Effect of Minimizing d-Limonene Compound on Anaerobic Co-digestion Feeding Mixtures to Improve Methane Yield

The purpose of this work was to assess the enhancement of the anaerobic digestion process of sewage sludge by the addition of orange peel as co-substrate. In this experimental study, an evaluation of the co-digestion process in a semi-continuous stirred tank reactor was carried out under mesophilic conditions (37 ± 0.2 °C) during an operation time up to 315 days. All trials were performed at a hydraulic retention time of 15 days and the AD reactor was fed daily with a mixture of sewage sludge and orange peel pulp, previously prepared. The organic loading rate was kept at 1.80 ± 0.31 g VS L −1 day −1 in all trials. A reference scenario (T0), mono-digestion, was performed using sewage sludge as substrate (primary sludge and activated sludge, 40:60, v/v). Thereafter, two different anaerobic co-digestion trials were carried out, one with pre-treated orange peel (T1) and the other with reduced d -limonene content (T2). Trials with pre-treated orange peel (T1 and T2) led to two- and threefold improvements in biogas and methane yields, compared with the reference scenario (T0). Furthermore, the reduction of d -Limonene content (T2) increased the methane yield by around 70% compared with T1. Therefore, it was proved that the sieving pre-treatment is effective to avoid the inhibitory effect of d -limonene.