Lecture.chemical Process in Waste Water Treatment

Chemical Unit Process Gyeongsang National University Enviromental Engineering Lab Ngoc Thuan Le ** 김 투안 **

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Chap.6|Chemical Unit Process

Overview •

The principal chemical unit processes used for wastewater treatment a. Basi Basic c defi defin niti itions ons b. Chem Chemic ical al prec precip ipit itat atio ion n c. Chemic Chemical al prec precipi ipitati tation on for for phosph phosphoru orus s remova removall d. Chem Chemic ical al oxid oxidat atio ion n e. Chem Chemic ical al neut neutra rali liza zati tion on

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Chap.6|Chemical Unit Process

Application of chemical unit processes

•

For the complete secondary treatment of untreated wastewater, including the removal of either nitrogen or phosphorus or both.

• • • •

To remove phosphorus by chemical precipitation To be used in conjunction with biological treatment. For the removal of heavy metals and specific organic compounds For advance treatment of wastewater, the disinfection of wastewater.

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Chap.6|Chemical Unit Process

Fundamental of chemical coagulation  Basic Definition

•

Coagulant is the chemical that is added to destabilize the colloidal Coagulant is  particles in wastewater.

•

A Flocculent   Flocculent is is a chemical, usually positive charge typically organic, added to enhance the flocculation process.

•

Microflocculation (perikinetic flocculation), aggregation is brought about  by the random thermal motion of fluid molecules.

•

Macroflocculation (orthokinetic flocculation), aggregation is brought about by inducing velocity gradients and mixing in the fluid containing the particles to be floccullated, (large particles overtake small particles to form larger particles)

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Chap.6|Chemical Unit Process

 Nature particles in the wastewater 

•

The size of colloidal particles: about 0.01-1µm, a net negative surface charge

•

The number of colloidal particles in untreated wastewater is from 106 to 1012 /ml.

•

Particles solvent interactions:

•



Hydrophobic or “water-hating”



Hydrophilic or “water-loving”



Association colloids

Ionization At high pH

At low pH

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At isoelectric point

Chap.6|Chemical Unit Process

 Particle destabilization and aggregation with polyelectrolytes

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Polyelectrolytes may be devided into two categories: 



•

 Natural: include polymers of biological origin such as cellulose derivatives  Natural: and alginates Synthesis: simple monomers that are polymerized into high molecular  Synthesis: weight substances.

The action of polyelectrolytes: 

Charge neutralization: neutralization: the cationic polyelectrolytes are used for this purpose



Polymer bridge formation: formation: polymers that are anionic and nonionic, a bridge is formed when two or more particles become absorbed along the length of   polymers



Charge neutralization and polymer bridge formation: formation : from using cationic  polyelectrolytes of extremely high molecular weight.

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Chap.6|Chemical Unit Process

 Particle destabilization and removal with hydrolyzed metal ions

•

Formation of hydrolysis products: products : in the past, it was thought that free Al +3 and Fe+3 were responsible for particles aggregation, now their hydrolysis products are responsible. OH2

H2O H2O H2O

Me

3+

OH2 OH2

H2O

In base: Al(OH)3(s) + OH-(aq)

Me

OH2

+

H+ Me: Cr, Al, Fe

OH2

H2O

In acid: Al(OH)3(s) + 6H3O+(aq)

2+

OH

H2O

Al3+

(aq)

+ 6H2O

Al(OH) 4- (aq)

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+ 6H2O

Chap.6|Chemical Unit Process

Chemical precipitation for improved plant performance

•

In the past, chemical precipitation was used to enhance the degree of TSS and BOD removal

•

In current practice, chemical precipitation is used for: 

Primary settling facilities



In the independent physical-chemical treatment of wastewater 



Removal of phosphorus



Removal of heavy metals

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Chap.6|Chemical Unit Process

Inorganic chemicals used most commonly for coagulation and precipitation processes in wastewater

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Chap.6|Chemical Unit Process

