1.

Introduction

1.1 Objectives of water treatment

• The principal objective of water treatment is to provide
potable water that is chemically and biologically safe for
human consumption. It should also be free from unpleasant
tastes and odors.
• water treatment objective is to produce both "potable" and
"palatable".
- Potable: - Water that can be consumed in any desired
amount without concern for adverse heath effects. Potable
dose not necessarily mean that the water tastes good.
- Palatable: - it is a water that is pleasing to drink but not
necessarily safe.

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 1. Introduction…cont’d

1.1 Objectives of water treatment..Cont’d

• Water treatment aims at producing water that satisfies
a set of drinking water quality standards at a reasonable
price to the consumers.
• Removal of solids in water. Solids maybe suspended,
dissolved or colloidal. Some of the dissolved solids
should stay in water at healthy concentrations.

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 1.3 Water treatment methods
1.3.1 Unit operations and Unit processes
Water treatment plants utilize many treatment
processes to produce water of a desired quality.
These processes fall into two broad divisions:-
A) Unit operations: (UO)
Removal of contaminants is achieved by physical
forces such as gravity and screening.
B) Unit processes (UP)
Removal is achieved by chemical and biological
reactions.

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 1.3 Water treatment methods
1.3.2 Most common treatment methods
• Coagulation and flocculation (UP)
• Softening (UP)
• Reverse osmosis RO (UP)
• electrodialysis (UP)
• ion exchange (UP)
• adsorption (UO)
• Precipitation (UP)
• disinfection (UP)
• sedimentation (UO)
• filtration (UO

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 1.4 Water Treatment Plants
1.4.1 Most common water treatment plants
Water treatment plants can be classified as:-
A) Simple disinfection:-
It is a direct pumping and chlorine injection. Used to treat
high quality water.
B) Filtration plants: (surface water)
• Removes: color, turbidity, taste, odor, and bacteria (filtration
plant)
• if the source water has better quality with lower solids,
flocculation and sedimentation can be omitted, this
modification is called direct filtration.
C) softening plants:- (ground water)

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 Surface Water Treatment

Sedimentation
Screen basin
Surface water
from supply
Rapid Flocculation
Rapid Mix Basin
Sand Filter Sludge
Disinfection To
Distribution
Storage System
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 Groundwater Treatment
Sedimentation
basin
Ground water
from wells
Rapid Flocculation
Recarbo- Mix Basin Sludge
nation
Disinfection
To Distri-
Storage bution
CO2 System

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 Treatment Flow Chart
PAC Coagulant
Capacity: 132 million m3/day

Chlorine Flash Flocculation Sedimentation

injectionStabilization mix tank tank
Intake tank

Reservoir

Filter

Finished water
storage tank Chlorine
injection
Clear well
Household Pump
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 Conventional Water Treatment Process

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 Sedimentation

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 Sedimentation Tank

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 Circular Clarifiers

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 Filtration

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 Flocculator

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2. WATER TREATMENT PROCESS

2.1 Process selection factors. The design of treatment facilities will be determined by
feasibility studies considering all engineering, economic, energy and environmental
factors. All legitimate alternatives will be identified and evaluated by life cycle cost
analyses. Additionally, energy use between candidate processes will be considered. For
the purpose of energy consumption, only the energy purchased or procured will be
included in the usage evaluation. All treatment process systems will be compared with a
basic treatment process system, which is that treatment process system accomplishing
the required treatment at the lowest first cost. Pilot or laboratory analysis will be used in
conjunction with published design data of similar existing plants to assure the optimal
treatment. It is the responsibility of the designer to ensure that the selected water
treatment plant process complies with Federal Environmental Agency, State or local
regulations, whichever is more stringent.

2.2 Preliminary treatment. Surface waters contain fish and debris which can clog or
damage pumps, clog pipes and cause problems in water treatment. Streams can
contain high concentrations of suspended sediment. Preliminary treatment processes
are employed for the removal of debris and part of the sediment load.

