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Degradation of dehydroabietic acid by bacterial isolates grown on CTMP effluents Zhang, Yi


Resin acids, which are a major component of softwood extractives, are released during the pulping of wood and are known to contribute to much of the toxicity of pulp mill-effluents. Usually biological treatment can efficiently remove resin acids. However, resin acid breakthroughs occasionally occur due to operational problems or seasonal variations in the wood chip furnish. Recently Bicho et al. (1994) isolated five dehydroabietic acid degrading bacterial strains from a BKME treatment system. These isolates were able to utilize high concentrations of abietanes. Most of the past and recent work on the microbes involved in resin acid degradation have utilized defined media and there has been no attempt to reintroduce these strains to mill effluents to see if they still functioned in this environment. This was the main focus of this study. Two strains (BKME5 and BKME 9) described by Bicho et al. were examined for their ability to grow and degrade DHA present in CTMP effluent. Initially, although both strains could grow on CTMP effluent, no DHA degradation was observed. COD measurements indicated that both strains used other organic material present in the CTMP effluent. When nutrients (ammonium, phosphate, mineral salts and vitamins) were added, both growth and DHA degradation increased significantly. A comparison of DHA degradation with COD removal indicated that these strains preferentially utilized other organic material present in the effluent before degrading DHA. The stimulated growth resulting from the use of other organic materials did not increase the rate of DHA degradation. This initial work showed that the CTMP effluent was nutrient deficient in terms of DHA degradation. It was found that strain BKME 5 required additional nitrogen before it could degrade DHA. However, strain BKME 9 required N, P, vitamins and minerals before efficient DHA degradation occurred. Magnesium was shown to be essential for effective DHA degradation by BKME 9. More than 12.5 mg/1 nitrogen had to be added to the CTMP effluent before either strain could effectively degrade DHA. To ensure complete degradation of DHA, at least 25 mg/1 of nitrogen had to be added for strain BKME 5 and 1000 mg/1 for BKME 9. A solid COD based method was developed as an alternative procedure for quantifying microbial biomass. This method was based on the same principals as are currently used for determining VSS. The advantages of the solid COD method over the VSS protocol include rapid analysis time, greater sensitivity and the use of smaller sample volumes. A good correlation between VSS and solid COD was found for a variety of sources of microbial biomass, including cultures at different stages of growth supported by different effluents. About 1.2-1.6 mg COD were measured per mg of VSS. Although the solid COD/VSS ratio was stable over a range of conditions, it was not constant when a large amount of non-biomass solids are present in samples such as a typical CTMP effluent. Both filtration and gamma irradiation proved to be effective sterilization methods that prevented growth of any contaminating microorganisms present in the CTMP effluent. However, both methods affected the COD concentration and the resin acid profile and concentration. Although filtration removed about 1/3 of the COD (particulates) and 1/2 of the total resin acids within the effluent, the gamma irradiation caused little change in effluent COD of the total resin acid concentration. However, the amount of the individual resin acids had changed significantly after irradiation. Neither the inorganic components nor the nutrient concentrations were changed by either sterilization method. This initial study indicated that these isolates required additional nutrients such as nitrogen and minerals before they could effectively remove DHA from CTMP effluent. Although they may not be fully representative of the microbial community responsible for most of the resin acid removal in mill waste water treatments, this initial work indicates the possible variability and selectivity of substrate utilization by resin acid degrading bacteria.

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