 Enhance removal of suspended solids in primary sedimentation

•

•

With chemical precipitation, it is possible to remove 

80-90% TSS



50-80% BOD



80-90% bacteria

Dependent factors 

Quantity of chemical used



Mixing time



Loading rates



Operator 

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Chap.6|Chemical Unit Process

Chemical precipitation for phosphorus removal

•

Phosphorus precipitation with calcium  

Using as form of lime, Ca(OH) C a(OH)2  pH of the wastewater increases beyond about 10

10Ca2+ + 6PO43- + 2OH-

Ca10 (PO4)6(OH)2 Hydroxylapatite

Dosage of lime depend on 

Amount of phosphate present



The alkalinity of wastewater 

The quantity of lime required is typically about 1.4 to 1.5 times the total alkalinity expressed as CaCO3

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Chap.6|Chemical Unit Process

•

Phosphate precipitation with aluminum and iron

Al3+ + HnPO43-n

AlPO4 + nH

Fe3+ + HnPO43-n

FePO4 + nH

•

There are many competing reactions because of the effects of alkalinity, pH, trace elements, and ligands in wastewater 

•

Dosages are established of bench scale test and occasionally by full scale tests.

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Chap.6|Chemical Unit Process

•

Typical alum dosage requirements for various levels of phosphorus removal Phosphorus reduction, %

Mole ratio, Al: P Range

Typical

75

1.25:1-1.5:1

1.4:1

85

1.6:1-1.9:1

1.7:1

95

2.1:1-2.6:1

2.3:1

Developed in part from US. EPA (1976)

•

Theoretically, the minimum solubility of AlPO4 occurs at pH 6.3, FePO4 occurs at pH 5.3. In practice, good phosphorus phosphorus removal removal anywhere in the range of pH 6.5 to 7.0

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Chap.6|Chemical Unit Process

•

Factors effecting the choice of chemical for phosphorus removal

a. Infl Influe uent nt pho phosp spho horu russ leve levell  b.  b. Wa Wast stew ewate aterr sus suspe pend nded ed solid solidss c. Alkalinity d. Chemica Chemicall cost cost (inclu (includin ding g transp transport ortatio ation) n) e. Reli Reliab abil ilit ity y of che chemi mica call supp supply ly f. Slud Sludge ge hand handli ling ng faci facili liti ties es g. Ulti Ultima mate te dis dispo posa sall meth method odss h. Compati Compatibili bility ty with with other other treat treatmen mentt proc process esses es

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Chap.6|Chemical Unit Process

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Phosphorus removal using metal salts and polymers

a. Iron and aluminu aluminum m salts can be be added added at a variety variety of of differen differentt points points in the treatment processes  b. Polyphos Polyphosphates phates and organic organic phosph phosphorus orus are less less easily easily removed removed than orthophosphorus c. Addin Adding g alumi aluminum num or iron iron salts salts after after seco seconda ndary ry treatme treatment nt (whe (where re organic phosphorus and polyphosphate are transformed into orthophosphorus) orthophosphorus) results in the best removal. d. Poly Polyme mers rs may may be adde added d (1) to the mixing zone zone of a highly mixed mixed or internally internally recirculated recirculated clarifier, (2) preceding preceding a static static of dynamic dynamic mixer, mixer, or  (3) to aerat aerated ed chan chanel el

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Chap.6|Chemical Unit Process

•

Phosphorus removal using lime

5Ca2+ + 3PO43- + OH-

Ca5(PO4)3(OH) Hydroxylapatite

a. Lime trea treatmen tmentt can be used used to to precipi precipitate tate a port portion ion of of the phos phospho phorus rus (about 65-80%)  b.  b. Prod Produc uctt of of prec precip ipita itatio tion: n: [Ca [Ca5(PO4)3(OH)]. c. pH is abou aboutt 11 for for high high lime syst systems ems,, about about 8.5-9. 8.5-9.5 5 for low low lime lime systems d. After precipitation, precipitation, the effluent effluent must must be be recarbon recarbonated ated before before biologic biological al treatment. e. In activated sludge sludge systems, systems, the pH pH of of the primary primary effluen effluentt should should not exceed 9.5 or 10

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Chap.6|Chemical Unit Process

