Effect of organic loading rate on anaerobic digestion of thermally pretreated Scenedesmus sp. biomass

doi:10.1016/j.biortech.2012.10.123

Bioresource Technology

Volume 129, February 2013, Pages 219-223

https://doi.org/10.1016/j.biortech.2012.10.123 Get rights and content

Abstract

Biogas production is one of the means to produce a biofuel from microalgae. Biomass consisting mainly of Scenedesmus sp. was thermally pretreated and optimum pretreatment length (1 h) and temperature (90 °C) was selected. Different chemical composition among batches stored at 4 °C for different lengths of time resulted in organic matter hydrolysis percentages ranging from 3% to 7%. The lower percentages were attributed to cell wall thickening observed during storage for 45 days. The different hydrolysis percentages did not cause differences in anaerobic digestion. Pretreatment of Scenedesmus sp. at 90 °C for 1 h increased methane production 2.9 and 3.4-fold at organic loading rates (OLR) of 1 and 2.5 kg COD m⁻³ day⁻¹, respectively. Regardless the OLR, inhibition caused by organic overloading or ammonia toxicity were not detected. Despite enhanced methane production, anaerobic biodegradability of this biomass remained low (32%). Therefore, this microalga is not a suitable feedstock for biogas production unless a more suitable pretreatment can be found.

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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There are more references available in the full text version of this article.

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Bioresource Technology

Abstract

Highlights

Introduction

Section snippets

Feedstock and inocula

Pretreatment: length and temperature selection

Conclusion

Acknowledgement

References (12)

Thermal pretreatment to improve methane production of Scenedesmus biomass

Biomass Bioenenergy

Recovery of dairy manure nutrients by benthic freshwater algae

Bioresour. Technol.

Evaluation of anaerobic codigestion of microalgal biomass and swine manure via response surface methodology

Appl. Energy

Life-cycle assessment of biodiesel production from microalgae

Environ. Sci. Technol.

Impact of microalgae characteristics on their conversion to biofuel. Part II: Focus on biomethane production

Biofuels, Bioprod. Biorefin.

Ernährungsphysiologisch-autökologische Untersuchungen an Oscillatoria rubescens

Z. Hydrologia

Cited by (74)

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Effects of different thermal and thermochemical pretreatments on the anaerobic digestion of algal biomass cultivated in urban wastewater and collected with and without chemical coagulants

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Potential applications of protein-rich waste: Progress in energy management and material recovery

Kinetic study of thermophilic anaerobic digestion of sugarcane vinasse in a single-stage continuous stirred tank reactor

Review on current approach for treatment of palm oil mill effluent: Integrated system