2.2.1 Screens.
2.2.1.1 Coarse screens or racks. Coarse screens, often termed bar screens or racks,
must be provided to intercept large, suspended or floating material. Such screens or
racks are made of l/2-inch to 3/4-inch metal bars spaced to provide 1- to 3-inch
openings.
2.2.1.2 Fine screens. Surface waters require screens or strainers for the removal of
material too small to be intercepted by the coarse rack. These may be basket-type in-
line strainers manually or hydraulically cleaned by backwashing, or of the traveling type
which are cleaned by water jets. Fine-screen clear openings should be approximately
3/8 inch. The velocity of the water in the screen openings should be less than 2 feet per

© J. Paul Guyer 2010 5

second at maximum design flow through the screen with minimum screen
submergence.
2.2.1.3 Ice clogging. In northern areas screens may be clogged by frazil or anchor ice.
Exposure of racks or screens to cold air favors ice formation on submerged parts and
should be avoided to the maximum practicable extent. Steam or electric heating,
diffusion aeration and flow reversal have been used to overcome ice problems.
2.2.1.4 Disposal of screenings. Project planning must include a provision for the
disposal of debris removed by coarse and fine screens.

2.2.2 Flow measurement. Water treatment processes (such as chemical application)

are related to the rate of flow of raw water. Therefore, it is essential that accurate flow-
rate measurement equipment is provided. Pressure differential producers of the Venturi
type are commonly used for the measurement of flow in pressure conduits. An
alternative selection for pressure conduits is a magnetic flow meter if the minimum
velocity through the meter is 5 feet per second or more. A Parshall flume can be used
for metering in open channels. Flow signals from the metering device selected should
be transmitted to the treatment plant control center.

2.2.3 Flow division. While not a treatment process, flow division (flow splitting) is an
important treatment plant feature that must be considered at an early stage of the
design. To ensure continuity of operation during major maintenance, plants are
frequently designed with parallel, identical, chemical mixing and sedimentation facilities.
No rigid rules can be given for the extent of duplication required because a multiplicity of
factors influences the decision. Normally, aerators are not provided in duplicate. Pre-
sedimentation basins may not require duplication if maintenance can be scheduled
during periods of relatively low raw water sediment load, or if the following plant units
can tolerate a temporary sediment overload. If it is determined that presedimentation is
essential at all times for reliable plant operation, then the flow division should be made
ahead of the presedimentation basins by means of identical splitting weirs. The weirs
should be arranged so that the flow over either weir may be stopped when necessary.
During normal operation, the weirs would accomplish a precise equal division of raw

© J. Paul Guyer 2010 6

water into parallel subsequent units (rapid-mix, slow-mix and sedimentation), regardless
of flow rate. The water would then be combined and distributed to the filters. If
presedimentation units are not provided, then the flow is commonly split ahead of the
rapid-mix units. If a single treatment train is to be provided initially with the expectation
of adding parallel units in the future, then the flow-splitting facilities should be provided
as part of the original design, with provision for Mocking flow over the weir intended to
serve future units.

2.2.4 Sand traps. Sand traps are not normally required at surface water treatment
plants. Their principal application is for the removal of fine sand from well water. The
presence of sand in well water is usually a sign of improper well construction or
development. If sand pumping cannot be stopped by reworking the well, the sand must
be removed. Otherwise, it will create serious problems in the distribution system by
clogging service pipes, meters, and plumbing. Centrifugal sand separators are an
effective means of sand removal. These cyclone-separator devices are available,
assembled from manufacturers and require no power other than that supplied by the
flowing water. They operate under system pressure; therefore, repumping is not
necessary. Water from the well pump enters tangentially into the upper section of the
cone and centrifugal force moves the sand particles to the wall of the cone. They then
pass downwater into the outlet chamber. Sand is periodically drained to waste from this
chamber through a valve that can be manually or automatically operated. The clarified
water is discharged from the top of the cone. These units are available in diameters of
6, 12, 18, 24, and 30 inches providing a capacity range from 15 to 4,500 gallons per
minute (gpm) and are suitable for operation up to 150 pounds per square inch (psi).
Pressure drop through the unit ranges from 3 to 25 psi, depending on unit size and flow
rate. These separators will remove up to 99 percent of plus 150 mesh sand and about
90 percent of plus 200 mesh. The units are rubber-lined for protection against sand
erosion.

