The role of natural wood constituents on the anaerobic treatability of forest industry wastewaters

Anaerobic treatment has been shown to be an efficient and energy conserving method for treating various types of readily biodegradable non-inhibitory forest industry wastewaters. However, the high toxicity of paper mill effluents derived from chemical wood processing operations has hampered the wide spread application of anaerobic treatment in the forest industry.This dissertation describes research on the anaerobic treatment of inhibitory wastewaters from the forest industry. The main objective was to determine the role of natural woodderived organic constituents on the methanogenic toxicity of these wastewaters.Lignocellulosic feedstocks were pulped by processes commonly applied in the forest industry, namely thermomechanical (TMP) and alkaline pulping processes, to determine which factors are responsible for the extraction of toxic substances. Batch anaerobic biodegradability and methanogenic toxicity assay results indicated that the pulping conditions applied had a significant effect on the anaerobic treatability of the resulting wastewaters. TMP effluents were highly biodegradable and non-inhibitory. Soda pulping liquors contain important fractions of recalcitrant organic matter and exerted severe toxicity. Wood resin constituents were shown to be the major inhibitors present in pulping wastewaters. Wood resin is composed of fatty constituents which are poorly soluble in water at neutral to acid pH values. The increased solubility of resin at high pH values, indicates that the contact of alkali with wood contributes strongly to producing toxic wastewaters by extracting resinous compounds. The alkali promotes lignin solubilization and thereby also contributes to a lowered biodegradability of the wastewater.Compounds representative of the major wood resin constituents were assayed for methanogenic toxicity. The high toxicity of a variety of resin compounds including volatile terpenes, resin acids and apolar phenols was demonstrated. Concentrations causing 50% inhibition ranged from 20 to 330 mg/l.Aside from the resinous wood constituents, lignin derived compounds are also potential sources of toxicity in pulping wastewaters. The methanogenic toxicity of lignin mixtures isolated from paper mill effluents was determined. Experiments with ultrafiltered lignins revealed that the toxicity of various wastewater lignins originated from the low molecular weight (MW) fraction. Studies with selected low MW lignin model compounds showed that their inhibitory activity 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 were distinctly less toxic.The effect of chemical structure on the methanogenic toxicity of aromatic compounds was investigated. Some basic structure- toxicity relationships were evident. In general, the toxicity increased with increasing the length of aliphatic side-chains and increasing the number of alkyl or chlorine groups. On the other hand, the toxicity decreased as polar 'functional groups were introduced on the alkylic side chains. The partition coefficient n-octanol/water, an indicator of hydrophobicity, was observed to be positively correlated with the methanogenic inhibition. The results indicate that hydrophobicity is an important factor contributing to the high toxicity of numerous aromatic compounds. Therefore, highly hydrophobic compounds such as resin constituents, apolar lignin derivatives and chlorinated aromatics and are the primary suspect toxicants in forest industry effluents.The susceptibility of important organic toxins in forest industry effluents to anaerobic biodegradation was assessed. The results indicated that anaerobic treatment technologies have a limited capacity to mineralize natural wood toxins. Although the degradation of a lignin derivative, guaiacol, and long chain fatty acids was demonstrated, other important wood toxins such as volatile terpenes and resin acids were persistent.Finally, the treatability of TMP and soda pulping effluents was evaluated in lab-scale upward-flow anaerobic sludge blanket (UASB) reactors. TMP wastewaters were found highly suitable for anaerobic treatment. Despite the inhibitory character of soda pulping liquors, anaerobic systems were feasible for removing the biodegradable COD if, prior to biological treatment, the wastewaters were diluted to subtoxic levels or detoxified by pretreatment with the adsorbent Amberlite-XAD-2.