2“Compare and choose the chemical treatment methods for waste treatment in industries. Calculate and design the basic structure of waste treatment unit operations”.Chemical treatment / Unit operation
3IntroductionChemical treatment usually are used in combination with the Physical Unit Operations;- Screening, coarse solids reduction, mixing and flocculation, gravity separation, grit removal, sedimentation, flotation, aeration, etc.and also with Biological Unit Operations.
5Role of Chemical Unit Processes in Wastewater Treatment Chemical coagulationChemical precipitationChemical disinfectionChemical oxidationAdvanced oxidation processIon exchangeChemical neutralization, scale control, and stabilization
6Application of chemical Unit Processes in wastewater treatment
7Considerations & Issues of Chemical Treatment…. “Chemical treatment – additive processes”Handling, treatment and disposal of the large volumes of sludge producedNet increase in dissolved constituentsIncrease in cost of energy & chemical costsTDS concentration increase;ex: chlorine additives
9Chemical Coagulation Colloidal particles found in wastewater : - net negative surface charge,to 1 µm in size- attractive body forces between particles < repelling forces- this stable conditions, Brownian motion (i.e., random movement) keeps the particles in suspension.Coagulation is the process of destabilizing colloidal particles so that particle growth can occur as a result of particle collisions.
11Basic definitionsChemical coagulation – all reactions and mechanisms involved in the chemical destabilization of particles and in the formation of larger particles through perikinetic flocculation (aggregation of particles in the size range from 0.01 to 1 µm )Coagulant – chemical that is added to destabilize the colloidal particles in wastewater so that floc formation can occur.Flocculant – chemical, typically organic, added to enhance the flocculation process.Coagulant & Flocculant : natural and synthetic organic polymers, metal salts, and prehydrolized metal salts (ex:alum, ferric sulfate, polyaluminum chloride (PACl) and polyiron chloride (PICl), etc )
12gravity sedimentation and filtration Flocculants are also used to enhance the performance of granular medium filters and dewatering of digested biosolids Filter aidsFlocculation: the process of increasing the size of particles as a result of particle collisions.Microflocculation(perikinetic flocculation)- Particle aggregation is brought about by the random thermal motion of fluid molecules known as Brownian motionMacroflocculation(orthokinetic flocculation)- Particle aggregation is brought about by inducing velocity gradients and mixing in the fluid containing the particles to be flocculatedReaching 1-10 µm size, then separated bygravity sedimentation and filtration
13Nature of particles in wastewater Suspended particles > 1.0 µm, can be removed by gravity sedimentationColloidal particles cannot be removed by sedimentation (need coagulants & flocculant aids)Important factors that contribute to the Characteristics of Colloidal Particles:a) Particle size and number0.01 to 1.0 µmnumber in untreated wastewater and after primary sedimentation= 106 to 1012 /mLb) Particle shape and flexibilityspherical, ellipsoids, disklike, various length, D, and random coils .shapes affect electrical properties, particle-particle interaction, andparticle-solvent interaction
14c) Particle-solvent interaction Hydrophobic – have relatively little attraction for waterHydrophilic – much greater attraction for waterAssociation colloids – made up of surface-active agents, ex: soaps, synthetic detergents, and dyestuff which form organized aggregates known as micelles.d) Surface properties including electrical characteristics (surface charge)e) Particle-particle interaction
15a) Isomorphous replacement Development of Surface Charge**surface charge is an important factor in the stability of colloids!**Develop through:a) Isomorphous replacement- occurs in clays and other soil particles, ions in lattice structure replaced with ions from solution, ex: Si4+ replaced with Al3+b) Structural imperfections- occurs in clay or similar particles, due to broken bonds on crystal edge (imperfections in crystal formation)c) Preferential Adsorption- when oil droplets, gas bubbles, or other inert substances are dispersed in water, they will acquire –ve charge through adsorption of ions (hydroxyl ions)d) Ionization- ionization of carboxyl and amino groups (at different level of pH)
16The Electrical Double Layer Electrical double layer or electrostatic interaction force
17Surface potential- depends on distance from particle surface
18Particle-particle interactions Involve 2 principal forces: repulsion force and van der Waals force of attraction*Refer page 483 in textbook
19Particle Destabilization Required to reduce particle charge or to overcome effect of this charge.Therefore aggregation of particles (microflocculation) can be achieved.Particle Destabilization and Aggregation with PolyelectrolytesActions of polyelectrolytes:Charge neutralizationPolimer bridge formationCharge neutralization and polimer bridge formation
20Charge neutralization - act as coagulants that neutralize or lower the charge of thewastewater particles- normally the wastewater particles are –ve charge, so,cationic (+ve charge) polyelectrolytes are used.- polyelectrolytes must be adsorbed to the particles usedsufficient and high intensity of mixing (prevent folding backof polyelectrolytes)
21b) Polymer bridge formation Anionic or nonionicpolyelectrolytes
22c) Charge neutralization and Polymer bridge formation - use cationic polyelectrolytes having extremely high molecular weight - can form both charge neutralization and polymer bridge
23a) Addition of potential determining ions 2) Particle Destabilization with Potential-determining Ions and electrolytesa) Addition of potential determining ions- add strong acids or bases to reduce charge of metal oxidesor hydroxides to near 0 so that coagulation can occur- not feasible due to massive concentrations of ions to be addedb) Use of Electrolytes- added to coagulate colloidal suspension- cause decrease in zeta potential and corresponding decrease in repulsive forces.- also not feasible in waste treatment.
