chem unit process.pdf

MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process

Chemical Unit Process

Gyeongsang National UniversityEnviromental Engineering Lab

Ngoc Thuan Le** 김 투안 **


• The principal chemical unit processes used for wastewater treatment

a. Basic definitions

b. Chemical precipitation

c. Chemical precipitation for phosphorus removal

d. Chemical oxidation

e. Chemical neutralization

Overview


• 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.

Application of chemical unit processes


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

• A Flocculent 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)

Fundamental of chemical coagulation

Basic Definition


• 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

Nature particles in the wastewater

At high pH At isoelectric pointAt low pH


• Polyelectrolytes may be devided into two categories:

Natural: include polymers of biological origin such as cellulose derivatives and alginates

Synthesis: simple monomers that are polymerized into high molecular weight substances.

• The action of polyelectrolytes:

Charge neutralization: the cationic polyelectrolytes are used for this purpose

Polymer bridge 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: from using cationic polyelectrolytes of extremely high molecular weight.

Particle destabilization and aggregation with polyelectrolytes


• Formation of hydrolysis 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.

Particle destabilization and removal with hydrolyzed metal ions

Me

H2O

H2O

H2O

OH2

OH2

OH2

3+

Me

H2O

H2O

H2O

OH

OH2

OH2

2+

+ H+

Me: Cr, Al, Fe

In acid: Al(OH)3(s) + 6H3O+(aq) Al3+ (aq) + 6H2O

In base: Al(OH)3(s) + OH-(aq) Al(OH)4

- (aq) + 6H2O


• 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

Chemical precipitation for improved plant performance


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


• 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

Enhance removal of suspended solids in primary sedimentation


• Phosphorus precipitation with calcium

Using as form of lime, Ca(OH)2

pH of the wastewater increases beyond about 10

Chemical precipitation for phosphorus removal

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


• 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.


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)

• Typical alum dosage requirements for various levels of phosphorus removal

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


• Factors effecting the choice of chemical for phosphorus removal

a. Influent phosphorus level

b. Wastewater suspended solids

c. Alkalinity

d. Chemical cost (including transportation)

e. Reliability of chemical supply

f. Sludge handling facilities

g. Ultimate disposal methods

h. Compatibility with other treatment processes


• Phosphorus removal using metal salts and polymers

a. Iron and aluminum salts can be added at a variety of different points in the treatment processes

b. Polyphosphates and organic phosphorus are less easily removed than orthophosphorus

c. Adding aluminum or iron salts after secondary treatment (where organic phosphorus and polyphosphate are transformed into orthophosphorus) results in the best removal.

d. Polymers may be added

(1) to the mixing zone of a highly mixed or internally recirculated clarifier,

(2) preceding a static of dynamic mixer, or

(3) to aerated chanel


• Phosphorus removal using lime

a. Lime treatment can be used to precipitate a portion of the phosphorus (about 65-80%)

b. Product of precipitation: [Ca5(PO4)3(OH)].

c. pH is about 11 for high lime systems, about 8.5-9.5 for low lime systems

d. After precipitation, the effluent must be recarbonated before biological treatment.

e. In activated sludge systems, the pH of the primary effluent should not exceed 9.5 or 10

5Ca2+ + 3PO43- + OH- Ca5(PO4)3(OH)

Hydroxylapatite


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 BOD and TSS removal, lowest degree of metal leakage, lime recovery demonstrated

Least efficient use of metal, polymer may be required for flocculation, sludge more difficult to dewater than primary sludge

Secondary lower cost, lower chemical dosage than primary, improved stability of activated sludge, polymer not required

overdose of metal cause low pH toxicity, with low alkalinity wastewaters, a pH control 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 precipitation lowest phosphorus effluent, most efficient metal use, lime recovery demonstrated

Highest capital cost, highest metal leakage

advanced single and two stage filtration

lower cost can be combineed wieth the removal of residual TSS

Length of filter run may be reduced with single-stage filtration additional expense with two stage filtration process

Adapted from US. EPA, 1976


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

• Dependent factors:

Solubility

pH

Chemical precipitation for removal of heavy metals and dissolved organic substances


• Oxidizing agents:

ozone (O3),

hydrogen peroxide (H2O2),

permanganate (MnO4),

chloride dioxide (ClO2),

chlorine (Cl2) or (HClO) and

oxygen (O2)

• For reduction of:

BOD,

COD,

ammonia,

nonbiodegradable organic compounds.

Chemical oxidation


• Oxidation-reduction reaction

Take place between oxidizing agent and reducing agent.

Cu2+ + Zn Cu + Zn2+

Zn - 2e Zn2+ (oxidation)

Cu2+ - 2e 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…


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


• Chemical oxidation of BOD and COD

Organic molecule (e.g., BOD)

Intermediate oxygenated molecules

Simple end products (e.g., CO2, H2O…)

Cl, O3,

H2O2

Cl, O3,

H2O2

• 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


• Chemical neutralization


• 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


Chemical storage, feeding, piping, and control systems.

Chemical feeders

Dry feeders Liquid feeders Gas feeders

gravimetric volumetric

BeltLoss in weightSelf-powered

BeltRevolving plateRotaryScrewShakervibratory

Slurry solution AmmoniaChlorineOxygenOzone sulfur dioxide


Schematic of typical dry chemical-feed system.


Typical volumetric chemical feeders

Conveyor belt Rotary

Screw Vibratory


Schematic of typical liquid chemical-feed system.


Chemical Application Recommentded mixing times, s

Alum, Al3+ , Ferric chloride, Fe3+ Coagulation of colloidal particles <1

Alum, Al3+ , Ferric chloride, Fe3+ Sweep floc precipitation 1-10

Lime Ca(OH)2 Chemical precipitation 10-30

Chlorine, Cl2 Chemical disinfection <1

Chloramine, NH2Cl Chemical disinfection 5-10

Cationic polymers Destabilization of colloidal particles <1

Anionic polymers particle bridging 1-10

Polymers, anionic Filter aids 1-10

Typical mixing times for various chemicals used in wastewater treatment facilities


Thank you for your attention!

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