 2H_2O$
• Three main reactions involved:
  1. Nitrite reduction to nitric oxide ($NO$) catalyzed by nitrite reductase enzyme
  • $NO_2^- + 2H^+ + e^- \rightarrow NO + H_2O$
  1. Hydrazine ($N_2H_4$) synthesis from $NO$ and $NH_4^+$ catalyzed by hydrazine synthase enzyme
  • $NO + NH_4^+ + 2H^+ + 3e^- \rightarrow N_2H_4 + H_2O$
  1. Hydrazine oxidation to $N_2$ catalyzed by hydrazine dehydrogenase enzyme
  • $N_2H_4 \rightarrow N_2 + 4H^+ + 4e^-$
• Electrons released during hydrazine oxidation used for cell carbon fixation and nitrite reduction
• Anammox process occurs in a specialized organelle called the anammoxosome

Microbial species in Anammox

• Anammox bacteria belong to the phylum Planctomycetes and the order "Candidatus Brocadiales"
  • Five genera identified: "Candidatus Brocadia", "Candidatus Kuenenia", "Candidatus Scalindua", "Candidatus Anammoxoglobus", and "Candidatus Jettenia"
• Unique characteristics of Anammox bacteria:
  • Slow growth rate with doubling time of 10-14 days
  • Optimal growth temperature between 30-40℃
  • Sensitive to oxygen and require anoxic conditions ($O_2$ < 0.1 mg/L)
  • Possess anammoxosome organelle where Anammox reactions occur
  • Contain ladderane lipids in cell membrane to maintain proton gradient for energy production

Anammox Process Conditions and Comparison

Environmental conditions for Anammox

• Anoxic conditions with dissolved oxygen concentration < 0.1 mg/L
• Temperature range: 30-40℃, optimum around 35℃
• pH range: 6.7-8.3, optimum around 8.0
• Sufficient $NH_4^+$ and $NO_2^-$ concentrations
  • Ideal $NO_2^-$:$NH_4^+$ ratio of 1.32:1 based on Anammox reaction stoichiometry
• Low organic carbon content to prevent growth of competing heterotrophic denitrifiers
• Adequate retention of Anammox biomass due to slow growth rate
  • Use of biofilm, granular, or carrier-based systems to retain biomass

Anammox vs conventional processes

• Conventional nitrification-denitrification:
  • Two-step process: aerobic nitrification followed by anoxic denitrification
  • Requires $O_2$ for nitrification and organic carbon for denitrification
  • Produces $NO_3^-$ as an intermediate
  • Higher sludge production due to growth of nitrifiers and heterotrophic denitrifiers
• Anammox process:
  • One-step process: directly converts $NH_4^+$ to $N_2$ using $NO_2^-$
  • Operates under anoxic conditions without need for $O_2$ or organic carbon
  • No $NO_3^-$ production
  • Lower sludge production due to slow growth rate of Anammox bacteria
  • Requires smaller footprint and has lower operational costs compared to conventional processes