Water quenched slag particles (WQSP) and haydite were applied to treat municipal wastewater in two lab-scale up-flow biological aerated filters (BAF) to compare their abilities to act as biofilm supports. The results showed that WQSP reactor brought a relative superiority to haydite reactor in terms of chemical oxygen demand (CODcr) and ammonia nitrogen (NH3-N) removal when hydraulic retention time (HRT) ranged from 1 h to 5 h.Compared with haydite, WQSP had higher total porosity, larger total surface area and lower bulk and apparent density. Tests of heavy metal elements in lixivium proved that WQSP were safe for wastewater treatment. In addition, the detection of the amount of hetero bacteria and nitrobacteria of two biological aerated filters in three HRTs also showed that WQSP medium was more suitable to the attached growth of nitrobacteria, which is helpful to the improvement of nitrification performance in WQSP BAF. Therefore, WQSP is a potential material for use as the filter media of BAF for wastewater treatment. WQSP application, as a novel process of treating wastes with waste, provides a promising way to use water quenched slag (WQS). (c) 2012 Elsevier Ltd. All rights reserved.