Environmental impacts of valorisation of crude glycerol from biodiesel production - A life cycle perspective

Biodiesel production produces significant quantities of impure crude glycerol as a by-product. Recent increases in the global biodiesel production have led to a surplus of crude glycerol, rendering it a waste. As a result, different methods for its valorisation are currently being investigated. This paper assesses the life cycle environmental impacts of an emerging technology for purification of crude glycerol - a multi-step physico-chemical treatment - in comparison to incineration with energy recovery commonly used for its disposal. For the former, three different acids (H3PO4, H2SO4 and HCl) are considered for the acidification step in the purification process. The results suggest that the H2SO4-based treatment is the best option with 17 net-negative impacts out of the 18 categories considered; this is due to system credits for the production of purified glycerol, heat and potassium salts. In comparison to incineration with energy recovery, the H2SO4-based process has lower savings for the climate change impact (-311 versus -504 kg CO2 eq./t crude glycerol) but it performs better in ten other categories. Sensitivity analyses suggest that that the impacts of the physico-chemical treatment are highly dependent on crude glycerol composition, allocation of burdens to crude glycerol and credits for glycerol production. For example, treating crude glycerol with lower glycerol content would increase all impacts except climate change and fossil depletion due to the higher consumption of chemicals and lower production of purified glycerol. Considering crude glycerol as a useful product rather than waste and allocating to it burdens from biodiesel production would increase most impacts significantly, including climate change (22-40 %), while fossil depletion, freshwater and marine eutrophication would become net-positive. The findings of this research will be of interest to the biodiesel industry and other industrial sectors that generate crude glycerol as a by-product.

Kinetics-based development of two-stage continuous fermentation of 1,3-propanediol from crude glycerol by Clostridium butyricum

BACKGROUND

Glycerol, as a by-product, mainly derives from the conversion of many crops to biodiesel, ethanol, and fatty ester. Its bioconversion to 1,3-propanediol (1,3-PDO) is an environmentally friendly method. Continuous fermentation has many striking merits over fed-batch and batch fermentation, such as high product concentration with easy feeding operation, long-term high productivity without frequent seed culture, and energy-intensive sterilization. However, it is usually difficult to harvest high product concentrations.

RESULTS

In this study, a three-stage continuous fermentation was firstly designed to produce 1,3-PDO from crude glycerol by Clostridium butyricum, in which the first stage fermentation was responsible for providing the excellent cells in a robust growth state, the second stage focused on promoting 1,3-PDO production, and the third stage aimed to further boost the 1,3-PDO concentration and reduce the residual glycerol concentration as much as possible. Through the three-stage continuous fermentation, 80.05 g/L 1,3-PDO as the maximum concentration was produced while maintaining residual glycerol of 5.87 g/L, achieving a yield of 0.48 g/g and a productivity of 3.67 g/(L·h). Based on the 14 sets of experimental data from the first stage, a kinetic model was developed to describe the intricate relationships among the concentrations of 1,3-PDO, substrate, biomass, and butyrate. Subsequently, this kinetic model was used to optimize and predict the highest 1,3-PDO productivity of 11.26 g/(L·h) in the first stage fermentation, while the glycerol feeding concentration and dilution rate were determined to be 92 g/L and 0.341 h-1, separately. Additionally, to achieve a target 1,3-PDO production of 80 g/L without the third stage fermentation, the predicted minimum volume ratio of the second fermenter to the first one was 11.9. The kinetics-based two-stage continuous fermentation was experimentally verified well with the predicted results.

CONCLUSION

A novel three-stage continuous fermentation and a kinetic model were reported. Then a simpler two-stage continuous fermentation was developed based on the optimization of the kinetic model. This kinetics-based development of two-stage continuous fermentation could achieve high-level production of 1,3-PDO. Meanwhile, it provides a reference for other bio-chemicals production by applying kinetics to optimize multi-stage continuous fermentation.

A systematic review on utilization of biodiesel-derived crude glycerol in sustainable polymers preparation

With the rapid development of biodiesel, biodiesel-derived glycerol has become a promising renewable bioresource. The key to utilizing this bioresource lies in the value-added conversion of crude glycerol. While purifying crude glycerol into a pure form allows for diverse applications, the intricate nature of this process renders it costly and environmentally stressful. Consequently, technology facilitating the direct utilization of unpurified crude glycerol holds significant importance. It has been reported that crude glycerol can be bio-transformed or chemically converted into high-value polymers. These technologies provide cost-effective alternatives for polymer production while contributing to a more sustainable biodiesel industry. This review article describes the global production and quality characteristics of biodiesel-derived glycerol and investigates the influencing factors and treatment of the composition of crude glycerol including water, methanol, soap, matter organic non-glycerol, and ash. Additionally, this review also focused on the advantages and challenges of various technologies for converting crude glycerol into polymers, considering factors such as the compatibility of crude glycerol and the control of unfavorable factors. Lastly, the application prospect and value of crude glycerol conversion were discussed from the aspects of economy and environmental protection. The development of new technologies for the increased use of crude glycerol as a renewable feedstock for polymer production will be facilitated by the findings of this review, while promoting mass market applications.

Valorisation of crude glycerol in the production of liquefied lignin bio-polyols for polyurethane formulations

Bio-polyols, produced by liquefying lignin with polyhydric alcohols, offer a promising alternative to conventional polyols for polyurethane production. To enhance the sustainability on the production of these bio-polyols, this study proposes the use of crude glycerol and microwave-assisted liquefaction as substitutes for conventional methods and commercial glycerol. This approach reduces the energy requirements of the reaction while also adding value to this by-product. The synthesis of bio-polyols with suitable properties to produce elastic and rigid polyurethane was carried out using previously optimised reaction conditions. Organosolv lignins obtained from Eucalyptus globulus and Pinus radiata were employed, using polyethylene glycol and crude glycerol as solvents and sulphuric acid as a catalyst. Several parameters of the bio-polyols were analysed, including hydroxyl number (IOH), acid number (An), and functionality (f), suggesting that the bio-polyols were suitable for polyurethane synthesis. Bio-polyols formulated to produce rigid polyurethanes exhibited IOH values of 554 and 383 (mg KOH/g), An values of 1.91 and 4.21 (mg KOH/g), and functionalities of 4.16 and 3.14 for Eucalyptus globulus and Pinus radiata lignin. In the case of bio-polyols for elastic polyurethanes, the values were 228 and 173 (mg KOH/g) (IOH), 20.94 and 25.09 (mg KOH/g) (An), and functionalities of 3.51 and 2.08.

YLGW01

Petrochemical-based plastics are hardly biodegradable and a major cause of environmental pollution, and polyhydroxybutyrate (PHB) is attracting attention as an alternative due to its similar properties. However, the cost of PHB production is high and is considered the greatest challenge for its industrialization. Here, crude glycerol was used as a carbon source for more efficient PHB production. Among the 18 strains investigated, Halomonas taeanenisis YLGW01 was selected for PHB production due to its salt tolerance and high glycerol consumption rate. Furthermore, this strain can produce poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co-3 HV)) with 17 % 3 HV mol fraction when a precursor is added. PHB production was maximized through medium optimization and activated carbon treatment of crude glycerol, resulting in 10.5 g/L of PHB with 60 % PHB content in fed-batch fermentation. Physical properties of the produced PHB were analyzed, i.e., weight average molecular weight (6.8 × 10⁵), number average molecular weight (4.4 × 10⁵), and the polydispersity index (1.53). In the universal testing machine analysis, the extracted intracellular PHB showed a decrease in Young's modulus, an increase in Elongation at break, greater flexibility than authentic film, and decreased brittleness. This study confirmed that YLGW01 is a promising strain for industrial PHB production using crude glycerol.

