A laboratory-scale aerobic activated sludge reactor (AASR) system was employed to investigate the effects of SRT on the removal of three less hydrophobic and six more hydrophobic PAHs in the presence of rhamnolipid (RD), emulsan (EM) and surfactine (SR) biosurfactants. Among the biosurfactants it was found that RD exhibits a better performance than the others in the removal of PAHs. At a RD of 15 mgl(-1) aerobic treatment for 25 days SRT was enough to remove over 90% of the total PAHs, 88% of the COD originating from the inert organics (COD(inert)) and 93% of the COD originating from the inert soluble microbial products (COD(imp)). At this SRT and RD concentration, about 96-98% of the RD was biodegraded by the AASR system, 1.2-1.4% was accumulated in the system, 1.1-1.3% was released in the effluent, and 1.2-1.4% remained in the waste sludge. The addition of electron acceptors (NO(3)(-1), SO(4)(-2)) and increasing of temperature up to 45 degrees C enhanced the PAH yields. The most effective PAH degradation occurred in high-oxygenated and neutral pH conditions. The PAH concentration affecting half of the Daphnia magna organism (EC(50) value) was reduced from EC(50) = 45.02 ng ml(-1) to the PAH concentration affecting only 6% of the live Daphnia magna (EC(6) = 5.30 ng ml(-1)) at the end of the aerobic treatment at a SRT of 25 days. Toxicity removals originating from the PAHs were 96%. (C) 2011 Elsevier B.V. All rights reserved.