AGSM 337/BAEN 465
Potable Water Treatment
Page 1 of 5
Overview

Purpose is to provide safe, aesthetically pleasing water

Water must be free of hazardous chemicals and pathogenic microorganisms

Water must have a pleasant taste, odor, and color

Systems vary depending on source of water, generally less treatment is required for ground
water than for surface water

Surface water treatment systems may include:
source, pumping, coagulation and flocculation, sedimentation, filtration, adsorption,
disinfection, storage, distribution

Ground water treatment systems may include:
source, pumping, aeration, filtration, disinfection, storage, distribution
Coagulation

Helps settling of smaller particles (colloids)

Most naturally occurring particles in water carry a negative charge (particularly clays) which
keep them from joining together and aid in their suspension in water

Coagulants are added to water to reduce this net negative surface charge

After charge is reduced, particles are more likely to come close together

When they are close, van der Waals forces pull them together

Coagulants containing divalent or trivalent cations may also form precipitates which further
aid the trapping of particles

Precipitating coagulants include ferric chloride, alum (aluminum sulfate), ferric sulfate, and
lime

Organic polymers – long chain organic chemicals – are also available for use as coagulants

The long chain polymer molecules attach to several colloidal particles and thus facilitate
settling
Flocculation

Gentle mixing aimed at forcing small particles to collide and stick together

Conducted during or after the addition of a coagulant

Mixing is intentionally gentle – anything stronger might break “flocs” apart

Flocculation of small particles increases the settling velocity (recall Stoke’s Law, v  d2)
dramatically
AGSM 337/BAEN 465
Potable Water Treatment
Page 2 of 5
Sedimentation

Removes solids – those that settle individually and those brought together during the
coagulation and flocculation process

Typically done in tanks referred to as clarifiers

Usually involves flocculant settling (recall 4 types of settling from lectures on sedimentation)
but sometimes may include hindered settling near bottoms of tanks

Solids (sludge) on the bottom of the tank are pushed to the tank center by gravity or a scraper
and are removed
Filtration

Removes solids that are too small to be removed in a timely fashion by sedimentation

Water is passed through a clean porous medium (sand or anthracite coal)

Solids are removed by straining, settling within the medium pores, and adhesion to medium
particles (aided by addition of coagulants and reduction of surface charge)
Slow sand filtration

Uses a low loading rate (340-3400 gal/ft2/d)

Relies on the accumulation of a sludge layer on the surface of the filter for efficient
operation, water passing through a newly cleaned filter is often wasted

When sludge buildup hinders flow, the filter is stopped and the sludge layer is removed
from the top of the medium

Widely used in Europe, but not so much in the US
Rapid sand filtration

Uses higher application rates (3400 - 26000 gal/ft2/d)

Water is ponded above the surface of the medium to drive flow through the filter

Filter usually contains a mixture of media and is graded by size with larger, less dense
particles near the surface and smaller, denser particles below

Filters are cleaned by “backwashing” or fluidizing the medium by forcing flow up from
beneath – trapped particles are washed away and discarded

The different densities of the mixed media, e.g., coal and sand, allow the mixture to
return to the original graded structure following backwash operations (sand, even though
smaller, will settle faster because of higher density)

Widely used in the US
AGSM 337/BAEN 465
Potable Water Treatment
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Taste and Odor Control
Gas Stripping (Aeration)

Removes dissolved gasses (hydrogen sulfide, methane) and helps oxidation and
precipitation of reduced metals (iron and manganese)

Sometimes a stronger oxidant (chlorine or ozone) may be used to oxidize metals if
concentrations are high – removed for taste and color control (iron causes brown stains
and manganese gray)

Since aeration may produce precipitates, it is performed prior to sedimentation and
filtration

Tray aerators pass water downward through a series of screened trays

Diffused aeration relies on air bubbled up from the bottom of the aerator
A batch process for transfer of oxygen into water is described as follows:
Cs  Ct
 e  K Lat
Cs  C0
Where Cs = saturated DO in water, mg/L
(depends on temperature and solute concentration)
Ct = DO at time t, mg/L
C0 = initial DO, mg/L
KLa = volumetric oxygen transfer coefficient, 1/time
t = time
Example
Water is aerated at a temperature of 20 ºC. The initial DO is 5.6 mg/L. What is the DO after 10
minutes if the oxygen transfer coefficient is 0.04 1/min?
Determine Cs = 9.1 mg/L
Cs  Ct
9.1  Ct
 e  K Lat 
 e 0.04 (10)
Cs  C0
9.1  5.6
Solve for Ct = 6.75 mg/L
AGSM 337/BAEN 465
Potable Water Treatment
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Carbon Adsorption

Water is passed through columns containing granular activated carbon (GAC) which are
similar to home water filter systems, but much larger

Removes trace organic compounds and some other compounds which do not dissolve
readily in water (lead, other heavy metals)

Dissolved constituents adsorb to (adhere to the surface of) the GAC

Not really effective on contaminants which readily dissolve in water (salts)

May be combined with filtration where one of the media used is GAC
Disinfection

Destruction or killing of pathogenic organisms

Intent for public water supply is to kill all pathogens currently present and leave a residual for
treatment of any organisms introduced in the distribution system

An ideal disinfectants should quickly kill (or deactivate) existing organisms, provide a
residual, be inexpensive, not create harmful byproducts, and be safe for the environment
Chlorine Gas

Effective and inexpensive, widely use in the US

Some taste and odor from residual

Leads to formation of trihalomethanes (THMs), which are carcinogenic, when it comes
into contact with organic compounds
Calcium Hypochlorite

Commonly used for disinfection in swimming pools, but sometimes used in small
treatment plants

Somewhat more expensive than gaseous chlorine

Works like chlorine gas and has similar benefits and problems
Chlorine Dioxide

Increasing in popularity because it forms fewer THMs

Some problems – must be produced on-site, explosive at elevated temperatures or when
exposed to light or organics, contains chlorine gas as an impurity leading to THM
formation
AGSM 337/BAEN 465
Potable Water Treatment
Page 5 of 5
Chloramines
 Formed from chlorine and ammonia
 Fewer problems with THMs
 Not as effective as chlorine gas, so higher concentrations must be used
Ozone

Fairly common in Europe

Benefits – no associated taste or odor (reverts back to O2 quickly), no THMs

Drawbacks – no residual disinfectant, must be produced on-site, expensive (requires a lot
of energy to produce)

Anticipated to have increased use in the US in the future, especially if chlorine
compounds are banned

May be used with low amounts of chlorine to provide residual
Chick-Watson Relationship

Can be used to determine the contact time necessary to achieve a desired level of
pathogen inactivation
N
 e  Ct
N0
Where

N = concentration of microorganisms at time t, count/mL
N0 = initial concentration of microorganisms, count/mL
λ = coefficient of specific lethality, L·min/mg
C = disinfectant concentration, mg/L
t = contact time, min
Note, normally you will want a % reduction in microorganism population
N/N0 is the fraction remaining, so 1-N/N0 would be the fraction reduction
Example
Determine the time required to obtain a 99% inactivation of the Polio virus using hypochlorite.
The specific lethality is 0.5 L·min/mg and the hypochlorite concentration is 0.5 mg/L.
The desired reduction rate is 99% or 0.99.
The fraction remaining, N/No, would be 1-0.99 = 0.01
N
 0.01  e ( 0.5)( 0.5)t
No
t = 18.42 min