Problem Statement and Objectives: The black liquor generated from Kraft pulping of wheat straw contains high levels of lignin, phenolic compounds, sulfur-containing substances, silica, and both dissolved and suspended solids. Due to their complex structures and high stability, these compounds are highly resistant to degradation, leading to significant increases in COD, BOD, and other pollution indices. Conventional treatment methods such as evaporation, incineration, and chemical recovery are feasible in large-scale mills, but for small and medium-sized units they are not economically viable because of high silica content, excessive costs, high energy demand, and limited efficiency. In this context, biological processes and microorganisms have gained importance as cost-effective, efficient, and environmentally friendly alternatives. The objective of this study was to compare the efficiency of the Phaenerochaete chrysosporium and the Bacillus subtilis in their free-cell form for reducing COD, BOD, and TDS in Kraft black liquor derived from wheat straw pulping.
Methodology: Samples of black liquor were obtained from the Kraft process using wheat straw. To reduce toxicity and create suitable conditions for biological activity, the liquor was diluted ten fold. The microorganisms used included P. chrysosporium and B. subtilis, both cultured in standard media before being added to the effluent under laboratory conditions. Treatment was conducted at pH 7, temperature 30 °C, shaker speed 80 rpm, and a treatment duration of 14 days. Sampling was performed at days 0, 3, 7, 11, and 14. COD, BOD, and TDS were measured according to APHA standard methods. All experiments were carried out in triplicate, and data were analyzed using independent T-tests and ANOVA to assess the significance of differences among treatments.
Results: Both microorganisms were effective in reducing pollution indices of Kraft black liquor, but P. chrysosporium achieved higher reductions in COD, BOD, and TDS by the end of the 14-day treatment period compared to B. subtilis. Due to its shorter lag phase, the bacterium showed higher initial reductions in COD and BOD during the early days of treatment. However, from day 7 onward, enzymatic secretion by the fungus (lignin peroxidase, manganese peroxidase, and laccase) enhanced degradation rates, allowing P. chrysosporium to outperform the bacterium. These findings indicate that in short-term treatments (approximately one week), B. subtilis may be more effective, while for longer and more sustainable pollutant removal, P. chrysosporium demonstrates superior performance. The observed slowdown in later stages of treatment was attributed to depletion of simple organic substrates, accumulation of inhibitory metabolites, nutrient limitation, and fungal autolysis.
Conclusion: This study demonstrated that both microorganisms hold considerable potential for the bioremediation of Kraft black liquor. However, P. chrysosporium—owing to its strong ligninolytic enzyme system and ability to degrade recalcitrant compounds—proved to be a more efficient and sustainable option for long-term pollutant reduction. At the same time, B. subtilis can accelerate the early stages of treatment and, in short-term operations (within one week), may even surpass fungal performance. Therefore, integrated fungal–bacterial treatment systems can achieve higher overall efficiency. Developing such combined biological systems offers a promising alternative to costly chemical and thermal methods in the pulp and paper industry, with the potential to reduce operational costs while improving environmental outcomes.