Abattoir wastewater has a complex.composition and is very harmful to the environment.rnIt causes eutrophication, suffocation of aquatic life and contamination of ground water.rnNitrogen is the main component of the abattoir wastewater. Because of environmentalrnand health problems they cause, nitrogen compounds should be reduced to thernacceptable discharge limit level before the wastewater is released to water bodies andrnthe environment. Biological methods of nitrogen removal are considered to be costrneffective and environmentally friendly alternatives than chemical treatment. Thernobjectives of this study were therefore: to evaluate the treatment processes of arnpredenitrification-nitrification activated sludge system for abattoir wastewater treatment;rnCharacterize the activated sludge involved in the treatment process through Ammoniumrnuptake rate and Nitrate uptake rate, and its seUlability by sludge volume index.rnAbattoir wastewater was sampled from Addis Ababa Abattoirs Enterprise, analyzed forrnselected parameters to check its biodegradability and fed into a predenitrificationrnnitrification activated sludge wastewater treatment system at varying operationalrnparameters. The removal efficiency for the selected parameters and the characteristicsrnof the sludge at each operational condition was evaluated. A removal efficiency of 94-rn98% for COD, 95-99% for BOD5, 85-97% for total nitrogen, 58-95% for AmmoniumNitrogenrnand 46-53% reduction in phosphate was obtained in the treatment processes.rnThe predenitrification-nitrification rates measured as Ammonium Uptake Rate andrnNitrate Uptake Rate ranged from 5.9-9.8 mg NH4-N/gVSSh and 8.2-12.1 mg N03-rnN/gVSSh respectively. The sludge volume index (SVI) ranged from 51.5 mUg to 87.6rnmUg. The abattoir wastewater was found to be suitable for biological wastewaterrntreatment. It is also showed that the predenitrification-nitrification system was efficient inrnorganic matter and nitrogen removal from abattoir wastewater.