Tehran Refinery is a large plant which produces several petroleum products. The wastewaters are generated from several different refinery processes and units. Because of the wastewaters uniqueness they need to be treated in each specific plant. Currently, an activated sludge system is the main biological wastewater treatment process in Tehran refinery plant. A study was initiated in order to find a more suitable and reliable process which can produce a better treated effluent which might, in case the process be successful, be reused for irrigation lands. 5 refs., 5 figs.