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Toxicity of anionic surfactants in a primary effluent : identification, characterization and removal Bradley, James Craig

Abstract

This research project was undertaken to identify, characterize and remove anionic surfactant induced toxicity from a primary effluent. The anionic surfactants, present in the primary effluent, were first separated into low, medium and high molecular weight fractions using solid phase extraction columns and gradient methanol elution. By separating the anionic surfactants, on the basis of molecular weight, the relative toxicities of each fraction could be determined. A colourimetric method was developed that was used to measure the concentration of anionic surfactants as methylene blue active substances (MBAS). This method required less time and less reagents than the conventional method outlined in Standard Methods (APHA et al, 1992). Finally, the Microtox™ system was used to measure the toxicity of the whole sample and of the different molecular weight fractions. Using the methods developed for the present study, the anionic surfactant concentration and associated toxicity in a primary effluent were determined. These determinations were performed on two sampling events at the Lions Gate Primary Wastewater Treatment Plant. In each sampling event primary effluent was collected in the morning, afternoon and night. MBAS concentrations in the primary effluent increased throughout the day and ranged from 1.20 mg/L MBAS to 9.34 mg/L MBAS. The anionic surfactant concentrations were highest in the medium molecular weight fraction and lowest in the high molecular weight fraction. The toxicity of the primary effluent was shown to increase as the concentration of anionic surfactants increased. The toxicity associated with anionic surfactants was highest in the high molecular weight fraction. While the high molecular weight fraction was the most toxic fraction, it contained the lowest anionic surfactant concentration. A screening study was conducted to provide a preliminary indication of the feasibility of using either partitioning to abiotic bio-solids, biological treatment, alum coagulation/flocculation with gravity settling, ozonation, or air flotation to remove anionic surfactants and the associated toxicity from a primary effluent. Partitioning to abiotic bio-solids and biological treatment produced anionic surfactant removals of 64% and 96%, respectively. Alum coagulation/flocculation with gravity settling removed 46% of the anionic surfactants, while ozone removed 95% of the anionic surfactants. Air flotation removed 77% of the anionic surfactants. Toxicity studies were conducted using the samples treated with ozone and air flotation only. The toxicity studies revealed that ozonation slightly increased the toxicity of the treated whole sample, possibly due to the formation of by-products from the oxidation process. On the other hand, air flotation reduced the toxicity of the whole sample significantly. In addition, air flotation removed all of the measurable toxicity, using the methods described in the present study, from the elution fraction corresponding to the high molecular weight fraction. These preliminary results indicate that air flotation may be an effective interim means of reducing both anionic surfactants and anionic surfactant induced toxicity from the Lions Gate Wastewater Treatment Plant primary effluent.

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