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8/3/2019 3- Standard Water Treatment
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Drinking water standards and therefore water
treatment depends on the water source:
Three choices:
Surface water
Groundwater
Groundwater under the direct influence of surface
water (GWUDI)
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The definition of the last source (GWUDI) is
groundwater that has physical evidence of surface
water contamination (e.g., insect parts, high
turbidity), or contains surface water organisms
(e.g., cryptospiridium, giardia), or has chemical
water quality parameters similar to surface water
(e.g., T, conductivity, TDS, pH, color).
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Surface water generally requires the most treatment
as shown in the following schematics.
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For surface waters and GWUDI:
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Groundwater requires much less treatment:
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Disinfection: selective killing or inactivation of
pathogens as opposed to sterilization (completeelimination of all microoganisms).
Chlorine is used because of it¶s relative ease of application and low cost.
At a minimum water treatment will involve
disinfection, usually by chlorination.
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Chemistry of Chlorination:
Chlorination can be accomplished by adding Cl2(gas),
NaOCl or Ca(OCl)2 (sodium or calcium hypochlorite).
When Cl2 is added to water:
2(gas) 2Cl H O HOCl H Cl
HOCl H OCl
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HOCl = hypochlorous acid
OCl- = hypochlorite ion
The ratio of HOCl/OCl- is a function of pH
This is an important concept because HOCl is a
better disinfectant than OCl-
HOCl and OCl- are called ³free residual chlorine´
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Free residual chlorine probably works by oxidizing
extracellular enzymes of bacterial cells.
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Chlorine Demand
Because Cl2 or HOCl are strong oxidizers reducing
agents will use up some of the chlorine before it
can disinfect. These materials exert a chlorine
demand .
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Some examples of chlorine demand:
2 24 2
2+ 3+
3
6 5 6 4 2
)
S SO (good to remove H S odors)
F
eF
e (good if trying to ppt.F
e(OH)
C H OH (phenol) HOCl C H ClOH(chlorophenol) H O
(chlorophenol has a medicinal odor).
p
p
p
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All of the above reactions consume the
disinfecting power of chlorine. There are some
reactions which do not entirely consume this
disinfecting power and in some cases the products
of these reactions are useful.
These reactions involve the reaction of HOCl with
NH3 to form chloramines as shown here.
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3 2 2
2 2 2
2 3 2
NH HOCl NH Cl H O (monochloramine)
NH Cl HOCl NHCl H O (dichloramine)
NHCl HOCl NCl H O (trichloramine)
p
p
p
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R elative ratio of the chloramine species is a function
of the Cl2/NH3 ratio, pH and temperature.
All of the chloramines retain the +I oxidation state
of HOCl but their oxidizing/disinfection capabilities
are reduced.
Because the chloramines retain disinfection power
They are called ³combined available chlorine´
2 2 3[NH Cl] [NHCl ] [NCl ]
Combined Available Chlorine R esidual
!
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Primary Drinking Water Standards for Disinfectants
Chloramines: MCL = 4 mg/L (as Cl2)
Chlorine: MCL = 4 mg/L (as Cl2)
(MCL = maximum contaminant level, so these numbers
represent upper limits of chlorination)
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Dosage requirements:
Disinfection effectiveness is a function of
concentration of disinfectant and contact
time. This results in the ³Ct´ concept.
Wherenk C t!
k = constantn = constant (usually = 1)
t = contact time.
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k = a constant for a particular % kill for a particular disinfectant, temperature, pH and microorganism.
(to attain a certain % kill the product of C and tmust equal this k).
The following table gives some
values
C t
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EPA drinking water standard for disinfection requires
water treatment systems to inactivate 99.9% of Giardia cysts and 99.99 % of enteric viruses ( 3 and 4
log reductions respectively).
These organisms were chosen as standards because of their resistance to disinfection.
³Ct´ concept used to determine required retentiontime and chlorine concentration to achieve these log
reductions. See Table 16.2.
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Photomicrographs of Cryptospiridium cysts:
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Photomicrographs of Giardia cysts:
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Note that ³C´ values are those are the effluent of
the chlorine contact tank.
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LogReduction
% Removal
1 Log 90
1.5 Log 96.84
2 Log 99
2.5 Log 99.68
3 Log 99.9
4 Log 99.99
Log R eduction Scale
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USEPA SWTR (for surface waters and
groundwater under the direct influence of
surface water):
2 log reduction assumed in conventional
treatment (with filtration). Therefore need 1
log reduction from chlorination.
Ot her fil t ers, such as membrane fil t ers, can get up t o 2.5 log reduct ions credit wit h demonst rat ion
of performance.
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R egardless of the filtration method used, the
water system must achieve a minimum of 0.5-
log reduction of Giardia lamblia from
disinfection alone after filtration treatment.
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Points of chlorination in water treatment plants
In many treatment plants chlorine is applied for
final disinfection at the storage well (wet well)
at the end of the treatment train. There is sufficient
contact time here and in the distribution system to
provide adequate ³Ct´.
