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CE3503 Environmental Engineering. Water Treatment. Dr. Martin T. Auer MTU Department of Civil & Environmental Engineering. Drinking Water Treatment Objectives. Potable: safe to drink - may be consumed with low risk of immediate or long term harm. Palatable: pleasant to taste. London.

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CE3503

Environmental

Engineering

Water Treatment

Dr. Martin T. Auer

MTU Department of Civil & Environmental Engineering


Drinking Water Treatment Objectives

Potable: safe to drink - may be consumed with low risk of immediate or long term harm.

Palatable: pleasant to taste


London

1854



Milwaukee

Cryptosporidium

1993 - 403,000 – 104 Dead


U.S. EPA Standards

National Primary Drinking Water Regulations

Microorganisms

Disinfectants & Disinfection Byproducts

Inorganic Chemicals

Organic Chemicals

Radionuclides

Enforceable


U.S. EPA Standards

National Primary Drinking Water Regulations

Microorganisms

Viruses: hepatitis A, gastroenteritis

Bacteria: cholera, dysentery, legionellosis, typhoid

Protozoa: Giardia and Cryptosporidium

Turbidity: standard is 1 NTU; microorganism contamination is associated with turbidity; particles also shield microorganisms from agents of disinfection.

Indicator organisms: standard for microbial contamination is based on E. coli , a species of bacteria originating from animal or human fecal material. Most strains of E. coli are not pathogenic, but their presence indicates the presence of fecal material and thus, potentially, pathogenic microbes. No E. coli may be present in finished drinking water.


U.S. EPA Standards

National Primary Drinking Water Regulations

Chemicals

Inorganic: arsenic, cadmium, copper, lead, mercury, nitrate

Organic: herbicides (e.g. atrazine), insecticides (methoxychlor), industrial residues (e.g. polychlorinated biphenyls, dioxin)

Radionuclides:  and  particles, radium, uranium

MCLGs: Maximum Contaminant Level Goals – level below which there is no know or expected risk to health. Allow a margin of safety and are not enforceable.

MCLs: Maximum Contaminant Levels – highest level allowed in drinking water. MCLs are enforceable and are set as close to MCLGs as feasible using BAT, taking into account economic considerations.


U.S. EPA Standards

National Primary Drinking Water Regulations

Disinfectants

Chlorine

Chlorine dioxide

Chloramines

Disinfectants & Disinfection Byproducts

Trihalomethanes


U.S. EPA Standards

Secondary Drinking Water Regulations

Cosmetic Effects (tooth color, excess fluoride)

Total Dissolved Solids (chloride, sulfate)

Taste, Odor, Color

Non-Enforceable (federally)


U.S. EPA Standards

National Primary Drinking Water Regulations

Microorganisms

Viruses: hepatitis A, gastroenteritis

Bacteria: cholera, dysentery, legionellosis, typhoid

Protozoa: Giardia and Cryptosporidium

Turbidity: standard is 1 NTU; microorganism contamination is associated with turbidity; particles also shield microorganisms from agents of disinfection.

Indicator organisms: standard for microbial contamination is based on E. coli , a species of bacteria originating from animal or human fecal material. Most strains of E. coli are not pathogenic, but their presence indicates the presence of fecal material and thus, potentially, pathogenic microbes. No E. coli may be present in finished drinking water.


Drinking Water Process Train

Basic treatment for turbidity and pathogens


Particle Settling Velocities

Source: Vesilind & Morgan

Stokes Law

Thus, the small, clay particles settle extremely slowly.


Coagulation

+

+

Particle populations are stable because their net negative charge repels one another.

Chemicals such as alum

are added to neutralize the negative charge and destabilize the particle populations and allow them to come together, i.e. coagulate.

Addition of the chemical occurs in a flash mix of 1-3 minutes where the chemical dissolves and mixes with the raw water.

+

+

+

+

+

+

+

+

+++

+++

+++

+++

+++

+++

+++


Flocculation

The alum reacts with calcium bicarbonate naturally present in most waters to form a precipitate or floc, aluminum hydroxide.

Destabilzied smaller particles can be attracted to the floc or simply swept up by the larger particles (sweep floc) as they settle and removed from the system.

Flocculation proceeds through a slow mix of 10-30 minutes.


