Fermentative energy conversion: Renewable carbon source to biofuels (ethanol) using Saccharomyces cerevisiae and downstream purification through solar driven membrane distillation and nanofiltration
Graphical abstract
Introduction
More than 80% of the energy consumed worldwide is supplied by fossil fuel produced by geological processes and such fossil fuel-derived energy consumption results in very massive global warming. If the fossil fuel consumption continues at this rate then oil reserves will be exhausted within 50 years [1]. Thus energy security and environmental safety push towards the development of new technologies in the area of renewable energy sources like solar, wind, biomass and geothermal [2]. To achieve the sustainable development goals, the future energy should be clean energy and should come from renewable sources only. Bioethanol fits in these parameters and is one of the promising alternatives to fossil fuels as it can be mixed with gasoline producing transport fuel with possibility of reduced CO and hydrocarbon emission and reduced greenhouse effect [3]. Bioethanol is an excellent fuel not only for transport and electricity but also has multiple advantages like use of natural bio-resources, energy security, environmental safety and socio-economic development of rural sector [4]. To reduce dependency on fossil fuel, large and hugely populated countries like India can use their enormous quantity of biomass for biofuel generation which is still very low (only 2%) vis-à-vis USA, European Union and Brazil where about 80% of world’s total bioethanol production takes place. World bioethanol production has sharply increased from 74 billion liters in 2009 to 100 billion liters in 2014 used as fuel as per report of World Bioenergy Association 2014.
Feedstock for the biofuel production should be easily available and cheap. Literature shows microwave-alkali-acid pre-treated rice straw [5], cheese whey, lactose [6], Madhuca indica (Mahua) [7], macroalgae Padina tetrastromatica, Scenedesmus dimorphus [8], [9], paperboard mill sludge [10] and waste cotton material [11] as ideal bio-sources. However, in India, sugarcane based product like sugarcane juice, molasses and bagasse are used as carbon source for the bioethanol production [12]. Carbon sources like molasses, cheese whey, bagasse, etc. are cheap and low cost material. However, these materials need additional pre-treatment to release the fermentable sugar for bioethanol production. Sugarcane juice being clean and renewable carbon source could be explored for bioethanol production in membrane-integrated fermenter system to avoid membrane clogging. Countries like India, Brazil and USA are major sugarcane producers where sugarcane crops can be found round the year [13]. Saccharomyces cerevisiae or yeast commonly used for the fermentation of sugarcane juice for industrial bioethanol production. In literature, batch [14], fed batch [15] and continuous fermentation [16], [17], [18] are available for ethanol production.
In chemical, biochemical, therapeutic and biotechnological production processes, separation of products and downstream purification play the critical role in its final purity and economic viability [19]. To economically produce bioethanol, problems emanating from the issues of high energy consumption and azeotropism need to be overcome. During refining, approximately 40% of the total energy consumption is involved in dehydration of ethanol from the fermentation broth (containing 14 wt% ethanol) to pure ethanol [20]. In addition to this, energy requirement to remove water by distillation from 92 wt% to 95.6% increases exponentially due to ethanol-water azeotropic formation. To avoid this, molecular sieves with zeolite beads technique has been used in purifying 91 wt% ethanol to 99 wt% ethanol [21]. Membrane separation processes can be successfully integrated with conventional fermentation system for purification of ethanol exploiting high selectivity of membranes. A membrane-based production technology has the potential to be viable alternative to many conventional technologies that involve various operational units like centrifugation, extraction or vacuum distillation along with high energy, high labour cost [22]. Such conventional processes invariably lead to generation of huge amount of wastewater also. The membrane systems offer high degree of process intensification which implies that the process being compact involves low material as well as energy consumption vis-à-vis a conventional process while offering environmentally benign operation [23]. In a conventional process, bioethanol production drops drastically after its concentration reaches 12% by volume due to product inhibition during batch fermentation [24] whereas continuous fermentation in membrane-integrated hybrid system, overcomes product inhibition and results in significant improvement in yield, cell-density (due to cell recycle), product purity and process control [25]. Continuous fermentation under high cell density has also been reported using two-tank system equipped with settler for yeast cell recycling to ensure enhanced productivity from 6.9 g L−1 h−1 to 7.5 g L−1 h−1 [16]. In some studies [17], [18], 0.25 vvm aeration rate has been maintained during ethanol fermentation where hollow fiber microfiltration membrane (pore size 0.65 μm) has been used for cell recycling to ensure high cell density for maximum productivity (7.94 g L−1 h−1) and maximum carbon source utilization. However, fouling was major problem for such membrane systems whereas in case of flat sheet cross flow membrane module of this study membrane fouling is quite low [23].
