Evaluation of the biodegradation of black liquor derived from soda process effluent using Phanerochaete chrysosporium in free and immobilized cell systems

Extended Abstract
Introduction
Large quantities of wastewater containing complex organic compounds, lignin, phenolics, suspended solids, and various toxic substances are generated by the pulp and paper industry. Effluents with elevated BOD, COD, TSS, and TDS pose serious environmental and public-health risks if discharged without adequate treatment. Black liquor produced from soda pulping particularly when using non-wood raw materials such as wheat straw also contains high levels of silica, making conventional treatment methods expensive, inefficient, and in many cases impractical.
Biological treatment using microorganisms has therefore gained attention as a more eco-friendly and cost-effective option, capable of degrading complex organic pollutants while reducing the need for chemicals and energy. However, free microbial cells often experience problems such as washout, gradual loss of activity, and sensitivity to toxic components. Immobilizing biomass on porous supports, especially polyurethane foam, helps retain the cells, stabilize enzyme activity, increase tolerance to environmental fluctuations, and enable repeated use. In this study, the performance of the white-rot fungus P. chrysosporium in both free and immobilized forms was evaluated for the treatment of soda black liquor. The focus was on reducing major pollution indicators, including COD, BOD, TDS, and TSS, to provide a sustainable approach for managing industrial wastewater.
Materials and Methods
Black liquor was obtained from laboratory-scale soda pulping of wheat straw. A 50-g oven-dry sample of wheat straw was cooked in a batch digester at 160 °C for 30 minutes using an active alkali charge of 16% NaOH based on oven-dry straw. After washing the pulp, the resulting black liquor was collected, filtered, and stored at 4 °C until needed. Before biological treatment, the liquor was diluted tenfold with distilled water. Fungal treatment was carried out at 30 °C using free and immobilized P. chrysosporium cells, with polyurethane foam (PUF) serving as the immobilization matrix. Experiments were conducted under near-optimal pH conditions (6.5–7) over treatment periods of 0, 1, 3, 7, 11, and 14 days. Pollution parameters COD, BOD, TDS, and TSS were measured at each interval. All experiments were performed in triplicate. Statistical analyses were conducted using SPSS software. Independent-samples t-tests were used to determine significant differences between treatment groups, F-tests were applied for variance analysis, and Duncan’s multiple range test was employed for comparing mean values.
Results and Discussion
In soda black liquor, both free and immobilized cells of P. chrysosporium substantially reduced the organic and dissolved solids load, but the immobilized fungus consistently showed superior performance for all monitored parameters. By day 14, the immobilized biomass achieved reductions of 78.03% in COD, 87.54% in BOD and 74.89% in TDS, whereas the free-cell system resulted in 58.05%, 71.54% and 56.22% reduction, respectively. The highest degradation rates for both systems occurred during the early stages of treatment, particularly up to day 7, after which the removal efficiency increased more slowly. This decline in the rate of pollutant removal can be attributed to the depletion of readily biodegradable organic matter, gradual limitation of nutrients and oxygen within the biomass, and partial autolysis or aging of fungal cells. The consistently higher performance of the immobilized fungus indicates that attachment on polyurethane foam improves contact between the biomass and soluble substrates, enhances local retention of enzymes and metabolites, and protects the cells against hydraulic wash-out and fluctuations in wastewater composition. The three-dimensional structure and high porosity of the carrier likely facilitate better mass transfer and provide additional active sites for adsorption and subsequent enzymatic degradation. Overall, the results demonstrate that immobilization not only increases the extent of COD, BOD and TDS removal, but also stabilizes fungal activity over time, thereby improving the robustness and overall efficiency of biological treatment for soda black liquor.
Conclusion
This study demonstrates that P. chrysosporium particularly in immobilized form on polyurethane foam—is an efficient, stable, and environmentally sound option for the primary biotreatment of soda black liquor. Both free and immobilized systems reduced COD, BOD, TDS, and TSS under controlled laboratory conditions, but the immobilized fungus consistently outperformed free cells. By day 14, the immobilized system achieved reductions of 78.03% (COD), 87.54% (BOD), and 74.89% (TDS), compared to 58.05%, 71.54%, and 56.22% for free cells, highlighting the improved treatment efficiency. The highest removal rates occurred during the first week, after which the process slowed, likely due to nutrient depletion and partial saturation of the immobilization matrix. These findings confirm the strong potential of P. chrysosporium to degrade complex organic pollutants in black liquor through its active ligninolytic enzymes. Immobilization enhances biomass stability, increases resilience to environmental changes, and improves enzymatic performance by ensuring sustained substrate access. Accordingly, immobilization on carriers such as polyurethane foam offers a cost-effective, environmentally friendly, and operationally stable strategy for primary treatment of effluents from the pulp and paper industry. Moreover, this approach can serve as an effective pretreatment before secondary processes such as activated sludge or advanced treatment, thereby supporting the development of semi-industrial and industrial-scale biotreatment systems based on immobilized microorganisms. Overall, the study highlights immobilized white-rot fungi as a scalable and sustainable alternative to conventional chemical or thermal treatment methods for industrial wastewater management.