The short- and long-term effects of formic acid on rapid nitritation start-up

The feasibility of achieving stable nitritation inoculating with activated sludge by adding formic acid was studied in this work. Short-term batch effects of formic acid on nitrification showed that the nitrite accumulation ratio (NAR) significantly increased from 0.3% to 83.7% with an increase of formic acid concentration from 0 to 50 mM at an initial ammonia concentration of 75 mg·L^-1, which was demonstrated to be due to the inhibition of nxrB transcription in nitrite oxidizing bacteria (NOB). The long-term effects of formic acid at 30 mM were constantly monitored in an aerobic sequencing batch reactor. During 27 days of operation, the NAR was rapidly raised and maintained approximately 90%. What's more, in the following 52 days without addition of formic acid, the NAR was kept above 91.3%. The sustained suppression of NOB genus Nitrospira coupling nxrB inhibition was the main reason to maintain stable nitritation. These results supported the feasibility of formic acid as an efficient nitritation regulator, thus providing a new approach for the development of the BNR process via nitrite pathway.