The synthetic effect on volatile fatty acid disinhibition and methane production enhancement by dosing FeCl3 in a sludge thermophilic anaerobic digestion system

Abstract

Anaerobic digestion (AD) is considered an efficient method for sludge reduction and biogas generation simultaneously, while the excessive accumulation of volatile fatty acids (VFAs) often results in process instability in the thermophilic AD system. In this study, ferric chloride (FeCl₃), as an additive, was supplemented into the sludge thermophilic AD system directly. A dosage gradient was adopted and 9.92 mg Fe per g DS was favorable for the disinhibition of VFAs, with the maximum methane production of 236.75 mL per g VS under the test conditions, which was 2.2 times higher than the control group. The results indicated that methane production could be successfully enhanced by disinhibiting the excessive VFAs. Further investigations of the composition of soluble organic compounds confirmed that acetic acid was the main inhibitor during the sludge thermophilic AD process, and was responsible for the acid inhibition. Meanwhile, dosing an appropriate amount of FeCl₃ into the thermophilic AD system would contribute to acetic acid utilization for methane production, which should be attributed to the synthetic effects, except for the direct removal of acetic acid by Fe(OH)(CH₃COO) precipitate. The dosage of FeCl₃ in the thermophilic AD system showed significant impact on the process performance and certain dosages at low concentration (<9.92 mg Fe per g DS) showed a positive effect on the substrate environment. Based on the fitting analysis, the optimum dosage of 8.95 mg Fe per g DS was recommended for the enhanced substrate utilization, with the Δcumulative methane production of 169.34 mL per g VS.