A biological aerated filter (BAF) was applied to treat textile wastewater. The performance of two lab-scale biofilters was monitored for 7 months to compare the effect of natural zeolite with sand as media. Under organic load varying from 1.2 to 3.3 kg COD/m(3) day, the overall chemical oxygen demand (COD) reductions of the biofilter supported by natural zeolite and sand averaged 88 and 75%, respectively, Higher nitrogen removal in the biofilter with natural zeolite was attributable to its ion exchange capacity with NH4+. The results of cell counting showed that the numbers of nitrifying bacteria within the biofilm were higher on natural zeolite than on sand. Both biofilters were able to remove about 97% of suspended solids (SS) with loading of 1-3 kg SS/m(3) day. A pilot-scale BAF supported by natural zeolite with capacity to treat up to 12 m(3)/day of textile wastewater was also monitored for 5 months. This system was able to remove about 99% biochemical oxygen demand, 92% COD, 74% SS, and 92% T-N with hydraulic load of 1.83 m(3)/m(2) h. Operation with a higher hydraulic load (2.3 m(3)/m(2) h) and at lower temperatures (range of 4-10 degreesC) resulted in decreasing the T-N removal efficiency while the reduction in terms of organic matter was less affected. Regardless of increasing hydraulic loading rate, colour removal (average of 78%) was effective in the biofilter with natural zeolite due to its adsorption capacity. (C) 2002 Elsevier Science Ltd. All rights reserved.