FLUORIDATION
ENGINEERING
Do not WEED. This document
should be retained in the EPA
Region 5 Library Collection.
ENVIRONMENTAL PROTECTION AGENCY
OFFICE qp WATER
HAZARDOUS MATERIALS
WATER SUPPLY DIVISION
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Fluoridation Engineering Manual
by
Ervin Bellack
ENVIRONMENTAL PROTECTION AGENCY
Office of Water and Hazardous Materials
Water Supply Division
1972
Reraint 1974
U.S. Invironmentai Protection Agency
He^ion 5, Library (PL-12J)
77 West Jackson Boufevard, 12th Ftooi
, !L 60604-3590
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Preface
The fluoridation of water supplies has been termed one of the most signifi-
cant health advances in recent years. This manual is intended to assist local
and state engineers in designing fluoridation installations, and water plant
personnel in operating them, so that the fullest advantage of the benefits of
fluoridation can be achieved.
No matter how well a fluoridation installation is designed, it is ultimately
the "man at the water plant" who insures its successful operation by his aware-
ness of the importance of his task and by his conscientious adherence to the
principles of good water plant practice. This manual is particularly dedicated
to his assistance.
The addition of fluorides to water supplies involves, more than the addition
of other chemicals, particular emphasis on accurate feed rates. The philosophy,
"if a little is good, more will be better," or the false economy in feeding less
than the optimum amount, have no place in the practice of fluoridation. The
optimum concentrations have been arrived at by countless studies involving
millions of people, and these same studies have shown that failure to conform
to the optimum concentration defeats the whole purpose of "controlled"
fluoridation. It is hoped that this manual will not only place the desired
stress on this aspect, but will provide assistance to the operator in maintaining
that optimum concentration.
The planner of a fluoridation installation should carefully consider the
options open to him, among them the choice of chemical and thus the choice
of equipment. The quantity of water pumped may affect these choices, as will
considerations of economy, convenience, and location. While it is the intention
of this manual to provide information on the criteria governing the selection of
an optimal system, there is no intent to take the place of expert advice where
such is needed.
Ervin Bellack
Chemist
Office of Water and Hazardous Materials
Water Supply Division
Environmental ftotection Agency
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Contents
I INTRODUCTION 1
Natural Fluoridation 1
Blending 2
Controlled Fluoridation 3
II COMPOUNDS USED IN CONTROLLED FLUORIDATION ... 4
Sodium Fluoride 4
Fluosilicic Acid 5
Sodium Silicofluoride 6
Other Fluoride Chemicals 7
III FEEDERS USED FOR ADDING FLUORIDES 9
Solution Feeders 9
Dry Feeders 12
Testing Procedures for Dry Feeders 18
Checking Particle Size 19
Auxiliary Equipment 19
Meters 20
Scales 20
Softeners 21
Mixers 21
Dissolving Tanks 22
Flow Meters 24
Day Tanks 24
Bag Loaders 25
Dust Collectors and Wet Scrubbers 25
Alarms 25
Vacuum Breakers 25
Hoppers - 26
Weight Recorders 26
Controllers 27
Eductors 27
Pumps 27
Timers 28
Hopper Agitators 28
Flow-Splitters 28
IV PREPARATION OF FLUORIDE SOLUTIONS 31
Manual Technique 31
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Automatic Devices 33
Calculations Involving Solutions 42
V SELECTING THE OPTIMAL FLUORIDATION SYSTEM 47
Location of Feeder 47
Fluoride Injection Point 50
Type of Feeder and Chemical 51
Chemical Availability 54
Chemical Storage and Handling 56
Cross-Connection Considerations 57
Automatic Proportioning (Pacing) 57
Installation 59
Solution Feeder Installations 59
Dry Feeder Installations 59
Valves and Meters 60
Installation Plans 60
VI CONTROL AND SURVEILLANCE 63
Analytical Procedures 63
The Standard Methods 63
Alizarin Visual Method 64
SPADNS Method 64
Electrode Method 65
Test Kits 66
Sampling 66
Calculations and Record-Keeping 66
Monitors 68
Trouble-Shooting 72
Low Fluoride Readings 73
High Fluoride Readings 73
Varying Fluoride Readings 74
Other Problems 75
VII MAINTENANCE 77
Cleaning and Lubrication 77
Spare Parts 77
Inspection and Re-Calibration 77
Leaks 78
Precipitates 78
Storage Area 79
Laboratory 79
VIII SAFETY AND HAZARDS IN HANDLING FLUORIDE
CHEMICALS 81
Safety Equipment and Chemical Handling 81
Ingestion 81
Inhalation 81
Safety Precautions 82
Acid Handling 82
VI
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First Aid 82
Fluoride Exposure Symptoms 82
Treatment 83
IX TECHNICAL PROBLEMS ATTRIBUTED TO
FLUORIDATION 85
Corrosion •. . . 85
Encrustations 85
Fluoride Losses 86
Compatibility 86
Tastes and Odors 87
Industrial Processes 87
Environmental Effects 87
X REFERENCES AND SUGGESTED READING 89
APPENDIX ABBREVIATIONS 93
vn
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Illustrations
Figure page
1 Positive-displacement Solution Feeders 10
2 Screw-Type Volumetric Dry Feeder 13
3 Roll-Type Volumetric Dry Feeder 14
4 Belt-Type Gravimetric Dry Feeder 16
5 Gravimetric Dry Feeder - "Loss-in-Weight" Type 17
6 Typical Arrangements of Fluoride Feeders and
Auxiliary Components 29
7 Typical Arrangement of Dry Feed Hoppers and
Dust Collectors 30
8 BIF Downflow Saturator 35
9 W & T Downflow Saturator 36
10 Precision Upflow Saturator 37
11 Checking Delivery Rate of a Solution Feeder 44
12 Fluoridation Check-List 48
13 Acid Feed Installation 51
14 Solution Feed Installation 52
15 Diluted Acid Feed System 53
16 Dry Feed Installation with Volumetric Feeder 54
17 Dry Feed Installation with Gravimetric Feeder 55
18 Fluoridation Nomograph 70
19 Fluoridation Alignment Chart 71
Table
1 Characteristics of Fluoride Compounds 8
2 Detention Time of Sodium Silicofluoride in Dissolving Tanks 23
3 Preparation of Solutions of Sodium Fluoride 34
4 Recommended Maximum Feed Rates for Downflow Sodium
Fluoride Saturator 38
5 Specific Gravity of Fluosilicic Acid Solutions at 17.5°C ... 45
6 Fluoride Calculation Factors 69
Vlll
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Chapter I
INTRODUCTION
Although the dental profession can claim the credit for first ascribing
dental mottling to some unknown constituent of drinking water, it was a
water plant chemist who first implicated the element fluorine. Initially, it was
thought that if fluorine was indeed the element responsible in some way for
tooth discoloration, it was a deficiency rather than an excess which brought
about the mottling. As analytical techniques were refined and data began to
accumulate, the true role of fluoride in water was revealed. Now we know that
a deficiency of fluoride can lead to extensive tooth decay, and that an excess
is the cause of mottling, or as it is now known, dental fluorosis.
