Biohydrogen production from dairy manures with acidification pretreatment by anaerobic fermentation

Abstract

Background, aim, and scope

Hydrogen is a clean and efficient energy source and has been deemed as one of the most promising carriers of new energy for the future. From an engineering point of view, producing hydrogen by mixed cultures is generally preferred because of lower cost, ease of control, and the possible use of organic waste as feedstock. The biological hydrogen production has been intensively studied in recent decades. So far, most investigates of biohydrogen production are still confined to using pure carbohydrates and carbohydrate-rich wastewater. Nowadays, the large amounts of livestock manure, which come from cattle feedlots, poultry, and swine buildings, are causing a major environmental issue because it has become a primary source of odors, gases, dust, and groundwater contamination. The increasingly stringent requirements for pollution control on livestock manures are challenging the scientific community to develop new waste treatment strategies. Thus, there is a pressing need to develop nonpolluting and renewable energy source utilizing the organic waste (e.g., livestock manure). It is well known that anaerobic digestion had successfully been used for the disposal of manures to produce methane in the last two decades. Recently, an alternative strategy has been developed to convert livestock manures (e.g., dairy manures) to biohydrogen as a high value-added clean energy source instead of methane. However, little information is available on hydrogen production from dairy manure via the mixed anaerobic microbe. As far as we know, the hydrogen production is habitually accompanied with production of volatile fatty acids (VFAs), such as acetate, butyrate, and propionate, which are also an optimal feedstock for production of methane by anaerobic digestion. Provided that the biohydrogen production from dairy manure is further combined with the anaerobic digestion of the effluent from the producing hydrogen reactor that would be a one-stone two-bird paradigm, it not only produces a clean and readily usable biologic energy but also cleans up simultaneously the environment in a sustainable fashion.

Materials and methods

Prior to use, the dairy manures as natural hydrogen-producing microflora/feedstock were pretreated by infrared radiation/boiling heat by 0.2% HCl, respectively. The batch experiments were preformed with 250 mL serum vials as batch reactors filled with 100 mL mixtures, comprising the inoculum from the pre-incubated dairy manures and the feedstock from acid pretreated dairy manures as stated in Sections 2.1 and 2.2. No extra nutrients were added into the serum vials. The scale-up test was performed in a 5-L continuous stirred anaerobic bioreactor. The concentration of hydrogen, carbon dioxide, and VFAs were measured by gas chromatograph equipped with a thermal conductivity detector and a flame ionization detector, respectively. All the experiments were carried out independently in triplicates.

Results and discussion

Dairy manures with acidification pretreatment had a maximum H₂ yield of 31.5 ml/g-TVS treating 70 g/L of substrate at operating pH 5.0. Meanwhile, the oxidation–reduction potential (ORP) value stayed stable at around −500 to −520 mV during the optimal hydrogen-producing period. The effluent was composed mostly of acetate and butyrate, which accounted for 78.2–81.4% of total VFAs. There was no significant methane observed in the tests. Experimental results indicated that the acidification pretreatment of dairy manure, substrate concentration, and operating pH and ORP level all had an individual significant influence on bio-H₂ production.

Conclusions

The feasibility of H₂ generation utilizing dairy manures as feedstock by anaerobic fermentation was demonstrated in this study. Biohydrogen production was found most effective utilizing acid pretreated dairy manures as feedstock at operating pH of 5.0 and substrate concentration of 70.0 g-TVS/L using pre-incubated dairy manures as inoculum. The maximal hydrogen yield of 31.5 mL H₂/g-TVS and corresponding hydrogen content of 38.6% were observed; the value was higher than previously reported.

Recommendations and perspectives

The biohydrogen production from organic wastes, such as dairy manures, is an attractive paradigm because it could produce clean biologic energy and simultaneously lean up the environment in an environmentally friendly fashion. In the present work, the biohydrogen production from dairy manures as the feedstock by mixed cultures was systematically investigated. This would provide ternary environmental benefits, viz., clean energy generation, effective method of organic waste treatment with simultaneously supplying an ideal feedstock for methane production. It is expected that the results obtained from this work could provide some valuable information for bio-H₂ production from livestock manure.