Assessing the Impact of Water Upflow Velocity on Nitrification in Submerged Aerated Filter

Nitrification has a pivotal role in water resource recovery facilities, forming also the rate-limiting step. Packed bed biofilm reactors-including biological aerated filter (BAF) and submerged aerated filter (SAF)-facilitate high-rate nitrification with volumetric rates > 1 kg N·m^-3·day^-1 (due to high specific surface areas) and are attractive for tertiary nitrification. Compared to SAFs, BAFs are susceptible to clogging, incur larger head losses, and demand frequent backwashing. While few works investigated the effect of water upflow velocity on nitrification in BAFs (using a single reactor), no such study has been performed with SAFs. This work attempted to systematically characterize the effect of water upflow velocity in nitrifying SAFs. Trials were performed in two test beds, each having five nitrifying SAFs (10 reactors in total) operated at water upflow velocities of about 1, 5, 10, 15, and 20 m/h. Volumetric ammonia loading rates (vALRs) from about 200-2000 and 300-1400 g N·m^-3·day^-1 were applied (in the test beds) over a period of about 230 and 157 days, respectively. While an increase in water upflow velocity from 4 to 15 m/h was observed to positively influence the nitrification rate in the previous studies using BAFs, water upflow velocity did not show any effect on nitrification rates in SAFs in this study. Nearly complete nitrification could be obtained at all five water upflow velocities even at vALR of about 2 kg N·m^-3·day^-1 (surface-specific loading rate ≈2.1 g N·m^-2·day^-1). Water velocity did not show any effect on the biofilm community structure (diversity, richness, and dominant organisms).