## 3 Results
### Conversion of Waste Glycerol with Free and Immobilized Cells of G. oxydans ATCC 621
oxydans_ cells, regardless of the initial concentration of substrate, resulted in a similar glycerol consumption rate except for the first day, in which this rate was nearly 3-fold higher in the medium with 3% of the waste (0.58 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\) for 3% of waste and 0.21 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\) for 5% of waste) (Table 1). The DHA production rate also developed almost identically in the process with the immobilized cells in the media with the initial concentration of glycerol at both 3% and 5%, and was on average 0.12, 0.20, and 0.15 at the individual measuring points (Table 1). Using free cells allowed us to obtain higher values of DHA yield (independent of initial glycerol concentration) compared to immobilized cells (Table 1). Regardless of the initial concentration of waste glycerol in the reaction medium, the rate of glycerol consumption in the reaction with a free cell extract was similar, and reached around 0.21 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\) after 24 h, 0.23 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\) after 72 h, and about 0.17 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\) after 96 h (Table 2). The only significant difference in these values was observed after 24 h of the process, when the glycerol consumption rate was 0.3 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\) in the medium with the lower initial waste content (30 g\(\cdot\)L\({}^{-1}\)), and almost 2-fold lower in the medium with the higher initial substrate concentration (50 g\(\cdot\)L\({}^{-1}\)) (Table 2). DHA production yield in the discussed variants was the same at the beginning and at the end of the bioconversion process (0.18 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\) after 24 h and 0.16 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\) after 96 h) (Table 2). However, after 48 and 72 h, significantly higher productivity was observed for the free cell extract in the medium with 3% of the waste, and it was 0.27 and 0.22 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\), respectively (while for 5% of the waste it was equal to 0.14 and 0.12 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\), respectively) (Table 2). Analyzing the obtained results, it can be noticed that the initial concentration of waste glycerol affected the glycerol consumption rate in the reaction with an immobilized cell extract. The obtained results clearly suggest that the consumption rates of the substrate (for each variant of biocatalyst) were lower in the solution initially containing 50 g\(\cdot\)L\({}^{-1}\) of glycerol compared to that with 30 g\(\cdot\)L\({}^{-1}\) of glycerol (Tables 1 and 2). Whereas, the DHA productivity rate was low independent of the initial concentration (30 or 50 g\(\cdot\)L\({}^{-1}\)), and ranged from 0.05 (Table 2) to 0.4 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\) (Table 1), regardless of the measurement time or of the initial composition of the reaction medium (Table 2). In the case of obtaining DHA with the use of a free cell extract (initial glycerol of 30 or 50 g\(\cdot\)L\({}^{-1}\)), the DHA yield was higher compared to data obtained in variants with free cells. The lowest DHA yield was observed in the medium with 50 g\(\cdot\)L\({}^{-1}\) of glycerol and immobilized cell extract as a biocatalyst (Table 2).