The element, fluorine, ranks thirteenth in abundance in the earth's crust,
and twelfth in the oceans. It is also thirteenth in abundance in the human
body, and although the concentrations vary widely, it is found in every
water supply used by man for drinking purposes. Because of the wide distri-
bution of the element, the difficulty in attempting to grow plants or animals
without it and its role in the formation of human bones and teeth, fluorine (as
the fluoride ion) is now considered to be essential to the normal growth and
development of man.
Fluorine, like chlorine a gaseous halogen, is never found in the free state but
always occurs in combination with other elements as fluoride compounds.
In water solution, these compounds dissociate into ions, and it is these fluoride
ions which are analytically determined and are the-form in which fluorine is
assimilated by man. The fluoride compounds most commonly found in the
earth's crust are fluorspar and apatite, calcium fluoride and a complex calcium
fluoride-phosphate, respectively. However, in water only the fluoride ion (F~)
can be detected without more than an educated guess as to the nature of the
compound from which the ion was derived. Thus, since there is no way one
fluoride ion can be distinguished from another, there is no difference between
fluoride ions dissolved from the earth's crust and fluoride ions occurring in
water due to the deliberate addition of fluoride compounds by man.
NATURAL FLUORIDATION
After the cause of dental fluorosis was ascertained, and the concomitant
observation was made that dental caries was relatively absent in the presence of
fluorosis, it followed logically that these observations should be verified by
examining the teeth of children in many areas and analyzing the drinking
water supplies in those areas. The examination of the teeth of many thousands
of children, and the fluoride analysis of hundreds of water supplies showed a
1
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remarkable relationship between the concentration of waterborne fluoride and
the incidence of dental caries. The relationship, or actually three distinct
relationships, are as follows:
1. When the fluoride level exceeds about 1.5 ppm, any further increase
does not significantly decrease the incidence of decayed, missing or filled
teeth, but does increase the occurrence and severity of mottling.
2. At a fluoride level of approximately 1.0 ppm, the optimum occurs —
maximum reduction in caries with no aesthetically significant mottling.
3. At fluoride levels below 1.0 ppm some benefits occur, but caries
reduction is not so great and gradually decreases as the fluoride levels
decrease until, as zero fluoride is approached, no observable improvement
occurs.
Since, as noted above, all water supplies contain measurable amounts of
fluoride, it can be said that all water supplies are fluoridated. However, since
only those water supplies containing fluoride concentrations in excess of 0.7
ppm (in the continental United States) have appreciable dental significance,
ordinarily these are the ones we refer to as being naturally fluoridated.
Thus, fluoridation is not something new, as many people suppose, but is
actually a process performed by nature for many centuries. What we now
refer to as "controlled fluoridation" is only man's attempt to overcome the
vagaries of nature in her random and sometimes inefficient process of putting
fluoride ions in our drinking water.
BLENDING
It was actually many years after the role of fluoride in water was first
determined that the idea of imitating the natural process was first suggested,
and the onset of World War II resulted in another delay before the first
demonstration project could begin. During the interval, hundreds of studies
were undertaken for the purpose of verifying the fluoride-caries relationships
and for determining the validity of proposed experimental controlled fluori-
dation of municipal water supplies.
Although it is generally accepted that the first controlled fluoridation
projects began in 1945 in the cities of Grand Rapids, Michigan, Newburgh,
New York, and Brantford, Ontario, it was several years earlier that at least one
city, without attendant fanfare, began efforts to adjust the fluoride content of
its water supply in order to bring the level to the optimum for dental benefits.
The feat was accomplished by blending the water from two sources — one
providing water with a natural fluoride content above the optimum, the other
providing water deficient in natural fluoride content. Since then, other cities
have adopted the blending process, either for raising the fluoride level in the
major source of water by adding water from a high-fluoride well, or for
reducing the overall fluoride level by adding water from a low-fluoride source
to an existing high-fluoride well supply. Unfortunately, the achievement of
controlled fluoridation by the blending of water from two sources is limited
to those areas where high-fluoride waters occur. When blending is possible, the
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water consumers gain more than dental benefits and economic advantages —
they also conserve the supply of drinking water by making use of quantities of
water which otherwise would not be considered acceptable.
CONTROLLED FLUORIDATION
The third type of fluoridation, in addition to natural and that achieved
by blending, is the one in which the fluoride content of a water supply is
adjusted by the deliberate addition of a chemical compound which provides
fluoride ions in water solution. This type of fluoridation, beginning with the
three cities mentioned above, is now practiced in approximately 5,000
communities serving over 80 million persons. With the residents of almost 3,000
additional communities consuming water containing at least 0.7 ppm fluoride
from natural sources, this means that 56% of the nation's population on public
water supplies (90 million persons) had access to water with a dentally signi-
ficant concentration of fluoride as of 1970.
The graduation of controlled fluoridation from an experimental procedure
into an established water plant practice has not been without problems. Few,
if any, of these problems were of an engineering nature, for the controlled
addition of a chemical to a water supply can hardly be considered something
new. For the most part, adding fluoride is just like adding any of several
different chemicals commonly added as part of the usual water plant
practices. The feeding equipment is in general the same equipment used for
feeding alum, soda ash, lime, or other chemicals. Only the nature of the chemi-
cal and the purpose for which it is added are different, and therein lies the
problem.
For some reason, fluoridation has aroused a controversy out of all propor-
tion to the simple premise of adjusting the concentration of a mineral in
which a particular water is deficient. Even the premise is not new - waters
which are deficient in alkalinity or hardness are often supplemented with
appropriate compounds at the water plant and no one seems to mind.
Apparently, it is just the word, "fluoride," which has stirred the imagination
of so many concerned citizens. The confusion with the word, "fluorine," is
understandable for fluoridation was originally termed, "fluorination," and
fluorine, of course, is a toxic and highly reactive gas. But the controversy goes
beyond this, to such areas as mass medication, legal rights, and a host of others
too numerous to mention. It may be of interest to point out that chlorination
received a similar reception, and that resistance to this type of water treatment
still exists today.
As far as the water plant operator or engineer need be concerned, the
addition of fluoride to a water supply is well within his province, and it is his
duty to follow the directives of the health officials and governing body of his
community in not just adding fluorides, but in doing the job right
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Chapter II
Compounds Used In
Controlled Fluoridation
SODIUM FLUORIDE
Theoretically, any compound which forms fluoride ions in water solution
can be used for adjusting upward the fluoride content of a water supply.
However, there are several practical considerations involved in selecting com-
pounds. First, the compound must have sufficient solubility to permit its use
in routine water plant practice. Second, the cation to which the fluoride ion is
attached must not have any undesirable characteristics. Third, the material
should be relatively inexpensive, and readily available in grades of size and
purity which are suitable for the intended use.
