The effect of hydraulic flush and pH control on hydrolysis, Volatile Fatty Acids (VFA) production and profile in anaerobic leach bed reactors was investigated for the first time. Six reactors were operated under different regimes for two consecutive batches of 28 days each. Buffering at pH similar to 6.5 improved hydrolysis (Volatile Solid (VS) degradation) and VFA production by similar to 50%. Butyric and acetic acid were dominant when reactors were buffered, while only butyric acid was produced at low pH. Hydraulic flush enhanced VS degradation and VFA production by similar to 15% and similar to 32%, respectively. Most Probable Number (MPN) of cellulolytic microorganisms indicated a wash out when hydraulic flush was applied, but pH control helped to counteract this. The highest VS degradation (similar to 89%), VFA yield (0.84 kg COD kg(-1) VSadded) and added, theoretical methane potential (0.37 m(3) CH4 kg(-1) VSadded) were obtained when pH control and hydraulic flush were applied, and therefore, these conditions are recommended. (C) 2012 Elsevier Ltd. All rights reserved.