Economic and environmental evaluation for a closed loop of crude glycerol bioconversion to biodiesel

Crude glycerol, a byproduct of biodiesel production, was utilized as a carbon source to produce microbial lipids by the oleaginous yeast Rhodotorula toruloides in this study. The maximum lipid production and lipid content were 10.56 g/L and 49.52%, respectively, by optimizing fermentation conditions. The obtained biodiesel met the standards of China, the United States, and the European Union. The economic value of biodiesel produced from crude glycerol increased by 48% compared with the sale of crude glycerol. In addition, biodiesel production from crude glycerol could reduce 11,928 tons of carbon dioxide emissions and 55 tons of sulfur dioxide emissions. This study provides a strategy for a closed loop of crude glycerol to biofuel and ensures sustainable and stable development of the biodiesel industries.

Characterization of crude glycerol and glycerol pitch from palm-based residual biomass

Crude glycerol (CG) and glycerol pitch (GP) are highly alkaline residues from biodiesel and oleochemical plants, respectively, and have organic content which incurs high disposal cost and poses an environmental threat. Characterization of these residues for composition and properties could provide insight into their quality for proper disposal and can help the biodiesel industry to adopt more sustainable practices, such as reducing waste and improving the efficiency of the production process, hence minimizing the impact of the biodiesel supply chain to the environment. These data also allow the identification and exploration of new ways for their utilization and transformation into highly value-added products. In this study, we evaluated four CG samples (B, C, D, and E) and two GP samples (F and G) obtained from Malaysian palm oil refineries, and the results were compared with pure glycerol (A). Spectroscopic analysis was performed using FTIR, 1H-, and 13C-NMR. All samples had similar density to A (1.26 g/cm3), except for F (1.31 g/cm3), while the density for E and G could not be determined due to their physical states. The pH and viscosity largely varied in the range of 7.26-11.89 and 43-225 cSt, respectively. The glycerol content of CG (B, C, D, and E) was high and consistent (81.7-87.3%) whereas GP F and G had 71.5 and 63.9% glycerol content, respectively. Major contaminants in CG and GP were water and matter organic non-glycerol (MONG), respectively. The water, ash, soap, and salt content were considerably low, which varied from 3.4 to 14.1%, 3.9 to 13.0%, 0.1 to 5.7%, and 4.1 to 9.2% respectively. Thermal analysis of CG and GP exhibited four phases of decomposition attributed to the impurities compared to the single phase in A. All samples had calorific values lower than A (18.1 MJ/kg) between 9.0 and 17.7 MJ/kg. Based on the results, CG and GP have high glycerol content which reveals their potential to be used as feedstock in bioconversion and chemical or thermal treatment while impurities may be removed by pre-treatment if required. As palm oil is one of the main feedstocks for the oleochemical industry, this work underlines the importance of characterization of the residue generated to provide additional data and information on palm-based agricultural industry wastes, minimize the impact of palm oil supply chain on the environment, and explore its potential usage for value-addition.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13399-023-04003-4.

Polyhydroxybutyrate production by Cupriavidus necator in a corn biorefinery concept

The high costs of bioplastics' production may hinder their commercialization. Development of new processes with high yields and in biorefineries can enhance diffusion of these materials. This work evaluated the production of polyhydroxybutyrate (PHB) from the combination of milled corn starchy fraction hydrolysate and crude glycerol as substrates by the strain Cupriavidus necator LPB 1421. After optimization steps, maximum accumulation of 62 % of PHB was obtained, which represents 11.64 g.L^-1 and productivity of 0.162 g.Lh^-1. In a stirred tank bioreactor system with 8 L of operational volume, 70 % of PHB accumulation was reported, representing 14.17 g.L^-1 of the biopolymer with 0.197 g.Lh^-1 productivity. PHB recovery was conducted using a chemical digestion method, reaching >99 % purity. Therefore, the potential application of milled corn as substrate for PHB production was confirmed. The developed bioplastic process could be coupled to a bioethanol producing unit creating the opportunity of a sustainable and economic biorefinery.

Second generation Pichia pastoris strain and bioprocess designs

Yeast was the first microorganism used by mankind for biotransformation processes that laid the foundations of industrial biotechnology. In the last decade, Pichia pastoris has become the leading eukaryotic host organism for bioproduct generation. Most of the P. pastoris bioprocess operations has been relying on toxic methanol and glucose feed. In the actual bioeconomy era, for sustainable value-added bioproduct generation, non-conventional yeast P. pastoris bioprocess operations should be extended to low-cost and renewable substrates for large volume bio-based commodity productions. In this review, we evaluated the potential of P. pastoris for the establishment of circular bioeconomy due to its potential to generate industrially relevant bioproducts from renewable sources and waste streams in a cost-effective and environmentally friendly manner. Furthermore, we discussed challenges with the second generation P. pastoris platforms and propose novel insights for future perspectives. In this regard, potential of low cost substrate candidates, i.e., lignocellulosic biomass components, cereal by-products, sugar industry by-products molasses and sugarcane bagasse, high fructose syrup by-products, biodiesel industry by-product crude glycerol, kitchen waste and other agri-food industry by products were evaluated for P. pastoris cell growth promoting effects and recombinant protein production. Further metabolic pathway engineering of P. pastoris to construct renewable and low cost substrate utilization pathways was discussed. Although, second generation P. pastoris bioprocess operations for valorisation of wastes and by-products still in its infancy, rapidly emerging synthetic biology tools and metabolic engineering of P. pastoris will pave the way for more sustainable environment and bioeconomy. From environmental point of view, second generation bioprocess development is also important for waste recycling otherwise disposal of carbon-rich effluents creates environmental concerns. P. pastoris high tolerance to toxic contaminants found in lignocellulosic biomass hydrolysate and industrial waste effluent crude glycerol provides the yeast with advantages to extend its applications toward second generation P. pastoris strain design and bioprocess engineering, in the years to come.

A co-fermentation strategy with wood hydrolysate and crude glycerol to enhance the lipid accumulation in Rhodosporidium toruloides-1588

Wood hydrolysate has been regarded as sustainable and renewable substrate to produce microbial lipids, a potential feedstock for the biodiesel industry. Moreover, the major by-product of biofuel industries is crude glycerol but its implementation as a carbon source is still constrained due to the presence of impurities resulting in low biomass production and low lipid titer. Thus, this study investigates the effect of different carbon ratios of hydrolysate and crude glycerol on R. toruloides-1588. Hydrolysate to crude glycerol ratio of 60:40 resulted in maximum lipid accumulation of 49% (w/w), more than 90% of sugars and glycerol consumption. Further, scale-up to bench-scale fermenter resulted in 12% higher lipid accumulation (56.3% w/w, 0.15 g/L∙h) in 50% less time than flask fermentation. Hence, the ability of R. toruloides-1588 to flourish on different carbohydrates and accumulate high lipid content will be beneficial for the further development of biorefinery industries.

Technological advancements in valorization of second generation (2G) feedstocks for bio-based succinic acid production

Succinic acid (SA) is used as a commodity chemical and as a precursor in chemical industry to produce other derivatives such as 1,4-butaneidol, tetrahydrofuran, fumaric acid, and bio-polyesters. The production of bio-based SA from renewable feedstocks has always been in the limelight owing to the advantages of renewability, abundance and reducing climate change by CO₂ capture. Considering this, the current review focuses on various 2G feedstocks such as lignocellulosic biomass, crude glycerol, and food waste for cost-effective SA production. It also highlights the importance of producing SA via separate enzymatic hydrolysis and fermentation, simultaneous saccharification and fermentation, and consolidated bioprocessing. Furthermore, recent advances in genetic engineering, and downstream SA processing are thoroughly discussed. It also elaborates on the techno-economic analysis and life cycle assessment (LCA) studies carried out to understand the economics and environmental effects of bio-based SA synthesis.

