Pilot-scale demonstration of self-enrichment of anammox bacteria in a two-stage nitrification-denitrification suspended sludge system treating municipal wastewater under extremely low nitrogen loading rate

In suspended sludge system, efficient enrichment and retention of anammox bacteria are crucial obstacles in mainstream wastewater treatment by anammox process. In this study, anammox bacteria was self-enriched in a pilot-scale suspended sludge system of two-stage nitrification–denitrification process serving municipal wastewater treatment. With the low ammonia (NH₄⁺-N) of 9.3 mg/L, nitrate (NO₃^–-N) of 15.6 mg/L and COD/NO₃^–-N of 2.2 under extremely low nitrogen loading rate of 0.012 kg N/m³/d, anammox activity bloomed after its abundance increasing from 5.9 × 10⁷ to 4.6 × 10⁹ copies/g dry sludge. Significant NH₄⁺-N removal was occurred and maintained stably in the denitrification reactor with anammox bacteria accounting for 1.13%, even under temperature decreasing to 20.0℃. The adequately anoxic environment, efficient retention with the static settlement, and NO₂^– production via NO₃^– reduction provided favorable environment for anammox bacteria. This study demonstrated the feasibility and great potential in mainstream anammox application without seeding specific sludge.