3- Standard Water Treatment

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)

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).

Surface water generally requires the most treatment

as shown in the following schematics.

For surface waters and GWUDI:

Groundwater requires much less treatment:

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.

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

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´

Free residual chlorine probably works by oxidizing

extracellular enzymes of bacterial cells.

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 .

Some examples of chlorine demand:

2 24 2

6 5 6 4 2

S SO (good to remove H S odors)

e (good if trying to ppt.F

C H OH (phenol) HOCl C H ClOH(chlorophenol) H O

(chlorophenol has a medicinal odor).

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.

NH HOCl NH Cl H O (monochloramine)

NH Cl HOCl NHCl H O (dichloramine)

NHCl HOCl NCl H O (trichloramine)

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

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)

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.

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

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.

Photomicrographs of Cryptospiridium cysts:

Photomicrographs of Giardia cysts:

Note that ³C´ values are those are the effluent of

the chlorine contact tank.

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

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.

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.

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.

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

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:

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.

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.

Filter media

Suspended solids

Top of filter

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.

Suspended solid

Filter media

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.

Attachment

Suspended solid

Filter media

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.

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

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.

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

Filtration rate

1 - 8 gpm/ft2 = acceptable range.

2-3 gpm/ft2

= average flow loading rates.

4-5 gpm/ft2 = peak flow loading rate

Terminal headloss

Commonly 3 - 5 ft for water treatment

Filter run = T = f(floc strength, Q and suspended

solids concentration in influent).

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.

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

Bolton Point Water Treatment Plant (Ithaca):

3- Standard Water Treatment

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