Bacterial decolorization and detoxification of black liquor from rayon grade pulp manufacturing paper industry and detection of their metabolic products
Highlights
▸ A novel bacterial consortium comprising Serratia marcescens, Citrobacter sp. and Klebsiella pneumoniae showed decolorization and detoxification of black liquor at high pollution load (COD: 18700 mg l-1; Lignin: 100 mg l-1) discharged from rayon grade pulp manufacturing industry. ▸ The consortium has broad range of pH tolerance and ligninolytic activity (MnP and laccase), reduced COD and color up to 83% and 85%, respectively. ▸ The bacterial decolorized black liquor showed formation of new metabolic product and reduces the toxicity up to 70%.
Introduction
In pulp paper industry, manufacturing of pulp from wood chips called pulping process where, wood chips are cooked in the solution of sodium hydroxide and sodium sulfite at elevated temperature and pressure to break into fiber mass. In order to manufacture rayon grade pulp (RGP), only high quality fiber containing wood chips are used with an extra chemical process that involves prehydrolysis of wood chips at elevated temperature and pressure followed by alkaline digestion. This process ensures the removal of hemicellulose and remaining fibers with high cellulose content around 92%. Thus, the effluent generated from pulping stage mainly contains lignins, hemi-cellulose, phenolics, resins, fatty acids and tannins which mixed together and make dark black alkaline wastewater known as black liquor (Zaied and Bellakhal, 2009). Black liquor (BL) contains about 40–48% lignin, which is a mixture of polypenolic compounds having a complex chemical structure consisting only 10–15% of total wastewater, but contributes almost above 90–95% of the total pollution load of pulp paper mill wastewater, which make it significantly toxic to the environment (Pokhrel and Viraraghavan, 2004).
In many developing countries including India, farmers are irrigating their crop plants with industrial effluents having high level of several toxic compounds due to the non-availability of alternative sources of irrigation. Hence, the adequate treatment of BL prior to its final discharge into the environment is necessary. Though, recent developments in physical and chemical methods (i.e. electrocoagulation, ozonation, ultrafiltration) or combination of different methods in series for the treatment of lignin containing wastewater has shown some encouraging results (Pokhrel and Viraraghavan, 2004). To date the most effective method for the management of BL is the alkali recovery process in which BL is first concentrated and then incinerated. In this way organic substances in wastewater is burn and alkali can be recovered (Yang et al., 2008). However, medium and small scale paper mills are gradually closing due to their incapability of alkali and energy recovery from black liquor. Acid precipitation is another alternative for lignin removal, which facilitates the secondary treatment greatly. Large consumption of mineral acids and secondary pollution of chlorine and sulfur make the technology unsatisfactory. Therefore, more practical method is needed for the treatment process for small and medium scale industries.
Biological treatment is another practical choice in BL treatment. Among biological methods tried so far, most of the literature confined to a few genera of white rot fungi because of their non specific extracellular enzymatic system (LiP, MnP and Laccase). But, bacteria seem to be more effective than fungi for the bioremediation of environmental pollutants due to their immense environmental adaptability and biochemical versatility. Bacteria isolated from compost soil, viz. Azotobacter and Serratia marcescens, were capable of degradation and decolorization of lignin (Morii et al., 1995). Bacteria such as Bacillus subtilis and Bacillus sp. have also been tested for kraft-lignin degradation (El-Hanafy et al., 2008, Abd-Elsalam and El-Hanafy, 2009). Previously, three potential bacterial strains of Panibacillus sp. Aneurinibacillus aneurinilyticus and Bacillus sp. were isolated from pulp paper sludge for degradation and decolorization of synthetic lignin at 500 mg l−1 and characterized their metabolic products by GC–MS analysis (Chandra et al., 2007, Raj et al., 2007). However, all the above studies have been carried out on synthetic/model compounds with very low pollution load (COD less than 8000 mg l−1) which do not directly explain the degradation of lignin present in pulp wastewater. However, the literature on bacterial degradation of BL discharged from RGP manufacturing industry is lacking at high pollution load. Hence, the present investigation has been focused on the decolorization of BL at high pollution load discharged from RGP manufacturing industry which will be useful for the management of lignin containing pulp paper mill wastewater.
Section snippets
Sample collection and isolation of bacteria
The RGP black liquor used in biodegradation study was collected from wash machine section of M/s. Century Pulp Paper Mill, Lalkuan, Nainital, Uttrakhand, India located (79°10′E longitude and 29°3′N latitude) at foot hills of Himalayas. The industry produces 90–100 ton per day of rayon grade pulp and discharges approximately 100 m3 per day of wastewater. For the isolation of potential lignin degrading bacterial strains, sludge samples were collected from the disposal site of M/s Century Pulp Paper
Characteristics of RGP-BL and screening of potential bacterial strains
Black liquor sample (10%) used for the bacterial decolorization study showed following characteristics i.e. pH 9.0, color 6100 Co.pt, lignin 1000 mg l−1, BOD 7360 mg l−1, COD 18,700 mg l−1. Phosphate, Cr, and Cd were noted below the detection limit (Table 1). The high COD/BOD ratio (approximately 2.8) in BL is due to presence of high molecular weight compounds, i.e. lignin and its derivatives which contribute high COD and color instead of BOD (Esposito et al., 1991). The source of sulfate ions in
Conclusion
This study revealed that the developed bacterial consortium was capable of 85% decolorization of BL from RGP plant having 18,700 mg l−1 COD at optimized conditions. Further, the HPLC and GC–MS analysis of control and bacterial degraded sample showed that the consortium utilized the BL rather than biotransformation. Hence, the developed bacterial consortium was capable for the effective decolorization and detoxification of pulp paper effluent for environmental safety.
Acknowledgement
The financial assistance from Department of Biotechnology under GAP-192 project and CSIR under SIP-08 is highly acknowledged.
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