1x107

Alum Sweep Floc

8x106

6x106

Particle Volume Distribution

(m3/mL)

4x106

2x106

0

0.0 0.4 0.8 1.2 1.6

log of particle diameter measured in m

Courtesy Dr. Desmond Lawler, University of Texas


Sedimentation

Sedimentation takes place over a period of 1-4 hours.


Filtration

Water is filtered until significant head loss is experienced, then the filter is backwashed.


Disinfection

Chlorination

Thus above HOCl dominates below pH 7.5

and OCL- dominates above pH 7.5

sorption


Disinfection

Design

Chick’s Law

sorption

The Surface Water Treatment Rule

Requires a 4-log or 99.99% removal

and


Disinfection

The Surface Water Treatment Rule

Requires a 4-log or 99.99% removal

Design Ct = concentration, time It = irradiance, time


Disinfection

Residual


Disinfection

Disinfection By-products (DBPs)

Formed through reaction of chlorine and natural organic matter (NOM) e.g. trihalomethanes such as chloroform,

Design – reduce NOM through pre-oxidation with ozone.



Hardness

Primarily Ca2+, Mg2+

Limestone: calcium carbonate, CaCO3

Dolomite: calcium magnesium carbonate, CaMg(CO3)2

The limestone formation underlying much of Miami, Florida


Lime – Soda Ash Process

Adding lime, Ca(OH)2

Adding soda ash, Na2CO3


Granular Activated Carbon

The most commonly used adsorbent is granular activated carbon (GAC). These irregular particles, 0.2-5 mm in diameter, are a char of carbon material (wood or coal). They are ‘activated’ or made more porous by exposure to steam at high temperature. Activated carbon has 1000 m2 of adsorbing surface area per gram (~ 1 teaspoon) or equivalent to that of a 40 acre farm in one handful!

Source:

Sontheimer et al. 1988

Source:

Chemviron Carbon

Source:

Millenium Inorganic Chemicals


The Adsorption Process

Organic chemicals are typically removed from a water supply prior to distribution through the process of adsorption:

the physical-chemical attraction of a solid material for a chemical in solution.

In adsorption, the chemical being adsorbed is termed the adsorbate and the solid to which it sorbs is the adsorbent.

Effluent

stream

Influent

stream


6

(mg/L)

4

2

Concentration

0

0

2

4

6

8

Time (d)

… tendency to sorb

For adsorption to be effective, the chemical must sorb strongly. Poorly soluble (hydrophobic) compounds (e.g. the components of gasoline) adsorb more strongly than highly soluble (hydrophilic) compounds (e.g. table salt).

Poorly sorbed

Strongly sorbed


… application in water treatment

In drinking water treatment, adsorption with GAC is accomplished using a packed bed column. The untreated water is introduced at the top of the column and trickles down through the GAC. Contaminants are removed en route and clean water emerges at the bottom of the column. In application, columns 6 feet in diameter and 30 feet in height are not uncommon.

Carbon

Bed

GAC columns


… column operation

Water flows thru the column and contaminants are adsorbed. With time, the GAC becomes saturated (sorption capacity is reached) and contaminants exit the bed (breakthrough). The exhausted carbon must then be replaced.

Cin

exhaustion

Cout

Ceq

breakthrough


Asbestos/Arsenic/Metals Removal

Sorption with Ferric sulfate: asbestos, arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, selenium, silver

Coagulation with Alum: asbestos, nickel, uranium



Ultrafiltration

polypropylene fiber

300 µm ID

500 µm OD


Ultrafiltration

0.2 µm nominal pore size


permeate flow

epoxy seal

Ultrafiltration

raw

water

in


Ultrafiltration

contaminants

banks of fiber bundles

backwashing


Ultrafiltration

contaminants

backwashing


Home Water Treatment

Reverse osmosis unit (salt)

Softening by

ion exchange

(hardness)


Home Water Treatment

  • Three step process:

  • sieve and bottom filter – rust, sand, turbidity

  • activated carbon filter – chlorine, color and SOCs

  • ion exchange resin – metals

Bottled Water: $8 /gallon

Tap Mount: $0.25 / gallon

Municipal: $0.0015 / gallon

Achieves 99.99% removal of Giardia

and Cryptosporidium cysts, but does not

remove all pathogenic organisms.


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