In the present study, the degradation of Procaine Penicillin G (PPG) formulation effluent by Fenton-like (Fe3+/H2O2) and UV-A light assisted Fenton-like (Fe3+/H2O2/UV-A) processes have been investigated at pH = 3. The effects of different process variables such as the initial Fe3+ and H2O2 concentrations, reaction time and UV-A light on PPG formulation effluent degradation rates have been evaluated. Chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD5), total organic carbon (TOC) and acute toxicity towards the water flea Daphnia magna were selected as the environmental sum parameters to follow the performance of Fenton-like and photo-Fenton-like advanced oxidation processes on the PPG (Procaine Penicillin G) formulation effluent (COD0 = 600 mg/l; BOD5 53 mg/l; TOC0 = 450 mg/l). At optimum reaction conditions (i.e. [Fe3+] = 1.5 mM and [H2O2] = 25 mM, pH = 3), 56% COD and 42% TOC removal where achieved by the photo-Fenton-like process after 30 min treatment time, whereas the removal efficiencies of the dark Fenton-like process were limited to 44% COD and 35% TOC for the same treatment period. The superiority of the UV-A light-assisted Fenton-like process over the dark Fenton-like reaction was more evident in terms of their effect on biodegradability improvement; the BOD5/COD ratio increased from 0.10 to 0.45 and 0.10 to only 0.24 after application of the photo-Fenton-like and Fenton-like processes, respectively. Based on the results of acute toxicity tests it could be inferred that the photo-Fenton-like process is a suitable method for complete detoxification and partial oxidation of PPG formulation effluent. COD removal efficiency decreased from 56 to 17% for the photo-Fenton-like process and from 44 to 14% for the dark Fenton-like process in the presence of the 1214 mg/l chloride ion (used as a free radical probe compound at acidic pH) after 30 min oxidative treatment, indicating that in both treatment processes the hydroxyl radical ((OH)-O-.) was the major oxidizing agent for PPG. (C) 2004 Elsevier B.V. All rights reserved.