A study of the anaerobic digestion process of synthetic domestic sewage (total COD: 705 mg/L) was carried Out. The digestion was conducted in an upflow anaerobic filter with corrugated plastic rings as packing media at psychrophilic temperature (15-17 degrees C). For HRTs of between 10.0 and 17.1 h, the total COD removal efficiency was almost constant and independent on the HRT, achieving an average value of around 80%. However, when the HRT decreased from 7.0 to 3.2 h the efficiency diminished from 77% to 65%. This decrease in removal efficiency was parallel to the increase in the VFA/Alkalinity ratio for this HRT range. The flow pattern observed in the reactor studied was intermediate between plug-flow and CSTR systems, although the plug-flow Was predominant. It can also be observed that Young and McCarty's model almost coincided with the CSTR model, when the biodegradable COD was used for fitting tile data. The Manariotis equation allowed a better fit of the experimental data (total COD removal efficiency with influent substrate concentration and HRT) than the Young model. The methane yield coefficient obtained was 0.15 L CH(4)/g COD consumed. (C) 2009 Elsevier Ltd. All rights reserved.