The effects of short-term organic shock loads on the performance of a laboratory scale two-stage activated Sludge (AS)-biofilm reactor working at 6 h HRT and treating medium strength domestic wastewater was studied by increasing the influent chemical oxygen demand (COD) to 2-4 times the normal values. Each shock load was applied for a period of 6 h, after which normal loading conditions were resumed. The maximum effluent COD concentration obtained was 169,169,250 and 617 mg/L under the shock loads of 808, 1170, 1358 and 1900 mg COD/L, respectively. The COD removal rate increased with increasing effective OLR. The system recovered quickly from shock loads: recovery time proportional to the magnitude of shock loads. The optimum loading rate of the reactor was found at effective OLR of 170 mg COD/m(3)/h. Shock loads changed the dominant bacterial type in the reactor from Gram-positive rods to Gram-negative oval shaped bacteria. (C) 2009 Elsevier Ltd. All rights reserved.