24- Addition of alum or ferric sulfate (Fe3+ & Al3+) 3) Particle destabilization and removal with hydrolyzed metal ions- Addition of alum or ferric sulfate (Fe3+ & Al3+)- Complex formation of metal ion hydrolysis productsLetterman, 1991
25Action of hydrolyzed metal ions: i) Adsorption and charge neutralization- mononuclear and polynuclear metal hydrolysis species adsorb on the colloidal particles.ii) Adsorption and interparticle bridging- involve the adsorption of polynuclear metal hydrolysis species and polymer species which in turn will form particle-polymer bridges- if enough coagulant requirement & charge neutralization, metal hydroxides precipitates and soluble metal hydrolysis products form- if sufficient metal salts added, large amount of metal hydroxide floc will form settleiii) Enmeshment (trapped) in sweep floc- floc particles settle and sweep through water containing colloidal particles- colloidal particles enmesh in the floc – removed by sedimetation.
26CHEMICAL PRECIPITATION FOR IMPROVEDPLANT PERFORMANCE
27Chemical Precipitations -involves the addition of chemicals to alter the physical state of dissolved and SS , and facilitate removal by sedimentation
29The quantity of alkalinity (as CaCO3 having Mw = 100) required to react with 10 mg/L of alum is; !! Note: If less than this amount of alkalinity is available,it must be added, ex: Lime
302) Lime- Reactions for carbonic acid – clarification;- Alkalinity;
313) Ferrous sulfate and lime Ferrous sulfate alone added to wastewater;FeSO4 ·7H2O + Ca(HCO3) Fe(HCO3)2 + CaSO4 + 7H2OAddition of Ferrous sulfate & limeFe(HCO3) Ca(OH) Fe(OH)2 + 2CaCO3 + 2H2OFerrous bicarbonate(soluble)Ferrous sulfate(soluble)Calcium carbonate(soluble)Calciumsulfate(soluble)Ferrousbicarbonate(soluble)Calciumcarbonate(somewhatsoluble)Calciumhydroxide(slightly soluble)Ferroushydroxide(very slightlysoluble)1782 x 56
32Fe(OH)2 + 1/4O2 + 1/2H2O Fe(OH)3 Ferrous hyroxide Oxygen Water Ferric hydroxide (insoluble) 89.9¼ x 32½ x 18The alkalinity required for 10 mg/L dosage of ferrous sulfate,10 mg/L x (100/278) = 3.6 mg/LThe lime required,10 mg/L x 2(56)/278 = 4 mg/LThe oxygen required,10 mg/L x 32/(4x278) = 0.29 mg/LBecause the formation of ferric hyroxide is dependent on the presence of O2, ferrous sulfate is not used commonly.replace with ferric chloride (equations in page 496).
