The Methanogenic Toxicity of Bark Tannins and the Anaerobic Biodegradability of Water Soluble Bark Matter Available to Purchase

The principal methanogenic toxins of bark soluble matter were identified as the tannins. The tannins, which were measured with a selective tannin adsorbant called polyvinylpyrrolidone, accounted for about half of the aqueous extractable COD of tree bark. The 50% inhibitory concentration of bark tannins averaged approximately 600 mg COD L⁻¹ (350 mg tannin solids L⁻¹). The toxicity caused by the resin fraction of bark was demonstrated not to be very important to the methanogenic toxicity of aqueous extracts. While tree resin compounds were found to be very toxic to methanogenic bacteria, the solubility of the resin fraction was very poor due to the low natural pH during the aqueous extraction. Additionally, those compounds aqueous extracted from tree resin at the natural pH were not as toxic as all the tree resin compounds solubilized by alkali and supplied at similar concentrations. Anaerobic biodegradability results indicated that 30 to 50% of the bark water soluble COD can be acidified to methanogenic substrates during short term digestion. Pine and birch bark water soluble COD acidified up to 70% after long term batch digestion of 7 weeks. The high level of bark aqueous extract UV absorbance elimination by anaerobic digestion and the appearance of intermediates indicated that at least some of the bark phenolic compounds were included in the methanogenic substrate.