Bio-hydrogen production from sewage sludge: Screening for pretreatments and semi-continuous reactor operation

The high volumes of sewage sludge produced have raised interests for simultaneous treatment and clean energy production, e.g. in the form of hydrogen. Pretreatment of sewage sludge is required to enhance microbial degradation and in turn hydrogen yield from sewage sludge. The potential of five substrate pretreatments, individually and in combinations, to increase biohydrogen production from mixed primary and secondary sewage sludge at four incubation pH (5, 7, 9, and 11) was studied in batch assays. Alkali + ultrasonication pretreatment increased the hydrogen production almost seven times (0.35 mmol H2/ g VS) compared to untreated sewage sludge at initial pH 11. In general, higher hydrogen yields and lower acetate concentrations were obtained under alkaline conditions (pH 9 and 11), being more favorable for protein degradation and not favorable for hydrogen consumption via homoacetogenesis. Subsequently, fermentation of alkali + ultrasonication pretreated sewage sludge in a semi-continuous stirred tank reactor (CSTR) produced a maximum hydrogen yield of 0.1 mmol H2/ g VS, three times higher than the yield obtained from alkali pretreated sludge. The gas produced in the CSTRs contained a low concentration of CO2 (< 5%), and is thus easily upgradable to biohydrogen.