(753a) Biofuel Generation from Cheese Whey In a Two –Stage Anaerobic Process

Global energy and environmental concerns due to the increasing use of fossil - derived fuels, lead many scientists to the search of for alternative energy sources. Among others, much attention has been paid to the hydrogen gas, and its potential use as a fuel for transport purposes and electricity generation. Hydrogen is a clean and environmentally friendly fuel, which produces water instead of greenhouse gases, when combusted. It possesses a high-energy yield (122 kJ/g) due to its light weight. Hydrogen can be produced by renewable raw materials, such as biomass, through biological or thermochemical processes. Anaerobic fermentative hydrogen production, has received special attention during the last decade. It is well founded that carbohydrates are the main source of hydrogen during fermentative processes and therefore wastes/wastewater or agricultural residues rich in carbohydrates can be considered as potential sources of hydrogen.

From the wastewater treatment point of view, fermentative hydrogen production mainly includes sugars transformation into volatile fatty acids, without much effect on the organic content of the wastewater. Thus, a complementary stage would be necessary for chemical oxygen demand elimination. It is well known that the formation of volatile fatty acids during the acidogenesis of the organic matter is actually the precursor to methanogenesis. Therefore, the hydrogen production process could be efficiently coupled with a subsequent anaerobic digestion step with the conversion of the remaining organic content to biogas (mainly methane and carbon dioxide), which may also be used as a fuel for the production of electricity.

As it has already been reported, wastewaters rich in carbohydrates can be used as raw materials for bio hydrogen production. Cheese whey is the lactose-rich watery by-product of cheese manufacturing and could be an excellent candidate for fermentation processes. Cheese whey represents about 85?95% of the milk volume and contains nutrients, such as lactose (4.5-5% w/v), soluble proteins (0.6-0.8% w/v), lipids (0.4-0.5% w/v) and mineral salts (8-10% of dried extract) (Sizo, 1996). Because of its high organic content, cheese whey disposal constitutes a serious environmental problem with lactose being mainly responsible for its high COD values.

The present study focuses on hydrogen and subsequent methane generation from cheese whey in a two-stage anaerobic process. The fermentative hydrogen production process was conducted in a mesophilic continuous stirred tank type bioreactor with an active volume of 3L, which was operated at hydraulic retention time (HRT) of 24 h and was started up and fed with cheese whey. For start-up, the reactor was filled up with cheese whey and operated anaerobically at a batch mode for 24 h in order to activate the indigenous microflora contained in the waste and was subsequently switched to the continuous mode at the an HRT of 24 h.

It has been proved that continuous fermentative hydrogen production is possible and stable using the indigenous microflora contained in cheese whey. The percentage of hydrogen in the gas phase was 29.3 ± 1.6 % while the hydrogen production rate reached 7.53 LH2 per day. The efficiency of glucose consumption was 68.6% since the concentration of carbohydrates in the mixed liquor was 11.30 ± 0.5 g /L. Based on the above observations, the yield of hydrogen produced per mole of glucose consumed was approximately 0.9 ± 0.1 mol H2 / mol glucose consumed and, consequently, that of hydrogen produced per L of cheese whey was 2.49 L H2/ L cheese whey, respectively. This corresponds to an energy yield of 24.85 kJ/ L cheese whey (assuming that the energy yield from hydrogen is 122000 kJ/ kg).The obtained yield of hydrogen production is lower than the theoretical yield of 2 or 4 mol H2 / mol glucose consumed, which corresponds to the maximum theoritical production of butyric or acetic acid, from glucose degradation

The effluent from the hydrogenogenic reactor was subsequently fed to a Periodic Anaerobic Buffled Reactor (PABR), an innovative reactor, initially developed by Skiadas and Lyberatos (1998) and Skiadas et al 2000. The anaerobic digester was operated at mesophilic conditions at hydraulic retention times of 20, 10 and 4.4 d. The biogas and methane production rates were higher at the HRT of 4.4 d and were equal to 105.9 L biogas /d and 75.6 L CH4 /d respectively. The percentage of methane in the gas phase lied between 71.4 and 74.9 %, while the culture pH was approximately stable at all HRTs. The COD removal efficiency was more than 94 % at all HRTs, implying that the performance of the anaerobic digester was not kinetically limited.

Consequently, this work demonstrated that continuous fermentative hydrogen production is possible and stable using the indigenous microflora contained in cheese whey. In addition, it was proven that biohydrogen production can be very efficiently coupled with a subsequent step of methane production and that cheese whey could be an ideal substrate for a combined gaseous biofuels generation.

Literature cited

Gonzalez Siso, M. I (1996). The biotechnological utilization of cheese whey: a review. Biores. Technol. 57, 1-11

Skiadas, I.V.; Lyberatos, G. (1998) The periodic anaerobic baffled reactor. Wat. Sci. Technol. 38, 401-408