Advanced wastewater treatment

Gyeongsang National UniversityDepartment of Biological and chemical EngineeringEnvironmental Engineering Lab

Ngoc Thuan Le

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Need for advanced wastewater treatment

1. Remove organic matter and TSS to meet more stringent discharge and reuse requirements.

2. Remove TSS for more effective disinfection.

3. Remove nutrients contained to limit eutrophication of sensitive water bodies.

4. Remove specific inorganic (e.g., heavy metals) and organic constituents (e.g., MTBE) to meet more stringent discharge and reuse requirements both surface water and land-based effluent dispersal and for indirect potable reuse application.

5. Remove specific inorganic and inorganic constituents for industrial reuse (e.g., cooling water, process water…).

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Technologies used for advanced treatment

1. Removal of organic and inorganic colloidal and suspended solids (suspended solids, organic matters…), using filtration

• Depth filtration• Surface filtration• Membrane filtratration

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Depth filtration Surface filtration

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2. Removal of dissolved organic constituents (total organic carbon, refractory organic, volatile organic compounds)

• Carbon adsorption• Reverse osmosis• Chemical precipitation• Chemical oxidation• Advanced chemical oxidation• Electrodialysis• Distillation

3. Removal of dissolved inorganic constituents (ammonia, nitrate, nitrite, phosphorus, total dissolved solids)

• Chemical precipitation• Ion exchange• Ultrafiltration• Reverse osmosis• Electrodialysis• Distillation

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4. Removal of biological constituents (bacteria, protozoan cysts and oocysts, viruses)

• Depth filtration• Micro and ultrafiltration• Reverse osmosis• Electrodialysis• Distillation

Because the effectiveness of the unit operations and processes listed is

variable, disinfection of the treated effluent is required for most application

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Introduction to depth filtration

a. Flow during filtration cycle

b. Flow during backwash cycle

• Grain size is the principal filter medium characteristic that affects the filtration operation

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Particle removal mechanisms

a. By straining

b. By sedimentation or inertial impaction

c. By interception

d. By adhesion

e. By flocculation

Other phenomena: chemical/physical adsorption or biological growth

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Selection and design considerations for depth filters

1. Selection and design filter technologies must be based on:• Knowledge of the types of filters that are available• A general understanding of their performance

characteristics• An appreciation of the process variables controlling depth

filtration

2. Design for effluent filtration systems include:• Influent wastewater characteristics• Design and operation of the biological treatment process• Type of filtration technology to be used• Available flow-control options• Type of filter backwashing system• Filter control systems and intrumentation

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Available filtration technologies

a. Conventional mono-medium downflow filter

b. Conventional dual-medium downflow filter c. Conventional mono-medium

deep-bed downflow filterd. Continuous backwash deep-bed upflow filter

e. Pulse-bed filter f. Traveling-bridge filter

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Synthetic-medium filter High pressure filter

Slow sand filter

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Two-stage filtration

A large size sand diameter is used in the first filter to increase the contact time and to minimize clogging

A smaller sand size is used in the second filter to remove residual particles from the first stage filter

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Effluent filtration with chemical addition

• To achieve specific treatment objectives including removal of specific contaminants

• Phosphorus • Metal ions• Humic substances

• Chemicals commonly used in effluent filtration• Organic polymers (cationic, anionic, or nonionic (no

charge)• Alum and ferric compounds (chloride)

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Surface filtration

Materials: woven metal fabrics, cloth fabrics of different weaves, and variety of synthetic materials

Surface filters have openings in size range from 10 to 30µm. In membrane filters the pore

size can vary from 0.0001 to 1.0µm

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Membrane filtration

Materials: different organic or inorganic materials: polypropylene, cellulose acetate, aromatic polyamides, and thin film composite (TFC).

Membrance process

Membrane driving force

Typical separation mechanism

Operating structure (pore size)

Typical operating range, µm

Permeate description

Typical constituents removed

Microfiltration Hydrostatic pressure difference

sieve Macropores (>50nm)

0.08-2.0 Water+dissolved solutes

TSS, turbidity, protozoan, some bacteria and viruses

Ultrafiltration Hydrostatic pressure difference

sieve Mesopores (2-50nm)

0.005-0.2 Water+small molecules

Macromolecules, colloids, most bacteria, some viruses, protein

Nanofiltration Hydrostatic pressure difference

sieve+solution/diffusion+exclusion

Micropores (<2nm)

0.001-0.01 Water+very small molecules, ionic solutes

Small molecules, some harness, viruses

Reverse osmosis

Hydrostatic pressure difference

solution/diffusion+exclusion

Dense (<2nm)

0.0001-0.001

Water+very small molecules, ionic solutes

very small molecules, color hardness, sulfates, nitrate, sodium, other ions

Dialysis Concentration difference

Diffusion Mesopores (2-50nm)

