Use of a packed-bed biofilm reactor to achieve rapid formation of anammox biofilms for high-rate nitrogen removal

Anammox is an efficient and carbon-saving process for biological nitrogen removal; however, the long start-up period and poor stability of anammox bioreactors hinder their application in wastewater treatment. An innovative technical strategy was developed in this study that used a packed-bed biofilm reactor (PBBR) to grow healthy anammox biofilms for the fast start-up of bioreactors. With a greatly reduced fluid turbulence in the PBBR, the attachment and growth of anammox biofilms were found to be accelerated and the specific anammox biofilm formation rate was 60% higher than that in the conventional moving-bed biofilm reactor (0.032 vs. 0.020/d). In the PBBR operation, maintaining a low free ammonia (<2.7 mg/L) and free nitrous acid (<5.8 mu g/L) concentrations in the early stage and stepwise increasing the nitrogen load also assisted the biofilm formation. With the rapid anammox biofilm formation and growth, the nitrogen removal capacity of the PBBR increased from 77.6 to 876.8 mg N/(L center dot d) in 2 months. The anoxic condition and prolonged sludge retention time in the PBBR also benefited the growth of fermentative bacteria and denitrifiers, which induced the partial denitrification-anammox process and decreased the nitrate accumulation by 50%.