Bioengineering of anaerobic digestion for volatile fatty acids, hydrogen or methane production: A critical review

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.

MBR-Assisted VFAs Production from Excess Sewage Sludge and Food Waste Slurry for Sustainable Wastewater Treatment

“…The average composition of the VFAs was as follows: acetic acid 42.63%, propionic acid 17.29%, iso-butyric acid 1.89%, butyric acid 19.55%, isovaleric acid 15.26%, and valeric acid 3.34% of total VFAs. The composition of the VFAs is linked to the microbial structure in the bioreactor and the established microbial pathways [8]. In the prevailing operating conditions, the dominating bacteria community seemed to favor the biosynthesis of mainly acetic, butyric, and isovaleric acids.…”

“…These compounds can be further converted into several high-value products, such as bioplastics, butanol and biodiesel [9]. The VFAs, such as acetic, butyric, and propionic acids, produced through AD, can be used directly or indirectly as chemical building blocks in, for instance, textile and plastic production industries [8]. Along with VFAs, hydrogen is produced during acidogenesis, which has attracted great attention as a clean energy source.…”

“…The biochemical reactions in the AD process take place in four interdependent stages namely: hydrolysis, acidogenesis, acetogenesis, and methanogenesis ( Figure 2) [7]. Biomethane is produced in the last stage of the AD process, while other metabolites such as volatile fatty acids (VFAs) and hydrogen are formed in the intermediate stages [8]. Biogas suffers low commercial value; however, a promising but hitherto not well-developed line of research is to extract and transform the intermediate products of AD (VFAs and H 2 ).…”

Combined freezing-thawing pretreatment and microbial electrolysis cell for enhancement of highly concentrated organics degradation from dewatered sludge

“…One way to increase the hydrolysis rate, as stated, was introducing the pretreatment method. To date plenty of studies focused on the pretreatment to enhance the digestion performance [8][9][10], few researches were reported for the influence of the pretreatment method on the MEC in terms of sludge [11,12]. Our previous study [12] demonstrated the efficiency of alkaline pretreatment for DS as this process affected soluble organic matter content and microbial community structures during electrogenesis.…”

Glucose fermentation with biochar-amended consortium: microbial consortium shift

“…Acid-producing strains produce volatile fatty acids (VFA), hydrogen (H 2 ) gas and carbon dioxide (CO 2 ) gas from complicated organic molecules via fermentation reactions [1,2]. In fermentation, dark fermentation presents an effective pathway to obtain valuable chemicals from organic waste [3,4].…”