Water Pollution Control: Physical Methods AWPPCE RPI Fall 2013
Water Pollution Control Processes Water and Waste Water Treatment are usually carried out in specially designed vessels (reactors) under controlled conditions. Classification of Water Pollution Control Techniques Physical Methods Chemical Methods Biological Methods
Water Treatment Reactors Classification of Reactors Mode of Operation Batch Continuous Semi-Continuous Phases Present Single-phase Multi-phase Continuous Flow Reactors Plug Flow Perfectly Stirred General
Age and Residence Time Distributions in Reactors Reactor Characteristics V =Volume Q =Volumetric Flow Rate a = age of a fluid element inside the reactor at any moment t e = the age of a fluid element at the moment of exiting the reactor Fraction of Fluid Exiting the Reactor whose Residence Time Inside is less than t F(t) = 0 t E(t e ) dt e Fraction of Fluid exiting the Reactor with ages between t e and t e + dt e E(t e ) dt e Fraction of Fluid inside Reactor with age between a and a + da I(a) da
Three Physical Methods for Water Pollution Control Screening Sedimentation Type I (Non-aggregative) Type II (Flocculant) Type III (Zonal) Aeration
Screening Obstacles designed to remove coarse debris from water flows Design Characteristics: Screen Opening/Spacing Coarse ~ 50 to 150 mm Medium ~ 20 50 mm Fine < 10 mm Micro ~ 20-60 µm Water velocity v > 0.6 m/s Depth/width ~ 1-2 Head loss (coarse screen) h = (1/2gC d2 )(v s2 v 2 ) h = difference in flow depth before after the screen C d = discharge coefficient ~ 0.84 v s = fluid velocity through screen Head loss with time (micro screen) h = h 0 exp(k Q t/a) h 0 = head loss of clean strainer Q = volumetric flow rate A = surface area of strainer k = head loss coefficient
Sedimentation Forces Acting on a Particle Settling in Water at its Terminal Velocity F g =F b + F d = m p g = m w g + (1/2) C d A p ρ w v 2 Drag Coefficient Cd = 24/Re for Re<1 (Stokes) C d = 18.5/Re 0.6 for 1 < Re < 1000 (Transition) C d ~ 0.4 for 1000 < Re (Turbulent) where Re = ρ w d p v/µ w Terminal Velocity v t = (ρ p ρ w ) d p2 g/18 µ w (Stokes) v t = complex function (Transition) v t = ((4/3) g d (ρ p ρ)/(c d ρ)) 1/2 (Turbulent)
Sedimentation Basin Water Flows gently from inlet to outlet at average velocity v f = Q/(L*H) The average residence time of fluid particles in the basin is t d =(L*H*W)/Q Turbulent flow zones form next to inlet and outlet and particles are uniformly distributed there Settling velocity required to capture a particle entering at the top of basin v o =H/t d = Q/(L*W) Particles entering the basin at height h < H and settling at velocity v t < v o will still be captured as long as h < (v t /v o ) H The separation efficiency is given by η= v t /v o and is thus a function of particle size d p Overall separation efficiency: η o = η(d p ) d p3 f(d p ) dd p / d p3 f(d p ) dd p
Type II Sedimentation Settling particles grown by natural or induced coalescence (flocculation) makes the size distribution function f change with time Settling velocity distribution determined in a laboratory using a settling column with sampling ports. Objective is to find the time required to achieve a desired separation efficiency at any given depth in the column
Type III Sedimentation High concentrations of particulates lead to hindered settling with formation of zones
Filtration Sand filters have long been used for filtration of particulates in water. Other media are also used. Filter Media Characteristics Particle size Particle size distribution (log-normal) Effective size = P 10 = Sphericity: Ratio (surface area/volume) of a particle of arbitrary shape to the (surface area/volume) of a spherical particle of the same volume Porosity: Void volume/total volume Typical media filter: Particle size ~ 100-500 µm ; log-normal distribution Sphericity ~ 0.7-1 Porosity ~ 0.38 0.55
Aeration Some Pollutants in water can be removed through the water-air interface Two-film model dm/dt = k l A (C l,i Cb l,b ) = k g A (p g,i -p g,b ) dm/dt = time rate of mass transfer k l, k g = mass transfer coefficients A = water-air interface C l,b, C l,i = bulk and interface concentration of pollutant in water p g,i, p g,b = partial pressure of pollutant in air at the interface and in the bulk