Chemical Chemical Treatment Treatment

Processes of Processes of Industrial WasteIndustrial Waste

“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

Introduction Chemical 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.

Role of Chemical Unit Processes in Wastewater

Treatment Chemical coagulationChemical precipitationChemical disinfectionChemical oxidationAdvanced oxidation processIon exchangeChemical neutralization, scale control, and

stabilization

Application of chemical Unit Processes in wastewater treatment

Considerations & Issues of Chemical Treatment….

“Chemical treatment – additive processes”

Net increase in dissolved constituents

TDS concentration increase;ex: chlorine additives

Handling, treatment and disposal of the large volumes of sludge produced

Increase in cost of energy & chemical costs

CHEMICAL CHEMICAL COAGULATIONCOAGULATION

Chemical Coagulation• Colloidal particles found in wastewater :

- net negative surface charge,- 0.01 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.

Brownian motion (random movement)

Basic definitions- Chemical 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 )

• Flocculants are also used to enhance the performance of granular medium filters and dewatering of digested biosolids Filter aids

Flocculation: 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 motion

Macroflocculation (orthokinetic flocculation)

- Particle aggregation is brought about by inducing velocity gradients and mixing in the fluid containing the

particles to be flocculated

Reaching 1-10 µm size, then separated by gravity sedimentation and filtration

Nature of particles in wastewater• Suspended particles > 1.0 µm, can be removed by gravity

sedimentation• Colloidal particles cannot be removed by sedimentation (need

coagulants & flocculant aids)

Important factors that contribute to the Characteristics of Colloidal Particles:

a) Particle size and number 0.01 to 1.0 µm number in untreated wastewater and after primary sedimentation = 106 to 1012 /mL

b) Particle shape and flexibility spherical, ellipsoids, disklike, various length, D, and random coils . shapes affect electrical properties, particle-particle interaction, and particle-solvent interaction

c) Particle-solvent interaction Hydrophobic – have relatively little attraction for water Hydrophilic – much greater attraction for water Association 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

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)

The Electrical Double Layer

Electrical double layer or electrostatic interaction force

Surface potential- depends on distance from particle surface

Particle-particle interactionsInvolve 2 principal forces: repulsion force and van der Waals force of attraction

*Refer page 483 in textbook

1) Particle Destabilization and Aggregation with Polyelectrolytes

Actions of polyelectrolytes:a) Charge neutralization b) Polimer bridge formationc) Charge neutralization and polimer bridge formation

Particle Destabilization Required to reduce particle charge or to overcome effect of this charge.

Therefore aggregation of particles (microflocculation) can be achieved.

a) Charge neutralization

- act as coagulants that neutralize or lower the charge of the wastewater particles

- normally the wastewater particles are –ve charge, so, cationic (+ve charge) polyelectrolytes are used.

- polyelectrolytes must be adsorbed to the particles used sufficient and high intensity of mixing (prevent folding back of polyelectrolytes)

b) Polymer bridge formation

Anionic or nonionic polyelectrolytes

c) Charge neutralization and Polymer bridge formation- use cationic polyelectrolytes having extremely high molecular weight- can form both charge neutralization and polymer bridge

2) Particle Destabilization with Potential-determining Ions and electrolytes

a) Addition of potential determining ions- add strong acids or bases to reduce charge of metal oxides

or hydroxides to near 0 so that coagulation can occur- not feasible due to massive concentrations of ions to be added

b) 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.

3) Particle destabilization and removal with hydrolyzed metal ions- Addition of alum or ferric sulfate (Fe3+ & Al3+)- Complex formation of metal ion hydrolysis products

Letterman, 1991

Action 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 settle

iii) Enmeshment (trapped) in sweep floc- floc particles settle and sweep through water containing colloidal particles - colloidal particles enmesh in the floc – removed by sedimetation.