Systematic engineering of Bacillus amyloliquefaciens for efficient production of poly-γ-glutamic acid from crude glycerol

Microbial production of poly-γ-glutamic acid (γ-PGA) from non-food raw materials is a promising alternative to food feedstocks-based biosynthesis. A superior cell factory of Bacillus amyloliquefaciens for the efficient synthesis of γ-PGA from crude glycerol was constructed through systematic metabolic engineering. Firstly, some phase-dependent promoters were screened from B. amyloliquefaciens, which can be used for fine regulation of subsequent metabolic pathways. Secondly, the glycerol utilization pathway and the γ-PGA synthesis pathway were co-optimized utilizing the above-screened promoters, which increased the titer of γ-PGA by 1.75-fold. Then, the titer of γ-PGA increased to 15.6 g/L by engineering transcription factors degU and blocking competitive pathways. Finally, combining these strategies with an optimized fermentation process, 26.4 g/L γ-PGA was obtained from crude glycerol as a single carbon source (a 3.72-fold improvement over the initial strain). Overall, these strategies will have great potential for synthesizing other products from crude glycerol in B. amyloliquefaciens.

New value from food and industrial wastes - Bioaccumulation of omega-3 fatty acids from an oleaginous microbial biomass paired with a brewery by-product using black soldier fly (Hermetia illucens) larvae

Research on bioconversion based on insects is intensifying as it addresses the problem of reducing and reusing food and industrial waste. To reach this goal, we need to find more means of pairing waste to insects. With this goal, brewers' spent grains (BSG) - a food waste of the brewing industry - paired with the oleaginous biomass of the thraustochytrid Schizochytrium limacinum cultivated on crude glycerol - a major waste of biodiesel production - were successfully used to grow Hermetia illucens larvae. Combining BSG and S. limacinum in the diet in an attempt to design the lipid profile of H. illucens larvae to contain a higher percentage of omega-3 fatty acids is novel. Insect larvae were grown on three different substrates: i) standard diet for Diptera (SD), ii) BSG, and iii) BSG + 10% S. limacinum biomass. The larvae and substrates were analyzed for fatty acid composition and larval growth was measured until 25% of insects reached the prepupal stage. Our data showed that including omega-3-rich S. limacinum biomass in the BSG substrate promoted an increase in larval weight compared to larvae fed on SD or BSG substrates. Furthermore, it was possible, albeit in a limited way, to incorporate omega-3 fatty acids, principally docosahexaenoic acid (DHA) from BSG + S. limacinum substrate containing 20% of DHA into the larval fat (7% DHA). However, H. illucens with this level of DHA may not be suitable if the aim is to get larvae with high omega-3 lipids to feed carnivorous fish.

Potential use of industrial by-products as promising feedstock for microbial lipid and lipase production and direct transesterification of wet yeast into biodiesel by lipase and acid catalysts

This work attempted the conversion of crude glycerol to lipid and lipase by Yarrowia lipolytica and the direct transesterification of wet yeast by its lipase into biodiesel via response surface methodology to enhance the cost-effectiveness of biodiesel production from the lipids. The yeast grew better and accumulated a high amount of lipids on the waste combined with fish waste hydrolysate, but only exhibited high lipase activity on the waste supplemented with surfactants (i.e., gum Arabic, Tween 20, Tween 80). However, the combination of both wastes and Tween 80 further improved growth, lipid productivity, and lipase activity. More importantly, lipase-direct transesterification under optimal conditions (wet cell concentration of 17.97 mg-DCW, methanol loading of 8.21 µL, and hexane loading of 10.26 µL) followed by acid-catalyst transesterification (0.4 M H₂SO₄), offered high FAME yields (>90%), showing the efficiency of the process when applied for the industrialization of biodiesel production from microbial lipids.

Rewiring the microbial metabolic network for efficient utilization of mixed carbon sources

Carbon sources represent the most dominant cost factor in the industrial biomanufacturing of products. Thus, it has attracted much attention to seek cheap and renewable feedstocks, such as lignocellulose, crude glycerol, methanol, and carbon dioxide, for biosynthesis of value-added compounds. Co-utilization of these carbon sources by microorganisms not only can reduce the production cost but also serves as a promising approach to improve the carbon yield. However, co-utilization of mixed carbon sources usually suffers from a low utilization rate. In the past few years, the development of metabolic engineering strategies to enhance carbon source co-utilization efficiency by inactivation of carbon catabolite repression has made significant progress. In this article, we provide informative and comprehensive insights into the co-utilization of two or more carbon sources including glucose, xylose, arabinose, glycerol, and C1 compounds, and we put our focus on parallel utilization, synergetic utilization, and complementary utilization of different carbon sources. Our goal is not only to summarize strategies of co-utilization of carbon sources, but also to discuss how to improve the carbon yield and the titer of target products.

Characterisation of bacterial nanocellulose and nanostructured carbon produced from crude glycerol by Komagataeibacter sucrofermentans

Bacterial nanocellulose (BNC), which has tunable properties, is a precursor of nanostructured energy storage materials; however, the cost of BNC production is challenging. This study uses crude glycerol from the biodiesel industry as a carbon nutrient and first-time carbonised BNC from K. sucrofermentans that is applied in energy storage. From crude glycerol in static cultivation, 6.4 g L^-1 BNC was produced with a high crystallinity index (85%) and tensile properties in comparison to conventionally used pure carbon substrates. Carbon materials were derived from the BNC retained fibrous and crystalline features with disordered porous structures. The electrochemical properties of the carbon materials have a specific capacitance of 140 F g^-1. This study highlights the valorisation of waste glycerol from the biodiesel industry as a substrate for efficient BNC production and the energy storage potential of carbon derived from BNC as renewable energy materials.

High value-added products derived from crude glycerol via microbial fermentation using Yarrowia clade yeast

BACKGROUND

Contemporary biotechnology focuses on many problems related to the functioning of developed societies. Many of these problems are related to health, especially with the rapidly rising numbers of people suffering from civilization diseases, such as obesity or diabetes. One factor contributing to the development of these diseases is the high consumption of sucrose. A very promising substitute for this sugar has emerged: the polyhydroxy alcohols, characterized by low caloric value and sufficient sweetness to replace table sugar in food production.

RESULTS

In the current study, yeast belonging to the Yarrowia clade were tested for erythritol, mannitol and arabitol production using crude glycerol from the biodiesel and soap industries as carbon sources. Out of the 13 tested species, Yarrowia divulgata and Candida oslonensis turned out to be particularly efficient polyol producers. Both species produced large amounts of these compounds from both soap-derived glycerol (59.8-62.7 g dm-3) and biodiesel-derived glycerol (76.8-79.5 g dm-3). However, it is equally important that the protein and lipid content of the biomass (around 30% protein and 12% lipid) obtained after the processes is high enough to use this yeast in the production of animal feed.

CONCLUSIONS

The use of waste glycerol for the production of polyols as well as utilization of the biomass obtained after the process for the production of feed are part of the development of modern waste-free technologies.

Effective bioconversion of 1,3-propanediol from biodiesel-derived crude glycerol using organic acid resistance-enhanced Lactobacillus reuteri JH83

Organic acids produced during the fermentation of lactic acid bacteria inhibit cellular growth and the production of 1,3-propanediol (1,3-PDO). Lactobacillus reuteri JH83, which has an increase of 18.6% in organic acid resistance, was obtained through electron beam irradiation mutagenesis irrelevant to the problem of genetically modified organisms. The maximum bioconversion of 1,3-PDO in fed-batch fermentation using pure glycerol by L. reuteri JH83 was 93.2 g/L at 72 h, and the productivity was 1.29 g/L·h, which achieved an increase by 34.6%, compared to that of the wild-type strain. In addition, the result of fed-batch fermentation for the production of 1,3-PDO using crude glycerol was not significantly different from that of pure glycerol. Additionally, transcriptome analysis confirmed changes in the expression levels of sucrose phosphorylase, which is a major facilitator superfamily transporter, and muramyl ligase family proteins, which protect lactic acid bacteria from various stressors, such as organic acids.

