Potential Use of Industrial Wastes for Low-cost Production of Single Cell Oils and Analysis of Techno-Economical Feasibility and Environmental Impact

Abstract

This study aimed to evaluate the potential use of agro-industrial wastes for low- cost production of single cell oil. Different types of abundant wastes from soft drinks industry, brewery industry and biodiesel industry were compared. Expired soft drinks (ES) is the waste from soft drinks industry. The sugar compositions in the ES were: fructose (59.80%), glucose (27.46 %) and sucrose (12.73 %), which are good nutrient sources for yeast cultivation. Among oleaginous yeasts tested, Trichosporonoides spathulata JU4-57, Yarrowia lipolytica TISTR 5151 and a red yeast Rhodotorula mucilaginosa G43 could grow on ES and accumulate considerably high lipid content >30%. Brewers' spent grain (BSG) and spent yeast cells (SYC) are solid wastes from malting process and final yeast fermentation, respectively. Both wastes have to be acid hydrolyzed into fermentable sugars before use as nutrient sources for yeast cultivation. The BSG hydrolysate was composed of xylose (45.83±1.53%) and arabinose (32.13±2.3%) from arabinoxylan in hemicellulose and glucose (22.02+0.8%) from cellulose. While main sugars found in the SYC hydrolysate, were mannose (69.57+1.04%) and glucose (31.43+0.38%). All the selected yeasts could grow on BSG and SYC hydrolysate but accumulated low amount of lipids (8-17%) possibly due to the high nitrogen content, which stimulated cell growth rather than lipid accumulation. When cultivated on crude glycerol (CG) from biodiesel industry, only yeast R. mucilaginosa G43 grew well on CG and accumulated lipids >50%. While T. spathulata JU4-57 and Y. lipolytica TISTR 5151 could not grow well and gave relatively low biomass. In addition to lipids, R. mucilaginosa G43 also contained red pigment called carotenoids at 3.78+0.29 mg/g-cell and protein at 16.46±0.17%w/w. It then has high potential not only as single cell oil but also as a source of pigment and protein. Fed-batch fermentation of ES by Y. lipolytica TISTR 5151 was performed. The ES was intermittently added every 24 h to increase the carbon source availability. The lipid content increased up to the maximum level of 40-46% during 24-48 h. Fed-batch fermentation of CG by R. mucilaginosa G43 was also attempted by adding CG every 36 h. However, the fed-batch fermentation of CG did not significantly increase yeast cell growth and lipid production possibly due to the accumulation of inhibitors from CG. From the above experiment, it could be concluded that crude glycerol and R. mucilaginosa G43 are suitable nutrient source and yeast strain for production of single cell oil. As fed-batch fermentation did not increase the lipid production by the yeast, the repeated batch fermentation was then chosen for scaling up in 2 L-bioreactor with 1-L working volume and the culture broth was replaced with the fresh medium at 80% replacement rate every 48 h. Five cycles of batch cultivation were repeated and the fermentation proceeded for 228 h. The yeast could grow and accumulate lipids in four cycles and the highest lipid obtained was 2.52±0.03 g/L. It was of interest that the carotenoid content in the cells from the third cycle was highest at 21.85±0.58 mg/g- cell. The fatty acid compositions of single cell oil are long chain fatty acids with 16-18 carbon atoms which are similar to those of plant oils. It then has potential to be used as biodiesel feedstocks. In addition, this study also performed the techno-economic analysis (TEA) and assessments of environmental impact by greenhouse gas emission (GHG) from bioconversion of each agro-industrial waste into biodiesel. The process included waste preparation, yeast cultivation, harvesting and production of biodiesel via direct transesterification. TEA indicates that CG was the most suitable waste for yeast cultivation and biodiesel production because in 1,000 L production scale CG could give the highest biodiesel 2.02 kg followed by BSG (1.46 kg), SYC (0.68 kg) and ES (0.64 kg). The economic assessment shows the highest production costs involved in the case of SYC (9.34 $/kg-biodiesel) followed by SYG (4.34 $/kg-biodiesel) and ES (4.29 $/kg-biodiesel). While CG requires the lowest production costs of 1.36 $/kg-biodiesel. The highest CO2 emissions are from the case of SYC (52.84 kg CO2-eq) followed by BSG (24.60 kg CO2-eq), ES (21.90 kg CO2-eq) and CG (6.93 kg CO2-eq). The CO2 emissions depend on the steps involved in the process and the yield of biodiesel. As SYC and BSG needed acid hydrolysis step before use and gave low yield of biodiesel, the CO2 emission per 1 kg biodiesel was then higher than those from other wastes. This study has shown the promising approach for cost-effective production of microbial based biofuels and may also contribute to the techno-economic and environmental sustainability of the biofuel industries and the agro-bio industries in Thailand.