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 s uspended solids and protein was studied in continuous stirred reactors. The effluents employed in this study were obtained from a fish meal factory (3 0-120 g COD/1, 5-40 g VSS/1, 10-30 g protein/1). The effect of stirring on the anaerobic biodegradability was initially determined and, on the basis o f these results, the hydrolysis-acidification step was optimised in terms o f temperature and HRT. Thermophilic (55 degrees C) and mesophilic (37 degre es C) reactors were operated at HRT ranging from 6 to 48 h. No nutrients we re 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/m (3). d) with no significant methane production, obtaining maxi mum efficien cies of 44% for acidification, 58% for VSS removal and 80% for protein remo val at 55 degrees C and 24 h of HRT. However, acidification (2 kg COD-VFA/m (3) h) and VSS removal rates (0.4 kg VSSr/ m(3) h) were maximum at HRT of 1 2-24 h, operating at the same temperature. Most of protein was converted in to VFA and ammonia, even when working at the lowest HRT. As a consequence. the content of total ammonia in these reactors reached extremely high value s in both cases (15-17 g N-TA/l), which implies high concentrations of free ammonia (up to 0.66 g N-FA. 1 at 37 degrees C and 1.64 g N-FA/l at 55 degr ees C), these differences bring due to the effect of temperature on the dis sociation equilibrium. Although a more efficient operation was achieved at 55 degrees C, mesophilic operation is recommended if a two-phase system was considered for the overall treatment of these effluents, since toxic effec ts from free ammonia would impede a stable operation in tee methanogenic re actor at thermophilic conditions. (C) 1999 Elsevier Science Ltd. All rights reserved.