Gram-scale synthesis of ZnS/NiO core-shell hierarchical nanostructures and their enhanced H2 production in crude glycerol and sulphide wastewater

Design and development of the efficient and durable photocatalyst that generates H₂ fuel utilizing industrial wastewater under solar light irradiation is a sustainable process. Innumerable photocatalysts have been reported for efficient H₂ production, but their large-scale production with the same efficiency of H₂ production is a challenging task. In this study, a few gram-scale syntheses of ZnS wrapped with NiO hierarchical core-shell nanostructure via the surfactant-mediated process has been reported. Morphology and crystal structure analysis of ZnS/NiO showed spherical shaped hierarchical core-shell with cubic and face-centered cubic crystal structures. The surface examination confirmed the presence of Zn²⁺, S^2-, Ni²⁺ and O^2- ions in the nanocomposite. The photocurrent and photoluminescence studies of pristine and nanocomposites revealed that core-shell material is non-corrosive with a prolonged life-time of photo-excitons. Parametric studies on photocatalytic H₂ generation in lab-scale photoreactor using crude glycerol in water recorded a high rate of H₂ generation of 9.3 mmol h^-1.g^-1 of catalyst under the simulated solar light irradiation. Optimized reaction parameters are extended to a demonstrative photoreactor containing aqueous crude glycerol produced 18.5 mmol h^-1 of H₂ generation under the natural solar light irradiation. The same nanostructures were further tested with the simulated sulfide wastewater and the optimized catalyst showed H₂ production of 350 mL h^-1. The experimental results of time-on stream and catalytic stability demonstrated that ZnS/NiO hierarchical core-shell nanostructures can be recyclable and reusable for the continuous photocatalytic H₂ generation.

Glycerol waste to value added products and its potential applications

The rapid industrial and economic development runs on fossil fuel and other energy sources. Limited oil reserves, environmental issues, and high transportation costs lead towards carbon unbiased renewable and sustainable fuel. Compared to other carbon-based fuels, biodiesel is attracted worldwide as a biofuel for the reduction of global dependence on fossil fuels and the greenhouse effect. During biodiesel production, approximately 10% of glycerol is formed in the transesterification process in a biodiesel plant. The ditching of crude glycerol is important as it contains salt, free fatty acids, and methanol that cause contamination of soil and creates environmental challenges for researchers. However, the excessive cost of crude glycerol refining and market capacity encourage the biodiesel industries for developing a new idea for utilising and produced extra sources of income and treat biodiesel waste. This review focuses on the significance of crude glycerol in the value-added utilisation and conversion to bioethanol by a fermentation process and describes the opportunities of glycerol in various applications.

Graphic abstract

Effect of inoculum and organic loading on mixed culture polyhydroxyalkanoate production using crude glycerol as the substrate

Two different sources of activated sludge were inoculated to select and enrich polyhydroxyalkanoate (PHA) producing culture from crude glycerol. The results showed that the sludge taken from the wastewater treatment plant with higher microbial diversity could enrich PHA producing culture with higher PHA synthesis capacity (25.93%) and specific PHA storage rate (0.27 mg COD/(mg·h)) in a short enrichment time, comparing to the sludge taken from the enriched PHA-producing culture using VFAs as the substrate. The enrichment performance under different organic loadings were investigated and similar microbial community composition, good operating stability and high PHA accumulation (SBR#1, 36.59%; SBR#2, 36.33%) was observed at 2000 mg COD/(L·d) when crude glycerol was used as the substrate. The maximum content of PHA was affected by the concentration of glycerol. Gardnerella was for the first time found to be the dominant genus in the PHA production system using crude glycerol. The research would guide the application of using crude glycerol resources for PHA production.

Real effluents and fractionation in the supply of COD: Rapid adaptation and high efficiency to treat mine drainage combined with industrial by-products

The biological treatment of mine drainage (MD) using sulfate-reducing bacteria (SRB) is a technology in growing exploitation. The use of by-products as sources of electrons can make this treatment more environmentally and economically advantageous. However, the high chemical oxygen demand (COD) and the presence of recalcitrant molecules can lead to the accumulation of metabolic intermediates that acidify the system, thus interrupting the treatment. Besides, the adaptation of the inoculum to the establishment of sulfidogenesis with MD and by-product may be slow. This study aimed to investigate prompt adaptation and operation strategies that do not require additives to enable the sulfidogenic process to occur while maintaining a pH close to neutrality. The sources of electrons tested were trub (brewery residue) and crude glycerol - CG (residue from the biodiesel production). The inoculum from a methanogenic reactor was stored with a real MD for a month. The adapted inoculum was applied in a batch reactor for 168 h of hydraulic detention time, and promoted 75.8 ± 4.3% of sulfate removal from an MD with 3756.4 ± 258 mg.L^-1 of sulfate using CG in a COD/SO₄^2- ratio of 3 ratio. With higher initial substrate concentrations, acidification occurred and the treatment was interrupted. Using trub instead of CG, the acidification occurred at a COD/SO₄^2- ratio of 3. Acidification was prevented and the best efficiencies in sulfate removal were obtained when the amount of substrate corresponding to COD/SO₄^2- ratio of 3 was fractioned into equal parts and added over six days in the CG reactor. It was achieved 94.15 ± 1.76% of sulfate removal. With trub, the same procedure in which this COD was divided into seven parts, and resulted in a sulfate removal of 88.49 ± 1.02%. The removal of metals and metalloids were greater than 94.5% in all the systems in which the substrate supply was made fractionally, and the effluent generated presented alkalinity between 3370 and 4242 mg CaCO₃.L^-1, and pH between 6.8 and 7. The method of adaptation and operation applied allowed the realization of a MD treatment with quick establishment of sulfidogenesis and without the use of neutralizing additives. Finally, the effluent presented characteristics considered favorable for a later stage of post-treatment of the effluent with methane generation.

Microbial lipid production from crude glycerol and hemicellulosic hydrolysate with oleaginous yeasts

BACKGROUND

Crude glycerol (CG) and hemicellulose hydrolysate (HH) are low-value side-products of biodiesel transesterification and pulp-and paper industry or lignocellulosic ethanol production, respectively, which can be converted to microbial lipids by oleaginous yeasts. This study aimed to test the ability of oleaginous yeasts to utilise CG and HH and mixtures of them as carbon source.

RESULTS

Eleven out of 27 tested strains of oleaginous yeast species were able to grow in plate tests on CG as sole carbon source. Among them, only one ascomycetous strain, belonging to Lipomyces starkeyi, was identified, the other 10 strains were Rhodotorula spec. When yeasts were cultivated in mixed CG/ HH medium, we observed an activation of glycerol conversion in the Rhodotorula strains, but not in L. starkeyi. Two strains-Rhodotorula toruloides CBS 14 and Rhodotorula glutinis CBS 3044 were further tested in controlled fermentations in bioreactors in different mixtures of CG and HH. The highest measured average biomass and lipid concentration were achieved with R. toruloides in 10% HH medium mixed with 55 g/L CG-19.4 g/L and 10.6 g/L, respectively, with a lipid yield of 0.25 g lipids per consumed g of carbon source. Fatty acid composition was similar to other R. toruloides strains and comparable to that of vegetable oils.

CONCLUSIONS

There were big strain differences in the ability to convert CG to lipids, as only few of the tested strains were able to grow. Lipid production rates and yields showed that mixing GC and HH have a stimulating effect on lipid accumulation in R. toruloides and R. glutinis resulting in shortened fermentation time to reach maximum lipid concentration, which provides a new perspective on converting these low-value compounds to microbial lipids.

Identifying economical route for crude glycerol valorization: Biodiesel versus polyhydroxy-butyrate (PHB)

Crude glycerol, a by-product of biodiesel industry, has been used for production of biodiesel and polyhydroxy-alkanoates. But question is: which product is economically favorable using crude glycerol as substrate? In this study, energy balance and economic assessment has been carried out for crude glycerol valorization for B10 biodiesel and polyhydroxy-butyrate (PHB) production. For same quantity of crude glycerol utilized, energy ratio for B10 production was higher than PHB production while unit production cost for B10 was lower than that of PHB. For 50 million L plant capacity of biodiesel, unit production cost was 0.77 $/L B10 while for 2 million kg plant capacity of PHB, unit production cost was 4.88 $/kg PHB. Thus, in present scenario production of biodiesel seems economically better than production of PHA with crude glycerol as raw material. This study is useful for researchers, environmental scientists and industries in identifying effective route for crude glycerol valorization.

