Oxidative detoxification of aqueous bark extracts. Part II: Alternative methods

The most important problem associated with the high‐pH autoxidative detoxification of methanogenic toxins in debarking wastewater was its limited application to the aqueous extractives of bark from specific species. The autoxidative polymerization of coniferous bark extracts exhausted prior to complete removal of the toxic oligomeric tannins. As a result, highly toxic spruce bark extracts were only partly detoxified. Birch bark extracts were not detoxified, even though effective polymerization of the oligomeric tannins occurred. The non‐tannin toxins in birch bark extracts and toxic intermediates produced by high‐pH autoxidation of these extracts were responsible for the poor detoxification results. These problems can potentially be resolved by applying alternative oxidation methods. In this study a high level of detoxification was obtained for all bark species tested by short‐term destructive oxidations with H₂O₂, or by long‐term aerobic biological treatments which caused high levels of polymerization. The applicability of high‐pH autoxidation was also made feasible for all species of bark by applying a granular active carbon treatment after the autoxidation. The active carbon was able to adsorb the residual toxic oligomeric tannins of autoxidized spruce bark extracts and the non‐tannin toxins in autoxidized birch bark extracts. Additionally, a potential disadvantage of the autoxidative detoxification method is the formation of colored end products. Although these compounds are non‐toxic and are non‐biodegradable, their elimination from the wastewater would be necessary if discharge norms concerning color and non‐biodegradable COD must be fulfilled. The destructive oxidation with H₂ O₂ produced non‐toxic low‐molecular‐weight compounds which did not have much color. The autoxidation and long‐term aerobic biological treatments produced highly colored humic end products which could be eliminated by calcium precipitation.