Implications of reactor type and conditions on first-order hydrolysis rate assessment of maize silage

The biodegradability and first-order hydrolysis coefficient of maize silage have been assessed from batch experiments using different types of inoculum and substrate to inocula (S/I) ratios, and from CSTRs working at different hydraulic retention times (HRTs). In the batch experiments, the assessed maximum biodegradability of the maize silage was 68 (±2.7)% and 73(±2.9)% while the first order hydrolysis was 0.26 (±0.01) and 0.27(±0.02) d⁻¹, using granular and a mixture of granular and suspended inoculum, respectively. In the CSTR experiment biodegradability ranged from 41–65% depending on the HRT applied whereas the calculated first order hydrolysis coefficient was 0.32 d⁻¹. It is concluded that batch experiments can be used to assess first order hydrolysis constants and biodegradability provided that a well balanced inoculum is guaranteed. Further, it is shown that CSTR reactors digesting maize silage and operating at HRTs as low as 20 days can attain 88% of maximum biodegradability as long as pH fluctuations are minimized. 2 mmol NaHCO₃ per gram maize silage was calculated to suffice for the purpose.