Co-digestion of palm oil mill effluent with chicken manure and crude glycerol: biochemical methane potential by monod kinetics

In Thailand, the palm oil industry produces a huge amount of palm oil mill effluent (POME), mostly used for electricity generation through biogas production. Co-digestion with other waste can further improve biogas yield and solve waste management problems. Most previous studies relied on biochemical methane potential (BMP) assay or batch co-digestion to obtain the optimal mixing ratio, ignoring the kinetic part or treat it for sole discussion of the results. This work directly uses mechanistic models based on Monod kinetics to describe the experimental results obtained from the co-digestion of POME (40 ml, BMP = 281.2 mlCH₄/gCODadded)) with chicken manure (CM) (0–50 g) and crude glycerol (Gly) (0–10 ml). The best mixing ratio between CM and POME was 5 gCM: 40 mlPOME (BMP = 276.9 mlCH₄/gCODadded). The best ratio for Gly and POME was 2 mlGly: 40 mlPOME (BMP = 211.9 mlCH₄/gCODadded). Adding Gly only 2 mlGly/40 mlPOME doubled the amount of biogas. Hence, crude glycerol is a good substrate for on-demand biogas output. The co-digestion increases the methane output but with a decreased yield. A multi-substrate Monod model was developed based on the levels of digestion difficulty. A partial-least squared fitting was used to estimate its main parameters. All parameters included in the model passed the significant tests at a 95% confidence level. The model can describe the experimental results very well, predict observable state variables of batch co-digestion, and allow a simple extension for continuous co-digestion dynamics. A limited continuous experiment was conducted to confirm the applicability of the model parameters of POME digestion obtained from BMP tests to predict a continuous AD. The results show good potential but must be carefully interpreted. It is generally possible and practical to directly obtain design and operational parameters from BMP assays based on only accumulated biogas curves and initial and final COD/VS.

Genome-scale metabolic modeling underscores the potential of Cutaneotrichosporon oleaginosus ATCC 20509 as a cell factory for biofuel production

BACKGROUND

Cutaneotrichosporon oleaginosus ATCC 20509 is a fast-growing oleaginous basidiomycete yeast that is able to grow in a wide range of low-cost carbon sources including crude glycerol, a byproduct of biodiesel production. When glycerol is used as a carbon source, this yeast can accumulate more than 50% lipids (w/w) with high concentrations of mono-unsaturated fatty acids.

RESULTS

To increase our understanding of this yeast and to provide a knowledge base for further industrial use, a FAIR re-annotated genome was used to build a genome-scale, constraint-based metabolic model containing 1553 reactions involving 1373 metabolites in 11 compartments. A new description of the biomass synthesis reaction was introduced to account for massive lipid accumulation in conditions with high carbon-to-nitrogen (C/N) ratio in the media. This condition-specific biomass objective function is shown to better predict conditions with high lipid accumulation using glucose, fructose, sucrose, xylose, and glycerol as sole carbon source.

CONCLUSION

Contributing to the economic viability of biodiesel as renewable fuel, C. oleaginosus ATCC 20509 can effectively convert crude glycerol waste streams in lipids as a potential bioenergy source. Performance simulations are essential to identify optimal production conditions and to develop and fine tune a cost-effective production process. Our model suggests ATP-citrate lyase as a possible target to further improve lipid production.

Production of 1,3-propanediol and lactic acid from crude glycerol by a microbial consortium from intertidal sludge

OBJECTIVES

To select a microbial consortium from intertidal sludge and evaluate its ability to convert crude glycerol from biodisel to high value-added products such as 1,3-propanediol (1,3-PDO) and lactic acid (LA).

RESULTS

A microbial consortium named CJD-S was selected from intertidal sludge and exhibited excellent performance for the conversion of crude glycerol to 1,3-PDO and LA. The composition of CJD-S was determined to be 85.99% Enterobacteriaceae and 13.75% Enterococcaceae by 16S rRNA gene amplicon high-throughput sequencing. In fed-batch fermentation with crude glycerol under nonsterile conditions, the highest concentrations of 1,3-PDO and LA were 41.47 g/L and 45.86 g/L, respectively.

CONCLUSIONS

The selected microbial consortium, CJD-S, effectively converted crude glycerol to 1,3-PDO and LA under nonsterile conditions and can contribute to the sustainable development of the biodiesel industry.

Rewiring of glycerol metabolism in Escherichia coli for effective production of recombinant proteins

BACKGROUND

The economic viability of a protein-production process relies highly on the production titer and the price of raw materials. Crude glycerol coming from the production of biodiesel is a renewable and cost-effective resource. However, glycerol is inefficiently utilized by Escherichia coli.

RESULTS

This issue was addressed by rewiring glycerol metabolism for redistribution of the metabolic flux. Key steps in central metabolism involving the glycerol dissimilation pathway, the pentose phosphate pathway, and the tricarboxylic acid cycle were pinpointed and manipulated to provide precursor metabolites and energy. As a result, the engineered E. coli strain displayed a 9- and 30-fold increase in utilization of crude glycerol and production of the target protein, respectively.

CONCLUSIONS

The result indicates that the present method of metabolic engineering is useful and straightforward for efficient adjustment of the flux distribution in glycerol metabolism. The practical application of this methodology in biorefinery and the related field would be acknowledged.

Exogenous l-proline improved Rhodosporidium toruloides lipid production on crude glycerol

BACKGROUND

Crude glycerol as a promising feedstock for microbial lipid production contains several impurities that make it toxic stress inducer at high amount. Under stress conditions, microorganisms can accumulate l-proline as a safeguard. Herein, l-proline was assessed as an anti-stress agent in crude glycerol media.

RESULTS

Crude glycerol was converted to microbial lipids by the oleaginous yeast Rhodosporidium toruloides CGMCC 2.1389 in a two-staged culture mode. The media was supplied with exogenous l-proline to improve lipid production efficiency in high crude glycerol stress. An optimal amount of 0.5 g/L l-proline increased lipid titer and lipid yield by 34% and 28%, respectively. The lipid titer of 12.2 g/L and lipid content of 64.5% with a highest lipid yield of 0.26 g/g were achieved with l-proline addition, which were far higher than those of the control, i.e., lipid titer of 9.1 g/L, lipid content of 58% and lipid yield of 0.21 g/g. Similarly, l-proline also improved cell growth and glycerol consumption. Moreover, fatty acid compositional profiles of the lipid products was found suitable as a potential feedstock for biodiesel production.

CONCLUSION

Our study suggested that exogenous l-proline improved cell growth and lipid production on crude glycerol by R. toruloides. The fact that higher lipid yield as well as glycerol consumption indicated that l-proline might act as a potential anti-stress agent for the oleaginous yeast strain.

Valorisation of crude glycerol to value-added products: Perspectives of process technology, economics and environmental issues

The enormous production of glycerol, a waste stream from biodiesel industries, as a low-value product has been causing a threat to both the environment and the economy. Therefore, it needs to be transformed effectively and efficiently into valued products for contributing positively towards the biodiesel economy. It can either be converted directly into competent chemicals or can be used as a feedstock/precursor for deriving valuable derivatives. In this review article, a technical evaluation has been stirred up, various factors and technologies used for producing value-added products from crude glycerol, Environmental and economic aspects of different conversion routes, cost factors and challenges of integration of the different routes for biorefinery have been reviewed and elaborated. There are tremendous environmental benefits in the conversion of crude glycerol via the biochemical route, the product and residue become eco-friendly. However, chemical conversions are faster processes, and economically viable if environmental aspects are partially ignored.

