Methods for Facilitating Microbial Growth on Pulp Mill Waste Streams and Characterization of the Biodegradation Potential of Cultured Microbes

The pulping of wood is a chemically intensive process that has been optimized over many years to create a system with minimal waste. However, some outputs of this process could be used to produce higher value product(s). Black liquor is one such example. It is generated from the kraft process, which is the dominant chemical pulping method, representing 85% of world lignin production¹. The kraft process (Figure 1) uses temperature (160-200 °C), pressure (120 psig), and the chemicals contained in white liquor (sodium hydroxide and sodium sulfide) to dissolve the lignin from the wood fibers^2,3. Black liquor contains lignin, organic acids, and polysaccharide degradation byproducts⁴. It is incinerated to produce steam and recover chemicals in the recovery boiler that provides thermal energy for downstream paper making and pulping processes. The volume of black liquor generated by pulping can exceed the amount that the recovery boiler can effectively process. Disposing of the black liquor as effluent negatively affects aquatic flora and fauna and thus is not an option. Application of microbial organisms that could use black liquor for growth would be beneficial in terms of increasing chemical recovery and generation of value-added product(s) that would improve the overall life cycle analysis of the pulping system. Chemical and biological conversion of lignin derived monomers has successfully produced vanillin and cinnamic acid for use as food sweeteners and fragrance additives, phenol used for plastic and resins, and cyclohexane, which could be used for fuel⁵.

Previous work on biodegradation of this pulp waste has been focused on lignin depolymerization. The International Lignin Institute (ILI) reports that between 40-50 million tons of lignin is produced each year (http://www.ili-lignin.com/aboutlignin.php). Only 1.5% of that lignin is used for commercial industrial processes⁶. Lignin depolymerization by laccase and peroxidase enzymes produced by white rot fungi of the Phanerochaete and Trametes genera has been studied at length⁷. Soil bacteria known to degrade aromatic compounds such as Nocardia and Rhodococcus⁸, Pseudomonas putida mt-2⁹, and Streptomyces viridosporus T7A¹⁰ have also been shown to be capable of lignin degradation. Bacterial degradation of pulping waste is promising because some bacteria can thrive in the saline and alkaline (pH 10-14) conditions that characterize the pulping waste effluents¹¹. While lignin is the main component of black liquor, microorganisms may also degrade the other components that make up black liquor. These techniques do not exclusively identify lignin degrading microorganisms, but serve to identify microorganisms that can be applied directly to the pulping waste black liquor instead of its further processed constituents.

Black liquor was collected and modified for microbial growth through neutralization and filter-sterilization. Microbial growth requirements were identified for an environmental microbial isolate by minimal media growth experiments on lignocellulosic components produced by a novel lignocellulose extraction protocol. Growth media were analyzed by gas chromatography-mass spectrometry (GC-MS) to determine the metabolic products of the environmental microbial isolate when grown on black liquor as the sole carbon source. The combination of these techniques provides an assessment tool to determine the metabolic capacity of a microorganism when grown on pulp mill wastes such as black liquor. Use of such techniques also offers insight into the value of application of specific microorganisms to pulp mill waste for the generation of byproducts.