2.2.5 Plain sedimentation. Plain sedimentation, also termed “presedimentation”, is

accomplished without the use of coagulating chemicals. Whether plain sedimentation is

© J. Paul Guyer 2010 7

essential, is a judgment decision influenced by the experience of plants treating water
from the same source. Water derived from lakes or impounding reservoirs rarely
requires presedimentation treatment. On the other hand, water obtained from notably
sediment-laden streams, such as those found in parts of the Middle West, requires
presedimentation facilities for removal of gross sediment load prior to additional
treatment. Presedimentation treatment should receive serious consideration for water
obtained from rivers whose turbidity value frequently exceeds 1,000 units. Turbidity
values of well over 10,000 units have been observed at times on some central U.S.
rivers.
2.2.5.1 Plain sedimentation basins. Plain sedimentation or presedimentation basins
may be square, circular, or rectangular and are invariably equipped with sludge removal
mechanisms.
2.2.5.2 Design criteria. Detention time should be approximately 3 hours. Basin depth
should be in the approximate range of 10 to 15 feet, corresponding to upflow rates of
600 to 900 gallons per day (gpd) per square foot for a detention period of 3 hours.
Short-circuiting can be minimized by careful attention to design of inlet and outlet
arrangements. Weir loading rates should not exceed approximately 20,000 gpd per foot.
Where presedimentation treatment is continuously required, duplicate basins should be
provided. Basin bypasses and overflows should also be included.

2.3 Aeration. The term “aeration” refers to the processes in which water is brought
into contact with air for the purpose of transferring volatile substances to or from water.
These volatile substances include oxygen, carbon dioxide, hydrogen sulfide, methane
and volatile organic compounds responsible for tastes and odor. Aeration is frequently
employed at plants treating ground water for iron and manganese removal.

2.3.1 Purpose of aeration. The principle objectives of aeration are:

2.3.1.1 Addition of oxygen to ground water for the oxidation of iron and
manganese. Ground waters are normally devoid of dissolved oxygen. The oxygen
added by aeration oxidizes dissolved iron and manganese to insoluble forms which can
then be removed by sedimentation and filtration.

© J. Paul Guyer 2010 8

2.3.1.2 Partial removal of carbon dioxide to reduce the cost of water softening by
precipitation with lime, and to increase pH.
2.3.1.3 Reduction of the concentration of taste-and-odor producing substances,
such as hydrogen sulfides and volatile organic compounds.
2.3.1.4 Removal of volatile organic compounds which are suspected carcinogens.

2.3.2 Types of aerators. Three types of aerators are commonly employed. These are:
waterfall aerators exemplified by spray nozzle, cascade, and multiple tray units;
diffusion or bubble aerators which involve passage of bubbles of compressed air
through the water; and mechanical aerators employing motor-driven impellers alone or
in combination with air injection devices. Of the three types, waterfall aerators
employing multiply trays are the most frequently used in water treatment processes.
The efficiency of multiple-tray aerators can be increased by the use of enclosures and
blowers to provide counterflow ventilation.

2.3.3 Design criteria.

2.3.3.1 Multiple-tray, tower aerators.
2.3.3.1.1 Multiple-tray aerators. Multiple-tray aerators are constructed of a series of
trays, usually 3 to 9, with perforated, slot or mesh bottoms. The water first enters a
distributor tray and then falls from tray to tray, then finally entering a collection basin at
the base. The vertical opening between trays usually ranges from 12 inches to 30
inches. Good distribution of the water over the entire area of each tray is essential.
Perforated distributors should be designed to provide a small amount of head,
approximately 2 inches on all holes, in order to ensure uniform flow. In aerators with no
provision for forced ventilation, the trays are usually filled with 2- to 6-inch media, such
as coke, stone, or ceramic balls to improve water distribution and gas transfer, and to
take advantage of the catalytic oxidation effect of manganese oxide deposits in the
media. The water loading on aerator trays should be in the range of 10 to 20 gpm per
square foot. Good and natural ventilation is a requirement for high efficiency. For
multiple tray aerators designed for natural ventilation, the following empirical equation
can be used to estimate carbon dioxide (CO2) removal:

© J. Paul Guyer 2010 9

supplies in conjunction with lime softening and for the removal of some VOCs. Surface
waters usually exhibit low concentrations of carbon dioxide, no hydrogen sulfide, and
fairly high dissolved oxygen. As a consequence, aeration is not required for the removal
or addition of these gases. However, surface waters contain higher levels of THM
precursors than groundwaters; and therefore, a need for aeration may arise to reduce
TTHM following chlorination. Water that is high in the bromine-containing THMs is
difficult to treat by aeration, so other methods of removal should be used; such as
coagulation and flocculation or contact with granular activated carbon.

