BACKGROUNDExploitation of oil-fields always faces problems with management of petroleum-contaminated wastewaters. Wastewater, severely contaminated with petroleum hydrocarbons: total petroleum hydrocarbons (TPH, 340.7 mg L-1), benzene, toluene, ethylbenzene, xylene (BTEX, 5.52 mg L-1), phenols (2.95 mg L-1), and polycyclic aromatic hydrocarbons (PAH, 29.2 mu g L-1), were pretreated on a flow-line installation. The following processes were applied: flotation, coagulation with flocculation, sedimentation and filtration of sediments. Then a biological module with an active biopreparation (microbial consortium, based on non-pathogenic indigenous bacteria and fungi) was used to remove extant hydrocarbons. RESULTSAfter wastewater pretreatment, TPH content decreased to 49.7 mg L-1. During biological treatment, petroleum contaminants (TPH, phenols, BTEX and PAH) were decreased by 89%, 69%, 62%, and 46%, respectively. Biodegradation reached 83-96% for nC(11)-nC(22) and 34-68% for nC(23)-nC(36) alkanes (more resistant). Microtox, MARA, Daphtoxkit and Ames test indicated that changes in toxicity were not strictly connected with the decrease in TPH contents, possibly due to the formation of toxic indirect metabolites during biodegradation. After treatment, wastewater had no toxic properties. CONCLUSIONAs results show, toxicological tests are an effective tool to monitor changes in toxicity during wastewaters treatment. This field scale study proved that combining physico-chemical processes and biological treatment has high effectiveness in petroleum hydrocarbons removal. (c) 2014 Society of Chemical Industry