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 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.