Inhibition of biogas production by substituted phenolic compounds in enriched methanogenic cultures

Gallic (GA), hydroxybenzoic (HBA) and protocatechuic (PTC) acids and tyrosol (TY) are important pollutants associated with the industrial production of olive oil, wine and both green and black tea. Their toxicity can be problematic for the anaerobic biodegradation of wastes discharged by such industries. This study aims at examining the toxicity of these compounds on biogas production from methanogenic digestion of industrial sludge and their apolarity-toxicity relationship. Anaerobic toxicity assays were carried out at different concentrations of aromatic compounds at 35 0 C. HBA was the most toxic compound and GA the least. 246, 397 and 480 mg g-1 VSS of HBA, TY and PTC inhibited gas production by 50%. Inhibition of GA was less than 17% in the range of concentrations utilized, indicating no threat to anaerobic biodegradation. Molecule apolarity, type, size or number of substitute group competed for toxicity effects. There was a direct apolarity-toxicity relationship between HBA and TY and between HBA, PTC and GA. The change of the carboxylic substitute by alkyl ethanol made TY less toxic than HBA but it was still more toxic than PTC and GA. Consequently, the type of substitution is more important than apolarity. This information is essential in estimating the toxicity of these compounds in anaerobic treatment systems. Results Conclusions • 246, 397 and 480 milligrams of HBA and TY and PTC per gram of VSS of sludge inhibited gas production by 50%. • GA is not a threat to anaerobic biodegradation, it is the least toxic compound (17% maximum). • There is a direct apolarity-toxicity relationship between HBA and TY; and between HBA, PTC and GA. • The type of the substitute was more important toxicity factor than polarity. • The toxicity of the aromatic compound decreased as the number of substitution increased. • Apolarity, type, size or number of substitute group compete over toxicity effects in anaerobic biodegradation. • This information is valuable for anaerobic biodegradability tests; and to estimate these substances toxicity operating an anaerobic digester.