2.4 Coagulation and flocculation. “Coagulation” means a reduction in the forces

which tend to keep suspended particles apart. “Flocculation” is the joining together of
small particles into larger, settleable and filterable particles. Thus, coagulation precedes
flocculation and the two processes must be considered conjunctively.

2.4.1 Purposes of coagulation and flocculation. Raw water supplies, especially

surface water supplies, often contain a wide range of suspended matter, including
suspended minerals, clay, silt, organic debris and microscopic organisms ranging in
size from about 0.001 to 1.0 micrometer. Small particles in this size range are often
referred to as “colloidal” particles. Larger particles, such as sand and silt, readily settle
out of water during plain sedimentation, but the settling rate of colloidal particles is so
low that removal of colloidal particles by plain sedimentation is not practicable.
Chemical coagulation and flocculation processes are required to aggregate these
smaller particles to form larger particles which will readily settle in sedimentation basins.
The coagulation-flocculation processes are accomplished step-wise by short-time rapid
mixing to disperse the chemical coagulant followed by a longer period of slow mixing
(flocculation) to promote particle growth.

2.4.2 Chemical coagulant. The most frequently used chemical coagulant is aluminum
sulfate (Al2 (SO4)3 14H2O). This aluminum coagulant is also called “alum” or “filter alum,”
and dissociates in water to form S04 = Al3+ ions and various aluminum hydroxide
complexes. Other aluminum compounds used as coagulants are potash alum and

© J. Paul Guyer 2010 13

peripheral speed of the mixing units should not exceed about 2.0 fps and provision
should be made for speed variation. To control short circuiting, 2 to 3 compartments are
usually provided. Compartmentation can be achieved by the use of baffles. Turbulence
following flocculation must be avoided. Conduits carrying flocculated water to
sedimentation basins should be designed to provide velocities of not less than 0.5 fps
and not more than 1.5 fps. Weirs produce considerable turbulence and should not be
used immediately following flocculation.

2.5 Sedimentation basins. Sedimentation follows flocculation. The most common

types of sedimentation basins in general use today are shown in Figures 2-1 and 2-2. A
recent innovation in clarifiers is a helical-flow solids contact reactor, consisting of an
aboveground steel conical basin. However, these aboveground basins require a high
head, and additional pumps may be required. A minimum of two basins should be
provided to allow one unit to be out of service for repair or maintenance. The design
must include arrangements that permit the use of a single basin when necessary.

2.5.1 Design criteria. The design of a sedimentation tank is based on the criterion as
listed in Table 2-1. The sedimentation basins should have adequate capacity to handle
peak flow conditions and to prevent excessive deteriorated effluent water qualities. The
above design data represent common conditions. Higher overflow rates may be used at
lime softening plants and at some plants employing upflow clarification units, as
indicated in the tables of Water Treatment Plant Design by ASCE, AWWA and CSSE.
Unusual conditions may dictate deviation from these general criteria. Detention time in
the range of 8 to 12 hours or more, provided in several stages, may be necessary for
treating highly turbid waters. On the other hand, conical clarifiers are more efficient in
softening and/or turbidity removal, and require a detention time of one hour or less. The
design data shall be examined by laboratory analysis or pilot plant studies especially for
larger plants.

© J. Paul Guyer 2010 16

2.5.3 Suspended solids contact basins. Basins of this type combine rapid-mixing,
flocculation, sedimentation and sludge removal in a single unit. Coagulation and
flocculation take place in the presence of a slurry of previously formed precipitates
which are cycled back to the mixing and reaction zone. Upflow rates at the point of
slurry separation should not exceed about 1.0 gpm per square foot for units used as
clarifiers following coagulation, and approximately 1.5 to 1.75 gpm per square foot for
units used in conjunction with lime softening.

