An ammonia loading rate of 4.1 kg N-NH4+/m3d was fully oxidized to nitrate using a nitrifying airlift without biomass carrier. The developed sludge, with a concentration of 12 gVSS/L and high density (100 gVSS/Lparticle), allowed particles to be retained with small diameter, having very low concentrations of solids in the effluent (5–10 mg VSS/L). The mean diameter of particles containing the majority of total biomass increased from 220 to 360 m during the operation time. The particle size analysis showed that the percentage of flocs with small diameter (1–3 m) is very high but the biomass content of these flocs is negligible with respect to the total biomass value. The oxygen transfer coefficient (KLa) was measured operating at different air flow rates, obtaining similar values during the overall operational period. The use of activated sludge without support gives high nitrifying rates with an easier fluidization and a lowercritical air flow rate than in a biofilm airlift reactor.

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