Abstract
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.
Highlights
Orange peel waste (OPW) is produced in large quantities in many parts of the world [1,2,3] but a fully sustainable solution for its valorization has not been found until now.Currently, part of OPW produced is used as animal feed
The use of granular activated carbon (GAC) during anaerobic digestion of citrus peel waste could be a suitable approach for the stabilization of the process [14]
In Experiments B1 and B2 (Table 2), in order to reduce d-Limonene concentration and to increase buffering capacity [2], lyophilized OPW was subjected to an alkaline pretreatment using NaOH (5% TS, 50% NaOH solution, Sigma-Aldrich) at room temperature for 24 h [33,34,35]
Summary
Orange peel waste (OPW) is produced in large quantities in many parts of the world [1,2,3] but a fully sustainable solution for its valorization has not been found until now. The surface area of the biochar is less efficient (less porous) than that of activated carbon [19] Due to these characteristics, the use of granular activated carbon (GAC) during anaerobic digestion of citrus peel waste could be a suitable approach for the stabilization of the process [14]. In Experiments B1 and B2 (Table 2), in order to reduce d-Limonene concentration and to increase buffering capacity [2], lyophilized OPW was subjected to an alkaline pretreatment using NaOH (5% TS, 50% NaOH solution, Sigma-Aldrich) at room temperature for 24 h [33,34,35]. Solution C contains MnCl2·4H2O, H3BO3, ZnCl2, CuCl2, Na2MoO4·2H2O, CoCl2·6H2O, NiCl2·6H2O, Na2SeO3 and distilled water and the amount to be used is 1/5 of the volume of Solution A
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