Raw dark fermentation effluent to support heterotrophic microalgae growth: microalgae successfully outcompete bacteria for acetate

International audienceCoupling dark fermentation (DF), which produces hydrogen from diverse effluents or solid waste, and heterotrophic cultivation of microalgae, which produces lipids, carbohydrates and proteins, is a promising and innovative solution for developing sustainable biorefineries. The use of a raw DF effluent, containing acetate and butyrate, to support the heterotrophic growth of Chlorella sorokiniana was investigated. All the acetate in sterilized and unsterilized DF effluent was exhausted in less than three days of heterotrophic cultivation, whereas butyrate was not used by the microalgae. The microalgae biomass reached 0.33 g L− 1 with a carbon yield on acetate of 55%. The algal yield was higher than previously reported for synthetic DF effluent. It was concluded that compounds other than volatile fatty acids were present in the DF effluent and these could be consumed by the microalgae. After the acetate had been exhausted, butyrate was consumed by facultative and strict aerobic bacteria originating from the DF effluent. The concentration of the bacterial community increased during the experiment but did not have any significant impact on heterotrophic microalgae growth. A high microalgal biomass yield was achieved without requiring the DF effluent to be sterilized

Formic acid pretreatment for enhanced production of bioenergy and biochemicals from organic solid waste

Organic solid waste is one of the most promising feedstocks for the implementation of the circular economy principles in waste management. Its anaerobic treatment can indeed promote organic matter conversion into a number of value-added products as well as energy carriers. However, the identification of sustainable strategies to handle organic solid waste in a biorefinery framework still poses technological as well as economic challenges. The aim of this study was in enhancing the potential of the organic fraction of municipal solid waste (OFMSW) to produce bioenergy and biochemicals by combining dark fermentation with a formic acid pretreatment. Hydrogen yields up to 31.6 ml/gVS were obtained pre-treating the OFMSW with 5% formic acid, at 80 °C for 70 min. Concomitantly, a wide range of metabolites of market significance, including acetic acid, butyric acid and ethanol, accumulated. The concentration of these metabolites further enhanced after the dark fermentation of the pretreated substrates. Experimental tests highlighted the influence of the different pretreatment operating conditions on the relative production of hydrogen and main metabolites as well as the related pathways. It was found that the acid concentration plays a key role in promoting the biological conversion of OFMSW and that the adjustment of the operating temperature and treatment time can be targeted towards the production of either building blocks or energy carriers, so as to ensure the viability of the process for its scale up

Impact of the microbial inoculum source on pre-treatment efficiency for fermentative H-2 production from glycerol

Hydrogen (H-2) production by dark fermentation can be performed from a wide variety of microbial inoculum sources, which are generally pre-treated to eliminate the activity of H-2 consuming species and/or enrich the microbial community with H-2-producing bacteria. This paper aims to study the impact of the microbial inoculum source on pre-treatment behavior, with a special focus on microbial community changes. Two inocula (aerobic and anaerobic sludge) and two pre-treatments (aeration and heat shock) were investigated using glycerol as substrate during a continuous operation. Our results show that the inoculum source significantly affected the pre-treatment efficiency. In aerobic sludge no pre-treatment is necessary, while in anaerobic sludge the heat pre-treatment increased H-2 production but aeration caused unstable H-2 production. In addition, biokinetic control was key in Clostridium selection as dominant species in all microbial communities. Lower and unstable H-2 production were associated with a higher relative abundance of Enterobacteriaceae family members. Our results allow a better understanding of H-2 production in continuous systems and how the microbial-community is affected. This provides key information for efficient selection of operating conditions for future applications

A review of dark fermentative biohydrogen production from organic biomass: process parameters and use of by-products

Dark fermentation of organic biomass, i.e. agricultural residues, agro-industrial wastes and organic municipal waste is a promising technology for producing renewable biohydrogen. In spite of its potential, this technology needs further research and development to improve the biohydrogen yield by optimizing substrate utilization, microbial community enrichment and bioreactor operational parameters such as pH, temperature and H2 partial pressure. On the other hand, the technical and economic viability of the processes need to be enhanced by the use of valuable by-products from dark fermentation, which mostly includes volatile fatty acids. This paper reviews a range of different organic biomasses and their biohydrogen potential from laboratory to pilot-scale systems. A review of the advances in H2 yield and production rates through different seed inocula enrichment methods, bioreactor design modifications and operational conditions optimization inside the dark fermentation bioreactor is presented. The prospects of valorizing the co-produced volatile fatty acids in photofermentation and bioelectrochemical systems for further H2 production, methane generation and other useful applications have been highlighted. A brief review on the simulation and modeling of the dark fermentation processes and their energy balance has been provided. Future prospects of solid state dark fermentation are discussed

Do furanic and phenolic compounds of lignocellulosic and algae biomass hydrolyzate inhibit anaerobic mixed cultures ? A comprehensive review

Nowadays there is a growing interest on the use of both lignocellulosic and algae biomass to produce biofuels (i.e. biohydrogen, ethanol and methane), as future alternatives to fossil fuels. In this purpose, thermal and thermo-chemical pretreatments have been widely investigated to overcome the natural physico-chemical barriers of such biomass and to enhance biofuel production from lignocellulosic residues and, more recently, marine biomass (i.e. macro and microalgae). However, the pretreatment technologies lead not only to the conversion of carbohydrate polymers (le cellulose, hemicelluloses, starch, agar) to soluble monomeric sugar (ie glucose, xylose, arabinose, galactose), but also the generation of various by-products (i.e. furfural and 5-HMF). In the case of lignocellulosit residues, part of the lignin can also be degraded in lignin derived by-products, mainly composed of phenolic compounds. Although the negative impact of such by-products on ethanol production has been widely described in literature, studies on their impact on biohydrogen and methane production operated with mixed cultures are still very limited. This review aims to summarise and discuss literature data on the impact of pre-treatment by-products on H-2-producing dark fermentation and anaerobic digestion processes when using mixed cultures as inoculum. As a summary, furanic (5-HMF, furfural) and phenolic compounds were found to be stronger inhibitors of the microbial dark fermentation than the full anaerobic digestion process. Such observations can be explained by differences in process parameters: anaerobic digestion is performed with more complex mixed cultures, lower substrate/inoculum and by-products/inoculum ratios and longer batch incubation times than dark fermentation. Finally, it has been reported that, during dark fermentation process, the presence of by-products could lead to a metabolic shift from H-2-producing pathways (i.e. acetate and butyrate) to non-H-2-producing pathways (i.e. lactate, ethanol and propionate) and whatever the metabolic route, metabolites can be all further converted into methane, but at different rates