AlgaeNRG: Anaerobic digestion tests using microalgae as substrate.

Microalgae cultivated with low cost techniques could be used as valid feed, or feed integrator, for anaerobic digestion process. In the last few years AD technique proved to be an affordable and sustainable treatment method for several agro-zootechnical types of biomass. It has been studied that a microalgae oil content lower than 40% of w/w, brings to a not economically affordable use of this biomass as biofuel source. A valid alternative is represented by the anaerobic digestion process where, with this kind of substrate, the methane quality inside the biogas could reach very high percentages (65-75%). Using the most promising microalgae strain, obtained from parallel experiments and conducted inside the AlgaeNRG research project, several tests under different feeding conditions were performed. In the experiments a self-made 25 L AD pilot plant created specifically for this kind of tests has been used. With this instrument the biogas and methane yield under batch, fed-batch and continuous regime can be correctly estimated. The tests were conducted starting from the simulation of a real anaerobic reactor working phase. The start-up stage was conducted loading half working volume of digestate and half working volume of microalgae biomass. When acceptable steady state condition was reached inside the reactor (methane quality exceeds 50% of the produced biogas), the second stage of the experience was performed. The biomass initially loaded has been gradually substituted with a new one made only of microalgae. COD behaviour, methane and biogas yields, pH, temperatures (inside the reactor and the gasometer), and pressure (relative and absolute) trends were analysed. Analytic evaluation and possible scale up use will be presented.