Lect09_CEP381.pptx

Lect09_CEP381.pptx

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  Environmental engineering “My Lord,Increase me in knowledge” Al-Quran 20:114 Prof. Md. Mohibul Alam B.Sc. (Engineering-SUST), M.Sc.(Engineering-Belgium) Joining as a faculty: February, 2006 Course teacher Department of Chemical Engineering and Polymer Science 1
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  Water treatment ? Departmentof Chemical Engineering and Polymer Science 2
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  Department of ChemicalEngineering and Polymer Science 3 Coagulation/ flocculation • Charge neutralization processes • Building large flocs/particles from small one Sedimentation • Separate out solids from water • e.g., clarification tank or settling tank Filtration • Rapid gravity sand filtration • Membrane filtrtion,MBR Water treatment steps ?
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  Coagulation/flocculation process? Department ofChemical Engineering and Polymer Science 4
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  Sedimentation tank? Department ofChemical Engineering and Polymer Science 5
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  Filtration? Department of ChemicalEngineering and Polymer Science 6
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  Mechanism of Coagulation/flocculation process? Departmentof Chemical Engineering and Polymer Science 7 Colloidal particles size: 10−6 mm (1 nm) to 10−6 mm (1mm)
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  Colloidal characteristics? 8 Department ofChemical Engineering and Polymer Science Imperfections in crystal structure Adsorption of ions on particles surface Ionization of surface sites Electrical Charges on Colloidal Surfaces arises in 3 main ways: Silicon atoms in crystalline material: replaced by atoms with lower valence (such as aluminum ion) giving an excess negative charge to the crystal material Example :Clay particles  Many colloidal particles acquire a charge as a result of the preferential adsorption of either positive or negative ions on their surface.  Colloidal particles in aqueous media usually adsorb anions and acquire a negative charge.  Many particles contain ionogenic groups,such as hydroxyl, carboxyl groups, which dissociate in water.  These ionogenic groups produce a surface charge depending on the solution pH.
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  Colloidal stability? Department ofChemical Engineering and Polymer Science 9 Attractive Forces: results from Van der Waal forces of intermolecular attraction Repulsive Forces: result from electric double layer (EDL) Forces acting on particles
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  EDL theory 10 Fixed (stern)layer: negative ions attract opposite charge ion from surrounding water to its surface Diffusion layer: Outside the fixed layer: electric potential decrease exponentially increasing with distance Electric double layer: Fixed layer +Diffusion layer; both layer contain (+) ve and (-) ve ions positive ions>>> negative ions Department of Chemical Engineering and Polymer Science
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  EDL 11 Department of ChemicalEngineering and Polymer Science
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  Zeta potential 12 Department ofChemical Engineering and Polymer Science The liquid layer surrounding the particle exists as 2 parts: an inner region (Stern layer) where the ions are strongly bound and an outer (diffuse) region where they are less firmly associated. Within the diffuse layer there is a notional boundary inside which the ions and particles form a stable entity. When a particle moves (e.g. due to gravity), ions within the boundary move it. Those ions beyond the boundary stay with the bulk dispersant. The potential at this boundary (surface of hydrodynamic shear) is the zeta potential
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  Coagulant 13 Department of ChemicalEngineering and Polymer Science