Advantages and disadvantages of chemical addition in various section of a treatment  plant for phosphorus removal Level of treatment

Advantages

Aisadvantages

Primary

applicable to most plant, increase Least efficient use of metal, polymer may be BOD and TSS removal, lowest degree required for flocculation, sludge more difficult of metal leakage, lime recovery to dewater than primary sludge demonstrated

Secondary

lower cost, lower chemical dosage overdose of metal cause low pH toxicity, with than primary, improved stability of  low alkalinity wastewaters, a pH control activated sludge, polymer not required system may be necessary, canot use lime  because of excess pH, inert solids added to activated sludge mixer liquor, reducing the  percentage of volatile solids

Advanced Advanced prec precipita ipitation tion lowest lowest phospho phosphorus rus efflue effluent, nt, most most efficient metal use, lime recovery demonstrated

Highest capital cost, highest metal leakage

advanced single and two lower cost can be combineed wieth the Length of filter run may be reduced with stage filtration removal of residual TSS single-stage filtration additional expense with two stage filtration process Adapted from US. EPA, 1976

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Chap.6|Chemical Unit Process

Chemical precipitation for removal of heavy metals and dissolved organic substances

•

Most of metal of interest in wastewater can be precipitated as hydroxides and sulfides

•

Dependent factors:  

Solubility  pH

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Chap.6|Chemical Unit Process

Chemical oxidation

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Oxidizing agents: 

ozone (O3),



hydrogen peroxide (H 2O2),



•

 permanganate (MnO 4),



chloride dioxide (ClO2),



chlorine (Cl2) or (HClO) and



oxygen (O 2)

For reduction of: 

BOD,



COD,



ammonia,



nonbiodegradable organic compounds.

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Chap.6|Chemical Unit Process

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Oxidation-reduction reaction 

Take place between oxidizing agent and reducing agent. Cu2+ + Zn Zn - 2e Cu2+ - 2e

Cu + Zn2+ Zn2+ (oxidation) Cu (reduction)



Oxidation-reduction reactions often require the presence of one or more catalysts to increase the rate of reaction.



Requirements: transition metal cations, enzymes, pH adjustment…

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Chap.6|Chemical Unit Process

Typical applications of chemical oxidation in wastewater collection, treatment and disposal.

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Chap.6|Chemical Unit Process

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Chemical oxidation of BOD and COD

Organic molecule (e.g., BOD)

Cl, O3, H2O2

Intermediate oxygenated molecules

Cl, O3, H2O2

Simple end  products (e.g., CO2, H2O…)

• Chemical oxidation of nonbiodegradable organic compounds For treatment of remaining after biological treatment: low molecular weight  polar organic compounds and complex organic compounds build around the  benzene ring structure

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Chap.6|Chemical Unit Process

•

Chemical neutralization

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Chap.6|Chemical Unit Process

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Scaling control 

Acidifying to reduce pH and alkalinity



Reducing calcium concentration by ion exchange or lime softening



Adding a scale inhibitor chemical (antiscalant) to increase the apparent solubility of CaCO3 in the concentrate stream



Lowering the product recovering rate

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Chap.6|Chemical Unit Process

Chemical storage, feeding, piping, and control systems.

Chemical feeders

Liquid feeders

Dry feeders

gravimetric

Belt Loss in weight Self-powered

volumetric

Slurry solution

Belt Revolving plate Rotary Screw Shaker  vibratory

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Gas feeders

Ammonia Chlorine Oxygen Ozone sulfur dioxide

Chap.6|Chemical Unit Process

Schematic of typical dry chemical-feed system.

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Chap.6|Chemical Unit Process

Conveyor belt

Screw

Rotary

Vibratory

Typical volumetric chemical feeders

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Chap.6|Chemical Unit Process

Schematic of typical liquid chemical-feed system.

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Chap.6|Chemical Unit Process

Typical mixing times for various chemicals used in wastewater treatment facilities Chemical

Application

Recommentded mixing times, s

Alum, Al3+ , Ferric chloride, Fe3+

Coagu oagula lati tion on of coll olloida oidall par particl ticlees