Sustainable mass production of algal biofuels requires a reduction in nutrient demand and efficient conversion
into fuels of all biomass including lipid-extracted algal residues (LEA). This study evaluated
methane production, nutrient recovery and recycling from untreated and enzymatically pretreated
Nannochloropsis LEA using semi-continuous anaerobic digestion (AD). Additionally, this process was compared
to methane generation from whole Nannochloropsis alga (WA) and thermally pretreated WA. The
methane production from untreated LEA and WA reached up to 0.22 L and 0.24 L per gram of biomass
volatile solids (VS), respectively, corresponding to only 36–38% of the theoretical potential.
Additionally, observed VS reduction was only 40–50% confirming biomass recalcitrance to biodegradation.
While enzymatic treatment hydrolyzed up to 65% of the LEA polysaccharides, the methane production
increased by only 15%. Alternatively, WA thermal pretreatment at 150–170 C enhanced methane
production up to 40%. Overall, an integrated process of lipid conversion into biodiesel coupled with
LEA conversion into methane generates nearly 40% more energy compared to methane production from
WA, and about 100% more energy than from biodiesel alone. Additionally, the AD effluent contained up to
60–70% of the LEA phosphorus content, 30–50% of the nitrogen, sulfur, calcium and boron, 20% of the iron
and cobalt, and 10% of manganese, zinc and copper, which can partially replace chemical fertilizers during
algal cultivation. Consequently, supplementation of Nannochloropsis cultures with 5% AD effluent was
optimal for a high algal growth rate. Therefore, coupling biodiesel and methane production provides significant
energy advantages along with sustainability and economic benefits from nutrient recycling.
Applied Energy 154 (2015) 718–731