Pilot study of the influence of stirring and pH on anaerobes converting high-solid organic wastes to hydrogen

Ten and 100 L bioreactors were utilized to evaluate the effect of stirring and pH on anaerobes converting spent brewery grains to hydrogen. Experimental results of 10-L bioreactor demonstrated that both stirring speed and pH significantly affected the hydrogen conversion and the chemical oxygen demand reduction of the spent grains. Highly reproducible metabolic data confirmed that the anaerobes from grass compost could convert the spent grains to fatty acids and alcohols as well as hydrogen. Multiple analyses indicated that stirring mainly affected the hydrogen production rate. pH level was a more important factor than the stirring on shortening hydrogen production lag and increasing hydrogen yield. Optimum conditions of pH = 6.0 and stirring speed=120 rpm were obtained for 10-L bioreactor converting 3% grains to hydrogen by observing its production rate together with yield, found to be 161mLH2/gTVS/d and 13mLH2/gTVS, respectively. However, the rate and the yield decreased along with stirring speeds increased above 120 rpm. When the stirring speeds of 10-L bioreactor were greater than this value, its mixing condition changed from laminar flow to turbulent flow. The 100-L experiment with a sequencing batch mode operation confirmed that the hydrogen production rate obtained with laminar flow was significantly more stable and reproducible than with turbulent flow.