CHEMICAL CHEMICAL PRECIPITATION PRECIPITATION FOR IMPROVED FOR IMPROVED

PLANT PERFORMANCEPLANT PERFORMANCE

Chemical Precipitations-involves the addition of chemicals to alter the physical state of dissolved and SS , and facilitate removal by sedimentation

1) Alum

Alkalinity(or Magnesium bicarbonate)

Precipitate

The 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

2) Lime- Reactions for carbonic acid – clarification;

- Alkalinity;

3) Ferrous sulfate and limeFerrous sulfate alone added to wastewater;FeSO4 ·7H2O + Ca(HCO3)2 Fe(HCO3)2 + CaSO4 + 7H2O

278 100

- Addition of Ferrous sulfate & limeFe(HCO3)2 + 2Ca(OH)2 Fe(OH)2 + 2CaCO3 + 2H2O

Ferrous sulfate(soluble)

Calcium carbonate(soluble)

Ferrous bicarbonate(soluble)

Calcium sulfate(soluble)

Ferrous bicarbonate(soluble)

Calcium hydroxide(slightly soluble)

Ferrous hydroxide(very slightly soluble)

Calcium carbonate(somewhat soluble)

178 2 x 56

Fe(OH)2 + 1/4O2 + 1/2H2O Fe(OH)3

Ferrous hyroxide Oxygen Water Ferric hydroxide (insoluble)

89.9 ¼ x 32 ½ x 18

The alkalinity required for 10 mg/L dosage of ferrous sulfate,10 mg/L x (100/278) = 3.6 mg/L

The lime required,10 mg/L x 2(56)/278 = 4 mg/L

The oxygen required,10 mg/L x 32/(4x278) = 0.29 mg/L

Because 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).

Example 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:

1. Wastewater flow rate = 1000 m3 /d2. Wastewater TSS = 220 mg/L3. Wastewater alkalinity as CaCO3 = 136 mg/L4. Ferric chloride (FeCl3) added = 40 kg/1000m3

5. Raw sludge properties:Specific gravity = 1.03Moisture content = 94 %

6. Chemical sludge properties:Specific gravity = 1.05Moisture content = 92.5 %

Solutions:1) Compute the mass of TSS removed without chemicals2) Compute the mass of TSS removed with chemicals3) Using Equation (6-16), determine the mass of ferric hyroxide produced from addition of ferric chloride4) Determine the mass of lime required using Eq (6-17)5) Determine total amount of sludge (TSS + Fe (OH)3)

6) Determine the total volume of sludge (use specific gravity and moisture content info) for i) from chemical precipitationii) without chemical precipitation

Answer : refer page 499-500, textbook

Recommended design for Primary Sedimentation

Types of Precipitation Percentage removal %

TSS BOD Bacteria

Chemical Precipitation 80 – 90 % 50 – 80 % 80 – 90 %

Precipitation without chemical additives 50 – 70 % 25 – 40 % 25 – 75 %

Surface Loading Rate (SLR) or “surface settling rate” or “surface overflow rate” : is a hydraulic loading factor expressed in terms of flow per surface area.

CHEMICAL CHEMICAL PRECIPITATION PRECIPITATION

FOR FOR PHOSPHORUS PHOSPHORUS

REMOVALREMOVAL

IntroductionThe removal of phosphorus from

wastewater involves the incorporation of phosphate into TSS and the subsequent

removal of these solids.

Incorporation into biological solids (during biological treatment)

Incorporation into Chemical precipitates

Phosphate 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.

2) 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.

Shaded area: pure metal phosphates are precipitatedSolid lines: Conc. of residual soluble phosphates after precipitation

Example 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 12 000 m3/d. the following data are for the liquid alum supply.

1. Formula for liquid alum Al2(SO4)3 ·18H2O2. Alum strength = 48 %3. Density of liquid alum solution = 1.2 kg/L

Solution (page 503):1)Determine the weight of Al/L2)Determine the weight of Al required per unit

weight of P3)Determine the amaunt of alum solution

required per kg P4)Determine the amount of alum solution

required per day5)Determine the required alum solution

storage capacity.

CHEMICAL CHEMICAL OXIDATIONOXIDATION

Chemical Oxidation- Use 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.

Fundamentals of chemical oxidation1)Oxidation-reduction reactions (redox)

Cu2+ + Zn Cu + Zn2+

2) Half-reaction potentials +ve(tendency to proceed to the right

-ve (tendency to the left)

Reaction potential• To predict whether a reaction comprised of 2 half

reactions will proceed as written.

• Equation: E°reaction = E°reduction – E°oxidation

• Example:

Cu2+ + Zn Cu + Zn2+

E°reaction = E°reduction – E°oxidation

= 0.34 – (-0.763)

= +1.103 voltsTherefore, the reaction will proceed as written.

Potential of overall reaction,

CHEMICAL NEUTRALIZATIONCHEMICAL NEUTRALIZATIONIs the removal of excess acidity or alkalinity by treatment with a chemical of opposite composition Chemical selection depends on suitability for particular application and economic consideration.

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