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In a view to uplift the biodiesel industry as a major energy carrier, new approach of minimizing the waste and utilizing generated waste needs to be explored. The spent media generated from the co-culture system is cost effective and can serve as renewable supplement for mixed-culture based hydrogen (H 2 ) production and lipid production. Direct conversion of spent media along with crude glycerol (CG) at 20 g/L using heat-shock pretreated wastewater sludge resulted in 38.12 ± 0.84 mmol/L of H 2 . In another approach, the spent media was used as co-supplement along with fresh media at 3:2 for algal growth, resulting in 0.098 ± 0.007 g/L of lipid. The spent media contained dead biomass, residual media nutrients, biomolecules and unutilized glycerol together acting as supplementary source during H 2 and lipid production. According to the closed system results, the H 2 produced (1.47 × 10 9 L of H 2 ) can be converted into energy (1.87 × 10 4 GJ) for electricity (1.77 × 10 4 GJ) and heat (4.32 × 10 3 GJ). The produced H 2 can be used as in-house energy source and the lipids can be used as third generation feedstock. The study explores the utilization of CG and spent media valorization towards an efficient closed system approach for a competitive biodiesel industry.

Hydrogen (H 2 ), a possible future clean energy carrier, requires process-based improvement routes for cutting down the production cost. The impact of nitrogen (N 2 ) sparging on H 2 production during co-culture system of Enterobacter aerogenes and Clostridium butyricum from crude glycerol (CG) was studied to reduce the overall process cost. H 2 production using 1% CG under nitrogen sparged medium before autoclaving resulted in 1.2 mmol-H 2 /mol of glycerol in comparison to 1.5 mmol-H 2 /mol of glycerol without sparging. In the presence of air ranging from 5 mL to 75 mL in the headspace volume, H 2 production increased to a maximum of 26.14 mmol/L with 1.4 g/L of ethanol production. The concentration of 1,3-propanediol with N 2 sparging was around 3.0 g/L and decreased to 0.5 g/L due to presence of 75 mL of air in the headspace. This observation can be attributed to a shift from reductive to oxidative metabolism of glycerol. A process-based improvement strategy to optimize H 2 formation resulted in metabolic pathway shift from reductive to oxidative with increase in H 2 production. Synergistic influence of co-culture system in absence of expensive reducing agent and without nitrogen sparging step can offer a better process-based economic strategy for H 2 production, minimize the metabolite production and increase field-scale application of biodiesel plant.

Organic acids produced by commercial phosphate solubilizing bio-fertilizer (PSB) can chelate the cations of insoluble phosphates (such as Ca3 (PO4)2) present in soil to solubilize the phosphate and make it available for plants. Meanwhile, during H2 production (dark fermentation) different organic acids are produced as unwanted by-products. Interestingly, the acid-rich liquid waste of the process has excellent phosphate solubilizing ability (36.12 folds increase) and it could be considered as a potential alternative for PSB. Studies have demonstrated that biodiesel manufacturing waste (crude glycerol) could be used as a substrate for hydrogen production and the secondary waste could be used as a phosphate solubilizing agent. Thus, this novel approach could: (i) offer a green fertilizer; (ii) improve economic sustainability of bio-hydrogen production and; (iii) could become a financial booster for biodiesel manufacturers by offering them a profitable waste management strategy.

Crude glycerol (CG) has several advantages over a range of conventional substrates used for biohydrogen production by Enterobacter aerogenes NRRL B 407. Meanwhile, high process cost due to requirement of expensive supplementary media component is a concern. Therefore, different less expensive (or wastes) materials have been evaluated as supplementary nutrient for H 2 production by CG (meat processing and restaurant waste based biodiesel derived) bioconversion. Among the materials selected, slaughterhouse liquid waste (SL), brewery waste biomass (BWB) and urea was found to improve the production by 18.81 ± 3.56, 27.30 ± 3.54 and 38.57 ± 3.66%, respectively. Further, in the case of urea (10 mg/L), cumulative production as high as 116.41 ± 3.72 mmol H 2 /L media has been achieved; which is comparable to other reports available on CG bioconversion. Thus, present study demonstrates successful replacement of large amount (∼5–6 g/L) of expensive nutrients/buffering agents by negligible amount (∼10 mg/L) of different waste materials, without compromising the cumulative H 2 yield. Further, the strain used in the present study was found to grow at an acidic pH as low as 3.3, indicating its prospective application for dark fermentative H 2 production.

Hydrogen is a clean source of energy with no harmful byproducts produced during its combustion. Bioconversion of different organic waste materials to hydrogen is a sustainable technology for hydrogen production and it has been investigated by several researchers. Crude glycerol generated during biodiesel manufacturing process can also be used as a feedstock for hydrogen production using microbial processes. The possibility of using crude glycerol as a feedstock for biohydrogen production has been reviewed in this article. A review of recent global biodiesel and crude glycerol production and their future market potential has also been carried out. Similarly, different technical constraints of crude glycerol bioconversion have been elaborately discussed and some strategies for improved hydrogen yield have also been proposed. It has been underlined that use of crude glycerol from biodiesel processing plants for hydrogen production has many advantages over the use of other organic wastes as substrate. Most importantly, it will give direct economic benefit to biodiesel manufacturing industries, which in turn will help in increasing biofuel production and it will partially replace harmful fossil fuels with biofuels. However, different impurities present in crude glycerol are known to inhibit microbial growth. Hence, suitable pretreatment of crude glycerol is recommended for maximum hydrogen yield. Similarly, by using suitable bioreactor system and adopting continuous mode of operation, further investigation of hydrogen production using crude glycerol as a substrate should be undertaken. Furthermore, isolation of more productive strains as well as development of engineered microorganism with enhanced hydrogen production potential is recommended. Strategies for application of co-culture of suitable microorganisms as inoculum for crude glycerol bioconversion and improved hydrogen production have also been proposed.