Two parallel biological aerated filters (BAF)

named as BAF1 and BAF2 (feeding with FeSO4·7H2O)

were used to treat petrochemical secondary effluent. The performance of the reactors

especially the phosphorus removal ability

was investigated. In addition

the biomass and the biofilm activity were also studied. The results showed that the phosphorus removal could be enhanced obviously with the suitable dosage of FeSO4·7H2O (3—15 mg·L-1). The phosphorus removal rate increased with the increase dosage of FeSO4·7H2O. However

the increasing trend became slowly when the dosage of FeSO4·7H2O was over 9 mg·L-1. The TP removal rate was over 57.0% in BAF2 when the dosage of FeSO4·7H2O was 9 mg·L-1

while it was only 7.1% in BAF1. The dosage of FeSO4·7H2O had no adverse effect on COD removal and the average COD removal rate in BAF2 was 5.4%

which was higher than that in BAF1. In addition

organic pollutants with high molecular weight were removed more obviously in BAF2 than that in BAF1. The dosage of FeSO4·7H2O had slightly effect on removal when the dosage was lower than 15 mg·L-1. However

it decreased when the dosage was 15 mg·L-1 due to the decrease of biofilm activity. There was almost no difference in TN removal in the two BAFs. The dosage of FeSO4·7H2O can decrease the amount of biofilm

but the biofilm reduction was not obvious when the dosage was lower than 12 mg·L-1. The activity of the biofilm increased when the FeSO4·7H2O dosage was between 3 to 9 mg·L-1

weakening the effect of the slightly biomass reduction

and therefore the performance of BAF2 was not affected.