The aim of this work was to investigate the resistance to hydraulic shock loads of flocculent versus granular sludge used in UASB reactors treating sewage with high solids content. Step-wise shock loads were conducted through decreasing HRT to examine the extent of reducing this parameter without significantly changing COD removal efficiency of the reactor. The lowest HRT of 4 h resulted in only 3-4% reduction in the COD removal efficiency and the effluent contained low VFAs. Both sludge types have been also tested under transient hydraulic shock loads, which represent the wide variations between peak and average sewage flows occurring in small communities (rural areas). Up to 6 times the average flow no significant impact was observed on reactor performance except during and few hours after applying the shock loads. (C) 2007 Published by Elsevier Ltd.