The present study compares the influence of organic and hydraulic shock loadings on the performance of submerged aerated filters (SAFs) packed with wool, as a novel media and Kaldnes rings media in terms of total organic carbon (TOC), suspended solids (SS) and ammonia removal. The SAFs continued to achieve an average of more than 95% TOC removal, 100% of SS removal efficiency and 99.8% ammonia removal even when subject to shock loads. The ammonia removal was more sensitive than the other parameters, which was attributed to the slow growth of nitrifying bacteria, which are poor competitors for space and substrate. SAF packed with wool had a better capacity to overcome hydraulic shock than the SAF with Kaldnes, attributed to the better filtration properties, however, both SAFs exhibited rapid return to normal operation conditions after both short and longer shock loading. On doubling the organic load, the reactors responded differently, rapidly coping with the shock but were less stable. The longer-term organic shock loading caused a greater disturbance until sufficient biomass had grown to compensate, suggesting that mass transfer is less important than growth kinetics. (C) 2010 Elsevier B.V. All rights reserved.