Substrate strategy optimization for polyhydroxyalkanoates producing culture enrichment from crude glycerol

Crude glycerol is by-product produced from biodiesel industry and can be converted directly by mixed microbial culture (MMC) into polyhydroxyalkanoates (PHAs). This study investigated the effects of the reverse (SBR_A) and positive (SBR_B) glycerol gradient substrate strategy on PHA-accumulating culture enrichment and the maximum PHA accumulating stability under substrates with different glycerol and volatile fatty acid (VFA) proportion. The results showed that crude glycerol was mainly used for PHA production rather than biomass growth in SBR_A. The maximum q_PHA was 0.65 g COD/g X^-1·h^-1 under sole crude glycerol condition in SBR_A, which was 2.41 times higher than that of SBR_B. Moreover, the PHA accumulating ability of the biomass from SBR_A was more stable than SBR_B. Saccharibacteria_genera_incertae_sedis was for the first time found to be the dominant genus using crude glycerol for PHA production. This research provides an insight into enrichment strategy to effectively enrich PHA-accumulating culture from crude glycerol.

Roles of Co-solvents in hydrothermal liquefaction of low-lipid, high-protein algae

Valorization of algal biomass is often limited by its low lipid content. Here, different alcohols: ethanol, isopropanol, and glycerol, were studied as co-solvents to improve the conversion efficiency of a lipid-poor microalgae, Galdieria sulphuraria, by hydrothermal liquefaction. Bio-crude oil yield increases, from 13 to 73 wt% (on dry algae basis), were attributed to the alcohols facilitating the transfer of algal protein-derived fragments from the aqueous phase into the oil phase. A series of characterization results showed that bio-crude oil formation was mainly the result of alcohols reacting with algal fragments via Maillard reactions, alkylation, and esterification, respectively. Insights into the synergistic effect of low-lipid feed and alcohol provide mechanistic support for choosing an alcohol-rich waste, crude glycerol, to improve bio-crude oil production from HTL of wastewater-grown G. sulphuraria. Promising improvements in yield and energy recovery indicates competitive economics for a low-lipid biomass waste-to-biofuel conversion technique.

Effect of utilization of crude glycerol as substrate on fatty acid composition of an oleaginous yeast Rhodotorula mucilagenosa IIPL32: Assessment of nutritional indices

This work studied the use of crude glycerol obtained from biodiesel industry as substrate to generate yeast lipid from Rhodotorula mucilagenosa IIPL32 MTCC 25056. Crude glycerol is a low value by product obtained from biodiesel industry. Rhodotorula mucilagenosa IIPL32 MTCC 25056 was evaluated for its potential to produce lipid using crude glycerol as sole source of carbon. Under nitrogen limiting condition a lipid and biomass content of 5.6 g/L and19.7 g/L were obtained from crude glycerol. The fatty acid profile was found to be interestingly rich in oleic acid (61.88%), linoleic acid (16.17%) and linolenic acid (1.03%) comprising ~80% of MUFA and PUFA of total lipid. Further, evaluations were attempted to compare MUFA rich yeast lipid against different plant-borne edible oils commonly used in India. In this study, nutritional indices were calculated to check feasibility of using yeast oil as a plausible blend to edible oil.

Investigation of fermentation conditions of biodiesel by-products for high production of β-farnesene by an engineered Escherichia coli

Recently, the research on conversion of biodiesel by-products to high value-added products has received much attention, due to the adverse effects of large accumulations of biodiesel by-products caused by the rapid increase in biodiesel production. Herein, this study investigated the utilization of by-products crude glycerol (CG-1 and CG-2) from two different industrial methods of biodiesel production and the favorable fermentation conditions for the high yield of β-farnesene by an engineered Escherichia coli F4, which harbored an optimized mevalonate pathway. Through analyzing by-products' components and fermentation performance, we found that CG-2 did not contain harmful impurities such as methanol and black solid impurities, and the β-farnesene production was up to 2.7 g/L from CG-2, which was similar to that from pure glycerol (2.5 g/L) and higher than that (2.21 g/L) from CG-1. Therefore, CG-2 was more suitable for β-farnesene production than CG-1, which might provide a reference for choosing a more suitable method on practical biodiesel production. Afterward, a variety of important fermentation conditions were explored using CG-2 as a substrate in shaken flasks. Under the optimal conditions (including induced cell density 1.0, initial cell density 0.25, temperature after induction 33 °C, initial medium pH 6.5), the yield of β-farnesene from CG-2 reached 10.31 g/L in a 5-L bioreactor, which was 2.8-fold higher than initial conditions in shake flasks and was the highest yield of β-farnesene produced from biodiesel by-products by fermentation as well. The recommended fermentation conditions in this work will provide a valuable reference for the industrial production of β-farnesene utilizing biodiesel by-products.

BM2 with enhanced lipid accumulation using crude glycerol as alternative carbon source

Aiming to improve the economy and sustainability of biodiesel production, the scale-up of lipid production by heterotrophic Thraustochytrium sp. BM2 utilizing crude glycerol as a low cost carbon source was optimized in stirred tank fermenter. The issues of impurities such as excess ions, methanol, soap and other organic impurities as well as different pretreatment techniques were explored and tackled for industrial application of crude glycerol as carbon source. For process engineering strategies to enhance lipid production, semi-batch operation outperformed fed-batch cultivation and achieved higher lipid yield and overall lipid productivity primarily due to shorter fermentation time. The two-step esterification/transesterification method achieved high fatty acid methyl ester (FAME) conversion rate up to 91.8%, which was two to three folds higher compared with the one-step process.

Biochemical conversion of biodiesel by-product into malic acid: A way towards sustainability

Crude glycerol, one of the ever-growing by-product of biodiesel industry and is receiving the closest review in recent times because direct disposal of crude glycerol may emerge ecological issues. The renewability, bioavailability and typical structure of glycerol, therefore, discover conceivable application in serving the role of carbon and energy source for microbial biosynthesis of high value products. This conceivable arrangement could find exploitation of crude glycerol as a renewable building block for bio-refineries as it is economically as well as environmentally profitable. In this review, we summarize the uptake and catabolism of crude glycerol by different wild and recombinant microorganism. The chemical and biochemical transformation of crude glycerol into high esteem malic acid by various microbial pathways is also additionally discussed. An extensive investigation in the synthesis of high-value malic acid production from various feed stock which finds applications in cosmeceutical and chemical industries, food and beverages, and to some extent in the field of medical science is also likewise studied. Finally, the open doors for unrefined crude glycerol in serving as a promising abundant energy source for malic acid production in near future have been highlighted.

Enhancement of 1,3-propanediol production from industrial by-product by Lactobacillus reuteri CH53

Background

1,3-propanediol (1,3-PDO) is the most widely studied value-added product that can be produced by feeding glycerol to bacteria, including Lactobacillus sp. However, previous research reported that L. reuteri only produced small amounts and had low productivity of 1,3-PDO. It is urgent to develop procedures that improve the production and productivity of 1,3-PDO.

Results

We identified a novel L. reuteri CH53 isolate that efficiently converted glycerol into 1,3-PDO, and performed batch co-fermentation with glycerol and glucose to evaluate its production of 1,3-PDO and other products. We optimized the fermentation conditions and nitrogen sources to increase the productivity. Fed-batch fermentation using corn steep liquor (CSL) as a replacement for beef extract led to 1,3-PDO production (68.32 ± 0.84 g/L) and productivity (1.27 ± 0.02 g/L/h) at optimized conditions (unaerated and 100 rpm). When CSL was used as an alternative nitrogen source, the activity of the vitamin B12-dependent glycerol dehydratase (dhaB) and 1,3-propanediol oxidoreductase (dhaT) increased. Also, the productivity and yield of 1,3-PDO increased as well. These results showed the highest productivity in Lactobacillus species. In addition, hurdle to 1,3-PDO production in this strain were identified via analysis of the half-maximal inhibitory concentration for growth (IC50) of numerous substrates and metabolites.

