The methanogenic toxicity of wastewater lignins and lignin related compounds

Lignin derivatives are major components of wastewater streams generated in the chemical processing of wood. The objective of this study was to evaluate the inhibitory effect of various lignins isolated from forest industry wastewaters, and selected lignin model compounds, on methanogenic bacteria. The methanogenic inhibition was determined at 30°C in standard toxicity assays, utilizing anaerobic granular sludge as inoculum. The wastewater lignins differed considerably in their inhibitory activity. Some lignin samples were nontoxic, whereas others caused 50% inhibition at concentrations ranging from 3320 to 5950 mg COD dm⁻³. Experiments with ultrafiltered lignins revealed that the toxicity of the inhibitory lignin samples originated from the low molecular weight fraction. In additional studies with low molecular weight lignin model compounds, it was observed that the inhibitory activity of these compounds was related to the functional groups on the aromatic ring. Compounds with aldehyde groups or apolar substituents were highly toxic, whereas those with carboxylic groups only caused significant inhibition at high concentrations. These results indicate that low molecular weight lignin derivatives in forest industry wastewaters are potential inhibitors of anaerobic treatment systems.