Recovery of acerbic anaerobic digester for biogas production from pomegranate shells using organic loading approach

Abstract

Anaerobic digestion of pomegranate shells was conducted in 25 L bioreactor operating at 35±0.5°C. The digester showed a reasonable amount of biogas (0.71 m3/kg VS fed) and methane (55.7%) with stable pH and acid: alkali profiles when operated at organic loading rate (OLR) from 1.0 to 3.0 kg VS/day/m−3. The reactor exhibited stable performance with methane yield of 0.44 m3/kg VS fed and reduction of 38.5% volatile solids (VS) As organic loading rate increased to 3.5 kg VS/day/m−3, accumulation of volatile fatty acid (VFA; 2797 ppm), mainly propionic acid (1617 ppm) was noticeable. The digester turned sour (pH 4.32) with lower biogas (2.5 Ld−1) and methane (30.80%) production, reflecting the case of overloading. Reversal of organic loading rate from 3.5 to 3.0 kg VS/day/m−3 gradually restored the upset anaerobic digester to normal profile in 4 weeks as judged from a gradual increase in biogas (6.5 Ld−1) and methane (58.4%).