Modeling of anaerobic digestion with a focus on estimation of hydrolysis constants at 35, 55, and 60°C

Hydrolysis constants of mixed sludge at 35°C, 55°C, and 60°C were found to be 0.32 d-1, 0.44 d-1, and 0.50 d-1, respectively in pilot-scale semi-continuously operated anaerobic digesters. The hydrolysis constants as well as estimated chemical oxygen demand (COD) fractions in the feed were introduced to a mathematical model for anaerobic digestion published by Siegrist et al. (2002) which is similar to Anaerobic Digestion Model No. 1 (ADM1). First-order and Monod-type kinetics were tested for estimation of hydrolysis constants. The applied kinetics were found to affect the outcome of the regression study. Moreover, the free ammonia inhibition model was excluded for both propionate oxidation and acetate conversion, thanks to the apparent acclimatized biomass. No substantial accumulation of volatile fatty-acids (VFA) was observed in the reactors at 35, 55 and 60°C corresponding to free ammonia nitrogen (FAN) concentrations of about 20, 110 and 130 g N/m3, respectively.