The first fluoride compound used in controlled fluoridation was sodium
fluoride, selected not only on the basis of the above criteria, but also because
its toxicity and physiological effects had been so thoroughly studied. In
addition, sodium fluoride is the reference standard used in measuring fluoride
concentration. Once fluoridation became an established practice, other com-
pounds came into use, but sodium fluoride, because of its unique physical
characteristics in addition to its other advantages in some situations, is still
one of the most widely used chemicals.
Sodium fluoride is a white, odorless material available either as a powder or
in the form of crystals of various sizes. Its formula weight is 42.00, specific
gravity 2.79, and its solubility practically constant at 4.0 grams per 100
milliliters of water at temperatures generally encountered in water-treatment
practice. The pH (hydrogen-ion concentration) of solution varies with the type
and amount of impurities, but solutions prepared from the usual grades of
sodium fluoride exhibit pH's near neutrality. It is available in purities ranging
from 90 to over 98 percent, the impurities consisting of water, free acid or
alkali, sodium silicofluoride, sulfites and iron, plus traces of other substances.
Powdered sodium fluoride is produced in different densities, the light
grade weighing less than 65 pounds per cubic foot and the heavy grade
weighing about 90 pounds per cubic foot. A typical sieve analysis of powdered
sodium fluoride shows 99 percent through 200 mesh and 97 percent through
325 mesh. Crystalline sodium fluoride is produced in various size ranges,
usually designated as coarse, fine and extra-fine, but some manufacturers can
furnish lots in specific mesh sizes. The crystalline type is preferred when manual
handling is involved, since the absence of fine powder results in a minimum of
dust.
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Sodium fluoride is manufactured by Allied Chemical Corporation, Industrial
Chemicals Division; J.T. Baker Chemical Company; Chemtech Corporation;
and Olin Chemicals, but is usually sold through distributors who are located in
most cities. In 1970, sodium fluoride sold for 18 to 25 cents per pound, F.O.B.
point of manufacture, the price depending largely on the quantity purchased.
Normal packing is in 100 Ib. multi-ply paper bags or fiber drums holding up to
400 pounds.
Sodium fluoride has a number of other industrial uses, one of which
requires that the material be tinted a blue color. Some state regulations specify
that only tinted sodium fluoride be used in order to distinguish it from other
water-treatment chemicals.
FLUOSILICIC ACID
Fluosilicic Acid, also known as hydrofluosilicic, hexafluosilicic, silicofluoric,
or even "silly" acid, is a 20 to 35% aqueous solution of H2SiF6 with a formula
weight of 144.08. It is a colorless (when pure), transparent, fuming, corrosive
liquid having a pungent odor and an irritating action on the skin. Upon
vaporizing, the acid decomposes to form hydrofluoric acid and silicon tetra-
fluoride, and under conditions where an equilibrium between the fluosilicic
acid and its decomposition products exist, such as at the surface of strong
solutions, etching of glass will occur. All solutions of fluosilicic acid exhibit a
low pH, and even at a concentration low enough to produce 1 ppm of
fluoride ion there can be a significant depression of the pH of poorly buffered
waters. (Example: Water containing 30 ppm IDS, pH 6.5. After H2SiF6 was
added to produce 1 ppm F, pH dropped to 6.2.)
Fluosilicic acid is manufactured by two different processes, resulting in
products having differing characteristics. The largest proportion of the acid is a
by-product of phosphate fertilizer manufacture, and this type is relatively
impure and seldom exceeds 30% strength. A smaller amount of acid is pre-
pared from hydrofluoric acid and silica, resulting in a purer product at a
slightly higher strength. Acid prepared from phosphate rock contains colloidal
silica in varying amounts, and while this is of little consequence when the acid
is used as received, dilution results in the formation of a visible precipitate of
the silica. Some suppliers of fluosilicic acid sell a "fortified" acid, which has
had a small amount of hydrofluoric acid added to it to prevent the formation
of the precipitate. Acid prepared from hydrofluoric acid and silica does not
normally form a precipitate when it is diluted.
Fluosilicic acid is produced by many fertilizer manufacturers, but only a
limited number of these recover the acid for sale as such. Among the latter
group are Stauffer Chemical Company, Fertilizer Division; U.S. Industrial
Chemicals Company, Division of National Distillers and Chemical Corporation
(through Conservation Chemical Company of Illinois); USS Agri-Chemicals
Division of U.S. Steel Corporation; Agrico Chemical Company, Division of
Continental Oil Company; W.R. Grace, Agricultural Products Division; Sobin
Chemicals, Inc.; and Kerr-McGee Chemical Corporation. Harshaw Chemical
Company, Division of Kewanee Oil Company, produces acid made by the
hydrofluoric acid-silica process.
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Since fluosilicic acid contains a high proportion of water, shipping large
quantities can be quite expensive. Larger users can purchase the acid directly
from the manufacturers in bulk (tank car or tank truck) lots, but smaller users
must obtain the acid from distributors who usually pack it in drums or
polyethylene carboys. In 1970, fluosilicic acid in bulk sold for $51.00 to
$58.00 per ton, 23% basis, F.O.B. point of manufacture. Smaller quantities
sold for 8 to 15 cents per pound, 30% basis. This method of pricing comes
about due to the variable characteristics of the acid as it comes from fertilizer
plants. Rather than attempt to adjust the acid strength to some uniform
figure, producers sell the acid as it comes, and the price is adjusted to com-
pensate for acid strength above or below the quoted figure. Note that the
"23% basis" type of pricing applies only to bulk quantities. It is the usual
practice for the supplier to furnish assay reports of the acid strength of each
lot.
Like other fluoride compounds, fluosilicic acid has a number of industrial
uses, one of which is in the manufacture of hydrofluoric acid, and thus in an
indirect way, other fluoride compounds.
SODIUM SILICOFLUORIDE
Fluosilicic acid can readily be converted into various salts, and one of
these, sodium silicofluoride, is the most widely used chemical for water
fluoridation. Undoubtedly, the principal reason for the popularity is one of
economics, for sodium silicofluoride is the cheapest of the compounds
currently in use. The conversion of fluosilicic acid, essentially a low-cost
by-product which contains too much water to permit economical shipping, to a
dry material containing a high percentage of available fluoride, results in a com-
pound having most of the advantages of the acid with few of its disadvantages.
Once it was shown that silicofluorides form fluoride ions in water solution as
readily as do simple fluoride compounds, and that there is no difference in
physiological effects, the silicofluorides (and fluosilicic acid) were rapidly
accepted for water fluoridation, and in some cases displaced the use of sodium
fluoride.
Sodium silicofluoride is a white, odorless crystalline powder. Its molecular
weight is 1 S8.06 and its specific gravity is 2.679. Its solubility varies from 0.44
grams per 100 milliliters of water at 0° Centigrade to 2.45 grams per 100 mini-
liters at 100°C. The pH's of solutions are definitely on the acid side, saturated
solutions usually exhibiting a pH between 3.0 and 4.0. Sodium silicofluoride
is available in purities of 98% or better, the principal irnpuiities being water,
chlorides and silica.
Sodium silicofluoride is sold in two commercial forms — regular and fluffy.
