The methanogenic toxicity and anaerobic degradability of a hydrolyzable tannin

Gallotannic acid was found to be highly toxic to methanogenic activity. Concentrations, representing 50% inhibition, approximated 700 mg l⁻¹. The toxicity was persistent despite the rapid degradation of gallotannic acid to volatile fatty acids and methane. A 72.5% loss of sludge activity was associated with a 1 day exposure of methanogenic granular sludge to 1000 mg l⁻¹ gallotannic acid. The toxicity of gallotannic acid was persistent over 2 month assay periods. The monomeric derivatives of gallotannic acid, gallic acid and pyrogallol were much less toxic. The 50% inhibition concentration of the monomers approximated 3000 mg l⁻¹ and their toxicities were not persistent. No activity losses were evident after sludge was exposed to 3000 mg l⁻¹ gallic acid for 19 days.The lower toxicities of the monomers compared to the gallotannic acid polymer suggests that the mechanism of toxicity was “tanning”, since data in the literature indicate that tannin polymers are more effectively adsorbed and precipitated with proteins compared to their monomeric counterparts. Functional proteins (enzymes) located at accessible sites in or on the methane bacteria are most likely disturbed by the tanning action.