33Example 6.1 – Estimation of sludge volume from chemical precipitation of untreated wastewater a) Estimate the mass and volume of sludge produced from untreated wastewater without and with the use of ferric chloride for the enhanced removal of TSS.b) Also estimate the amount of lime required for the specified ferric chloride dose.- Assume that 60% of the TSS is removed in the primary settling tank without the addition of chemicals, and that the addition of ferric chloride results in an increased removal of TSS to 85%.- Also, assume that the following data apply to this situation:
35Solutions: Compute the mass of TSS removed without chemicals Compute the mass of TSS removed with chemicalsUsing Equation (6-16), determine the mass of ferric hyroxide produced from addition of ferric chlorideDetermine the mass of lime required using Eq (6-17)Determine total amount of sludge (TSS + Fe (OH)3)Determine the total volume of sludge (use specific gravity and moisture content info) fori) from chemical precipitationii) without chemical precipitationAnswer : refer page , textbook
36Recommended design for Primary Sedimentation Types of PrecipitationPercentage removal %TSSBODBacteriaChemical Precipitation80 – 90 %50 – 80 %Precipitation without chemical additives50 – 70 %25 – 40 %25 – 75 %
37Surface Loading Rate (SLR) or “surface settling rate” or “surface overflow rate” : is a hydraulic loading factor expressed in terms of flow per surface area.
39IntroductionThe removal of phosphorus from wastewater involves the incorporation of phosphate into TSS and the subsequent removal of these solids.Incorporation intobiological solids (during biological treatment)Incorporation intoChemical precipitates
40Phosphate Precipitation Addition of salts of multivalent metal ions, ex: Ca(II), Al(III), and Fe(III).1) Phosphate precipitation with Calcium- Calcium is added in the form of lime Ca(OH)2.- usually, when lime is added, it reacts with natural bicarbonate alkalinity to precipitate CaCO3.- As pH > 10, excess calcium ions will react with phosphate, to precipitate hydroxylapatite [Ca10(PO4)6(OH)2].- Quantity of lime required to precipitate P - independent of phosphate amount present, but dependent of wastewater alkalinity (about 1.4 – 1.5 times total alkalinity as CaCO3)- because need high pH- not feasible.
412) Phosphate precipitation with Aluminum and Iron Because many competing reactions and effects of pH, alkalinity, trace elements, etc., Eqs 6-20 & 6-21 cannot be used to estimate the required chemical dosages.So, achieved by bench-scale tests Figure 6-12.
42Solid lines: Conc. of residual soluble phosphates after precipitation Shaded area: pure metal phosphates are precipitatedSolid lines: Conc. of residual soluble phosphates after precipitation
44Example 6.2 – Determination of Alum Dosage for Phosphorus Removal Determine the amount of liquid alum required to precipitate phosphorus in a wastewater that contains 8 mg P/L.Also determine the required alum storage capacity if a 30-d supply is to be stored at the treatment facility. Based on laboratory testing, 1.5 mole of Al will be required per mole of P. The flow rate is m3/d. the following data are for the liquid alum supply.Formula for liquid alum Al2(SO4)3 ·18H2OAlum strength = 48 %Density of liquid alum solution = 1.2 kg/L
45Solution (page 503): Determine the weight of Al/L Determine the weight of Al required per unit weight of PDetermine the amaunt of alum solution required per kg PDetermine the amount of alum solution required per dayDetermine the required alum solution storage capacity.
50Chemical OxidationUse of Ozone (O3), Hydrogen peroxide (H2O2), Permanganate (MnO4), Chloride dioxide (ClO2), Chlorine (Cl2) or (HOCl), and Oxygen (O2)To reduce /degrade BOD, COD, ammonia, nonbiodegradable organic compounds.
51Fundamentals of chemical oxidation Oxidation-reduction reactions (redox)Cu2+ + Zn Cu + Zn2+2) Half-reaction potentials+ve(tendency to proceed to the right-ve (tendency to the left)
52E°reaction = E°reduction – E°oxidation Reaction potentialTo predict whether a reaction comprised of 2 half reactions will proceed as written.Equation: E°reaction = E°reduction – E°oxidationExample:Cu2+ + Zn Cu + Zn2+Potential of overall reaction,E°reaction = E°reduction – E°oxidation= 0.34 – (-0.763)= voltsTherefore, the reaction will proceed as written.
53CHEMICAL NEUTRALIZATION Is the removal of excess acidity or alkalinity by treatment with a chemical of opposite compositionChemical selection depends on suitability for particular application and economic consideration.