Methane-rich biogas production from waste-activated sludge with the addition of ferric chloride under a thermophilic anaerobic digestion system

Abstract

Thermophilic anaerobic digestion for methane production could realize simultaneously both energy recovery from waste-activated sludge and pollution control; however, low methane production and an imbalance between the hydrolysis and methanogenesis processes are often encountered. Three ferric salts, Fe(NO₃)₃, Fe₂(SO₄)₃ and FeCl₃, were introduced to test the potential effects on sludge anaerobic digestion performance under a thermophilic system. Enhanced methane production was achieved using FeCl₃ as the additive: this had a cumulative methane production of 117.44 mL CH₄ per g of volatile solid (VS), an increase of 98.9% over that in the control experiment (59.05 mL CH₄ per g VS). Both Fe(NO₃)₃ and Fe₂(SO₄)₃ caused some negative effects, meaning that the type of anion should also be considered. The introduction of FeCl₃ created a favorable environment resembling positive precipitation and biocatalysis. The succession of microbial communities before and after the introduction of ferric salts was investigated and compared through pyrosequencing analysis, and in particular the dominance of Methanosarcina increased from 1.3% to 63.2% in effective reads with the addition of FeCl₃.