Dark fermentative H2H2 production from xylose and lactose—Effects of on-line pH control

Although xylose and lactose, two major carbohydrates existing in lignocellulosic and dairy wastes, are produced in huge amounts every year, their potentials in dark fermentative H2H2 production were not investigated sufficiently and the optimum fermentation pH is still unclear particularly at thermophilic temperatures. Therefore, in this study, the influence of pH on fermentative H2H2 production from xylose and lactose was investigated using a mixed culture compost inoculum. Completely mixed thermophilic (55∘C) bioreactors were operated in sequential batch mode at constant pH levels between 4.7 and 6.5. The highest H2H2 yield in fermentation of xylose was achieved as 1.70 mol/mol at pH 5.0. The H2H2 yield was strongly affected by the level of pH and butyrate fermentation. The unsatisfactory yields were associated with lactate fermentation at pH 4.7 and without pH control and with ethanol formation at pH 6.0. The optimal pH for stable H2H2 yield from lactose was suggested to be 5.3 considering the molar yields increased to a maximum of 3.45molH2/molhexosesugar. Another indication of efficient H2H2 production at pH 5.3 was the metabolic shift from ethanol to butyrate fermentation. Acetate fermentation yielding H2H2 below stoichiometric ratios especially in low H2H2 yielding runs revealed that some portion of the H2H2 was consumed by acetogenic bacteria.