PRINCIPLES OF ENVIRONMENTAL ENGINEERING AND SUSTAINABILITY LECTURE NINE
PRINCIPLES OF ENVIRONMENTAL ENGINEERING AND SUSTAINABILITY LECTURE NINE: WATER TREATMENT PLANT DEPARTMENT OF ENVIRONMENTAL ENGINEERING
Water treatment plant (WTP) Conventional WTP: Detailed components: ü Low lift pumps unit. ü Coagulation unit. ü Sedimentation unit. ü Flocculation unit. ü Chlorination unit. ü High lift pumps unit.
Conventional Surface Water Treatment Raw water Filtration Screening sludge Alum Polymers Coagulation sludge Cl 2 Disinfection Flocculation Storage Sedimentation Distribution sludge
Water treatment plant layout (capacity 4000 m 3/hr)
WTP Low lift pumps unit To WTP
Screening Ø Removes large solids Ø Ø logs branches rags fish Ø Simple process Ø may incorporate a mechanized trash removal system Ø Protects pumps and pipes in WTP
Coagulation Ø Coagulation is a physicalchemical process whereby particles are destabilized Ø Several mechanisms Ø adsorption of cations onto negatively charged particles Ø decrease thickness of the layer of counter ions Ø sweep coagulation Ø interparticle bridging
Coagulant introduction with rapid mixing Ø The coagulant must be mixed with the water Ø Retention times in the mixing zone are typically between 1 and 10 seconds Ø Types of rapid mix units Ø Ø Ø pumps hydraulic jumps flow-through basins with many baffles In-line blenders In-line static mixers
Flocculation • Coagulation has destabilized the particles by reducing the energy barrier • Now we want to get the particles to collide • We need relative motion between particles
Mechanical Flocculation ü Shear provided by turbulence created by gentle stirring ü Turbulence also keeps large flocs from settling so they can grow even larger! ü Retention time of 10 - 30 minutes ü Advantage is that amount of shear can be varied independent of flow rate ü Disadvantage is the tanks are far from plug flow
Sedimentation WHAT IS? ? Sedimentation is a physical water treatment process using gravity to remove suspended solids from water. Solid particles entrained by the turbulence of moving water may be removed naturally by sedimentation in the still water of lakes and oceans. Settling basins are ponds constructed for the purpose of removing entrained solids by sedimentation
Types:
Ø Ø Ø Sludge zone Minimal turbulence (inlet baffles) Uniform velocity (small dimensions normal to velocity) No scour of settled particles Slow moving particle collection system Q/As must be small (to capture small particles) This will be one of the ways you can improve the performance of your water treatment plant. Outlet zone Inlet zone Design Criteria for Sedimentation Tanks Settling zone
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