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Occurrence and treatment of plant sterols in pulp and paper mill effluents Mahmood-Khan, Zahid

Abstract

Pulp and paper mill effluents (PPMEs) contain plant sterols or phytosterols that may exert adverse effects on growth, physiology and reproduction of aquatic life by acting as endocrine disrupting chemicals (EDCs) or hormonally active agents (HAAs). Phytosterols form a part of wood extractives and structurally resemble steroid hormones. A suggested procedure was modified for isolation and analysis of PPME sterols. The technique involved liquid-liquid extractions with methyl-t-butyl ether (MTBE), trimethylsilylation by BSTFA (N, O-Bis tri-methylsilyl-triflouroacetamide) and gas-chromatography coupled with mass-spectrometry. Emulsion formation, incomplete silylation and chromatographic peak overlapping were the major problems encountered. The emulsion problems were resolved by additional amounts of solvents (MTBE and ethanol) and centrifugation. The silylation was improved by increasing the reaction temperature from 20 to 70°C. This shortened the incubation time from 12 to 4 h. The chromatographic separation was achieved through carrier gas flow and oven temperature adjustments. The modifications improved reproducibility and method sensitivity, and reduced the total time required for analysis. The modified technique was successfully used for analyzing sterols in PPMEs collected from two British Columbia pulp and paper mills. Six different sterols were quantitatively analyzed. B-sitosterol (B-Sito), B-sitostanol (B-Sitosta) and campesterol (Campe) were the major phytosterols that accounted for about 70% of the total sterols in PPMEs. Cholesterol (Chole), stigmasterol (Stigma) and ergosterol (Ergo) collectively accounted for about 30% or less. Total sterol concentrations were about 800-4,000 u,g/L in primary effluents, 250-1,200 p.g/L in final effluents and 12,000-40,000 pg/L in recycle and waste activated sludges. A more detailed survey, at two mills, revealed a general removal of sterols from PPMEs across the UNOX-AST (pure oxygen-activated sludge treatment) systems. The sterols removal efficiencies were variable. About 72% of the sterols were removed at Mill A and about 66% at Mill B. Bio-adsorption and selective biodegradation were the suggested mechanisms of sterols removal during secondary treatment. Sterol mass balance calculations across the UNOX-AST systems, revealed that about 30% of the incoming sterols were being discharged to the receiving waters with final effluents. Another 40% or more of the sterols left the treatment systems with excess sludge, indicating that 70% or more of the sterols may leave the treatment systems without biodegradation. Thus, a typical pulp mill (producing 1000 air dried tones /d) may discharge about 20 kg/day of sterols in treated effluents only. Two lab-scale suspended growth bioreactors, treating PPMEs, were used to investigate the fate and behavior of sterols during secondary treatment. A removal efficiency of about 90% was achieved and maintained for major sterols: B-Sito, B-Sitosta and Campe, with a solids retention time of 11-13 d and a hydraulic retention time of 10-12 h. The biological treatment was sensitive to process pH, solids detention time and hydraulic retention time. Under suitable process conditions, biodegradation/transformation appeared to be the major mechanism of sterols removal. During the conditions sub-optimal for sterols biodegradation, bio-adsorption became the major mechanism of removal and a larger portion of the influent sterols was discharged with waste sludge as well as final effluent. The investigation of sterols adsorption to inactivated sludge, revealed comparable adsorption kinetics for all three major sterols tested: B-Sito, B-Sitosta and Campe. Isotherms were generated by fitting a linear Freundlich adsorption model that suggested two adsorption regions for each sterol. In the high adsorption region, the inactivated biomass appeared to have the highest capacity for Campe, the lowest for B-Sito, and intermediate for B-Sitosta. The adsorption capacity of the inactivated biomass showed a biphasic behavior and increased with increasing equilibrium concentration of sterols, suggesting that bio-adsorption may be effectively used for sterols removal from PPMEs at higher concentrations. The results of sterols biodegradation and bio-adsorption studies presented, can be used in secondary treatment process modeling and design to incorporate specific organic pollutant removal.

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