Improved organic loading rates for anaerobic bioconversion of cellulos
ic feedstocks are possible through high-solids processing. Additionall
y, the reduction in process water for such a system further improves t
he economics by reducing the overall size of the digestion system. How
ever, mixing of high-solids materials is often viewed as an energy-int
ensive part of the process. Although the energy demand for high-solids
mixing may be minimized by improving the agitator configuration and r
educing the mixing speed, relatively little information is available f
or the actual horsepower requirements of a mechanically mixed high-sol
ids digester system. The effect of sludge total solids content and dig
ester fill level on mixing power requirements was evaluated using a no
vel NREL laboratory-scale high-solids digester. Trends in horsepower r
equirements are shown that establish the optimum parameters for minimi
zing mixing energy requirements, while maintaining adequate solids ble
nding for biological activity. The comparative relationship between la
boratory-scale mixing energy estimates and those required for scaleup
systems is also established.