In some treatment plants chlorine is applied just before filtration.
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Typical Chlorine Dosages at Water Treatment Plants
Calcium hypochlorite 0.5 ± 5 mg/L
Sodium hypochlorite 0.2 ± 2 mg/L
Chlorine gas 1 ± 16 mg/L
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Definition: R emoval of colloidal (usually
destabilized) and suspended material from
water by passage through layers of porous
media ----- turbidity removal
The backbone of most water treatment plants is:
Porous Media Filtration:
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Deep Granular Filters
Deep granular filters are made of granular material
(sand, anthracite, garnet) arranged in a bed to
provide a porous media as shown in the figure below. Filter bed is supported by gravel bed as also
shown. Flow is typically in the downflow mode.
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Mechanisms of suspended solids removal
S urface removal (st raining)
Mechanical straining caused by a layer of suspended
solids (from the feed water) which builds up on theupper surface of the porous media. This type of
removal is to be avoided because of the excessive
headloss that results from the suspended solidslayer's compressibility.
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Filter media
Suspended solids
Flow
Top of filter
media
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Dept h removal
Depth removal refers to SS removal below the
surface of the filter bed. There are two types of
³depth removal´.
Interstitial straining
Larger particles become trapped in the void
space between granular media particles.
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Suspended solid
Filter media
Flow
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Attachment
Suspended solids are typically flocculent by design
(filter often follows coagulation/flocculation) or bynature (clays, algae, bacteria). Therefore, attachment
or adsorption of suspended solids is a good
possibility. Attachment can be electrostatic, chemical bridging or specific adsorption. Attachment is
enhanced by addition of small amount of coagulant
and as the filter bed becomes coated with suspendedsolids ("ripened" filter). It is easier for suspended
solids to attach to other SS that are already attached
to the filter media.
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Attachment
Suspended solids are typically flocculent by design
(filter often follows coagulation/flocculation) or bynature (clays, algae, bacteria). Therefore, attachment
or adsorption of suspended solids is a good
possibility. Attachment can be electrostatic, chemical bridging or specific adsorption. Attachment is
enhanced by addition of small amount of coagulant
and as the filter bed becomes coated with suspendedsolids ("ripened" filter). It is easier for suspended
solids to attach to other SS that are already attached
to the filter media.
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Attachment
Suspended solids are typically flocculent by design
(filter often follows coagulation/flocculation) or bynature (clays, algae, bacteria). Therefore, attachment
or adsorption of suspended solids is a good
possibility. Attachment can be electrostatic, chemical bridging or specific adsorption. Attachment is
enhanced by addition of small amount of coagulant
and as the filter bed becomes coated with suspendedsolids ("ripened" filter). It is easier for suspended
solids to attach to other SS that are already attached
to the filter media.
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Suspended solid
Filter media
Flow
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Filter Cycle
As filter run proceeds deposits build up in the
upper portion of the filter bed. As a consequence
void volume decreases, interstitial flow velocity
increases with more hydraulic shear on the trapped
and attached SS. This drives some of the filtered
SS deeper into the filter bed. Ultimately the SS
get washed into the effluent. At this point thefilter must be backwashed to clean the filter bed
surfaces.
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S ingle media:
Sand : 24"-30" depth
Effective size = 0.4-1.0 mm. (d10)
Uniformity coefficient < 1.65 (d60/d10)
Density = 2.65. porosity = 0.43
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Dual media:
To compensate for the unfavorable gradation that
occurs in the single media filters we can use dual
media (reverse graded) filters. Place a less dense,
larger diameter media on top of sand. This results
in a higher porosity (0.55) at top of filter. Sand has
porosity of about 0.4. Lower density also allows
the less dense media to remain on top after
backwashing.
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Media Depth (in) Eff size(mm) Uniform Coeff
Anthracite 12 ±20 0.9 ±1 < 1.8
Sand 12- 16 0.5- 0.55 <1.65
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Filtration rate
1 - 8 gpm/ft2 = acceptable range.
2-3 gpm/ft2
= average flow loading rates.
4-5 gpm/ft2 = peak flow loading rate
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Terminal headloss
Commonly 3 - 5 ft for water treatment
Filter run = T = f(floc strength, Q and suspended
solids concentration in influent).
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Backwash sequence
Bed expansion is between 15-30 %. This is
accomplished by applying a backflow rate of about
15 gpm/ft2 for about 5 - 10 mins. Hydrodynamic
shear cleans the media particles (attached, as well asstrained). Optimum shearing occurs at about 50 %
expansion but this tends to require excessive
backwash velocities with the coarser media particles
and these high flow backwashs could fluidize the
gravel underdrain.
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Head applied above sand: 3-5 ft.
Depth of sand is also about 3- 5 ft.
Loading rates: 0.05 - 0.1 gpm/ft2
T: 1-6 months
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Bolton Point Water Treatment Plant (Ithaca):