Results from standard tests for BAF media have been compared to data from pilot-scale operation, indicating that some tests may require modifications. Materials that showed 3.0% and 5.2% weight loss after attrition testing exhibited no attrition during nine months of operation, possibly due to biofilm protection. Comparing the performance of BAFs containing media with similar dimensions, but different surfaces, showed the importance of measuring surface roughness. With an influent of 0.3 l min(-1) settled sewage, media with high surface, roughness achieved an extra 29%, 15% and 70% removal of suspended solids, soluble chemical oxygen demand and ammonia respectively. Tracer studies on empty beds gave a good approximation to operating beds, highlighting potential problems such as channelling and excess liquid hold-up. For three media beds which were equivalent to 3, 4 and 8 tanks in series, the nitrification results were superior in those reactors in which the flow was closest to plug flow. The ideal properties of a BAF medium depend on the application and operating conditions. Therefore, media that can be manufactured with controlled properties such as size and density provide definite advantages.