Drinking water standards and therefore water treatment depends on the water source:

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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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At a minimum water treatment will involve
disinfection, usually by chlorination.
Disinfection selective killing or inactivation
of pathogens as opposed to sterilization
(complete elimination of all microoganisms). Chlo
rine is used because of its relative ease of
application and low cost.
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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
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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 free 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
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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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Relative 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 combined available
chlorine
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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 Ct concept. Where
k constant n 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
t must equal this k).
The following table gives some
values
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Required disinfection levels Goal to reduce
microorganism level to the Primary Drinking Water
Standards Total Coliforms 5 - This means
that less than 5 of the samples taken per
month can be positive (i.e., show the presence of
coliforms)
Cryptosporidium, Giardia, HPC, Legionella,
Viruses are all regulated by TT standards
(resulting from disinfection and filtration)
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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.
• Ct concept used to determine required retention
  time 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 C values are those are the effluent
  of the chlorine contact tank.

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Log Reduction 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.
Other filters, such as membrane filters, can
get up to 2.5 log reductions credit with
demonstration of performance.
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Regardless 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 Ct.
• 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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The backbone of most water treatment plants
is Porous Media Filtration
Definition Removal of colloidal (usually
destabilized) and suspended material from water
by passage through layers of porous media -----
turbidity removal
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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        
Surface removal (straining) Mechanical straining
caused by a layer of suspended solids (from the
feed water) which builds up on the upper surface
of the porous media. This type of removal is to
be avoided because of the excessive headloss that
results from the suspended solids layer's
compressibility.
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Flow
Suspended solids
Top of filter media
Filter media
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 Depth removal Depth removal refers to SS
removal below the surface of the filter bed.
There are two types of depth removal.
Interstitial straining Larger particles become
trapped in the void space between granular media
particles.
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Flow
Suspended solid
Filter media
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Attachment Suspended solids are typically
flocculent by design (filter often follows
coagulation/flocculation) or by nature (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 suspended solids ("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 by nature (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 suspended solids ("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 by nature (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 suspended solids ("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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Flow
Suspended solid
Filter media
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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 the filter must
be backwashed to clean the filter bed surfaces.
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Single media Sand 24"-30" depth Effective
size 0.4-1.0 mm. (d10) Uniformity
coefficient lt 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 lt
1.8 Sand 12- 16 0.5- 0.55
lt1.65
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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 as strained). 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)