Sequential co-processing of olive mill wastewater and organic residues by anaerobic co-digestion and pyrolysis for the generation of bioadsorbent and low-cost media for microalgae cultivation

This investigation aims to evaluate the co-valorization of olive mill wastewater with other organic residues involving acid pretreatment, co-digestion, pyrolysis, and microalgae cultivation. Phosphoric acid pretreatment (5%, 60°C for 1h) was used to solubilize the organic matter in a mixture of coffee grounds, orange peel, olive mill wastewater, and cardboard. The acidic treatment led to the production of 328 Nml of biogas/gVS, while the untreated mixture provided 257 Nml of biogas/gVS. The solid digestates were subjected to pyrolysis (500°C for 1h), providing a biochar yield of approximately 44% for both digestates. Adsorption tests using methylene blue revealed an adsorption capacity of the biochar generated from the digestate of the acid pre-treated mixture of 4mg/g, which was higher than that of the biochar from the untreated mixture (3.5mg/g). The liquid digestates were also used as growth media for microalgae. Compared to the reference media (BG11), the use of the digestates-based media resulted in a lower growth rate (0.04 d-1) and biomass (247mg/L) but significantly higher lipid content (49%). The study shows, therefore, that the use of acid pretreatment had a positive effect on biogas production (solubilization of sugars), on the adsorption efficiency of the biochar (improvement of the porosity of the material), and on the production of lipids from the microalgae which can be destined to biodiesel.