Abstract
ABSTRACT Anaerobic digestion (AD) is a well-established technology used for producing biogas or biomethane alongside the slurry used as biofertilizer. However, using a variety of wastes and residuals as substrate and mixed cultures in the bioreactor makes AD as one of the most complicated biochemical processes employing hydrolytic, acidogenic, hydrogen-producing, acetate-forming bacteria as well as acetoclastic and hydrogenoclastic methanogens. Hydrogen and volatile fatty acids (VFAs) including acetic, propionic, isobutyric, butyric, isovaleric, valeric and caproic acid and other carboxylic acids such as succinic and lactic acids are formed as intermediate products. As these acids are important precursors for various industries as mixed or purified chemicals, the AD process can be bioengineered to produce VFAs alongside hydrogen and therefore biogas plants can become biorefineries. The current review paper provides the theory and means to produce and accumulate VFAs and hydrogen, inhibit their conversion to methane and to extract them as the final products. The effects of pretreatment, pH, temperature, hydraulic retention time (HRT), organic loading rate (OLR), chemical methane inhibitions, and heat shocking of the inoculum on VFAs accumulation, hydrogen production, VFAs composition, and the microbial community were discussed. Furthermore, this paper highlights the possible techniques for recovery of VFAs from the fermentation media in order to minimize product inhibition as well as to supply the carboxylates for downstream procedures.
Highlights
The growing global population has led to an increase in the amounts of organic wastes that require appropriate treatment
The preceding sections of this paper have provided a description of how the Anaerobic digestion (AD) process can be employed for the production of intermediate metabolites with high market value, namely volatile fatty acids (VFAs) and hydrogen, instead of biogas
Due to the vast application potentials of these intermediate products, it is envisioned that existing AD facilities would possibly be converted into biorefineries
Summary
The growing global population has led to an increase in the amounts of organic wastes that require appropriate treatment. Research in the application of the anaerobic digestion process is increasingly shifting to other intermediate metabolites of higher value than biogas that includes hydrogen and carboxylic acids such as lactic, succinic and volatile fatty acids (VFAs) [3]. Anaerobic digestion (AD) for production of renewable energy in the form of biogas or biomethane and even hydrogen, through the socalled dark fermentation, is well documented in the literature and there are several good reviews on these topics [10–12]. AD can generally be considered as a three-phase process that entails; (1) the breakdown of complex organic matter in the raw substrate into soluble substances, (2) the formation of VFAs and hydrogen and (3) the consumption of the VFAs and hydrogen for biogas production.
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