## 1 Introduction  
Biodiesel can be an alternative to environmentally-unfriendly fossil fuels, whose global resources have depleted rapidly over the past several years. On the other hand, replacement of conventional fuels with biofuels (i.e., biodiesel) entails generation of high amounts of waste glycerol (crude glycerol), which is the major by-product from biodiesel production. According to literature data, more than 10 pounds of crude glycerol are generated per every 100 pounds of biodiesel produced [1; 2]. Following expert opinions, biodiesel production in European countries and around the globe will be developing very dynamically in the upcoming years, and the amount of crude glycerol will be growing with it [3]. Studies on the utilization of the waste glycerol fraction are carried out in many research centers. Many methods are being developed which foster a hope for economical and environmentally-friendly solutions in this area [4]. Crude glycerol can be used as the starting material for the production of some valuable products to be applied in many sectors of industry [5]. One of the main hurdles in adapting this waste fraction for further use is the high cost of it's purification to a level that enables it's application in industry. Purified waste glycerol can be used as an additive to feedstuff for animals due to its non-toxicity and high calorific value [6; 7]. It can also be used as an additive to wood pellets [8]. The possibilities of its industrial application also include production of chemicals, fuel additives, hydrogen, ethanol, or methanol [9]. One of the concepts for using unpurified waste glycerol is it's microbial bioconversion into dihydroxyacetone (DHA), with the use of free or immobilized biocatalysts. DHA is a ketotriose, produced on the industrial scale with a microbiological method using specific species of free acetic acid bacteria, characterized by a high activity of membrane-bound glycerol dehydrogenase (GlyDH) [10]. Immobilization of microbial cells in fermentation holds several advantages over free cells in terms of higher cell density, easy separation of product from the reaction mixture, repeated use of biocatalyst for several cycles, reduced cell washout problems, less substrate or product inhibition effects, and control over biomass growth. Several studies have reported the use of immobilized cells over various supports for effective utilization of glycerol. The merits and demerits of the fermentation process rely on the choice of immobilization method. Previous studies also reported the use of various carried material for immobilization of microbial cells for DHA production from glycerol. [PERSON] et al. [11] reported the use of a novel carrier material (Ralu rings) for immobilization of _G. oxydans_ cells, and the experiments were carried out in a repeated-fed-batch process in a packed-bed bubble-column bioreactor. In another study, [PERSON] and [PERSON] [12] used commercial reticulated polyurethane foam as a support for _G. oxydans_, which provides a high porous surface area for increasing cell density. In the present study, we have selected calcium alginate beads as the support for immobilization of _G. oxydans_ cells. The major advantages of using alginate beads in the immobilization process are their low cost, easy operation, and mild operating conditions [13]. Calcium alginate beads are most widely supported for immobilization of cells and enzymes. Oxidation of glycerol by acetic acid bacteria may follow two pathways (Figure 1). The first pathway (independent from adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NAD)) proceeds at pH 5.0-5.5 in periplasmic space and is catalyzed by membrane-bound GlyDH. The only product of this pathway is DHA, which is directly released out of the membrane into the outside environment. The second pathway of glycerol transformation proceeds at pH 8.5-10.0 and requires the presence of ATP and Mg\({}^{2+}\) ions, in addition to an enzyme--glycerol kinase.