Integrated fixed-biofilm activated sludge reactor as a powerful tool to enrich anammox biofilm and granular sludge

• Nitritation–anammox process was obtained in a pilot-scale IFAS reactor.
• Nitrogen loading rate of 1.2 kg m−3 d−1 was achieved with the removal efficiency of 80%.
• Anammox bacteria was attached grew and enriched on the surface of the carrier.
• Anammox granular sludge was gradually obtained due to the detachment of biofilm.

A pilot-scale activated sludge bioreactor was filled with immobile carrier to treat high ammonium wastewater. Autotrophic nitrogen elimination occurred rapidly by inoculating nitrifying activated sludge and anammox biofilm. As the ammonium loading rate increased, nitrogen removal rate of 1.2 kg N m−3 d−1 was obtained with the removal efficiency of 80%. Activated sludge diameter distribution profiles presented two peak values, indicating simultaneous existence of flocculent and granular sludge. Red granular sludge was observed in the reactor. Furthermore, the results of morphological and molecular analysis showed that the characteristics of granular sludge were similar to that of biofilm, while much different from the flocculent sludge. It was assumed granular sludge was formed through the continuous growth and detachment of anammox biofilm. The mechanism of granular sludge formation was discussed and the procedure model was proposed. According to the experimental results, the integrated fixed-biofilm activated sludge reactor provided an alternative to nitrogen removal based on anammox.