The former has a density of about 85 pounds per cubic foot while the latter
has a density of about 65 pounds pei cubic foot. A typical sieve analysis of the
regular grade shows more than 99% through a 200-mesh sieve and more than
10% through a 325-mesh sieve. For best feeding characteristics, other size
specifications may be selected, experience having shown that a low moisture
content plus a relatively narrow si/e distribution results in a material which is
handled better by dry feeders.
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Sodium silicofluoride is manufactured by Agrico Chemical Company,
Division of Continental Oil Company; Kerr-McGee Chemical Corporation; Olin
Chemicals; Tennessee Corporation, (Cities Service), Industrial Marketing
Division, and possibly other manufacturers of fluosilicic acid. Considerable
quantities of the material are imported, and numerous distributors handle it. In
1970, prices ranged from 8 to 10 cents per pound F.O.B. point of manufacture.
Sodium silicofluoride is normally packed in bags and drums similar to those
used for sodium fluoride. Blue-tinted material is sometimes available.
OTHER FLUORIDE COMPOUNDS
Ammonium silicofluoride, magnesium silicofluoride, potassium fluoride and
calcium fluoride (fluorspar) either are being or have been used for water
fluoridation, and at one time hydrofluoric acid was also used. Each has
particular properties which make the material desirable in a specific application,
but none of these have wide-spread application.
Ammonium silicofluoride has the peculiar advantage of supplying all or
part of the ammonium ion necessary for the production of chloramines when
this form of disinfectant is preferred to chlorine in a particular situation.
Magnesium silicofluoride and potassium fluoride have the advantage of
extremely high solubility, of particular importance in such applications as
school fluoridation when infrequent refills of the solution container are
desired. In addition, potassium fluoride is quite compatible with potassium
hypochlorite, so a mixture of the two solutions can be used for simultaneous
fluoridation and chlorination.
Calcium fluoride (fluorspar) is the cheapest of the compounds ever used
for fluoridation but it is also the least soluble. It has been successfully fed by
first dissolving it in alum solution, and then utilizing the resultant solution to
supply both the alum needed for coagulation and the fluoride ion. Some
attempts have been made to feed fluorspar directly in the form of ultra-fine
powder, on the premise that the powder would eventually dissolve or at least
remain in suspension until consumed.
Hydrofluoric acid, although low in cost, presents too much of a safety and
corrosion hazard to be acceptable for water fluoridation, although it has been
used in a specially designed installation.
A number of other fluoride compounds have been suggested for use in watei
fluoridation, among them ammonium and sodium bifluoride. These latter
materials have advantages of solubility and cost, but their potential corrosive-
ness has hindered acceptance.
The chemical and physical characteristics, costs, shipping containers,
storage requirements, etc., of the three commonly-used fluoride compounds
are summarized in Table 1. The selection of a compound for a particular
application can be based at least in part on the information in this table, but
such intangible factors as personal preference often influence the ultimate
decision.
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The manufacturers of the various fluoride compounds can provide technical
data and specification sheets which will assist the prospective purchaser in
selecting the appropriate grade of chemical for the type of feeder used.
Another helpful adjunct is a sieve shaker and a set of testing sieves, so the user
of a dry chemical can identify a product which feeds to his satisfaction and so
he can then specify that product in future purchases. Sieve shakers sell for less
than $100 (for the hand-powered models) up to over $500 (for motor-driven
models equipped with timers). Sieves are variable in price, depending on such
factors as material of construction and size of openings. A sieve shaker and set
of sieves will find many other applications in a water plant, ranging from
grading filter sand to testing many of the treatment chemicals.
Table 1. CHARACTERISTICS OF FLUORIDE COMPOUNDS
Hem
Form
Molecular Weight
Commercial Purity, %
Fluoride Ion, %
(100% pure material)
Lb. required per MG for 1.0
ppm F at indicated purity
pH of Saturated Solution
Sodium Ion contributed at
1.0 ppm F, ppm
Storage Space, cu. ft. per
1000 Ib. F ion
Solubility g per 100 g Water,
at 25°C
Weight Ib pei cu ft
Cost:
Cents per Ib
Cents per Ib available F
Shipping containers
Sodium Fluoride
NaF
Powder or Crystal
42.00
90-98
45.25
18.8 (98%)
7.6
1.17
22-34
4.05
65-90
18-25
41-57
100-lb bags
125-400-lb
fiber drums, bulk
Sodium Silicofluoride
Na2SiF6
Powder or
Very Fine Crystal
188.05
98-99
60.7
14.0 (98.5%)
3.5
0.40
23-30
0.762
55-72
8-10
13-17
100-lb bags
125-400-lb
fiber drums, bulk
Fluosilicic Acid
H2SiF6
Liquid
144.08
22-30
79.2
35.2 (30%)
1.2 (1% solution)
0.00
54-73
Infinite
10.5 Ib/gal (30%)
2'/2 - 15
14-63
13-gal carboys
55-gal drums, bulk
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Chapter III
Feeders Used For Adding Fluorides
A chemical feeder, such as the type used for adding fluorides or other
substances to a water supply, is a mechanical device which measures out
quantities of the chemical and administers them to the water at the pre-set
rate. Feeders are designated as either solution or dry types, depending on
whether the chemical is measured as volumes of solution or as volumes or
weights of dry chemical. In either case, the chemical is in the form of a solution
when it is introduced into the water. Dry feeders are further sub-divided into
two categories, volumetric and gravimetric, depending on whether the chemical
is measured by volume or weight. It should be again noted that fluoride feeders
are not something new, but are the same as feeders which have been used for
many years for measuring and administering chemicals not only in water
plants, but also in many industrial applications.
SOLUTION FEEDERS
In general, a solution feeder is nothing more than a small pump, of which
there are almost unlimited varieties. For feeding fluoride solutions, almost
every type which has ever been used for feeding other water treatment
chemicals has fJund application, with at most only minor modification in
construction details.
If there is indeed any requirement for a fluoride solution feeder which
distinguishes it from feeders for other purposes, it is accuracy and constancy
of delivery, for the optimum fluoride level has been prescribed between very
narrow limits and thus requires that the fluoride be added in precise proportion
to the quantity of water being treated. This requirement favors the so-called
positive-displacement pump or feeder, defined as a feeder which delivers a
specific volume of liquid for each stroke of a piston or rotation of an impeller.
Of course, very few feeders deliver replicate volumes under all conditions, for
such factors as pressure and viscosity can affect the volume displaced by the
driving member of the pump. However, by using fluoride solutions of fixed
strength and by feeding against a fixed pressure, the positive displacement
feeder has shown sufficient reliability for this purpose.