Direct contact membrane distillation (DCMD) may be successfully applied for bioethanol separation after fermentation using porous and hydrophobic membranes [26]. During DCMD, the hot stream of ethanol solution and deionized water as cold stream flow along the two opposite surfaces of the hydrophobic membranes where generated ethanol vapour permeate through the pores of the membrane and condensed directly by cold distillate. Thus the vapour pressure gradient between the two sides of the membrane resulting from the temperature difference is the necessary driving force for the desired mass transfer [27]. Due to high partial pressure (compared to that of water), ethanol vapour preferentially passes through the pores of the hydrophobic membrane [28].
So long two broad approaches have been adopted for bioethanol production and these are either separate saccharification (hydrolysis of complex carbon source), fermentation and distillation [2] or simultaneous saccharification, fermentation and membrane distillation to get high yield, productivity and concentration in comparison to fermentation without membrane distillation [26], [29]. However, direct coupling of fermentation with pervaporation/MD could significantly deteriorate the membrane permeability, selectivity and its efficiency due to high concentration of yeast cells, residual carbon source and byproduct like proteins present in fermentation broth [30]. It has been confirmed Tomaszewska and Białończyk [6] through FTIR analysis that during simultaneous fermentation and membrane distillation for ethanol recovery, the hydrophobic membranes are seriously damaged/fouled due to infiltration or adsorption of yeast cells and its by-products in the pores on the surface of the membrane, resulting in fouling as well as partial or full wetting of the hydrophobic membranes. Such rapid fouling of membrane surface and its wettability may limit the industrial scale application of membrane distillation. However, module design plays significant role heat and mass transfer coefficients and concentration polarization during DCMD. So, when clear fermentation broth after microfiltration and nanofiltration is obtained as feed for DCMD in good module design, membrane fouling and membrane wettability problems can be largely overcome during membrane distillation. Most of the reported studies have been either using direct fermentation broth or model solution for ethanol separation by MD/DCMD leading to membrane wetting and hence low flux [6], [31].
In this experimental investigation, we have tried to evaluate the application of membrane-integrated fermenter with MF-NF-DCMD system for the continuous production, downstream purification and concentration enrichment of bioethanol from fermentation broth using clean, cheap and renewable carbon source viz. sugarcane juice. The development of sustainable and green process for continuous production with instant separation and enrichment of ethanol using optimized membranes for MF, NF and DCMD using flat-sheet cross-flow membrane module have been successfully carried out. The novel system represents small, compact, flexible, environmentally benign plant representing high degree of process intensification for ethanol production. A similar study has not been reported.
Section snippets
Microorganism, medium, reagents and membranes
The lyophilized powder of yeast cells, Saccharomyces cerevisiae (NCIM 3205) has been procured from National Collection of Industrial Microorganism (NCIM), National Chemical Laboratory (Pune, India). It was first inoculated on agar plate of composition, yeast extract, 20 g L−1; Maltose, 3.0 g L−1, peptone, 5.0 g L−1, glucose, 10 g L−1 and agar, 20 g L−1 to induce spore formation and maintained at 4 °C by weekly sub-culturing. The seed culture was made in 1 L flask with 250 mL of seed medium having same
Microfiltration for long term operation for cell recycle during continuous fermentation
Microfiltration was done to separate the yeast cells from the fermentation broth so that clear permeate could be obtained as feed to the nanofiltration unit. The cross flow MF module recycled the retained cells back to the fermenter thereby increasing the density of cells in the reactor to achieve a significant increase in ethanol productivity from 7.1 g L−1 h−1 to 9.2 g L−1 h−1. Permeate flux and rate of fouling were studied during microfiltration using two different membranes (PVDF - 0.45 µm and
Conclusions
A new type of membrane based fermenter integrated with downstream microfiltration, nanofiltration and direct contact membrane distillation (MF, NF and DCMD) has shown the potential to be a sustainable technology for continuous production and purification of ethanol from cheap, renewable carbon source by Saccharomyces cerevisiae. The unconverted sugars and useful ions are recycled back to the fermenter through membrane (NF2) filtration that permeates ∼99% bioethanol while retaining and recycling
Acknowledgements
Authors are thankful to the University Grant Commission, Government of India for financial support under Dr. D.S. Kothari Post-Doctoral Fellowship sanctioned No. F.4-2/2006 (BSR)/EN/16-17/ 0001, September 01, 2016 (65th List).
Conflict of interest
The authors declare no conflict of interest.
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