Abstract
Bench scale experiments in 2.51 mechanically stirred glass bioreactors at 24 °C and 35 °C with cattle slurry (VS = 77.5 + 3.1 gl−1) and pig slurry (VS = 31.5 + 2.1 gl−1) showed a linear decrease of methane yield yCH4 (lg-1 VS added) with increasing organic volumetric loading rate BV (gVS l-1d-1) over a wide range. This relationship was used to develop model equations for yCH4 and methane production rate rCH4 (ll−1d−1) in terms of ultimate methane yield at infinite hydraulic retention yCH4 MAX time, hydraulic retention time (HRT), critical hydraulic retention time for wash-out of methane bacteria (HRTc), VS slurry feed concentration (c0) and a temperature-term calculated from a derived ARRHENIUS- equation (fT). yCH4 MAX values for cattle and pig slurry were 0.132 – 0.143 and 0.351 – 0.406 lg−1 respectively. Calculated HRTc values for cattle slurry at 24 °C and 35 °C were 7.75 d and 2.76 d, while corresponding values for pig slurry were 9.66 d and 2.57 d. This resulted in a temperature term fT values for cattle slurry and pig slurry of 1.098 and 1.127 respectively. Plots of yCH4 and rCH4 as a function of HRT at different temperatures are presented and the calibrated model equations are applicable to other slurries in fully mixed anaerobic digesters. The influence of high VS-concentrations on yCH4 and rCH4 for cow slurry digestion is described by means of three dimensional plots.
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