Effect of organic loading rate on anaerobic digestion of thermally pretreated Scenedesmus sp. biomass
Highlights
► Increasing the organic loading rate did not affect anaerobic digestion performance. ► Scenedesmus sp. biofuel production is hindered by its cell wall. ► Storage time of Scenedesmus sp. biomass affected organic matter hydrolysis. ► Scenedesmus sp. thermal pretreatment was beneficial for anaerobic digestion.
Introduction
Microalgae are a promising feedstock for the production of biodiesel and bioethanol; however, the generation of these biofuels only extracts part of the energy stored in the biomass. According to Lardon et al. (2009), oil cakes discarded after biodiesel production still contained 35–73% of the energy present in the biomass. Therefore, anaerobic digestion of these cakes would decrease costs by providing heat and electricity. Likewise, bioethanol production only ferments the carbohydrate fraction while lipids and proteins can still be used for other purposes.
Production of biogas requires pretreatment of the biomass in order to disrupt the cell wall of the microalgae (González-Fernández et al., 2012a). In the present study, thermal pretreatment was evaluated for enhancing anaerobic biodegradation. Methane production at organic loading rates of 1 and 2.5 kg COD m−3 day−1, was applied to a semi-continuously fed reactor. In addition to organic matter degradation, nitrogen mineralization and potential inhibitions by organic overloading and free ammonia were assessed.
Section snippets
Feedstock and inocula
Biomass containing mainly Scenedesmus sp. microalgae was grown at 20 °C in synthetic Z-8 nutrient broth (Staub, 1961). Continuous illumination with fluorescent lamps (Edison, 58 W) was employed. The photobioreactor consisted of an open plastic bag of 0.2 m3. Air flow of 12 L min−1 was provided for mixing. When the absorbance (680/800 nm) of the culture reached a plateau (after approximately two weeks), the biomass was allowed to settle by gravity.
The inoculum for anaerobic digestion was granular
Pretreatment: length and temperature selection
Fig. 1 shows the sCOD released upon pretreatments at 70, 80 and 90 °C. sCOD increased with time. The same pattern in terms of organic matter hydrolysis was achieved with treatments at 70 and 80 °C. Maximum COD hydrolysis yield at these temperatures was approximately 6% of initial tCOD. Thermal treatment at 90 °C achieved this value within the first 15 min of heating. Increasing the length of the pretreatment to up to 4 h led to a hydrolysis yield of 8%. A temperature of 90 °C is sufficient to break
Conclusion
Hydrolysis of biomass consisting mainly of Scenedesmus sp. was affected by prolonged storage. Thermal pretreatment at 90 °C was selected to improve methane production of Scenedesmus sp. biomass. Feeding the reactor with thermally pretreated biomass at OLR of 1 kg tCOD m−3 day−1 improved methane production by 3-fold. Raising OLR to 2.5 kg tCOD m−3 day−1 did not cause organic overloading or ammonia toxicity. Despite the pretreatment, Scenedesmus sp. was refractory to anaerobic degradation. For biofuel
Acknowledgement
This research was financially supported by the French National Research Agency for the Symbiose project (ANR-08-BIO-E11).
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