For delivery against pressure, there are two general types of solution
feeders — the piston feeder and the diaphragm feeder. In the former type, a
reciprocating piston alternately forces solution out of a chamber and then,
on its return stroke, refills the chamber by pulling solution from a reservoir. In
the latter type, a flexible diaphragm driven either directly or indirectly by a
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STROKE-POSITION
CONNECTING ROD
ECCENTRIC
OUTLET
INLET
DIAPHRAGM
POSITIVE-DISPLACEMENT
DIAPHRAGM PUMP
POSITIVE-DISPLACEMENT PLUNGER PUMP
CAM AND PISTON
GEAR
VANE SLIDING
VANE SWINGING
Figure 1. Positive Displacement Solution Feeders
10
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mechanical linkage performs a similar function. When the mechanical drive for
the feeder is an electric motor, a gear box or system of belts and pulleys
determines the number of strokes in a given time interval. Pneumatic or
hydraulic drives are also available, and these permit the use of a meter con-
tactor to provide stroking which is in direct proportion to water flow instead
of being at a fixed rate.
For gravity feed, there are several types of solution feeders which operate
on the paddle-wheel principle. A rotating wheel equipped with small buckets
dips solution from a constant-head tank and discharges the solution into the
water to be treated. Rate of feed can be varied by changing the size of the
buckets or their number, by changing the rate of rotation of the wheel, or by
varying the proportion of the bucket contents which is emptied.
Ordinarily, such solution feeding devices as centrifugal pumps, pot feeders,
or head tank and orifice are not used for fluoridation because of their relative
inaccuracy. However, in addition to the piston and diaphragm feeders
mentioned above, there are several types of rotary pumps which qualify as
positive-displacement feeders. These include gear, swinging-vane, sliding-vane,
oscillating screw, eccentric and cam pumps and various modifications of
these. Figure 1 illustrates various types of solution feeders.
The criteria used in selecting a feeder are capacity, corrosion resistance,
pressure capability, and of course accuracy and durability. A point to consider
is that most feeders perform most accurately near mid-range of both stroke
length and stroking frequency and should be selected accordingly. At the
extremely low feed rates required by small installations, the diaphragm feeders
are superior to the piston types. Most feeders come equipped with plastic
heads and resilient check-valves, both of which are satisfactory for fluoride
solutions unless the pressure is over 100#/sq. in. For higher pressures,
corrosion-resistant alloys, such as 316 stainless steel or Carpenter 20 alloy, are
required for feeder head construction.
Since most solution feeders are adjustable both for stroke length, which
determines the volume of liquid delivered per stroke, and stroke frequency,
usually expressed in strokes-per-minute (SPM), both factors should be consid-
ered in selecting the size of feeder for a particular application.
An example of feeder selection follows:
Problem: Select a solution feeder for the following application:
Water flow - 200 gpm @ 75 psi
Fluoride source — saturator (produces a 4% sodium fluoride solution,
18,000 ppm as F-)
Desiied fluoride level — 1.0 ppm
Calculated solution feed rate: Rj XCj = R2 X C2
where R! = water rate, in gal/min
Ci = fluoride level, in ppm
R2 = solution feed rate, in gal/min (the unknown quantity, in this
case)
C2 = solution strength, in ppm
11
-------
then: 200 gal/min X 1.0 ppm = (x) X 18,000 ppm
_ 200 gal/min X 1.0 ppm = 0.011 gal/min
W= 18,000 ppm
0.011j£X60min = 0.67 gal
mm ~EF nr
Feeders available:
Manufacturer A, Model 1203, three-step pulley drive.
Delivery rate: @ 13 SPM (Strokes per Minute), 0.02 - 0.3 gph @ 100 psi
@ 26 SPM, 0.04 - 0.6 gph
@ 46 SPM, 0.06-1.06 gph
Manufacturer B, Model 5701-111, single speed (37.5 SPM)
Delivery rate: 0.5 - 5 gph maximum
Manufacturer C, Model 12000, electronic stroking control (3 - 72 SPM)
Delivery rate: 0.01 -1.6 gph
Selection: The required delivery rate falls within the range of all three feeders,
so all are possibly acceptable. However, the delivery rate would require the
highest stroke frequency of the feeder from Manufacturer A, a situation
which, while not unacceptable, is not preferred. Similarly, the delivery rate is
too close to the minimum of the feeder from Manufacturer B to be completely
satisfactory. The feeder from Manufacturer C appears to be the best choice,
since the delivery rate is approximately in the middle of its range. A further
investigation into the feeder characteristics should be made in order to ascertain
the combination of output per stroke and stroke frequency which would be
required, and to verify that neither of these is near the extremes of the feeder
capability.
In addition to the feeder specifications and operating data contained in
manufacturers' bulletins, assistance in feeder selection can be obtained directly
from most manufacturers, some of whom supply a questionnaire which, when
completed by the prospective purchaser, provides detailed information for
more thorough evaluation of requirements.
DRY FEEDERS
A dry feeder is a device for metering a dry, powdered chemical at a predeter-
mined rate. It can be based on volume or weight measurements, the former type
being known as a volumetric while the latter is known as a gravimetric feeder.
Generally, volumetric dry feeders are simpler, less expensive, deliver slightly
smaller quantities and are slightly less accurate. Gravimetric dry feeders are
capable of delivering extremely large quantities in a given time period, are
extremely accurate, more expensive, and are readily adapted to recording and
automatic control. By attaching a weighing and controlling mechanism to a
volumetric feeder, it is possible to convert one to a gravimetric type.
12
-------
MOTOR
GEAR REDUCER
FEED RATE
REGISTER AND FEED
ADJUSTING KNOB
SOLUTION
CHAMBER
HOPPER
ROTATING AND
RECIPROCATING
FEED SCREW
VACUUM BREAKER
WATER INLET
JET MIXER
Figure 2. Screw-Type Volumetric Dry Feeder
-------
MOTOR-DRIVEN
AGITATOR
FLOAT
HOPPER
GUIDE VANES
FEED SLIDE
FEED ROLLS
TO DISSOLVING CHAMBER '
Figure 3. Roll-Type Volumetric Dry Feeder
14
-------
Volumetric dry feeders are available in several types, distinguished by the
means used for measuring and delivering the dry chemical. Among the types
are the rotating roller, rotating disc, rotating and/or reciprocating screw, star
wheel, moving belt, vibratory pan, oscillating hopper and combinations of
these principles. See Figures 2 & 3 for illustrations of some of the types of
volumetric dry feeders.
There are two general types of gravimetric dry feeders — those based on
loss-in-weight of the feeder hopper and those which are based on the weight of
material on a section of moving belt. Many gravimetric dry feeders also
incorporate some of the features of volumetric feeders, in that they have a
rotary feed mechanism between the hopper and the weighing section or use a
mechanical vibrator to move chemicals out of the hopper. Since ultimately it is
the weight of material per unit of time that is measured and regulated, such
variables as material density or consistency have no effect on feed rate. This
accounts for the extreme accuracy of which these feeders are capable. Gravi-
metric dry feeders are illustrated in Figures 4 and 5.
The stream or ribbon of dry material which is discharged from a dry feeder
falls into a dissolving chamber where it is dissolved in water (see the section on
Auxiliary Equipment). The solution thus prepared either falls by gravity into an
open flume or clear well, or is transferred by a continuously-running pump or
eductor into a pressure main.
