Improvement of third generation bioethanol production from photoheterotrophically cultivated Chlorella vulgaris in the presence of carrot pomace and onion peel waste.

Microalgae are promising feedstocks, holding great potential for bioethanol production. However, microalgal bioethanol production requires the use of different approaches in order to optimize the process. In this study, the bioethanol production capacities of local yeasts (Saccharomyces cerevisiae, Candida boidinii and Kluyveromyces marxianus) from Chlorella vulgaris biomass cultivated with 0.5 g/L carrot pomace sugar was investigated. Onion peel, a common waste product, was used for investigating its potential in improving yeast growth and bioethanol production by K. marxianus. Parameters such as biomass loading (50-200 g/L), onion peel waste concentration (1%-5%), and supplement types (nitrogen sources and mineral salts such as peptone, yeast extract, MgSO4, KH2PO4, CaCl2, ZnSO4) were optimized. Among local yeasts, K. marxianus demonstrated the highest bioethanol production and productivity with a measured value of 4.29 ± 0.25 g/L and 0.36 g/L.h, respectively. Bioethanol concentration and productivity increased to 6.98 ± 0.12 g/L and 0.58 g/L.h, respectively, when 200 g/L biomass loading and 3% onion peel concentration was used. This study demonstrates the high potential of K. marxianus in metabolizing microalgal carbohydrates and the utilization potential of food waste as an inexpensive nutrient source in the fermentation medium.