The effect of tannic compounds on anaerobic wastewater treatment

Anaerobic wastewater treatment is an alternative to the conventional aerobic treatment processes for the removal of easily biodegradable organic matter in medium to high strength industrial wastestreams. Anaerobic treatment has several advantages, however one important disadvantage is the high sensitivity of the anaerobic bacteria (ie. methanogenic bacteria) to toxic compounds. The anaerobic technologies were initially developed for the treatment of non-toxic organic wastewaters. As the technology matured, the limits of its application to toxic wastewaters were studied. Past research has been mostly directed towards the toxic effects of compounds introduced by man into the industrial process rather than natural constituents present in agricultural wastewaters.This dissertation investigates the role of natural polar phenolics (ie. tannins and related compounds) on anaerobic digestion. Tannins are important constituents of certain types of agro-industrial wastewaters such as vegetable tannery effluent; olive oil mill effluent; wine vinasse; coffee pulp water; debarking wastewater; and masonite (fiber board) wastewater. A distinct feature of highly hydroxylated phenolics is that they are readily oxidized to darkly colored humic compounds. Such transformations can generate products which differ in toxicity and biodegradability compared to the original tannic compounds. Industrial process waters are often exposed to conditions which promote phenol oxidation, therefore the role of humus forming processes was a major consideration included in this study.The toxicity of tannin compounds to anaerobic bacteria was determined. The concentration of tannins found to cause 50% inhibition to methanogenic bacteria was 350 and 700 mg L -1of condensed and hydrolyzable tannins, respectively. The condensed tannins were the major inhibitors present in wastewater derived from the debarking of wood at pulping factories.The effects of oxidation treatments on the methanogenic toxicity of phenolic compounds was evaluated. The initial polymerization of monomers led to a higher toxicity due to an increase in tannic qualities. The oligomers formed have stronger hydrogen bonds with proteins than the monomers. They are thus more likely to react with the functional proteins of bacteria. If the polymerization was continued, a decrease in toxicity occurred due to a lower effectiveness of high MW compounds to penetrate bacteria.These results indicated that toxic oligomeric tannins can be detoxified by oxidative polymerization. The application of autoxidation (aeration at a high pH) as a pretreatment prior to anaerobic digestion of tannin containing wastewater was tested. Debarking wastewaters of coniferous trees were successfully detoxified by autoxidation pretreatments. The tannins were converted to poorly degradable humic compounds that were non-toxic. During anaerobic treatment, no inhibition occurred and the fermentable fraction of the wastewater was converted to methane. The high MW humic products were non-toxic for aquatic organisms and thus could be discharged to the surface waters with considerably less environmental impact as compared to the unoxidized tannins.Up to date, methods of combatting toxic organic pollutants have been largely based on microbial degradation or physical-chemical removal. A viable alternative approach to these methods that potentially is applicable for certain aromatic compounds, could be polymerizing these inhibitory compounds to non-toxic humus. The humus forming process is a natural mechanism in the forest environment that detoxifies tannic compounds before such compounds are released into the surface waters. The humus forming reactions were imitated in this study and were an effective method for eliminating the environmental impact of tannins in wastewater. Research should be continued to determine the extent to which humus forming processes can be applied for the treatment of other toxic organic contaminants.

Bibliographic Details

Main Author:	Field, J.A.
Other Authors:	Lettinga, G.
Format:	Doctoral thesis biblioteca
Language:	English
Published:	Landbouwuniversiteit Wageningen
Subjects:	anaerobic treatment, derivatives, organic acids, tannins, waste water treatment, water treatment, afvalwaterbehandeling, anaërobe behandeling, derivaten, organische zuren, tanninen, waterzuivering,
Online Access:	https://research.wur.nl/en/publications/the-effect-of-tannic-compounds-on-anaerobic-wastewater-treatment
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