Cattle manure is the most susceptible feed for anaerobic digestion among organic waste materials. Cyclic batch reactors with periodical feeds and discharges are often used in anaerobic digestion of cattle manure. On the other hand, mixing is an important factor for optimal perforn1ance of cyclic batch reactors and its modeling is essential for performance prediction of real reactors. In this study a mathematical model for anaerobic digestion of cattle manure was developed to describe the dynamic behavior of non-ideally mixed cyclic batch reactors. The microbial kinetic model includes an enzymatic hydrolysis step and four bacterial steps, together with the effects of substrate inhibition, pH and thermodynamic considerations. The biokinetic expressions were linked to a simple two-region liquid mixing model, which considered the reactor volume split into two sections, the flow-through and the retention regions. Changing the relative volume of the flow-through region and the flow rate of the internal exchange to the feed flow rate ratio represented deviations from an ideal completely mixed regime. The dynamic model described the effects of retention time and reactant's distribution, resulting from the mixing conditions, on the process performance. The evaluation of the impact of the characteristic mixing parameters on anaerobic digestion showed that the relative volume of the flow-through region has a more significant effect than the turnover time. However, retention time and reactant's distribution and consequently, the methane yield were complex functions of both mixing parameters. In addition, the simulation results showed that the feeding interval is efficient on reactants'
distribution of both regions. So that the methane yield is improved when the cyclic batch reactor is approached to a continuously reactor.
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