Impact of phenol shock loads on the performance of a combined activated sludge-moving bed biofilm reactor system

- The effect of phenol shock loads (100-3000 mg/L) on an AS-MBBR system was assessed.
- Bacteria acclimation allowed full phenolic COD removal at 2000 mg phenol/L.
- Nitrifiers, especially NOB, showed higher sensitivity to phenol than heterotrophs.
- At 3000 mg/L, phenol toxicity limited microbial activity and system functioning.
- Development of filamentous bacteria at high phenol loads hindered biomass settling.

The effect of phenol shock loads (100-3000 mg phenol/L) on the performance of an activated-sludge (AS) - moving bed biofilm reactor (MBBR) system was assessed. The AS-MBBR system could well withstand phenol shocks up to 500 mg phenol/L (organic load of 2.72 kgCOD/(m3d)), above which its performance was disturbed, more or less intensely depending on the phenol load. Nevertheless, acclimation of microorganisms to the increasing phenol levels was observed, and full phenolic COD removal was reached at 2000 mg phenol/L. Ammonium removal occurred by both bacterial assimilation and nitrification. Nitrifiers showed higher sensitivity to phenol than heterotrophs, being already impaired at 250 mg phenol/L. However, further adaptation of ammonium oxidizers allowed stable ammonium oxidation activity to be reached up to 1500 mg phenol/L. Nevertheless, nitrite oxidizers were severely affected above 250 mg phenol/L, leading to nitrite build-up. At 3000 mg phenol/L, phenol toxicity strongly limited microbial activity. Meanwhile, COD and ammonium removal performance was dramatically impaired. Moreover, the increasing phenol loads led to the development of filamentous organisms, deteriorating the biomass settling properties. The MBBR, employed downstream of the AS reactor, was frequently subjected to low (sometimes nil) phenol loads, not being therefore acclimated to this compound. Once phenol reached this reactor during some shocks, it accounted for less than 23% of the overall COD removal. The results indicated that the use of a non-acclimated biofilm reactor as a polishing step under phenol stress conditions did not always render improvements to the overall treatment and its implementation is only recommended under certain shock loads.