Enhanced hydrogen accumulation from waste activated sludge by combining ultrasonic and free nitrous acid pretreatment: Performance, mechanism, and implication.

• Ultrasonic aids FNA pretreatment enhanced the yield of hydrogen. • Ultrasonic aids FNA pretreatment accelerated sludge solubilization. • Ultrasonic aids FNA pretreatment enhanced the release of biodegradable organics. • Ultrasonic aids FNA pretreatment severely inhibited hydrogen consumers. This study presents a novel and effective method, i.e., adding nitrite into acidic fermentations after ultrasonic (US) pretreatment to form free nitrous acid (FNA), to further enhance hydrogen yield. Experimental results showed that when 180 mg/L nitrite was added into the US (2 W/mL, 15 min) pretreated waste activated sludge (WAS), the maximal hydrogen yield of 24.81 ± 1.24 mL/g VSS (volatile suspended solids) was obtained under acidic fermentation (1.0 mg/L FNA was initially formed under this condition), which was 2.21-folds (or 1.36-folds) of that from US pretreatment (or FNA treatment) alone. This combination approach caused a positive synergy on sludge disintegration and enhanced the transformation of the released organics from non-biodegradable to biodegradable. Further study showed that the inhibiting effect of this combination method on hydrogen consuming microorganism was severer. Considering its pollution free, this combination strategy is an attractive technology for hydrogen recovery from WAS. [ABSTRACT FROM AUTHOR]