Impact of gradually-achieved high phenol loads on the nitrification and COD removal performance of an MBBR fed with synthetic wastewater

Phenol is a noteworthy pollutant, found in effluents of many industrial processes, like oil refining and drugs production, which can impair the treatment efficiency of bioreactors. This study evaluated the performance of phenol, COD, and nitrogen removal of an aerobic bench-scale Moving Bed Biofilm Reactor (MBBR) exposed to gradually increasing phenol content over 233 days. The reactor had Hydraulic Retention Time (HRT) set at 3 h and 40% filling degree (K1 media), and was fed with synthetic wastewater containing phenol (10, 20, 50, 100, 250 and 400 mg/L), glucose (400 mgCOD/L), and 40 mgN-NH₃/L. Phenol, COD, and ammoniacal nitrogen removal averages were high - above 88%, 81%, and 82%, respectively -, even when the MBBR was exposed to the greatest phenol loads, indicating that the biofilm was able to acclimate and resist high phenol concentrations. However, the intense EPS production revealed the impact caused by phenol to the biofilm from the concentration of 250 mg/L onwards. Even though, at this concentration, the average removals of COD and phenol were 87.2% and 89.7%. The removal of ammoniacal nitrogen by nitrification was compromised, being 91.6% of the ammoniacal nitrogen removed by assimilation and only 0.35% removed by nitrification. At 400 mg phenol/L, the reactor provided COD and phenol average removals equal 88.6% and 80.9%, respectively. On the last day of operation, the removal of COD dropped to 55.4% and phenol removal was equal 49.0%. Novel microscopical evaluation of the MBBR's biofilm revealed some negative effects of the phenol on the microbiota composition.