The impact of mechanical pretreatment on biogas production from waste materials of the chemical and brewing industries

Respirometric tests, carried out in OxiTop system, were used to determine biogas production (BP) from two waste materials, willow bark residue (W) from the chemical industry and brewer’s spent grain (BSG) from the brewing industry. Moreover, the kinetics of BP and the loss of organic compounds (expressed as COD) were investigated. In this investigation, W and BSG were used both in their unchanged forms and after mechanical pretreatment (grinding to a diameter of 1 mm) (W_G and BSG_G). The initial organic load in the bioreactors was 4 kg OM/m³. The BP from W was 154.1 dm³/kg DM (166.6 dm³/kg OM), and from BSG, it was 536.9 dm³/kg DM (559.5 dm³/kg OM). This probably resulted from the fact that the content of lignin that was hard to biodegrade was higher in W than in BSG. Mechanical pretreatment increased BP from W_G to 186.7 dm³/kg DM (201.9 dm³/kg OM), and from BSG_G to 564.0 dm³/kg DM (588.7 dm³/kg OM). The net biogas yield from W and BSG increased by 17% (35 dm³/kg OM) and 5 % (29 dm³/kg OM), respectively. The kinetic coefficient of BP (k_B) and the rate of BP (r_B) of W were lower than those of BSG. Mechanical pretreatment increased the k_B and r_B of biogas production from both waste materials.