Section 9.
5 Surface Water Treatment Processes 207
Wells
Flow measurement
pH adjustment
Oxidant/disinfectant
Antiscalant
Catridge filters or
microscreens
Reverse osmosis
Concentrate to disposal
modules
Base and corrosion inhibitor
Oxidant/disinfectant and fluoride
Aeration
Clear well
High-service pumps Figure 9.4
Flow diagram of reverse
osmosis water treatment
To distribution system plant.
removing color and natural organic matter (NOM), which are precursors for disin-
fection and disinfection by-products (DDBPs). Figure 9.4 presents a flow diagram
of a typical reverse-osmosis (RO) WTP equipped for hardness removal. One major
drawback to using any type of membrane process is the disposal of concentrate gen-
erated during treatment. Regulatory agencies are sometimes unwilling to permit
deep-well injection of these waste streams.
9.5 SURFACE WATER TREATMENT PROCESSES
This section describes major unit operations and processes used for treating surface
water for drinking. The coagulation process is discussed first. Then, examples are
given to illustrate the design procedure for the following systems: mixing, floccula-
tion, water softening, sedimentation, filtration, backwashing, and filter sizing. Disin-
fection and treatment of water treatment plant residuals are briefly introduced.
9.5.1 Coagulation and Flocculation
Coagulation is a unit process consisting of the addition and mixing of a chemical
reagent (coagulant or flocculant) to destabilize colloidal and fine solids suspended
in water. Figure 9.5 is a schematic of the coagulation process. Flocculation involves
slow stirring or gentle agitation to promote agglomeration of the destabilized par-
ticles formed during coagulation, so that heavy, rapid-settling flocs are formed.
The destabilized particles are subsequently removed through sedimentation and
filtration.
208 Chapter 9 Water Treatment
Coagulation process Solids/Liquid separation
Coagulant Direct Filtration
Rapid Flocculation
mixing Agglomeration Settling Filtration
Figure 9.5 Destabilization
Schematic of coagulation
process. Sludge
Colloidal Particles
A large portion of the solids in surface water and groundwater cannot be removed by
sedimentation because they are colloidal particles. Colloidal particles are extremely
small and have negligible mass and large surface area per unit volume. Colloidal parti-
cles are defined by their size, generally ranging from 1 nm 110-9 m2 to 1 mm 110-6 m2.
Because of their large surface area, colloidal particles tend to acquire a negative sur-
face charge.
Turbidity in water is caused primarily by colloidal and suspended matter such
as clay, silt, nonliving organic particles, plankton, and microbes. Clays are a major
portion of natural turbidity. Kaolinite, montmorillonite, and other clay suspensions
can be removed by adding alum.
Color in water is usually attributed to colloidal forms of iron and manganese
or to organic compounds (humic and fulvic acids) resulting from the decay of veg-
etation. Coagulation is effective in removing color and many organic macromole-
cules and particles from water. Iron (III) and aluminum (III) salts are effective at
removing humic acids, but not as effective with fulvic acids.
Coagulation and filtration readily remove algae and amoebic cysts and can
achieve bacterial removals of 99%. Toxic organic compounds such as polychlori-
nated biphenyls (PCB) and dichloro-diphenyl-trichloro-ethane (DDT) and many
inorganic toxic materials are adsorbed, and thus removed, on naturally occurring
inorganic and organic particulates. Removal of the particulates removes the toxic
substances.
Electrical Double Layer
According to the double-layer electrical theory, a negative colloidal particle will be
surrounded by a layer of counter positive ions known as the fixed or Stern layer.
Surrounding this is another layer consisting primarily of counter-ions and co-ions
called the Diffuse layer. Figure 9.6 is a schematic diagram of the double layer. The
electrical potential decreases as the distance from the colloidal particle increases.
The electrical potential at the colloid surface is called the Nerst potential and at the
surface of shear it is called the zeta potential. The electrical potential drops linearly
across the Stern layer and exponentially from the Stern layer through the Diffuse
layer on into the bulk solution. The greater the magnitude of the zeta potential, the
greater the stability of the colloidal particles.
Colloidal Destabilization
Stability of the colloidal particles depends both on the repulsive forces associ-
ated with the electrical charge of the double layer and on the attractive forces
associated with van der Waals forces. If the electrostatic charge on the particles
Section 9.5 Surface Water Treatment Processes 209
Fixed or stern
layer ! __ Diffuse layer
!
__ !
! - ! __
- -
! !- -!
- - - !
! !
__ ! __
Colloid __ !
!
Plane of shear
Electrical
potential
Zeta potential
Figure 9.6
Schematic of the electrical
Distance from surface double layer.
can be reduced or neutralized, the attractive van der Waals forces cause the desta-
bilized colloids to agglomerate or coalesce when particles get close enough
together.
Chemicals called coagulants or flocculants are added to water for destabilizing
colloidal particles. Destabilization can be accomplished by one of four methods:
1. lowering the surface potential by compressing the double layer;
2. lowering the surface potential by adsorption and charge neutralization;
3. enmeshment in precipitate; and
4. adsorption and interparticle bridging.