Conclusions

We used CSL as a low-cost nitrogen source to replace beef extract for 1,3-PDO production in L. reuteri CH53. These cells efficiently utilized crude glycerol and CSL to produce 1,3-PDO. This strain has great promise for the production of 1,3-PDO because it is generally recognized as safe (GRAS) and non-pathogenic. Also, this strain has high productivity and high conversion yield.

Enhanced polyunsaturated fatty acid production using food wastes and biofuels byproducts by an evolved strain of Phaeodactylum tricornutum

This study investigates the prospective of utilizing kitchen wastewater and food wastes, biofuels industry byproducts as alternative water and carbon sources. Kitchen wastewater did not impede cellular growth rate of the evolved Phaeodactylum strain E70, which indicates its potential as an alternative to freshwater resources. Among the organic wastes assessed, food waste hydrolysate significantly increased cell growth. Supplement of crude glycerol in cultivation medium enhances the total fatty acid content. Mixed food waste hydrolysate and crude glycerol remarkably increased both the cell density and total fatty acid content. Also, the supplement of butylated hydroxytoluene alleviated the oxidative stress induced by impurities in organic wastes and concomitantly increased microalgal total fatty acids and polyunsaturated fatty acids content. The experimental results reported in this study show that a waste-based biorefinery could lead to utilization of organic waste resources for the efficient production of value-added products.

Exploring Dual-Substrate Cultivation Strategy of 1,3-Propanediol Production Using Klebsiella pneumoniae

1,3-Propanediol (1,3-PDO) has numerous industrial applications in the synthesis of the monomer of the widely used fiber polytrimethylene terephthalate. In this work, the production of 1,3-PDO by Klebsiella pneumoniae is increased by dual-substrate cultivation and fed-batch fermentation. Experimental results indicate that the production of 1,3-PDO can be elevated to 16.09 g/L using a dual substrate ratio (of glucose to crude glycerol) of 1/30 and to 20.73 g/L using an optimized dual-substrate ratio of 1/20. Ultimately, the optimal dual-substrate feeding for a 5 L scale fed-batch fermenter that maximizes 1,3-PDO production (29.69 g/L) is determined. This production yield is better than that reported in most related studies. Eventually, the molecular weight and chemical structure of 1,3-PDO were obtained by FAB-MS, ¹H-NMR, and ¹³C-NMR. Also, in demonstrating the effectiveness of the fermentation strategy in increasing the production and production yield of 1,3-PDO, experimental results indicate that the fermentation of 1,3-PDO is highly promising for commercialization.

Evaluation of synthetic formaldehyde and methanol assimilation pathways in Yarrowia lipolytica

Background

Crude glycerol coming from biodiesel production is an attractive carbon source for biological production of chemicals. The major impurity in preparations of crude glycerol is methanol, which is toxic for most microbes. Development of microbes, which would not only tolerate the methanol, but also use it as co-substrate, would increase the feasibility of bioprocesses using crude glycerol as substrate.

Results

To prevent methanol conversion to CO₂ via formaldehyde and formate, the formaldehyde dehydrogenase (FLD) gene was identified in and deleted from Yarrowia lipolytica. The deletion strain was able to convert methanol to formaldehyde without expression of heterologous methanol dehydrogenases. Further, it was shown that expression of heterologous formaldehyde assimilating enzymes could complement the deletion of FLD. The expression of either 3-hexulose-6-phosphate synthase (HPS) enzyme of ribulose monosphosphate pathway or dihydroxyacetone synthase (DHAS) enzyme of xylulose monosphosphate pathway restored the formaldehyde tolerance of the formaldehyde sensitive Δfld1 strain.

Conclusions

In silico, the expression of heterologous formaldehyde assimilation pathways enable Y. lipolytica to use methanol as substrate for growth and metabolite production. In vivo, methanol was shown to be converted to formaldehyde and the enzymes of formaldehyde assimilation were actively expressed in this yeast. However, further development is required to enable Y. lipolytica to efficiently use methanol as co-substrate with glycerol.

A review on variation in crude glycerol composition, bio-valorization of crude and purified glycerol as carbon source for lipid production

Crude glycerol (CG) is a by-product formed during the trans-esterification reaction for biodiesel production. Although crude glycerol is considered a waste stream of the biodiesel industry, it can replace expensive carbon substrates required for lipid production by oleaginous micro-organisms. However, crude glycerol has several impurities, such as methanol, soap, triglycerides, fatty acids, salts and metals, which are created during the trans-esterification process and may affect the cellular metabolism involved in lipid synthesis. This review aims to critically present a variation in crude glycerol composition depending on trans-esterification process and impact of impurities present in the crude glycerol on the cell growth and lipid accumulation by oleaginous microbes. This study also draws comparison between purified and crude glycerol for lipid production. Several techniques for crude glycerol purification (chemical treatment, thermal treatment, membrane technology, ion-exchange chromatography and adsorption) have been presented and discussed with reference to cost and environmental effects.

A simple strategy to effectively produce d-lactate in crude glycerol-utilizing Escherichia coli

BACKGROUND

Fed-batch fermentation has been conventionally implemented for the production of lactic acid with a high titer and high productivity. However, its operation needs a complicated control which increases the production cost.

RESULTS

This issue was addressed by simplifying the production scheme. Escherichia coli was manipulated for its glycerol dissimilation and d-lactate synthesis pathways and then subjected to adaptive evolution under high crude glycerol. Batch fermentation in the two-stage mode was performed by controlling the dissolved oxygen (DO), and the evolved strain deprived of poxB enabled production of 100 g/L d-lactate with productivity of 1.85 g/L/h. To increase productivity, the producer strain was further evolved to improve its growth rate on crude glycerol. The fermentation was performed to undergo the aerobic growth with low substrate, followed by the anaerobic production with high substrate. Moreover, the intracellular redox of the strain was balanced by fulfillment of the anaerobic respiratory chain with nitrate reduction. Without controlling the DO, the microbial fermentation resulted in the homofermentative production of d-lactate (ca. 0.97 g/g) with a titer of 115 g/L and productivity of 3.29 g/L/h.

CONCLUSIONS

The proposed fermentation strategy achieves the highest yield based on crude glycerol and a comparable titer and productivity as compared to the approach by fed-batch fermentation. It holds a promise to sustain the continued development of the crude glycerol-based biorefinery.

Exploring the performance limits of a sulfidogenic UASB during the long-term use of crude glycerol as electron donor

SOx contained in flue gases and S-rich liquid effluents can be valorized to recover elemental sulfur in a two-stage bioscrubbing process. The reduction of sulfate to sulfide is the most crucial stage to be optimized. In this study, the long-term performance of an up-flow anaerobic sludge blanket (UASB) reactor using crude glycerol as electron donor was assessed. The UASB was operated for 400 days with different sulfate and organic loading rates (SLR and OLR, respectively) and a COD/S-SO₄^2- ratio ranging from 3.8 g O₂ g^-1 S to 5.4 g O₂ g^-1 S. After inoculation with methanogenic, granular biomass, the competition between sulfate-reducing and methanogenic microorganisms determined to what extent dissolved sulfide and methane were produced. After the complete washout of methanogens, which was revealed by next-generation sequencing analysis, the highest S-EC was reached in the system. The highest average sulfate elimination capacity (S-EC = 4.3 kg S m^-3d^-1) was obtained at a COD/S-SO₄^2- ratio of 5.4 g O₂ g^-1 S and an OLR of 24.4 kg O₂ m^-3d^-1 with a sulfate removal efficiency of 94%. The conversion of influent COD to methane decreased from 12% to 2.5% as the SLR increased while a large fraction of acetate (35% of the initial COD) was accumulated. Our data indicate that crude glycerol can promote sulfidogenesis. However, the disappearance of methanogens in the long-term due to the out competition by sulfate reducing bacteria, lead to such large accumulation of acetate.

