Anaerobic hydrolysis and acidogenesis of wastewaters from food industries with high content of organic solids and protein

The anaerobic hydrolysis and acidification of wastewaters rich in organic suspended solids and protein was studied in continuous stirred reactors. The effluents employed in this study were obtained from a fish meal factory (30–120 g COD/l, 5–40 g VSS/l, 10–30 g protein/l). The effect of stirring on the anaerobic biodegradability was initially determined and, on the basis of these results, the hydrolysis–acidification step was optimised in terms of temperature and HRT. Thermophilic (55°C) and mesophilic (37°C) reactors were operated at HRT ranging from 6 to 48 h. No nutrients were added, and pH remained in the range 7.2–7.7 due to the high alkalinity. Extremely high organic loading rates (OLR) were applied (up to 400 kg COD/m3 d) with no significant methane production, obtaining maximum efficiencies of 44% for acidification, 58% for VSS removal and 80% for protein removal at 55°C and 24 h of HRT. However, acidification (2 kg COD-VFA/m3 h) and VSS removal rates (0.4 kg VSSr/m3 h) were maximum at HRT of 12–24 h, operating at the same temperature. Most of protein was converted into VFA and ammonia, even when working at the lowest HRT. As a consequence, the content of total ammonia in these reactors reached extremely high values in both cases (15–17 g N-TA/l), which implies high concentrations of free ammonia (up to 0.66 g N-FA/l at 37°C and 1.64 g N-FA/l at 55°C), these differences being due to the effect of temperature on the dissociation equilibrium. Although a more efficient operation was achieved at 55°C, mesophilic operation is recommended if a two-phase system was considered for the overall treatment of these effluents, since toxic effects from free ammonia would impede a stable operation in the methanogenic reactor at thermophilic conditions.