Importance of Microalgae Speciation on Biogas Production and Nutrient Recovery from Anaerobic Digestion of Lipid-Extracted Microalgae Biomass

Abstract

Abstract

While the recovery of nutrients (nitrogen and phosphorus) from anaerobic digestion (AD) of lipid-extracted microalgae biomass has been suggested to increase microalgae biofuel sustainability, most studies use assumed biogas and nutrient recovery values that do not take into consideration how microalgae speciation and biomass composition may affect these recovery values. This study is among the first to examine how microalgae speciation and biomass composition influence the recovery of biogas and nutrients from the AD of lipid-extracted microalgae. Batch AD of lipid-extracted Chlorella vulgaris (a model freshwater microalgae) and lipid-extracted Cyclotella sp. (a model marine diatomic microalgae with a high silica composition) resulted in similar 30-day biogas yields of 0.37 ± 0.02 L-biogas/g-microalgaeVS and 0.38 ± 0.02 L-biogas/g-microalgaeVS, respectively. However, biogas production rates varied significantly between species with lipid-extracted Cyclotella producing 79% ± 1% of its biogas within the first 5 days compared to 49% ± 1% for Cyclotella. In addition, microalgae speciation and composition significantly influenced nutrient recovery with an average nitrogen recovery of 46% ± 9% for lipid-extracted Cyclotella, while a negligible nitrogen recovery (6.3% ± 7.6%) was observed for lipid-extracted Chlorella. Interestingly, for phosphate, the opposite trend occurred with an average phosphorus recovery of 78% ± 14% for lipid-extracted Chlorella and a negative phosphorus recovery from lipid-extracted Cyclotella, possibly due to its high salt content, which can result in the precipitation of phosphorus-containing salts. Thus, unlike biogas yields, nutrient recovery appears to be highly sensitive to microalgae speciation and composition.