In this study, the effect of increasing p-nitrophenol (PNP) concentrations on the performance of anaerobic baffled reactor (ABR) (chemical oxygen demand (COD), removals, volatile fatty acid (VFA), p-aminophenol (PAP) and methane gas productions) was investigated through 240 days. The PNP concentrations were raised to 700 from 10 mg/L corresponding to PNP loading rates of 0.97 and 67.9 g/m(3) day. The PNP and COD removal efficiencies were 99 and 90% at PNP loading rates as high as 33.9 g/m(3) day, respectively, through the acclimation of anaerobic granular sludge. After this loading rate, the removal efficiencies decreased to 79%. The COD removal efficiencies were high in compartment 1 (E = 78-93%) while a small amount of COD removal was achieved in compartments 2 and 3. The PNP removal efficiencies were approximately 90% in all PNP loading rates except for loading rate of 0.97 g/m(3) day. The maximum PNP removal efficiency was measured as 99% at a loading rate of 8.32 g/m(3) day. The optimum PNP loading rate for maximum COD, PNP removals and methane yield was 8.32 g/m(3) day. The total, methane gas productions and methane percentages were approximately 2160-2400 mL/day and 950-1250 mL/day and 44-52% for the PNP loading rates varying between 4.36 and 33.9 g/m(3) day, respectively. For PNP loading rates varying between 33.9 and 67.9 g/m(3) day, the total, methane gas productions and methane percentages were approximately 2160 and 960 mL/day and 44%, respectively. The highest total volatile fatty acid (TVFA) concentrations were found in the first compartment with fluctuated values varied between 50 and 200 mg/L indicating the acidogenesis. p-Aminophenol was found as the main intermediate through anaerobic degradation of PNP which later was broken down to phenol and ammonia. (c) 2005 Elsevier Inc. All rights reserved.