A MODEL FOR ANAEROBIC-DIGESTION OF ANIMAL WASTE SLURRIES

Bench scale experiments in 2.51 mechanically stirred glass bioreactors at 24 degrees C and 35 degrees C with cattle slurry (VS = 77.5 + 3.1 gl(-1)) and pig slurry (VS = 31.5 + 2.1 gl(-1)) showed a linear decrea se of methane yield y(CH4) (lg-1 VS added) with increasing organic vol umetric loading rate BV (gVS l-1d-1) over a wide range. This relations hip was used to develop model equations for y(CH4) and methane product ion rate r(CH4) (ll(-1)d(-1))) in terms of ultimate methane yield at i nfinite hydraulic retention y(CH4)(MAX) time, hydraulic retention time (HRT), critical hydraulic retention time for wash-out of methane bact eria (HRTc), VS slurry feed concentration (cO) and a temperature-term calculated from a derived ARRHENIUS equation (fT). y(CH4)(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 degrees C and 35 degrees C were 7.75 d and 2.76 d, while corresponding values f or pig slurry were 9.66 d and 2.57 d. This resulted in a temperature t erm fT values for cattle slurry and pig slurry of 1.098 and 1.127 resp ectively. Plots of y(CH4) and r(CH4) as a function of HRT at different temperatures are presented and the calibrated model equations are app licable to other slurries in fully mixed anaerobic digesters. The infl uence of high VS-concentrations on y(CH4) and r(CH4) for cow slurry di gestion is described by means of three dimensional plots.