The anaerobic biodegradability and methanogenic toxicity of pulping wastewaters

The objective of this study was to evaluate the effect of various pulping conditions and different lignocellulosic feedstocks on the anaerobic treatability of pulping wastewaters. Wastewaters were prepared from lignocellulosic feedstocks commonly used in the forest industry, namely, pine, spruce and birch wood, and wheat straw. The pulping conditions used were representative of those applied in TMP and soda pulping processes. The anaerobic biodegradability and the methanogenic toxicity of the various wastewaters were evaluated in standardized batch bioassays using anaerobic granular sludge. The acidification of the TMP wastewaters (conversion to CH₄ and VFA) ranged from 68 to 87% of the total COD, indicating their high anaerobic biodegradability. TMP wastewaters were non-toxic to methane bacteria at concentrations expected in paper mill wastewaters. No inhibition was observed at 10 g COD/l. In contrast, wastewaters prepared in alkaline conditions were poorly biodegradable (approx. 50% acidification) and they caused severe inhibition of the methanogenic activity. The 50% inhibitory concentrations ranged from 2.1 to 5.4 g COD/l. Additional experiments showed that wood resin components, poorly solubilized at acidic to neutral pH, but easily extractable in alkali, are responsible for most of the methanogenic toxicity observed in alkaline pulping wastewaters. These results indicated that contact of wood with alkali contributes significantly to increase the methanogenic toxicity of the pulping wastewater.