Coagulants decrease the magnitude of the zeta potential by compressing
the double layer or by adsorption and charge neutralization. At certain coagu-
lant dosages, precipitates will form, and the colloidal particles will either serve
as condensation nuclei for the precipitates or become enmeshed in the precipi-
tate. The addition of polymers results in floc formation due to interparticle
bridging. Overdosing of coagulants and polymer can lead to restabilization of
the colloidal suspensions.
Coagulants
Aluminum and iron salts are the major types of coagulants used in water treatment,
primarily aluminum sulfate, ferrous sulfate, ferric chloride, ferric sulfate, and sodium
aluminate. Alkalinity is consumed with the addition of inorganic metallic coagulants.
Stoichiometric equations showing the formation of hydroxide precipitates and the
consumption of alkalinity for the various coagulants may be found elsewhere (MWH,
2005; Viessman and Hammer, 2005). Only aluminum sulfate will be discussed in this
section, since its low cost makes it the coagulant of choice in the United States
(MWH, 2005).
Aluminum sulfate or filter alum 3Al2 1SO423 # 14 H2O4 is the major coagulant
used for coagulating turbidity in surface waters. Alum readily dissolves in water, with
sulfate ions (SO 2"
4 ) being dispersed throughout the liquid. Aluminum ions exist in
water as hydrated ions. There are no free aluminum ions in the form of Al3+. The sim-
plest hydrated form of aluminum is Al(H 2O)3! 6 . Other forms of aluminum include:
Al1OH2 , Al 21OH22 , Al7(OH)17 , Al13(OH)5!
2+ 4+ 4! 1"
34 , and Al(OH) 4 . Al(OH)
2!
and
-
Al1OH24 are called monomers, since they have only one Al atom. Al1OH231S2 is a
solid, amorphous, gelatinous precipitate that forms. Each mg/L of alum decreases
210 Chapter 9 Water Treatment
water alkalinity by 0.50 mg/L as CaCO3 and produces 0.44 mg/L of CO2 (Viessman
and Hammer, 2005).
Alum causes coagulation in two ways: adsorption and charge neutralization
or “sweep coagulation.” Adsorption and charge neutralization results when posi-
tively charged aluminum monomers and polymers are adsorbed onto negatively
charged colloids, neutralizing the charge. The alum dose required is less than the
solubility limit of the metal hydroxide. For “sweep coagulation,” a sufficient alum
dose is added such that the solubility limit of the metal hydroxide is exceeded,
allowing the precipitation of Al1OH23 . Colloidal matter and suspended matter
become covered with a gelatinous, sticky sheath, causing them to settle out of
solution.
Coagulant Aids
Coagulant aids include acids and bases that may be added to the water to maintain a
specific pH. Alkalinity is often added in the form of lime (CaO), soda ash 1Na2CO32,
or sodium hydroxide (NaOH) to increase the buffering capacity of the water. Recall
that alkalinity is consumed when inorganic metallic coagulants are used. Therefore,
during water treatment, engineers must design chemical feed and storage facilities
to provide for alkalinity addition during treatment. Other coagulant aids include
anionic, cationic, or nonionic polymers, powdered activated carbon (PAC), and clays.
PAC is added for removing color and the sorption of specific organic compounds.
Clays may be added to provide turbidity or targets for coagulants. Clays should be
added before alum addition. Polymers are normally added after alum or coagulant,
if they are used together.
Polymers are long chains of small subunits or monomers. Polymers may be
cationic or positively charged 1 + 2, anionic or negatively charged 1 - 2, or non-ionic
(neutral). The charge is related to functional groups and pH. Major functional groups
associated with polymers include carboxyl (COOH); amines 1NH3+2, and hydroxyl
1OH-2. Polymers must be adsorbed onto the turbidity particles. Charge neutraliza-
tion normally occurs with low-molecular-weight polymers. High-molecular-weight
polymers are used to “bridge” between particles that would otherwise repel each
other. Polymer dosages are normally 6 1 mg/L. Polymers are ineffective at very low
turbidities; therefore, alum or clay is often added to the water to provide targets or
nuclei for precipitation reactions to occur. If used, the targets should be added prior
to adding the polymer.
The quantity of coagulant that must be added to a particular type of water
must be determined through experimentation. This may be accomplished by per-
forming jar tests in the laboratory using a 6-gang stirring mechanism. Optimum dose
is dependent on pH, temperature, turbidity, and alkalinity. Figure 9.7 is a photo of a
typical jar test apparatus.
9.5.2 Mixing
Mixing is the process whereby chemicals are added and instantaneously dispersed
into the water. Mixing of coagulants has primarily been accomplished by mechani-
cal units. Mechanical mixing is often called rapid or “flash” mixing, since the deten-
tion times used are generally less than 2 minutes and more frequently on the order
of 20 to 30 seconds. Actual chemical reaction times are usually less than 1 second.
Rapid-mix tanks are either square or circular. Stator blades and/or baffles along the
sides of the walls should be used to prevent vortexes from forming. The design of
rapid-mix units is based on detention time and the velocity gradient, G. Detention
