Waste water treatment: Sorption

Sorption and Ion Exchange Sorption Sorption is a process which involves the accumulation of substances at a surface or interface Sorption equilibrium is established when the concentration of the contaminant remaining in solution (C) is in dynamic balance with that at the surface (S) C S A common sorbent is activated carbon Sorbate: the chemical which is sorbed Sorbent: the solid surface where the chemical is sorbed Equilibrium Sorption Models The Linear model S=KC )The Fruendlich model (nonlinear sorption The Langmuir behavior S= kCn 1 1 = + S b ab C Note that K, k, n, a, and b are constants usually determined in the laboratory for each sorbate-sorbent combination Example Laboratory tests were conducted on a waste containing 50 mg/l phenol Five bottles containing liter of the waste were dosed with powdered activated carbon When equilibrium was reached, the contents of each bottle were analyzed for phenol The results are shown in the following table Determine the best sorption isotherm model to be used Bottle Carbon, g Equilibrium concentration of the aqueous phase (C), mg/l 0.1 13 0.2 6.0 0.9 1.0 1.6 0.25 2.7 0.08 Solution Determine the equilibrium concentration on the solid phase (on the carbon) using the mass balance approach: CoV=CV+MS where Co is the initial concentration (=50 mg/l) and V is the volume of solution (= liter for each bottle) Bottle M, g C, mg/l S, mg/g 0.1 13 370 0.2 6.0 220 0.9 1.0 54.4 1.6 0.25 31.1 2.7 0.08 18.5 Now plot • S versus C (linear model), • log S versus log C (Freundlich model), and • 1/S versus 1/C (Langmuir model) Based on r2 values, the Freundlich model would be the best as its r2 is 0.98 (closer to 1.0), then the linear (r2 =0.96) and then the Langmuir (r2=0.91) But the linear model has one parameter and the Freundlich model has two To account for differences in the number of model parameters, we use the corrected Akaike Information Criteria (AICc) to judge between the Linear and Freundlich models Corrected Akaike Information Criteria (AICc) 2( P + 1)( P + 2)  SSR  AICc = N ln  + 2( P + 1) + N −P  N  N= Number of data points SSR= Sum of squares residuals P= Number of model parameters Criterion: The smaller the AICc value, the better the model is Linear model Bottle Estimated S Predicted S 370 Freundlich model Residual Squares residuals Predicted S Residual Squares residual 390.7 20.7 429.6 333.4 36.6- 1338.9 220 180.3 39.6- 1573.2 210.2 9.8- 96.6 54.4 30.1 24.4- 594.8 72.1 17.7 313.1 31.1 7.5 23.6- 556.0 31.5 0.4 0.2 18.5 2.4 16.1- 258.7 15.9 2.5- 6.3 =SSR 3412.4 =SSR 1755.1 Model N P P+1 SSR AICc Linear 3412.4 39.6 Freundlich 1755.1 43.3 The linear model is better than the Freundlich model for this case Design of Carbon beds Co Q Carbon bed Cout Q Breakthrough Scale-up Approach Design criteria: The hydraulic retention time in the lab and field are the same  Vcolumn  V    =  column   Q  laboratory  Q  field Vcolumn = πr L Packed carbon density ρ= Moisture content of column M Vcolumn Vwater θ= Vcolumn Note that the moisture content and the bulk density are usually the same in the lab and the field  Vtreated   Vtreated  Design equation :  =    M laboratory  M  field Example A phenolic wastewater having a TOC of 200 mg/l is to be treated by a fixed bed granular activated carbon with a wastewater flow of 150 m3/d, and the allowable effluent concentration is 10 mg/l as TOC A breakthrough curve shown in the figure below has been obtained from an experimental pilot column The laboratory column has the following characteristics: Column diameter= 9.5 cm Length= 1.04 m Mass of carbon= 2.98 kg Flow rate= 12.39 liter/hr Bulk density= 400 kg/m3 Using the scale-up approach determine The design column volume The design mass of carbon The breakthrough time The breakthrough volume Solution Vcolumn −lab = πr L = π (4.75) (104) / 1000 = 7.37 liter V  Vcol − field =  col  × Q field  Q lab Vcol-field= [7.37 (liter)/12.39 (liter/hr)] [150 (m3/d)/24 (hr/d)]= 3.74 m3 M field M =   Vcol   × Vcol − field = ρ × Vcol − field lab Mfield= (400)(3.74)= 1500 kg From graph, Vtreated-lab= 2000 liters with carbon mass of 2.98 kg Using V  Vtreated − field =  treated  × M field  M lab Thus, the breakthrough volume (Vtreated) in the field= 1006 m3 Breakthrough time = Vtreated-field/Qfield= 1006/150= 6.7 days Ion Exchange Objective: To remove specific cations or anions by a chemical exchange reaction Resin Ca hardness Ca2+ + Na2R CaR +2Na+ Hard water Resin Soft water NO3 or SO4 + RCl (resin) Ca Ca Ca Ca Ca Na Na Na Ca Ca Na Na Na Ca Na Na Na RNO3 or RSO4 +Cl- • Resin: A naturally or chemically manufactured material A common resin is a polystyrene resin of small spheres (0.5 mm-dia) • The meq/l bar graph of water after ion exchange has the same number of meqs as the raw water Since ions are just exchanged • Maximum Na for softened water with ion exchange should have less than 100 mg/l Na, or less than 20 mg/l for people on restricted diet • The quantity of waste brine for regeneration is approximately 5% of water processed When the resin is exhausted it is regenerated using a concentrated solution of NaCl CaR + NaCl Excess NaCl Ca2+ + Na2R [...]... ρ= Moisture content of column M Vcolumn Vwater θ= Vcolumn Note that the moisture content and the bulk density are usually the same in the lab and the field  Vtreated   Vtreated  Design equation :  =    M laboratory  M  field Example A phenolic wastewater having a TOC of 200 mg/l is to be treated by a fixed bed granular activated carbon with a wastewater flow of 150 m3/d, and the allowable... Hard water Resin Soft water NO3 or SO4 + RCl (resin) Ca Ca Ca Ca Ca Na Na Na Ca Ca Na Na Na Ca Na Na Na RNO3 or RSO4 +Cl- • Resin: A naturally or chemically manufactured material A common resin is a polystyrene resin of small spheres (0.5 mm-dia) • The meq/l bar graph of water after ion exchange has the same number of meqs as the raw water Since ions are just exchanged • Maximum Na for softened water. .. ions are just exchanged • Maximum Na for softened water with ion exchange should have less than 100 mg/l Na, or less than 20 mg/l for people on restricted diet • The quantity of waste brine for regeneration is approximately 5% of water processed When the resin is exhausted it is regenerated using a concentrated solution of NaCl CaR + NaCl Excess NaCl Ca2+ + Na2R