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Near field mixing of a vertical buoyant jet in a shallow crossflow : implications on adsorption and flocculation Gomm, Leslie


The behaviour and movement of pulpmill pollutants discharged into the Northern Fraser River is of significant concern due to their potential impact on this valuable aquatic ecosystem. The shallow receiving water can influence the mixing and subsequent dilution of these discharges. The association of contaminants with suspended sediment, either by direct adsorption or flocculation of contaminated solids discharged with the effluent (biosolids), also affects pollutant fate. This study examined the effects of a shallow crossflow in the near field mixing of a vertical buoyant jet, specifically dilution and trajectory. Physical mixing experiments were carried out in a shallow ambient current over a range of conditions similar to those seen in the Fraser River, specifically peak and low flow conditions. The dilution and trajectory results were then compared to those predicted by CORMLX1. The mechanism of association of contaminants with suspended sediment under these near field conditions was also investigated. A jet classification scheme was developed based on the behaviour of the jets in the shallow crossflow. Jets were classified to be Bottom, Intermediate or Surface Jets. Bottom Jets were influenced primarily by interaction of the jet with the bottom boundary layer, resulting in significantly higher levels of dilution and possible bottom attachment. The mixing of Intermediate Jets was more complicated due to interaction with both the top and bottom boundaries: the free surface inhibited mixing while interactions with the boundary layer enhanced mixing. Surface Jets were drastically affected by the free surface, with a reduction in dilution due to impingement on the free surface. The C0RMLX1 model was found to be unsuitable for predicting the dilution in this application since it does not consider the effects of either the free surface or the bottom boundary layer on jet mixing. Adsorption was found to play a limited role in the near field region. Of greater importance, is the potential for flocculation of biosolids with suspended sediment. The most important parameter in predicting where the conditions for this increased flocculation will occur was the ratio of the number of biosolid particles to the number of suspended sediment particles.

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