2.6 Filtration. Filtration of water is defined as the separation of colloidal and larger
particles by passage through a porous medium; usually sand, granular coal, or granular
activated carbon. The suspended particles removed during filtration range in diameter
from about 0.001 to 50 microns and larger. Several different types of medium
arrangements and rates of flow through filters can be used. The filtration process most
commonly used is gravity filtration, but pressure filters and diatomite filters are used at
smaller installations. Recently high-rate filters have been developed which require less
space and have higher solids-loading capacity than conventional filters.

2.6.1 Rapid Sand Filters.

2.6.1.1 Filtration Rate. Rapid sand filters are those filters which commonly operate at
rates between approximately 2 and 8 gpm per square foot. The rate of filtration to be
employed at a specific plant can be determined only after careful consideration by the
designer of raw water quality and the efficiency of pretreatment that will consistently be
provided. Good quality water is not assured by low filtration rates. Adequate
pretreatment and filter design will allow application rates of up to 6 gpm per square foot
with little difference in water quality. It is emphasized that if high rates are to be used in
design, great care must be taken to ensure that all prefiltration treatment processes
including coagulation, flocculation, and sedimentation will perform satisfactorily and
consistently. High-rate filter operation definitely requires excellence in pre-filtration
treatment; especially in the case of surface waters. It is recommended that data from

© J. Paul Guyer 2010 19

backwash valves, storage tanks, or backwash pumps; therefore their operation is
greatly simplified.

2.7 Disinfection. Disinfection involves destruction or inactivation of organisms which

may be objectionable from the standpoint of either health or esthetics. Inasmuch as the
health of water consumers is of a principal concern to those responsible for supplying
water, design of facilities for disinfection must be carefully executed.

2.7.1 Chlorination. The application of chlorine to water is the preferred method of

disinfecting water supplies.
2.7.1.1 Definitions. Terms frequently used in connection with chlorination practice are
defined as follows:
2.7.1.1.1 Chlorine demand. The difference between the concentration of chlorine
added to the water and the concentration of chlorine remaining at the end of a specified
contact period. Chlorine demand varies with the concentration of chlorine applied, time
of contact, temperature, and water quality.
2.7.1.1.2 Chlorine residual. The total concentration of chlorine remaining in the water
at the end of a specified contact period.
2.7.1.1.3 Combined available residual chlorine. Any chlorine in water which has
combined with nitrogen. The most common source of nitrogen is ammonia, and
compounds formed by the reactions between chlorine and ammonia are known as
chloramines. The disinfecting power of combined available chlorine is about 25 to 100
times less than that of free available chlorine.
2.7.1.1.4 Free available residual chlorine. That part of the chlorine residual which has
not combined with nitrogen.

2.7.2 Chlorination practice.

2.7.2.1 Combined residual chlorination. Combined residual chlorination entails the
application of sufficient quantities of chlorine and ammonia (if ammonia is not present in
the raw water) to produce the desired amount of combined available chlorine
(chloramine) in water. If enough ammonia is present in raw water to form a combined

© J. Paul Guyer 2010 26

2.7.7.5 UV and Ozone. Recently there has been some experimentation in a combined
UV and ozone contactor. Results from these tests show promise; however, there is no
known water treatment plant operating with this method of disinfection.

2.8 Fluoride adjustment

2.8.1 Health effects. An excessive fluoride concentration will damage the teeth of
children using the water for extended periods. On the other hand, moderate
concentrations of 0.7 to 1.2 mg/L are beneficial to children’s teeth. Most natural waters
contain less than the optimum concentration of fluoride. Upward adjustment of the
fluoride concentration can be achieved by the application of a measured amount of a
fluoride chemical to the water.
2.8.2 Fluoridation chemicals. Chemicals most frequently used for fluoridation are
sodium silicofluoride, sodium fluoride, and fluosilcic acid. For a particular installation,
the choice of chemical will depend principally on delivered cost and availability.
2.8.2.1 Sodium fluoride. This chemical is commercially available as a white crystalline
powder having a purity of 95 to 98 percent. (Sometimes it is artificially colored nile blue.)
Volubility is approximately 4 percent at 770°F. The pH of a saturated solution is 6.6. The
100 percent pure material contains 45.25 percent fluoride. It is available in 100 pound
bags, 125 to 400 pound drums, and bulk.
2.8.2.2 Sodium silicofluoride. This compound is commercially available as a white
powder with a purity of 98 to 99 percent. Volubility is only about 0.76 percent at 770°F.
The pH of a saturated solution is 3.5. The 100 percent material contains 60.7 percent
fluoride. It is available in 100 pound bags, 125 to 400 pound drums, and bulk.
2.8.2.3 Fluosilicic acid. This chemical is commercially available as a liquid containing
22 to 30 percent by weight of fluosilicic acid. It is sold in 13 gallon carboys, 55 gallon
drums, and in bulk. The 100 percent pure acid contains 79.2 percent fluoride. The pH of
a 1 percent solution is 1.2, and the use of fluosilicic acid as a fluoridation agent in a
water of low alkalinity will significantly reduce the pH of the water. It should not be used
for fluoride adjustment of waters of this type unless pH adjustment is also provided.

