The effect of pH control and ‘hydraulic flush’ on hydrolysis and Volatile Fatty Acids (VFA) production and profile in anaerobic leach bed reactors digesting a high solids content substrate

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 ∼6.5 improved hydrolysis (Volatile Solid (VS) degradation) and VFA production by ∼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 ∼15% and ∼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 (∼89%), VFA yield (0.84 kg COD kg−1 VSadded) and theoretical methane potential (0.37 m3 CH4 kg−1 VSadded) were obtained when pH control and hydraulic flush were applied, and therefore, these conditions are recommended.