(655d) Two Phase Anaerobic Treatment Process for Hydrogen and Electricity Production from Biodiesel Glycerol Waste

This paper discusses a two phase energy efficient anaerobic process composed of dark fermentation and biologically catalyzed electrolysis to treat the biodiesel glycerol waste as well as produce hydrogen and/or electricity. Biodiesel is a promising renewable fuel which will presumably be produced more intensively in the coming years. Glycerol is the major by-product of biodiesel production however it is highly contaminated with salts formed during neutralization of the alkali used in transesterification process. For every 10 litre biodiesel produced of about 1 kg glycerol is generated. The rapid increase in biodiesel production will result in a great surplus of glycerol in near future. Besides, glycerol derived from animal fats and frying oils would not be favourable as a commercial product. Therefore biodiesel glycerol seems to rapidly become a waste material that should be managed in an energy efficient way. Although the generation of hydrogen through glycerol fermentation is known for decades, researches and practices on dark fermentative hydrogen production from biodiesel-glycerol waste are very limited. Another novel and energy efficient way to manage biodiesel glycerol waste is production of hydrogen and/or electricity using a bio-electrochemical process such as biologically catalyzed electrolysis. In a bio-electrolyser, hydrogen is produced through bacterial electrolysis of glycerol by only applying an extra electrical energy about 0.25V which is substantially lower than the energy (~1.5V) necessary for conventional water electrolysis. Although, both processes could be used separately, a two phase system composed of dark fermentation plus bio-electrolysis would provide higher energy yields and treatment efficiencies. Since, the two phase anaerobic system proposed here for the treatment of biodiesel glycerol waste present an innovative and promising technology, this study will blaze a trail in developing a new energy efficient treatment concept.

Dark H2 fermentation experiments are conducted in completely stirred fermenters inoculated with mixed culture compost materials. The H2 consuming methanogens in the inoculum are eliminated by keeping pH quite low at the start-up. To maintain the thermophilic conditions (55°C), fermenters are placed in hot water circulated water jackets and rigorously mixed with magnetic stirrers to maximize H2 stripping. To control the pH during fermentation, pH transmitters equipped with pH electrodes and pumps dosing NaOH and HCl are used. The biogas evolved is continuously measured using volumetric gas counters appropriate for small volumes and be instantly released from fermenter. The H2 gas is monitored on-line with H2 sensors and periodically with gas chromatograph analyses. To prevent decomposition in the feeding tank waste glycerol is kept at 4°C and fed to the fermenters with peristaltic pumps. Glycerol in the influent and effluent is analyzed with a HPLC equipped with an ELSD detector. VFAs, acetone, alcohols and lactate are analyzed using a GC-FID. The effluents of hydrogen fermenters are fed to the bio-electrolyser unit after appropriate dilutions.

Bio-electrolysis experiments are conducted in a two chamber electrochemical cell. Anode and cathode chambers are separated by a cation exchange membrane and both equipped with a Ag/AgCl reference electrode to measure the electrode potentials. Graphite felt is used as electrode material in anode and cathode and connected to a potentiostat via isolated electrical wires. The potentiostat is used to control the applied voltage on the total system (as the power supply) and to control the anode potential during enrichment of electrochemical bacteria after inoculation. The experiments are performed between 25 and 30°C. Current and applied voltage measurements are done by the potentiostat. Hydrogen and CO2 gases produced are monitored in the same way as in dark fermentation experiments.