Effect of pH on Continuous Biohydrogen Production from End-of-Life Dairy Products (EoL-DPs) via Dark Fermentation
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To investigate the effect of pH and thus determine the optimal pH value for biohydrogen production from a mixture of End-of-Life Dairy Products (EoL-DPs) through dark fermentation in view of increasing the yield of their subsequent co-digestion with agroindustrial wastes in a two-stage anaerobic digestion system.
A Continuously Stirred Tank Reactor (CSTR) was operated under mesophilic conditions (37 °C) and hydraulic retention time of 6 days in order to enhance biohydrogen production from a typical mixture of EoL-DPs (93 % milk–5 % yoghurt–2 % cheese, w/w). CSTR experiments were performed to investigate the effect of controlled pH (4.0, 4.5, 4.7, 5.0, 5.3, 5.7) on the production of biohydrogen and Volatile Fatty Acids (VFAs).
The maximum hydrogen yield and productivity (0.84 mol H2/mol carbohydrates consumed and 0.76 L L R −1 ·day−1 respectively) was obtained at pH 5.0, whereas the greatest degradation of soluble carbohydrates was observed at pH 4.7. Equally high concentration of total VFAs, 25.1 and 24.6 g/L, was recorded at pH 4.7 and 5.0 respectively. Acetic, butyric and propionic acid were the main volatile fatty acids detected, while lactic acid was identified as a major intermediate metabolite of the studied process which presented an intense accumulation prior to its conversion to butyric and/or acetic acid and hydrogen.
The optimum conversion of the studied EoL-DPs’ mixture to biohydrogen (40.6 mL H2/g VSadded or 24.3 mL H2/g CODadded) was identified at pH 5.0, with hydrogen production being primarily associated with the bioconversion of lactic acid to butyric acid.
KeywordsEnd-of-life dairy products Biohydrogen production Dark fermentation Acidogenesis Anaerobic digestion
Financial support from the European Commission Project LIFE10 ENV/CY/000721 (DAIRIUS) “Sustainable management via energy exploitation of end-of-life dairy products in Cyprus” is gratefully acknowledged.
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