Effect of phenol addition on COD and nitrate removal in an anoxic suspension reactor

In this study, a denitrifying culture was enriched in a continuously re-circulated anoxic suspension reactor fed with glucose and nitrate for about 8 months (stage I) under different organic loading rates (OLR). At the end of stage I, the removal efficiency for NO3-–N was 80% with 93% COD (5 g/l) removal at an OLR of 2.5 g/l d. The mean COD/N removal ratio during the whole enrichment was 3.3. The response to phenol as a toxic substance on glucose enriched culture for long time period was investigated by introducing phenol as a co-substrate in the reactor feed in stage II. Phenol was increased gradually to 753 mg/l till termination of the reactor operation. After increasing the OLR or the phenol concentration, fluctuations in removal efficiencies were observed which were partly reversible. At the end of the reactor operation, NO3-–N removal was 65% with 81% COD removal at a phenol degradation rate of 207 mg/l d phenol. The OLR of the reactor was 4.3 g/l d COD and a hydraulic retention time (HRT) of 1 day. Phenol degradation in batch assays under anoxic conditions and at low phenol concentrations (188 mg/l) proceeded a removal rate of 1.2 g/l which decreased to 0.67 mg/l d at high phenol concentration (847 mg/l).