The choice between a volumetric or gravimetric feeder is largely governed
by size and economics. The gravimetrics are more expensive but have greater
capacity, and are also basically more accurate. The selection of one of the
types of volumetric feeders involves several factors, among them size, cost,
accuracy, and personal preference.
Dry feeders are designed to handle powdered materials, but not all such
materials are handled with equal ease. For example, material which is too fine
will flow like a liquid right through the measuring mechanism ("flooding").
Some materials will form an arch in the hopper and when the arch collapses,
emit a cloud of dust and then flood the feeder. Other powders, either hydro-
scopic by nature (water-attracters) or produced with a significant moisture
content, tend to form lumps which will affect the feed rate or not be fed at all.
For most consistent feeding, a narrow size distribution and a low moisture
content is the best. A study of feeding problems with sodium silicofluoride
resulted in the development of a "feedability index" (F):
F = 100-(A+B+ IOC), where A is the percentage retained on a 100-mesh
sieve, B is the percentage passing through a 325-
mesh sieve, and C is the percentage of moisture.
In almost every case, a feedability index below 80 proved to be unsatisfactory,
80 - 90 good, 90-100 excellent.
Examples: A shipment of sodium silicofluoride was sampled and subjected
to particle-size analysis (see the section on particle size testing) with the
following results: 3% retained on 100-mesh sieve, 9% passed through the
15
-------
3
I'
•u
re3
I
ro
re
fr
FEED
SECTION
GATE
POSITIONER
GATE
YOKE
MAGNET
POISE
WEIGHT
SCALE BEAM
MERCURY
SWITCHES
WEIGH BELT
WEIGH DECK
FLEXURES
STATIONARY DECKS
-------
TRANSITION
HOPPER
HELIX TYPE
-FEEDER MECH.
VARIABLE RATE-
SPEED SET FROM
PHOTOCELL
CONTROL PANEL
RATE SETTER
-SCALE BEAM
VARIABLE SPEED
FEEDER MOTOR
Figure 5. Gravimetric Dry Feeder - "Loss-in-Weight" Type
325-mesh sieve. The moisture content was 0.05%. Applying the feedability
formula:
F = 100 - [3 + 9 + (10 X .05)] = 87.5
(Although all figures are percentages, the feedability index is an abstract
number.)
This material showed good feeding characteristics.
A second shipment of sodium silicofluoride, when sampled and tested
similarly, gave the following results: 1% retained on 100-mesh sieve, 20%
passed through the 325-mesh sieve. The moisture content again was 0.05%.
From the feedability formula:
F = 100 - (1 + 20 + 0.5) = 78.5
As could be expected, this material was troublesome, the excess of ultra-fine
powder (the portion passing through the 325-mesh sieve) causing arching,
flooding, and dust problems.
17
-------
The AWWA Standards for Sodium Silicofluoride (and Sodium Fluoride)
limit the moisture content of these materials, and although the particle sizes
are not precisely defined, the statement is made that the materials "shall be
suitable for feeding with a conventional dry-feed machine as used in water
treatment." In many cases, individual water plants have devised size specifica-
tions which assist them in purchasing chemicals handled adequately by the
feeders on hand. Ordinarily, chemicals specified as meeting AWWA Standards
produce minimal feeding problems, but such is not the case with some non-
standard or imported chemicals.
Other problems related to the operation of a dry feeder are mentioned in
the sections on auxiliary equipment and maintenance. As with solution feeders,
manufacturers' bulletins and direct technical assistance can be utilized in the
selection of appropriate equipment for a particular application. The criterion
used in selecting the size of a dry feeder is similar to that used in selecting a
solution feeder — they both operate most reliably when the delivery rate is near
mid-range. All too often, dry feeders are used on water supplies which are so
small that the feeder is operating at dead minimum or even below this rate, this
latter feat being accomplished by equipping the feeder with a cycle timer.
Obviously, accurate and uniform fluoride feed rates cannot be expected under
these conditions.
TESTING PROCEDURES FOR DRY FEEDERS
To determine the accuracy and reliability of a dry feeder, a small balance
or scales and a stop-watch, or a watch with a sweep-second hand, are required.
Insert a shallow pan or sheet of cardboard between the measuring mechanism
and dissolving chamber of the feeder while the feeder is operating, making sure
that all the chemical which feeds through will be collected. Collect the chemical
which is fed in several short periods, for example, five periods of five minutes
each. Weigh each of the amounts collected and the total. Provided the weigh-
ings and timings are accurate, the individual samples will indicate the uniformity
of feed, and the total will indicate the accuracy of feed rate.
Example: Weights of sodium silicofluoride collected in 5-minute periods:
35 grams
35
34
36
35
Total: 175 grams Average: 35 grams/minute
Uniformity: 35 i 1 gram in 5 minutes (about 3% variation).
^ . 35 grams X 60 min = 420 grams/hr or 0.925 Ibs/hr
Feed rate: g& . —r—
5 mm hr
The uniformity of feed in this case would be acceptable. If fluoride levels
are to be maintained within 10%, the feeder delivery rate should certainly be
maintained at the highest accuracy possible. Repeating a test, as above, but
with longer sampling periods, would tend to show smaller percentage of
variation, provided, of course, the feeder is in good order.
18
-------
CHECKING PARTICLE SIZE
Dry chemicals are often furnished in specified particle sizes, and to insure
freedom from handling or feeding problems, it can be helpful to determine the
degree of adherence to specifications. For example, crystalline sodium fluoride
for use in a down-flow saturator should be in the 20-60 mesh range, and sodium
silicofluoride should be relatively free from extremely coarse or extremely fine
particles for best feeding characteristics.
The size of particles is usually determined with a set of standard sieves,
the size designation then referring to the range of sieve sizes which best
describes those particles. For example, a 20-60 specification means that most
particles pass through a size 20 sieve but are retained by a size 60 sieve (the
larger the sieve number, the smaller the openings).
The testing procedure is determined somewhat by the apparatus used, but
generally a set of standard testing sieves is stacked, coarsest on top and progres-
sively finer toward the bottom, above a collecting pan. A weighed amount of
material is placed in the uppermost sieve, the set of sieves shaken gently
(mechanically) for about five minutes, and then the portion collected by each
sieve and the pan weighed separately.
Example: 100 grams of sodium silicofluoride, sieves:
100, 140, 200, 325, and pan.
Results: On 100 mesh 10 grams
on 140 25
on 200 30
on 325 30
in pan 5
Assuming the moisture content of this particular material was 0.05% or less,
applying the feedability formula would give the following results:
F= 100-(10 + 5 + 0.5) = 87.5
This result would indicate that the material should feed satisfactorily through a
dry feeder.
