Denitrification of synthetic concentrated nitrate wastes by aerobic granular sludge under anoxic conditions

The aim of the present work was to determine the denitrification potential of aerobic granular sludge for concentrated nitrate wastes. We cultivated mixed microbial granules in a sequencing batch reactor operated at a superficial air velocity of 0.8 cm s⁻¹. The denitrification experiments were performed under anoxic conditions using serum bottles containing synthetic media with 225-2250 mg L⁻¹ NO₃-N. Time required for complete denitrification varied with the initial nitrate concentration and acetate to nitrate-N mass ratio. Complete denitrification of 2250 mg L⁻¹ NO₃-N under anoxic conditions was accomplished in 120 h. Nitrite accumulation was not significant (<5 mg N L⁻¹) at initial NO₃-N concentrations below 677 mg L⁻¹. However, denitrification of higher concentrations of nitrate (?900 mg N L⁻¹) resulted in buildup of nitrite. Nevertheless, nitrite buildups observed in present study were relatively lower compared to that reported in previous studies using flocculent activated sludge. The experimental results suggest that acetate-fed aerobic granular sludge can be quickly adapted to treat high strength nitrate waste and can thus be used as seed biomass for developing high-rate bioreactors for efficient treatment of concentrated nitrate-bearing wastes.