© J. Paul Guyer 2010 33

2.8.5 Fluoride removal. Fluoride removal can be accomplished by passage of the
water through beds of activated alumina, bone char, or tri-calcium phosphate. When the
capacity of the bed to remove fluoride is exhausted, it can be regenerated by treatment
with a caustic soda solution followed by rinsing and acid neutralization of the residual
caustic soda. Other methods of fluoride removal include electrodialysis, reverse
osmosis and ion exchange. Some fluoride reduction can be obtained by water softening
using excess lime treatment. Fluoride reduction by this method is associated with
magnesium precipitation and the extent of fluoride removal is a function of the amount
of magnesium precipitated from the water. All removal processes produce liquid wastes
and suitable provision must be made for their disposal. Guidance as to the fluoride
removal process to be employed can be obtained from laboratory studies of process
effectiveness and fluoride removal capacity, using samples of the water that is to be
treated.

2.9 Taste and odor control. Most tastes and odors in surface water are caused by
low concentrations of organic substances derived from decomposing vegetation,
microscopic organisms, sewage and industrial waste pollution, etc. Treatment for taste
and odor removal involves destruction of the odorous substance by chemical oxidation
or its removal by aeration, adsorption or activated carbon.

2.9.1 Chemical oxidation. Chemical oxidizing agents which have been found effective
and which can be used in the treatment of potable water are chlorine, chlorine dioxide,
potassium permanganate, and ozone. No single chemical is completely effective under
all operating conditions.

2.9.2 Aeration. Aeration is helpful in eliminating odor caused by hydrogen sulfide, but is
ineffective in significantly reducing odor associated with dissolved organics.

2.9.3 Absorption. Powdered activated carbon is commonly used for removal of tastes,
odor and color by adsorption. The carbon can be applied to the water at any point in
the treatment plant prior to filtration, but it is usually advisable to apply it early in the

© J. Paul Guyer 2010 35

treatment process to prolong contact. For maximum effectiveness, carbon should be
applied well ahead of chlorine, and preferably in advance of lime softening. The influent
to a presedimentation basin is normally an effective carbon application point. Powdered
carbon dosages usually range from 5 to 10 mg/L, but as much as 50 mg/L may be
required. The use of powdered activated carbon adds more suspended solids and
increases the amount of sludge; thereby creating a sludge disposal problem. Powder
activated carbon is marginally effective in reducing TTHMs. Granular activated carbon
(GAC) has also been used for taste and odor removal. It has been employed as a
separate treatment step in the form of carbon columns and as a substitute for sand in
the filtration process. Used this way, the granular carbon serves in a dual capacity as a
filtration medium and for taste and odor removal. Granular activated carbon is also
excellent at reducing TTHMs. Granular activated carbon must be reactivated on a
regular basis to keep its absorptive abilities. Because of the cost of reactivation of GAC,
other methods of taste-and-odor control and reduction of TTHMs should be considered.
Aeration is generally more cost effective than GAC contractors.

2.10 Softening. Whether water softening is provided will depend entirely on the type of
project and the uses of the water to be made. Two general types of processes are used
for softening: The “lime-soda ash” process and the “cation-ion exchange” or “zeolite”
process.

2.11 Iron and manganese control.

2.11.1 Occurrence of iron and manganese. Dissolved iron and manganese are
encountered principally in groundwaters devoid of dissolved oxygen. Normal,
oxygenated surface waters do not contain significant concentrations of these metals;
however, stagnant water found in the bottom of thermally-stratified reservoirs
sometimes contain dissolved iron and manganese. Their presence in solution is
associated with anaerobic conditions near the bottom of the reservoir.