AUXILIARY EQUIPMENT
For the simplest system, one involving a manually prepared fluoride solution
and a proportioning pump feeding the solution into a water supply flowing at a
fixed rate, the requirements for auxiliary equipment are minimal. A dissolving
tank, either a paddle or electric mixer for stirring, a platform scale and the
feeder itself are all the equipment needed. A vacuum breaker, to prevent pulling
un-metered quantities of fluoride solution into the system in the event of a low-
pressure situation, can be incorporated into the design of the feeder. Extras
could include an alarm system for detecting and reporting low solution levels,
a softener for removing hardness constituents from the solution water, a small
meter for measuring the amount of water used in solution preparation, etc. As
the size and complexity of the fluoridation system grows, the number and
complexity of these pieces of auxiliary equipment increases, and their
desirability becomes transformed to necessity.
19
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METERS
The water meter, often absent in the smallest water plants, is one of the
primary requisites for accurate fluoride feeding. If the supply is un-metered,
calculation of fluoride feed rate will have to be by guesswork, and even the
selection of an appropriate feeder will have to be somewhat chancy. The type
of meter used for water flow depends largely on the flow rate; disc or piston
meters are used for low flows, and compound, propeller or magnetic meters
used for the higher flow rates. Unfortunately, meters are usually sold by pipe
size, not flow rate, so all too often the water meter is grossly oversized for the
flow rate through it. Since most meters are least accurate at the low end of
their measurement range, the result is that water flow is not accurately
measured. The remedy is to select a meter no larger than necessary to handle
the maximum flow rates expected, even if the pipe and meter sizes don't
match. In some cases this may involve the use of pipe reducers to adapt the
water main to the meter, a practice which is acceptable provided pressure loss
as a result of this arrangement is not excessive.
Other applications for water meters, besides the main supply meters dis-
cussed above, are on supply lines for solution make-up water. In the case of
the sodium fluoride saturator, a meter is a necessity, for without one it would
be impossible to calculate the fluoride feed rate. Since water usage rate in a
saturator installation is minimal, the meter must be the smallest available
(usually %").
SCALES
In any fluoridation installation, except one based on a sodium fluoride
saturator, scales are a necessity for either weighing the quantity of dry material
to be used in solution preparation, weighing the quantity of solution fed, or for
weighing the quantity of dry fluoride compound or fluosilicic acid delivered
by the appropriate feeder.
The type of scales can vary from a small household-type used for weighing
a pound or two of sodium fluoride to be used in solution preparation to the
complex built-in mechanism of a gravimetric dry feeder. The most generally
applicable type is the platform scale, on which can be placed a solution tank,
carboy of acid, or an entire volumetric dry feeder. Although the scales may be
specifically designed for the application, as are those supplied by manufacturers
with volumetric dry feeders, in many cases an ordinary hardware store type of
scales will be perfectly satisfactory. Some minor modifications, such as
removing the wheels or rotating the beam, may be necessary, but as long as
the scales have sufficient capacity and sensitivity there is no reason why they
cannot be used. Capacity and sensitivity are the only serious considerations,
and the points to remember are that the scale must be capable of weighing
the tank and its contents when full or the volumetric feeder and its hopper
when full, with measurements to the nearest pound or better. For small scales
used for measuring sodium fluoride to be used in manual solution preparation,
sensitivity to the nearest ounce should be sufficient.
20
-------
No particular problems should be encountered when mounting equipment
on platform scales, except when there is connection to a water line, or the
discharge line from the dissolving chamber of a volumetric dry feeder is fixed
in place. It should be remembered that this dissolving chamber, or solution pot,
is also mounted on the scale platform and thus all connections to it must be
flexible enough to permit the scale to operate.
SOFTENERS
When a fluoridation system involving the use of sodium fluoride solutions
is being considered, it should be remembered that, while sodium fluoride is
quite soluble, the fluorides of calcium and magnesium are not. Thus, the
fluoride ions in solution will combine with calcium and magnesium ions in the
make-up water and form a precipitate which can clog the feeder, the injection
port, the feeder suction line, the saturator bed, etc. For this reason, water used
for sodium fluoride dissolution should be softened whenever the hardness
exceeds 75 ppm, or even if the hardness is less than this figure but the amount
of labor involved in clearing stoppages or removing scale is objectionable.
Remember, the entire water supply need not be softened — only the water used
for solution preparation.
Two types of softening treatment are available - ion exchange and the use
of polyphosphates (calgon, micromet, etc.). Since the volume of water to be
softened is usually quite small, a household type of zeolite water softener is
usually adequate. This type of softener operates on the ion-exchange principle,
and can be installed directly in the pipeline used for solution make-up. When
softening capacity is exhausted, the zeolite (or synthetic resin) can be regener-
ated with brine made from common salt.
Polyphosphates can be used for sequestering (keeping in solution) calcium
and magnesium, the amount required usually amounting to 7 to 15 mg/1. The
polyphosphate may be added directly into the solution tank, but if an eductor
is used both the eductor water and the dissolving water should be treated.
(See the section on eductors.) In the latter case, some type of feeder will be
required for adding the polyphosphate.
MIXERS
Whenever solutions are prepared, whether it be manual preparation of
sodium fluoride solutions, dilution of fluosilicic acid, or the dissolution of the
output of dry feeder, it is particularly important that the solution be homo-
geneous. Slurries must not be tolerated in the feeding of fluorides, since
undissolved fluoride compound can go into solution subsequently, causing a
higher-than-optimum situation, or if the fluoride compound remains undis-
solved, a lower-than-optimum situation will result. Undissolved material can
also result in the clogging of feeders and other devices having small openings,
and if allowed to accumulate, results in considerable waste.
In the manual preparation of solutions, thorough mixing is a must. Even
when a solution is being diluted, as in the preparation of fluosilicic acid
21
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dilutions, the two liquids must be thoroughly mixed, since liquids of differing
specific gravities tend to stratify, and such stratification could result in feeding
a solution too concentrated, or at the other extreme, plain water. Sodium
fluoride is quite soluble, but even the preparation of the most dilute solutions
requires sufficient agitation. Undissolved material will remain in the bottom of
the dissolving tank while a too-dilute solution is being fed, and even if it
gradually dissolves, the strong solution formed at the bottom of the tank will
tend to remain in its own stratum.
While a paddle, accompanied by sufficient "elbow grease," (manual mixing)
will suffice for the preparation of the dilute solution, a mechanical mixer is
preferred. Mixers come in various sizes, with shafts and propellers made of
various materials. A fractional horse-power mixer with a stainless-steel shaft
and propeller will be satisfactory for sodium fluoride solutions, and a similar
mixer with a corrosion-resistant alloy or plastic-coated shaft and propeller will
handle fluosilicic acid.
The dissolution of sodium silicofluoride m the solution pot of a dry feeder
can be accomplished by a jet mixer, but again a mechanical mixer is preferred.
Because of the low solubility of sodium silicofluoride, particularly in cold
water, and the limited retention time available for dissolution, violent agitation
is a must to prevent the discharge of a slurry. Preferred materials of con-
struction are 316 stainless steel or plastic-coated steel.
