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The valorization of orange peel waste (OPW) is sought worldwide mainly via anaerobic digestion. A common problem encountered during the biological treatment is the seasonality of its production and the presence of d-Limonene. The latter is a typical anti-microbial compound. This work aims to evaluate the effect of the use of granular activated carbon (GAC) combined with alkaline pretreatment to enhance methane generation during semi-continuous anaerobic digestion of OPW. The experimental design consisted of two groups of experiments, A and B. Experiment A was designed to verify the maximum OPW loading and to assess the effect of pH and nutrients on the process. Experiment B was designed to study the effect of alkaline pretreatment alone and of alkaline pretreatment aided by biochar addition to the process. Apart from the methane yields, the d-Limonene contents were measured in all experiments. The preliminary results showed that OPW alkaline pretreatment after the addition of a moderate amount of GAC can render anaerobic digestion of OPW sustainable as long as the organic loading does not exceed 2 gVS·L−1·day−1 and nutrients are supplemented. The experiment in which GAC was added after alkaline pretreatment resulted in the highest methane yield and reactor stability.

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