Antibiotic formulation effluent is well known for its important contribution to environmental pollution due to its fluctuating and recalcitrant nature. In the present study, the chemical treatability of penicillin formulation effluent (average filtered CODo = 830 mg/l; average soluble CODo = 615 mg/l; pH(o) = 6.9) bearing the active substances penicillin Amoxicillin Trihydrate (C16H19N3O5S . 3H(2)O) and the P-lactamase inhibitor Potassium Clavulanate (C8H8KNO5) has been investigated. For this purpose, the penicillin formulation effluent was subjected to ozonation (applied ozone dose = 2500 mg/(1 x h)) at varying pH (2.5-12.0) and O-3 + H2O2 (perozonation) at different initial H2O2 concentrations (= 2-40 mM) and pH 10.5. According to the experimental results, the overall Chemical Oxygen Demand (COD) removal efficiency varied between 10 and 56% for ozonation and 30% (no H2O2) and 83% (20 mM H2O2) for the O-3 + H2O2 process. The addition of H2O2 improved the COD removal rates considerably even at the lowest studied H2O2 concentration. An optimum H2O2 concentration of 20 mM existed at which the highest COD removal efficiency and abatement kinetics were obtained. The ozone absorption rate ranged between 53% (ozonation) and 68% (perozonation). An ozone input of 800 mg/l in 20 min was sufficient to achieve the highest BOD5/COD (biodegradability) ratio (=0.45) and BOD5 value (109 mg/l) for the pre-treated penicillin formulation effluent. After the establishment of optimum ozonation and perozonation conditions, mixtures of synthetic domestic wastewater+raw, ozonated and perozonated penicillin formulation effluent were subjected to biological activated sludge treatment at a food-to-microorganisms (F/M) ratio of 0.23 mg COD/(mg MLSS x d), using a consortium of acclimated microorganisms. COD removal efficiencies of the activated sludge process were 71, 81 and 72% for pharmaceutical wastewater containing synthetic domestic wastewater mixed with either raw, ozonated or perozonated formulation effluent, respectively. The ultimate COD value obtained after 24-h biotreatment of the synthetic domestic wastewater+pre-ozonated formulation effluent mixture was around 100 mg/l instead of 180 mg/l which was the final COD obtained for the wastewater mixture containing raw formulation effluent, indicating that pre-ozonation at least partially removed the non-biodegradable COD fraction of the formulation effluent. (C) 2004 Elsevier Ltd. All rights reserved.