_ Water+very small molecules,

Macromolecules, colloids, most bacteria, some viruses, protein

Electrodialysis Electromotive force ion exchange with selective membranes

Micropores (<2nm)

_ Water, ionic solutes ionized salt ions

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Reverse osmosis (RO)

• When two solutions having different solute concentrations are separated by a semi permeable membrane, a difference in chemical potential will exist across the membrane

• RO is used for the removal of dissolved constituents from the wastewater remaining after advanced treatment with depth filtration of microfiltration.

a. Osmotic flow

b. Osmotic equilibrium

c. Reverse osmosis

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Electrodialysis (ED)• In the electrodialysis process, ionic components of a solution are separated

through the use of semipereable ion-selective membrane

• The current required for electrodialysis can be estimated by Faraday’s Laws of electrolysis

Where:

I = current, amp

F = Faraday’s constant

= 96,485amp.s/gram equivalent = 96,485 A.s/eq

n = number of cell in the stack

Ec = current efficiency expressed as a fraction

nEc

FQNI

ŋ

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Adsorption

• Adsorption is the process of accumulation substances that are in solution on a suitable interface

• Types of adsorbents: activated carbon, synthetic polymeric, and silica-based adsorbents

• Activated carbon: (1) powdered activated carbon (PAC), a diameter of less than 0.074mm (200 sieve), and (2) granular activated carbon (GAC), a diameter greater than 0.1mm (140 sieve)

Parameter UnitType of activated carbon

GAC PAC

Total surface area m2/g 700-1300 800-1800

Bulk density kg/m3 400-500 360-740

Particle density, wetted in water kg/l 1.0-1.5 1.3-1.4

Particle size range mm (µm) 0.1-2.36 (5-50)

Effective size mm 0.6-0.9 na

Uniformity coefficient UC ≤1.9 na

Mean pore radius  16-30 20-40

Iodine number   600-1100 800-1200

Abrasion number minimum 75-85 70-80

Ash % ≤ 8 ≤ 6

Moisture as packed % 2-8 3-10

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Fundamentals of adsorption

• Absorbent phase concentration data

Where:

qe= absorbent (solids) phase concentration after equilibrium, mg adsorbate/g adsorbent

Co = initial concentration of adsorbate, mg/L

Ce = final equilibrium concentration of adsorbate after absorption has occurred, mg/L

V = volume of liquid in the reactor, L

m = mass of absorbent, g

m

VCCq ee

)( 0

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Types of activated carbon contactors

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Gas stripping

• Gas stripping involves the mass transfer of a gas from the liquid phase to the gas phase.

• Considerable attention: remove ammonia, odorous gases and volatile organic compounds (VOCs)

Countercurrent flow Current flowCross flow

Typical water and airflow patterns for gas stripping towers

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Typical stripping towers for the removal of volatile gases from water

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ION EXCHANGE

• Ion exchange is a unit process in which ions of a given species are displaced from an insoluble exchange material by ions of a different species in solution.

• Domestic water softening: where sodium ions from a cationic-exchange resin replace the calcium and magnesium ions in the treated water.

• Ion exchange has been used in wastewater application for removal of nitrogen, heavy metals, and total dissolved solids

• Ion-exchange materials:

• Naturally, zeolites (complex of aluminosilicates with sodium)

• Synthetic ion-exchange material: resins or phenolic polymers

1. Strong-acid cation

2. Weak-acid cation

3. Strong-base anion

4. Weak-base anion

5. Heavy-metal selective chelating resins

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Typical ion-exchange reaction

• For natural zeolite (Z)

Ca2+ Ca2+

ZNa+ + Mg2+ Z Mg2+ + 2Na+

Fe2+ Fe2+

• For synthetic resin (R)

Strong acid cation exchange:

RSO3H + Na+ RSO3Na + H+

2RSO3Na + Ca+2 (RSO3)2Ca + 2Na+

Weak acid cation exchange:

RCOOH + Na+ RCOONa + H+

2RCOONa + Ca+2 (RCOO)2Ca + 2Na+

Strong-base anion exchange:

RR’3NOH + Cl- RR’3NCl + OH-

Weak-base anion exchange:

RNH3OH + Cl- RNH3Cl + OH-

2RNH3Cl + SO42- (RNH3)2SO4 + 2Cl-

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Application of ion-exchange

• Typical flow diagram for the removal of ammonia by zeolite exchange

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Application of ion-exchange

• Typical flow diagram for the removal of hardness and for the complete demineralization of water

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• 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

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• 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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Ozone/Hydrogen peroxide

H2O2 + 2O3 HO* + HO* +3O2

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DISTILLATION

• Distillation is a unit operation in which the components of a liquid solution are separated by vaporization and condensation.

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Thank you for your attention!