2.11.2 Effects of iron and manganese. Dissolved iron in excess of 1 or 2 mg/L will
cause an unpleasant taste, and on standing, the water will develop a cloudy

© J. Paul Guyer 2010 36

appearance. Iron concentrations appreciably greater than 0.3 mg/L will cause red
stains on plumbing fixtures and laundry. Similarly, manganese will cause black stains if
present to the extent of more than about 0.05 mg/L. Deposits of iron and manganese
can build up in water distribution systems, and periodic “flushouts” of these deposits
result in objectionable color and turbidity at the consumer’s tap.

2.11.3 Removal by oxidation and filtration. Oxidation can be accomplished with

dissolved oxygen added by aeration and by the addition of an oxidizing chemical, such
as chlorine, chlorine dioxide, potassium permanganate, or ozone. Manganese is more
difficult than iron to oxidize and precipitate. In the absence of manganese, iron can often
be removed with minimum treatment consisting of aeration followed by direct filtration.
In general, aeration alone will not oxidize manganese unless the pH is raised to about
9.5. Strong oxidants, such as chlorine or potassium permanganate, are effective at
lower pH values. To ensure oxidation, precipitation and agglomeration of iron and
manganese and their essential complete removal, at least three treatment steps are
usually necessary: aeration, contact time, and filtration. An aerator containing trays of
coke, limestone, etc., is commonly used. Reaction time is provided by a contact or
contact-sedimentation basin having a detention period of at least 30 minutes. Filtration
is accomplished by conventional single or multimedia filters designed for a filtration rate
of at least 3.0 gpm per square foot. The aeration step is frequently supplemented by a
chemical oxidant such as chlorine or permanganate. Flocculation is advantageous in
the contact basin; particularly if iron exceeds about 2 mg/L.

2.11.4 Removal by ion exchange. Under proper conditions, the cation exchange
(sodium zeolite) softening process is capable of removing limited amounts of dissolved
(unoxidized) iron and manganese. For application of this process, it is essential that the
raw water and wash water contain no dissolved oxygen and that the sum of the iron and
manganese concentrations not exceed about 0.5 mg/L. The presence of oxygen or
higher concentrations of iron and manganese will cause rapid fouling of the exchange
resin with consequent loss of removal capacity. If fouling occurs, treatment of the resin

© J. Paul Guyer 2010 37

with sodium bisulfite solution and dilute hydrochloric or sulfuric acid will be required to
restore capacity.

2.11.5 Removal by lime-soda softening. Lime-soda softening is an effective means

of removing both iron and manganese.

2.11.6 Stabilization of iron and manganese. Under some circumstances, stabilization

of iron and manganese by application of a polyphosphate compound may be
acceptable. The iron and manganese in the water are maintained in a dispersed state
through the completing action of a polyphosphate compound. Dosages of about 5 mg/L
of sodium hexametaphosphate for each mg/L of iron and manganese are reasonably
effective; however, the total polyphosphate dosage should not exceed 10 mg/L. The
polyphosphate stabilizing compound must be added to the water prior to chlorination. If
the chlorine is applied first, it will oxidize the iron and manganese to insoluble forms
rendering the stabilizing agent ineffective. Stabilization of concentrations of iron and
manganese in excess of approximately 1.0 mg/L is generally not satisfactory. Also,
stabilization will not persist if the water is heated, because heating converts
polyphosphates to orthophosphates which have no stabilizing power. Although helpful,
stabilization is not a substitute for iron and manganese removal, and in general, should
be viewed as a temporary expedient to be used pending installation of permanent
removal facilities.

2.12 Corrosion and scale control. “Corrosion” can be defined as the deterioration of
metal by direct chemical or electrochemical reaction with its environment. “Scale” refers
to an accumulation of solids precipitated out of the water. In water treatment, corrosion
and scale are closely associated. Both must be considered in connection with the
design and operation of treatment works. This scale may be desirable because it can
provide a measure of protection against corrosion. However, thick layers of scale are
detrimental in both hot and cold water systems. It is essential to produce “balanced”
water that is neither highly corrosive nor excessively scale forming.

© J. Paul Guyer 2010 38