Bioremediation of a bleach plant effluent from the pulp and paper industry

English: Bleach plant effluent was characterised by physico-chemical methods. The chemistry of the bleach plant effluent was examined to devise effective treatment methods. Effluent contained trace amounts of nitrogen as well as carbohydrates and no ortho phosphate could be detected in the wastewater. The best decolourisation activities were obtained using adsorption as treatment method, with activated carbon removing > 99% colour from effluent. Chitosan (81%) and chitin (77%) could remove appreciable levels of colour from bleach plant effluent, followed by biomass from Rhizomucor pusillus, a mucoraelean fungus (71%). Chitosan and chitin from the cell wall of R. pusillus might be involved in the fungus decolourisation ability. Effluent pH was inversely related to effluent decolourisation when R. pusillus, chitosan or chitin was used as adsorbents. This might in part be due to acid catalysis during nucleophilic addition reactions, where amino groups of chitin/chitosan react with carbonyl groups in Eo-effluent. Also, chitin and chitosan amino groups can be protonated under acidic conditions and acquire positive charges that can interact with the chromophores found in Eo-effluent. However, pH exerted no significant effect on decolourisation when activated carbon was employed as adsorbent of effluent colour. Decolourisation employing commercial adsorbents seemed to be mainly due to chemisorption. Adsorption experiments conducted at various ionic strengths indicated that coulombic interactions are responsible for a fraction of the decolourisation activity of chitosan and chitin. Nevertheless, decolourisation obtained with RM7 and activated carbon was unaffected by the ionic strength. Flocculation of coloured compounds from Eo-effluent by chitosan containing solutions resulted in a maximum decolourisation of 75%. Anion-exchange treatment removed 96% colour from Eo-effluent. Ultraviolet irradiation could decolourise the Eo-effluent by about 42 to 43%. Decolourisation using organic solvent extraction proved ineffective with a highest colour removal efficiency of only 21% being achieved. Biological methods used for effluent remediation were: 1) Trickling filters, 2) Activated sludge reactors and 3) Rotating biological contactor reactors (RBC). Treatment using one biological system was followed by treatment in another system With trickling filters containing immobilised white-rot fungi, the highest decolourisation (61%) was obtained with Coriolus versicolor. This fungus required a co-substrate to efficiently decolourise the effluent. Effluent treatment in an activated sludge reactor reduced toxicity, COD and chlorophenol levels. However, colour and high molecular mass compounds were not affected significantly by this method of treatment. Decolourisation was studied in a RBC using immobilised C. versicolor and R. pusillus, respectively. The decolourisation rate by both fungi was proportional to initial colour intensities. Decolourisation was not adversely affected by colour intensity, except at the lowest level tested. Decolourisation of 53 to 74% could be attained using a hydraulic retention time of 23 h. Rhizomucor pusillus, removed 55% of AOX compared to a 40% AOX reduction by C. versicolor. Treatment employing R. pusillus and C. versicolor, respectively, rendered the effluent essentially non-toxic. Addition of nutrients to the decolourisation media stimulated colour removal by C. versicolor, but not significantly in the case of R. pusillus. Ligninolytic enzymes (manganese peroxidase and laccase) were only detected in effluent treated by C. versicolor. Decolourisation mechanisms were investigated using gel permeation chromatography. Rhizomucor pusillus decolourised the effluent by adsorption and C. versicolor removed effluent colour by adsorption as well as by biodegradation. Coriolus versicolor could decolourise the effluent for a period of 34 d whereas R. pusillus decolourised the effluent up to 54 d. Further improvements in effluent quality could be attained when treatment using one system was followed by treatment in another system, possibly because of toxicity reduction in the pre-treatment steps.