Semi-continuous anaerobic co-digestion of orange peel waste and residual glycerol derived from biodiesel manufacturing

• Orange peel waste and residual glycerol were subjected to anaerobic co-digestion.
• Their mixture at 1:1 in COD was highly biodegradable (85% in volatile solids).
• The mesophilic co-digestion was stable for organic loads up to 2.10 g VS/L.
• The methane yield coefficient reached a mean value of 330 ± 51 mLSTP CH4/g VSadded.
• The mean OLR was found to be 17.59 ± 2.78 kg mixture/m3 d.

The manufacturing of orange juice generates high volumes of orange peel waste which should not be deposited in landfill according to current recommendations. Furthermore, glycerol is a compound co-generated in biodiesel manufacturing, but the volume generated is higher than the current demand for pure glycerol. The anaerobic co-digestion of orange peel waste with residual glycerol could reduce the inhibitory effect of some compounds and provide a correct nutrient balance. Under mesophilic temperature and semi-continuous conditions, a mixture of orange peel waste-residual glycerol of 1:1 (in COD) operated favorably for organic loads up to 2.10 g VS/L. At higher organic loads, the accumulation of volatile fatty acids (VFA) and a decrease in the pH caused process destabilization. The methane yield coefficient was quite constant, with a mean value of 330 ± 51 mLSTP/g VSadded, while the organic loading rate (OLR) reached a mean value of 1.91 ± 0.37 kg VS/m3 d (17.59 ± 2.78 kg mixture/m3 d) and the hydraulic retention time (HRT) varied in a range of 8.5–30.0 d.