Fruity flavors from waste: A novel process to upgrade crude glycerol to ethyl valerate

Valeric acid and its ester derivatives are chemical compounds with a high industrial interest. Here we report a new approach to produce them from crude glycerol, by combining propionic acid fermentation with chain elongation. Propionic acid was produced by Propionibacterium acidipropionici (8.49 ± 1.40 g·L^-1). In the subsequent mixed population chain elongation, valeric acid was the dominant product (5.3 ± 0.69 g·L^-1) of the chain elongation process. Residual glycerol negatively impacted the selectivity of mixed culture chain elongation towards valeric acid, whereas this was unaffected when Clostridium kluyveri was used as bio-catalyst. Valeric acid could be selectively isolated and upgraded to ethyl valerate by using dodecane as extractant and medium for esterification, whereas shorter-chain carboxylic acids could be recovered by using a 10 wt% solution of trioctylphosphine oxide (TOPO) in dodecane. Overall, our work shows that the combined fermentation, electrochemistry and homogeneous catalysis enables fine chemical production from side streams.

Homologous overexpression of hydrogenase and glycerol dehydrogenase in Clostridium pasteurianum to enhance hydrogen production from crude glycerol

This study reports engineering of a hypertransformable variant of C. pasteurianum for bioconversion of glycerol into hydrogen (H₂). A functional glycerol-triggered hydrogen pathway was engineered based on two approaches: (1) increasing product yield by overexpression of immediate enzyme catalyzing H₂ production, (2) increasing substrate uptake by overexpression of enzymes involved in glycerol utilization. The first strategy aimed at overexpression of hydA gene encoding hydrogenase, and the second one, through combination of overexpression of dhaD1 and dhaK genes encoding glycerol dehydrogenase and dihydroxyacetone kinase. These genetic manipulations resulted in two recombinant strains (hydA⁺⁺/dhaD1K⁺⁺) capable of producing 97% H₂ (v/v), with yields of 1.1 mol H₂/mol glycerol in hydA overexpressed strain, and 0.93 mol H₂/mol glycerol in dhaD1K overexpressed strain, which was 1.5 fold higher than wild type. Among two strains, dhaD1K⁺⁺ consumed more glycerol than hydA⁺⁺ which proves that overexpression of glycerol enzymes has enhanced glycerol intake rate.

Process optimization and kinetic analysis of malic acid production from crude glycerol using Aspergillus niger

In the present work, optimization of crude glycerol fermentation to produce malic acid by using Aspergillus niger was investigated using response surface methodology and artificial neural network. Kinetic investigation of bioconversion of crude glycerol into malic acid using Aspergillus niger was studied using Monod, Mosser, and Haldane-Andrew models. Crude glycerol concentration, initial pH and yeast extract concentration were found to be significant compounds affecting malic acid production by Aspergillus niger. Both dry cell weight and malic acid titre were found decreased with increase in crude glycerol concentration. Haldane-Andrew model gave the best fit for the production of malic acid from crude glycerol with µ_max of 0.1542 h^-1. The maximum malic acid production obtained under optimum conditions was 92.64 + 1.54 g/L after 192 h from crude glycerol using Aspergillus niger.

Occurrence of non-toxic bioemulsifiers during polyhydroxyalkanoate production by Pseudomonas strains valorizing crude glycerol by-product

While screening for polyhydroxyalkanoate (PHA) producing strains, using glycerol rich by-product as carbon source, it was observed that extracellular polymers were also secreted into the culture broth. The scope of this study was to characterize both intracellular and extracellular polymers, produced by Pseudomonas putida NRRL B-14875 and Pseudomonas chlororaphis DSM 50083, mostly focusing on those novel extracellular polymers. It was found that they fall into the class of bioemulsifiers (BE), as they showed excellent emulsion stability against different hydrocarbons/oils at various pH conditions, temperature and salinity concentrations. Cytotoxicity tests revealed that BE produced by P. chlororaphis inhibited the growth of highly pigmented human melanoma cells (MNT-1) by 50% at concentrations between 150 and 200 μg/mL, while no effect was observed on normal skin primary keratinocytes and melanocytes. This is the first study reporting mcl-PHA production by P. putida NRRL B-14785 and bioemulsifier production from both P. putida and P. chlororaphis strains.

Formate-removing inoculum dominated by Methanobacterium congolense supports succinate production from crude glycerol fermentation

We developed a formate-removing methanogenic inoculum (FRI) to facilitate succinate production from crude glycerol by Escherichia coli. FRI converted formate to methane, thereby enabling glycerol fermentation without additional electron acceptors under neutral pH. FRI was selectively enriched from sludge from the anaerobic digester of the Seonam sewage treatment plant (Seoul); this process was assessed via Illumina sequencing and scanning electron microscopy imaging. Methanobacterium congolense species occupied only 0.3% of the archaea community in the sludge and was enriched to 99.5% in complete FRI, wherein succinate-degrading bacteria were successfully eliminated. Co-culture with FRI improved glycerol fermentation and yielded 7.3 mM succinate from 28.7 mM crude glycerol, whereby FRI completely converted formate into methane. This study is the first to demonstrate methane production by M. congolense species, using formate. M. congolense-dominated FRI can serve as a renewable facilitator of waste feedstock fermentation and enable the production of commercially important compounds.

Two-stage microbial conversion of crude glycerol to 1,3-propanediol and polyhydroxyalkanoates after pretreatment

With increasing demand for biodiesel, crude glycerol as a by-product in biodiesel production has been generated and oversupplied. This study, therefore, explored the pretreatment and a subsequent two-stage microbial system to convert crude glycerol into high value-added products: 1,3-propanediol (1,3-PDO) and polyhydroxyalkanoates (PHAs). After pretreatment, long chain fatty acids (LCFAs) could be effectively removed from crude glycerol to eliminate the inhibition effects on subsequent microbial process. In the anaerobic fermentation, when fed treated crude glycerol increased from 20 g/L to 100 g/L, 1,3-PDO yield decreased from 0.438 g/g to 0.345 g/g and accompanied carboxylic acids shifted from acetate and lactate dominant to lactate overwhelmingly dominant. Meanwhile, the relative abundance of Clostridiales sustained around 50% but Enterobacteriales increased from 19% to 53%. Further fed glycerol increase to 140 g/L resulted in severe substrate inhibition, which could be relieved by intermittent feeding. In aerobic process, glycerol anaerobic digestion effluent (ADE) was fed to the consortium of Bacillus megaterium and Corynebacterium hydrocarbooxydans for selectively consumption of carboxylic acids and residual glycerol from 1,3-PDO to produce PHAs as a secondary high value-added product. The consortium accumulated maximum 8.0 g/L poly (3-hydroxybutyrate) (PHB), and 1,3-PDO purity increased from initial 27.7% to almost 100% when fed with 100 g/L glycerol ADE. Overall, this study provided comprehensive and insightful information on microbial conversion of crude glycerol to high value-added products after pretreatment.

Valorization of crude glycerol based on biological processes for accumulation of lipophilic compounds

Bacteria that are capable of accumulating lipids in their cells as storage compounds can also produce polyhydroxyalkanoates of high technological value, depending on the specific culture conditions. The objective of this study was to utilize crude glycerol from biodiesel (CGB) as a substrate, which is a major byproduct from biodiesel production, to produce lipophilic compounds. Bacillus megaterium INCQS 425 was cultivated and evaluated for the production of lipophilic compounds and the properties of these compounds were investigated. Cultivation of the bacteria in a medium with a C:N ratio of 0.60:1 favored the accumulation of lipids by (17.5%) comprising mainly palmitic acid (13.08%), palmitoleic (39.48%), and especially oleic acid (37.02%), which imparts good characteristics to biodiesel. Meanwhile, cultivation of the bacteria in a medium with a C:N ratio of 4:1 favored the accumulation of polyhydroxyalkanoates (PHA) (3.31gL^-1) mainly comprising medium and long chain PHA. Low crystallinity (<30%) and excellent thermal properties make them suitable for processes that demand high temperatures, such as extrusion. The lipids produced in the present study had satisfactory oxidative stability for the production of quality biodiesel. The polyhydroxyalkanoates produced in the study are of low cost and have promising thermal properties that justify its technological potential, thereby configuring highly competitive bioproducts.