Abstract
The biomass valorisation of the invasive brown alga Rugulopteryx okamurae (Dictyotales, Phaeophyceae) is key to curbing the expansion of this invasive macroalga which is generating tonnes of biomass on southern Spain beaches. As a feasible alternative for the biomass management, anaerobic co-digestion is proposed in this study. Although the anaerobic digestion of macroalgae barely produced 177 mL of CH4 g−1 VS, the co-digestion with a C-rich substrate, such as the olive mill solid waste (OMSW, the main waste derived from the two-phase olive oil manufacturing process), improved the anaerobic digestion process. The mixture improved not only the methane yield, but also its biodegradability. The highest biodegradability was found in the mixture 1 R. okamurae—1 OMSW, which improved the biodegradability of the macroalgae by 12.9% and 38.1% for the OMSW. The highest methane yield was observed for the mixture 1 R. okamurae—3 OMSW, improving the methane production of macroalgae alone by 157% and the OMSW methane production by 8.6%. Two mathematical models were used to fit the experimental data of methane production time with the aim of assessing the processes and obtaining the kinetic constants of the anaerobic co-digestion of different combination of R. okamurae and OMSW and both substrates independently. First-order kinetic and the transference function models allowed for appropriately fitting the experimental results of methane production with digestion time. The specific rate constant, k (first-order model) for the mixture 1 R. okamurae- 1.5 OMSW, was 5.1 and 1.3 times higher than that obtained for the mono-digestion of single OMSW and the macroalga, respectively. In the same way, the transference function model revealed that the maximum methane production rate (Rmax) was also found for the mixture 1 R. okamurae—1.5 OMSW (30.4 mL CH4 g−1 VS day−1), which was 1.6 and 2.2 times higher than the corresponding to the mono-digestions of the single OMSW and sole R. okamurae (18.9 and 13.6 mL CH4 g−1 VS day−1), respectively.
Highlights
The greatest biodiversity loss in the world is produced by the introduction of invasive alien species (IAS) (Convention on Biological Diversity 2017), with the seas the areas that suffer the most its consequences
The transference function (TF) model was used to fit the experimental data of methane production during biochemical methane production (BMP) tests (Eq (2))
This work demonstrated an improvement in biomass valorisation by anaerobic co-digestion of the macroalga with a C-rich substrate such as the olive mill solid waste (OMSW)
Summary
The greatest biodiversity loss in the world is produced by the introduction of invasive alien species (IAS) (Convention on Biological Diversity 2017), with the seas the areas that suffer the most its consequences. Rugulopteryx okamurae comes from temperate waters near China, Korea, the Philippines, and Japan This macroalga has been recently included in the Spanish catalogue of invasive alien species according to the ministerial order TED/1126/2020 of the. Only in 1 year after being observed for first time in the Strait of Gibraltar area in the autumn of 2015 in Ceuta, the beach cleaning machines of this city removed more than 5000 t of upstream macroalga biomass of the IAS (Ocaña et al 2016) It took only 4 years to occupy the entire Cadiz coastline and to expand along the coasts of Malaga and Granada, affecting very relevant spaces of great ecological value included in the Natura 2000 Network. This rapid expansion is unprecedented; the most similar case may be the spread of the seaweed Sargassum spp. in the Caribbean (van Tussenbroek et al 2017)
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