Transcript Disinfection

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
Philadelphia
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
MCLs: Maximum Contaminant Levels
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
Particle
Diameter (mm)
Velocity (m/s)
Sand
1.0
2x10-1
Fine sand
0.1
1x10-2
Silt
0.01
1x10-4
Clay
0.001
1x10-6
Source: Vesilind & Morgan
Stokes Law
v
d 2  g  (  p  l )
18  
Thus, the small, clay particles settle extremely slowly.
Coagulation
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Chemicals such as alum
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Addition of the chemical occurs
in a flash mix of 1-3 minutes
where the chemical dissolves
and mixes with the raw water.
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Al2 ( SO4 )3
are added to neutralize the
negative charge and destabilize
the particle populations and
allow them to come together,
i.e. coagulate.
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Particle populations are stable
because their net negative
charge repels one another.
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Flocculation
The alum reacts with calcium bicarbonate
naturally present in most waters to form a
precipitate or floc, aluminum hydroxide.
Al2 (SO4 )3  3Ca( HCO3 )2  Al (OH )3  3CaSO4  6CO2
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.
Sedimentation
Sedimentation takes place over
a period of 1-4 hours.
Disinfection
Chlorination
sorption
Cl2( g )  H 2O  HOCl  HCl
HOCl  OCl   H 
K a  107.5 ,
pK a  7.5
Thus above HOCl dominates below pH 7.5
and OCL dominates above pH 7.5
Disinfection
Design
sorption
Chick’s Law
dN
 k  N
dt
The Surface Water Treatment Rule
Requires a 4-log or 99.99% removal
Nt
 0.0001
N0
t
and
 ln(0.0001)
k
N t  N 0  exp  k t
Disinfection
Design
Ct = concentration, time
The Surface Water Treatment Rule
Requires a 4-log or 99.99% removal
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,
 Cl2
Design – reduce NOM through pre-oxidation with ozone.
Iron and Manganese
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
Add Ca(OH) 2 to raise pH and convert HCO3  CO32
Ca(HCO3 )2 + Ca(OH)2  2CaCO3 + 2H2 0
Mg  HCO3 2  2Ca  OH 2  MgCO3 + 2CaCO3  2H2O
But MgCO3 is soluble. Add more Ca(OH)2 to further raise pH.
MgCO3  Ca  OH 2  Mg  OH 2  CaCO3
Adding soda ash, Na2CO3
Sometimes there is insufficient HCO3 to convert to CO32 ; adding soda ash
Na 2 CO3  2Na 2  CO32
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:
Chemviron Carbon
Source:
Millenium Inorganic Chemicals
Source:
Sontheimer et al. 1988
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.
Influent
stream
Effluent
stream
… tendency to sorb
Concentration (mg/L)
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).
6
Poorly sorbed
4
2
Strongly sorbed
0
0
2
4
Time (d)
6
8
… 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
Cout
Ceq
exhaustion
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
Membrane Processes
Ultrafiltration
polypropylene fiber
300 µm ID
500 µm OD
Ultrafiltration
0.2 µm nominal pore size
raw
water
in
epoxy seal
permeate flow
Ultrafiltration
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
Achieves 99.99% removal of Giardia
and Cryptosporidium cysts, but does not
remove all pathogenic organisms.
Bottled Water: $8 /gallon
Tap Mount: $0.25 / gallon
Municipal: $0.0015 / gallon