Analysis of acclimation behavior against nitrification inhibitors in activated sludge processes

The acclimation behavior of activated sludge against nitrification inhibitors was explored by continuously adding thiourea and/or aniline to unacclimated sludge. Thiourea, which is difficult to biodegrade, severely inhibited the ammonia oxidation step, whereas aniline inhibited both the ammonia and nitrite oxidation steps and its biodegradation by nitrifying sludge was possible after a short acclimation period. Since thiourea-acclimated sludge showed higher tolerance than unacclimated sludge, the recovery of complete nitrification after being in contact with thiourea over a long period was attributed not only to the acquisition of biodegrading ability but also to the tolerance acquired by the sludge. Once both biodegrading ability and tolerance were acquired, they remained even after stopping the addition of thiourea for several weeks. In contrast, aniline-acclimated sludge was suspected to acquire only biodegrading ability.