Anaerobic digestion of microalgal biomass: effects of solid concentration and pre-treatment

Abstract

Microalgae have recently attracted considerable attention as a potential substrate for biofuel production. Through the anaerobic digestion process, microalgal biomass can be converted to biogas. Although the first work on anaerobic digestion of microalgae appeared in the 1950s, for a long time further information on AD of microalgae was scarce. This study assessed the potential for energy recovery from microalgal biomass grown in two large-scale systems in Spain: a closed tubular photobioreactor (PBR) and an open raceway.
A series of digestion trials was carried out using freeze dried microalgae (FDA) from the PBR and fresh frozen microalgae (FFA) from the open raceway system as feedstock. Results from biochemical methane potential (BMP) assays showed that both feedstocks were poorly degraded and gave low methane yields. The specific methane yields of FDA and FFA were 0.161 and 0.220 L CH₄ g^-1 VS , respectively, which is only about 30% and 44% of the Theoretical Methane Potential (TMP) of these substrates based on their elemental composition.
Digestion of FDA under semi-continuous conditions was stable at feedstock concentrations of up to 10% VS, equivalent to a hydraulic retention time of 20 days. Specific methane yields (SMY) were 0.11 - 0.12 L CH₄ g^-1 VS, corresponding to 69 - 75% of the value obtained from BMP. Digestion of FDA at 20% VS concentration gave only 0.09 L CH₄ g^-1 VS which is 56% of the value from BMP, or ~21% of the measured calorific value (CV). The digesters operating at 20% VS were able to achieve meta-stable operation at very high total ammonia nitrogen (TAN) concentrations of up to 12 g L^-1 while showing reasonable methane production. They therefore showed a degree of adaptation to high TAN, but no evidence of improved biomass degradation even after operating periods in excess of 800 days. Results from the isotope labelling experiment indicated that syntrophic methanogenic pathway was the major route in high TAN concentration digesters.
Digestion of FFA was stable at feedstock concentration of 4.33% VS and OLR up to 3.5 g VS L^-1 day^-1. SMY obtained under semi-continuous conditions was ~0.13 L CH4 g^-1 VS, corresponding to 23% of the measured CV. A series of pretreatments were carried out on FFA, and the combined thermal-alkaline pretreatment (dosage of 3% w/w NaOH and incubated in water bath at 80 oC for 2 hours) enhanced SMY by 42% compared with that of untreated FFA obtained under batch condition. The results from semi-continuous condition indicated that some improvement was achieved with the thermal alkaline pretreatment, but there were also signs of inhibition due to the high alkaline dosages of 3% NaOH (w/w) required. There is clearly scope for optimisation of the treatment of feedstock and adaptation of the inoculum.

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ORCID for Sonia Heaven: orcid.org/0000-0001-7798-4683

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