As is typically found for extreme thermophiles [19], maximal volumetric H 2 production rates were quite low, 9–10 mmol H 2 .l–1.h–1 with simple sugars, and even lower rates, 5 to 6 mmol H 2 .l–1.h–1, were found with the complex substrate indicating the possible presence of inhibiting components in paper sludge hydrolysate [59].