Comparison of energy recovery after anaerobic digestion of three Marchica lagoon algae (Caulerpa prolifera, Colpomenia sinuosa, Gracilaria bursa-pastoris)

The Marchica lagoon, which currently suffers from eutrophication caused by algae proliferation, is one of the largest lagoons in the Mediterranean. In this work, we valorised the three important algae (Caulerpa prolifera, Gracilaria bursa-pastoris, and Colpomenia sinuosa) that are responsible for pollution of the lagoon, by determining their biogas production and green energy capacities and studying their effects on anaerobic bacteria. The red alga Gracilaria bursa-pastoris produced the most biogas with 86.35 ml/g VS, which is equivalent to 514.99 kWh/t VS of green energy, whereas the brown alga Colpomenia sinuosa ranked second with 74.68 ml/g VS of biogas product, the equivalent to 445.39 kWh/tVS. Thus, these two algae were considered excellent substrates for anaerobic digestion. The green alga Caulerpa prolifera produced the least biogas with only 24.53 ml/g VS and green energy of 146.29 kWh/t VS. These results correlated with the anaerobic bacterial proliferation of the inoculum, where Gracilaria bursa-pastoris led to a large proliferation of anaerobic bacteria, from 0.19 × 106 CFU/g to 7 × 106 CFU/g, followed by the brown alga and the green alga. Therefore, we consider these three Marchica lagoon algae (Caulerpa prolifera, Gracilaria bursa-pastoris, and Colpomenia sinuosa) as an exploitable energy reservoir. Finally, we propose that a plant, comprising anaerobic digestion of the three algae, be installed to produce energy and a lagoon aerator to dissolve the atmospheric oxygen inside the lagoon. This will halt eutrophication of the Marchica lagoon.