In this study glycerol was treated in the batch-mode anaerobic serum bottles (volume: 160 ml). It focuses on two aspects: 1) the anaerobic treatment of biodiesel waste�glycerol and 2) the conversion of glycerol to useful energy source (hydrogen) and industrial materials (e.g. 1,3 propanediol). The glycerol used in this study involves two types of purification level (p2 and p3 glycerol) by removing impurities (e.g. methanol & KOH) from raw biodiesel glycerol. These two stages glycerol contain 30.5% and 32.4% methanol (mass %), respectively.  Hydrogen is the major energy source produced in the anaerobic fermentation of glycerol, and the hydrogen partial pressure (HPP) in the headspace of the reactors was expected to affect the metabolic pathways of anaerobic fermentations of glycerol and finally lead to the different hydrogen yields and liquid products. In this study, different HPP levels were built up by two approaches. First, the sampling interval duration (SID) between gas collections from the headspace of the reactors was varied (5 hrs-24 hrs). Second, the initial gas collection time point (IGCT) was changed over the batch cycle (20 hrs-72 hrs). The longer of SID and IGCT will increase the HPP in the reactors, and thus affected fermentation pathways of glycerol.The results showed that higher HPP inhibited hydrogen production and changed the composition of the liquid products. Because the metabolic cycle of the acidogenic and hydrogenic bacteria were short than 100 hrs, a SID of 5hrs substantially decreased the HPP and lead the hydrogen yields to 0.6~0.7mole hydrogen/mole glycerol consumed, which is the highest hydrogen yields by anaerobic fermentation of glycerol reported. In the meantime, ethanol was the only liquid fermentation product, with the yield of 0.7mole/mole glycerol consumed.  A SID of 8 hrs had the similar hydrogen yield and ethanol yield. However, with the increase in HPP, the hydrogen yields decreased and ethanol was no longer the major liquid product.  Instead, butyric acid (HBu) and 1, 3 propanediol (1, 3 PD) were the major liquid products.  By prolonging the ICT, the metabolic cycle of fermentation can also be prolonged. The highest yields of 1, 3PD and HBu were 0.3 and 0.2mole/mole glycerol consumed, respectively.The biodiesel glycerol of p2 and p3 stages were tested in three concentrations of 3g/L, 5g/L and 7g/L, respectively, and compared with pure glycerol. While the consumption of glycerol reached 95% and above in all the concentration levels, no clear correlation was found between hydrogen yields and glycerol concentrations. The results showed the clear change of hydrogen yield, acetate yield, 1,3 PD yield and ethanol yield with HPP.  This study demonstrates that the anaerobic fermentation metabolic pathways of biodiesel glycerol can be controlled through adjusting hydrogen partial pressure in order to obtain the desired bio-energy products.

Influence of hydrogen partial pressure (HPP) on the metabolic pathways of biodiesel glycerol anaerobic fermentation

244th National Meeting (2012)

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