Anaerobic digestion of marine microalgae

Abstract

Anaerobic digestion is a simple and energetically efficient way in comparison to some other biofuel methods of producing renewable energy from a range of biomass types. Although digestion of micro-algal biomass was first suggested in the 1950s, only a few studies have been conducted for assessment of its performance. This work assessed the potential for energy recovery from microalgae via anaerobic digestion for both freshwater and marine species.

This research screened seven laboratory-grown marine and freshwater microalgal species (Nannochloropsis. oculata, Thalassiosira . pseudonana, Dunaliella. salina, Rhododomas sp, Isochrysis. galbana, Chlorella. vulgaris and Scenedesmus sp) and two samples from large-scale cultivation systems for their suitability as a substrate for anaerobic digestion. Biochemical methane production and a theoretical maximum growth yield of each species were employed to offer a means of comparing methane productivity per unit of cultivation under standard conditions. The data generated were useful in determining suitable species to culture and digest under continuous operation.

A review of the literature highlighted a gap in the knowledge for the continuous digestion of different marine micro-algal species, as well as the potential inhibitory effect of high salinities on the anaerobic digestion process to non-acclimatised systems run under continuous operation. Addition of total salt ? 10g L-1 caused reactor failure, supporting the findings of the literature review. It was possible, however, to gradually adapt the inoculum to marine concentrations of chloride salts (31.1 g L-1) with <7% difference in specific methane production of controls. Addition of sulphate showed competition between methanogens and sulphate-reducing bacteria with further minor losses in methane yield. There was up to 60% reduction in SMP for the highest sulphate loaded reactors, however, the population successfully adapted to sulphate concentrations above those typically found in seawater and showed gaseous H2S productivity in proportion to the applied sulphate load. This suggests that the effects of marine concentrations of chloride and sulphate salts can be overcome by a gradual acclimatisation.

The selected algal species I. galbana and D. salina were continuously cultivated in a photobioreactor under low and high sulphate media and continuously digested using the salt adapted inoculum. The specific methane production for I. galbana and D. salina was 0.19 and 0.23 L CH4 g-1 VS, with a VS destruction of 32% and 50% respectively. Addition of a high SO4 grown D. salina as a feed resulted in a reduction of SMP to 0.19 L CH4 g-1 VS with an increase in H2S production. Loses in total solids and sulphur were observed under continuous study due to oxidation of H2S and struvite precipitation within the reactors, which was not observed under batch analysis. This highlights the importance in conducting continuous studies over batch, as these effects can be overlooked.

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ORCID for Charles Banks: orcid.org/0000-0001-6795-814X

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