Effects of partial maize silage substitution with microalgae on viscosity and biogas yields in continuous AD trials.

The microalga Acutodesmus obliquus was investigated as a feedstock in semi-continuously fed anaerobic digestion trials, where A. obliquus was co-digested with pig slurry and maize silage. Maize silage was substituted by both 10% and 20% untreated, and 20% ultrasonicated microalgae biomass on a VS (volatile solids) basis. The substitution of maize silage with 20% of either ultrasonicated and untreated microalgae led to significantly lower biogas yields, i.e., 560 dm³ kg-1 VScorr in the reference compared to 516 and 509 dm³ kg-1VScorr for untreated and ultrasonicated microalgae substitution. Further, the viscosities in the different reactors were measured at an OLR of 3.5 g VS dm-3 d-1. However, all treatments with microalgae resulted in significantly lower viscosities. While the mean viscosity reached 0.503 Pa s in the reference reactor, mean viscosities were 53% lower in reactors where maize was substituted by 20% microalgae, i.e. 0.239 Pa s, at a constant rotation speed of 30 rpm. Reactors where maize was substituted by 20% ultrasonicated microalgae had a 32% lower viscosity, for 10% microalgae substitution a decrease of 8% was measured. Decreased viscosities have beneficial effect on the bioprocess and the economy in biogas plants. Nonetheless, with regard to other parameters, no positive effect on biogas yields by partial substitution with microalgae biomass was found. The application of microalgae may be an interesting option in anaerobic digestion when fibrous or lignocellulosic substances lead to high viscosities of the digested slurries. High production costs remain the bottleneck for making microalgae an interesting feedstock.

Cyanobacteria Biorefinery - Production of poly(3-hydroxybutyrate) with Synechocystis salina and utilisation of residual biomass.

This study evaluates a biorefinery concept for producing poly(3-hydroxybutyrate) (PHB) with the cyanobacterial strain Synechocystis salina. Due to this reason, pigment extraction and cell disruption were investigated as pre-treatment steps for the harvested cyanobacterial biomass. The results demonstrated that at least pigment removal was necessary to obtain PHB with processable quality (weight average molecular weight: 569-988kgmol-1, melting temperature: 177-182°C), which was comparable to heterotrophically produced PHB. The removed pigments could be utilised as additional by-products (chlorophylls 0.27-1.98mgg-1 TS, carotenoids 0.21-1.51mgg-1 TS, phycocyanin 0-127mgg-1 TS), whose concentration depended on the used nutrient source. Since the residual biomass still contained proteins (242mgg-1 TS), carbohydrates (6.1mgg-1 TS) and lipids (14mgg-1 TS), it could be used as animal feed or converted to biomethane (348 mn3 t-1VS) and fertiliser. The obtained results indicate that the combination of photoautotrophic PHB production with pigment extraction and utilisation of residual biomass offer the highest potential, since it contributes to decrease the environmental footprint of the process and because biomass could be used in a cascading way and the nutrient cycle could be closed.