Microalgae to Bioenergy: Optimization of Aurantiochytrium sp. Saccharification.

Microalgae are a promising feedstock for bioethanol production, essentially due to their high growth rates and absence of lignin. Hydrolysis-where the monosaccharides are released for further fermentation-is considered a critical step, and its optimization is advised for each raw material. The present study focuses on the thermal acid hydrolysis (with sulfuric acid) of Aurantiochytrium sp. through a response surface methodology (RSM), studying the effect of acid concentration, hydrolysis time and biomass/acid ratio on both sugar concentration of the hydrolysate and biomass conversion yield. Preliminary studies allowed to establish the range of the variables to be optimized. The obtained models predicted a maximum sugar concentration (18.05 g/L; R2 = 0.990) after 90 min of hydrolysis, using 15% (w/v) biomass/acid ratio and sulfuric acid at 3.5% (v/v), whereas the maximum conversion yield (12.86 g/100 g; R2 = 0.876) was obtained using 9.3% (w/v) biomass/acid ratio, maintaining the other parameters. Model outputs indicate that the biomass/acid ratio and time are the most influential parameters on the sugar concentration and yield models, respectively. The study allowed to obtain a predictive model that is very well adjusted to the experimental data to find the best saccharification conditions for the Aurantiochytrium sp. microalgae.

Anaerobic co-digestion of marine macroalgae waste and fruit waste: Effect of mixture ratio on biogas production.

Marine macroalgae waste (MMW) was used at different mixing ratios with fruit waste (FW) to evaluate the potential of co-digestion in enhancing methane yield. The process was conducted at mesophilic conditions (37 °C) with a fixed amount of biomass (10 g, 3.5% TS) and inoculum (150 mL; digested sewage sludge) and using MMW:FW ratios from 40:60 to 70:30. The results showed inhibition of the process for most of the studied substrate ratios, and in the mono-digestion of both substrates, possibly due to the accumulation of volatile fatty acids. A maximum biogas yield of 295 mL/g VS with 72% of methane was however obtained for the 60MMW:40FW ratio, corresponding to an estimated maximum methane yield of 213 mL/g VS and around 46% of the theoretical maximum methane production (49% of organic matter removal). The results show that the co-digestion of MMW with FW enhances the methane yield of both independent substrates.

Biogas production from residual marine macroalgae biomass: Kinetic modelling approach.

Modelling the conversion of residual biomass to renewable fuels is of high relevance to promote the development of effective technological solutions. The present study compares the performance of five different kinetic models (pseudo-first-order kinetics, logistics, modified Gompertz, double-Gompertz, and multi-Gompertz) to describe the cumulative methane production during a low-solids anaerobic digestion of marine macroalgae waste. Different substrate concentrations were evaluated (0.9, 1.7 and 2.5% TS) with the best methane yield (105.2 mL CH4.g VS-1) being obtained at the highest amount of biomass. All models fitted the experimental data with R2 > 0.988. The innovative multi-Gompertz model herein proposed led to the best performance indexes for all tested experimental conditions, allowing to predict methane yields more accurately when the digestion occurs in two or more steps, as it was the case with marine macroalgae waste.

Marine macroalgae in a circular economy context: A comprehensive analysis focused on residual biomass.

The accumulation of Marine Macroalgal Waste (MMW) - drifted marine macroalgae - is a growing phenomenon. The lack of appropriate management protocols for this residual biomass leaves it mostly unmanaged or disposed of in landfills. Such inappropriate solutions lead to negative environmental impacts, health concerns and coastal degradation, representing an opportunity to develop the bioeconomy. Although a relevant number of studies exists on the use of edible marine macroalgae for several purposes, there is generally a lack of information concerning MMW valorisation, at both local and global scales. This review thus explores the ecological and biotechnological potential for the establishment of a new approach for MMW valorisation based in the circular and blue economies. It is supported in literature on the use of residual biomass whenever possible and also includes studies with fresh biomass to extrapolate, considering known characteristics of MMW; consequently, a roadmap for its utilisation is provided. The review includes characteristics of various seaweeds, MMW accumulation worldwide and traditional uses, together with the development of alternative and integrated valorisation strategies. By analysing a large number of studies, it was possible to perceive a clear potential to explore MMW to obtain renewable fuels, especially biogas and bioethanol and for use in agricultural activities. Allied to this, the extraction of value-added compounds, such as lipids, pigments, phenolic compounds and specific polysaccharides is highlighted, aiming to contribute to the development of an economically feasible zero-waste biorefinery approach.

Marine macroalgae waste: A potential feedstock for biogas production.

In the present study, marine macroalgae waste, mainly composed by Saccorhiza polyschides, was collected from a beach in northern Portugal and evaluated as feedstock for anaerobic digestion. Batch experiments (500 mL flasks, 300 mL working volume) were conducted at the following conditions: mesophilic temperature (37 °C); 80 rpm stirring speed; 150 mL inoculum (anaerobically digested sludge) and variable total solids content (0.9, 1.7, 2.5 and 3.5% TS). Methane concentration and volume of biogas obtained were monitored during up to 57 days by optical sensors and milligascounters, respectively. The results show that an increase in total solids content up to 2.5% TS led to the highest biogas volume and methane concentration. The maximum biogas yield was 227 ± 4 mL/g VS (2.5% TS, 53 operation days), with the maximum methane content in the biogas being 64.5 ± 0.6% (51 operation days). A maximum methane yield of 146 ± 2 mL/g VS was consequently estimated. At the end of the process (57 days), an average of 43% COD reduction and 46% VS reduction were observed. These results correspond to about 27% of the theoretical maximum methane production. Using 3.5% TS the inhibition of the process was observed, by the decrease in pH, most likely due to the accumulation of volatile fatty acids. The results indicate that marine macroalgae waste may be a good candidate as substrate for anaerobic digestion processes, most probably by co-digestion.