DISSOLVING TANKS
The dry material discharged from a volumetric or gravimetric feeder is
continuously dissolved in a chamber beneath the feeder, from whence the
clear solution falls or is pumped into the water to be treated. This chamber,
variously referred to as the solution pot, dissolver tank, solution tank, or
dissolving chamber, may be a part of the feeder or a separate entity. While
some chemicals can be fed directly into flumes or basins without using a
dissolving tank, the fluorides are not among them. The necessity for accurate
feed rates will not permit the possibility of slurry feed or the formation of
build-ups of undissolved dry material.
Dissolving tanks come in sizes from 5 gallons up, the size often determined
by the size of the feeder under which they are mounted. If there is a choice,
the largest size available should be used for fluoride compounds. Mixing of the
chemical with water is accomplished by a system of baffles, and agitation can
be provided by a paddle driven by jets of water, or, as mentioned above, a
mechanical mixer. Experience has shown that the jet mixer is not nearly so
dependable as a good mechanical mixer, even under ideal conditions.
The failure to produce a clear, homogeneous solution discharge fiom the
dissolving tank of a dry feeder indicates that (1) the dissolving chamber is too
small, (2) the detention time is too shoit, (3) too little solution water is being
provided, (4) agitation is insufficient, or (5) diy chemical is short-circuiting and
is not Deing adequately mixed wiin (lie water.
-------
Detention time, the length of time the fluoride compound remains in the
dissolving tank, has been determined experimentally to be a minimum of five
minutes to provide a concentration which is one-fourth the maximum solu-
bility, provided the water temperature is above 60°F and the chemical is in
the form of a fine powder. If the chemical is in the form of crystals or the
water temperature is below 60°F, the dissolving time should be doubled; if
both, the time should be tripled (i.e. 15 minutes). Table 2 gives some relation-
ships between detention time, dissolving-chamber capacity, and water flow
rate for sodium silicofluoride dry feed rates.
Table 2. DETENTION TIME OF SODIUM SILICOFLUORIDE
IN DISSOLVING TANKS
Feed Rate
#/hr
1
2
3
4
5
6
7
8
9
10
20
Min. Water Flow Rate
Required for Solution
1 gpm
2
3
4
5
6
7
8
9
10
20
5
5 min
Dissolving Tank Size (Gal.)
10
10 min
5
25
25 min
12.5
^.3
6.2
5
50
50 mm
25
16.7
12.5
10
8.3
7.1
6.2
5.5
5
100
100 min
50
33.3
25
20
16.7
14.3
12.5
11.1
10
5
In order to use the table, read across (horizontally) from the feed rate which comes
closest to your specific application. The figure in the next column will be the
minimum water flow rate required for dissolving the chemical. The next figure, or
figures, will be the detention time for each given dissolving tank size. Where no
figure is given, the detention time for the particular tank size would have been less
than five minutes and would therefore be inadequate. Remember, if the water flow
rate is increased above the specified minimum, the detention time is decreased
proportionately, and a larger tank may be required.
Short-cii cutting is essentially a function of the dissolving tank design, and
is more likely to occur in the smaller size tanks. The remedy, if short-circuiting
does occur, is to add baffles to the tank so that the path of the chemical to the
outlet of the chambei is sufficiently circuitous to provide the necessary
detention time for solution.
Since the usual atrangement for a dissolving tank is to have the water inlet
below the outlet, a cross-connection requiring adequate safety measures exists.
If the dissolving tank is not already equipped with a coirectly-placed vacuum-
breaker, one should be installed on the water inlet as ncai as possible to the
point of entry. If there is a solenoid or manually operated valve on the water
inlet line, the vacuum breaker must be installed between the valve and the tank
for adequate cross-connection protection.
-------
FLOW METERS
A flow meter, in contrast to an ordinary water meter, measures rate of
flow rather than volume of flow. While those used for measuring flow rates in
large pipelines operate on various differential-pressure principles, the flow
meters applicable to small flows are usually based on the lifting of a spherical
or cylindrical "float" by the hydraulic action of a flowing fluid in a vertical
tube.
In a water plant where the water output is variable, as in cases where more
than one pump is used, a flow-meter on the main serves two purposes: it will
indicate the flow rates on which the fluoride feeder or feeders must operate,
and if so designed, will provide an electrical, pneumatic or hydraulic signal
which can be used to adjust the feeder output to correspond to changes in
water flow rate. This type of flow meter is discussed further in the section on
system selection.
The flow meter used in small pipelines, often known as a rotameter, finds
application on the water supply to the dissolving chamber of a dry feeder.
Since, as mentioned above, detention time is a function of water flow rate.
among other things, the flow must be regulated and maintained at the pre-
scribed figure.
The flow meter must be selected on the basis of pipe size, nature of fluid
(water) and particularly, the range of flows expected. For greatest accuracy,
the range of the flow meter should coincide with the range of flows which will
be encountered in the particular installation.
DAY TANKS
A day tank is just what the name implies — a tank which holds a day's supply
of a particular water treatment chemical. It is a convenient, and often necessary,
means for isolating the supply of fluoride solution which will be fed during one
day or shift at the water plant.
The day tank is a necessity when feeding fluosilicic acid, particularly if the
acid is received and stored in a large tank. In order to provide a record of the
weight of acid fed, a small quantity of the acid is pumped or siphoned into a
small tank mounted on a platform scale, and it is from this day tank that the
fluosilicic acid is fed into the water system. A similar arrangement can be used
for sodium fluoride solutions or fluosilicic acid dilutions. A large batch of the
solution or dilution can be prepared, and a smaller amount transferred to the
day tank mounted on the platform scale. This system reduces the amount of
labor for preparing solutions or dilutions, and is an additional safeguard
against over-feeding.
Materials of construction for day tanks are determined by the chemical
being used, but plastics such as polyethylene are generally applicable. The tank
can be provided with graduations or some sort of gauge so that approximate
volume measurements can be used. Commercial mixing tanks are available with
the day tank mounted on the cover, a convenient means for preparing fluosilicic
acid dilutions, but this arrangement does not permit weighing the acid, so
volume measurements must be relied upon.
24
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BAG LOADERS
When the hopper of a dry feeder is directly above the feeder, that is, when
the operator has to lift the chemical to a considerable height in order to fill the
hopper, a bag loader is more a necessity than a convenience. A bag loader is
essentially a hopper extension large enough to hold a single 100-pound bag of
chemical. The front of the loader is hinged so that it will swing down to a more
accessible height. The operator places a bag in the hinged section, opens the
bag and then swings the section back into position. This device not only lessens
the labor required to empty a bag, but also eliminates a good part of the dust
resulting from bag emptying.
DUST COLLECTORS AND WET SCRUBBERS
The handling of powdered dry chemicals invariably results in the generation
of various quantities of dust. When the quantities of fluoride compounds being
handled are small, ordinary care will minimize the dust problem, and good
housekeeping plus an exhaust fan will keep the storage and loading area
relatively dust-free. However, when larger quantities (i.e., more than one bag
at a time) are handled, dust prevention and collection facilities should be
provided.
A dust canopy, completely enclosing the hopper-filling area, and provided
with an exhaust fan, will prevent the dissemination of dust throughout the
loading area. To prevent the escape of dust into the atmosphere and thence into
the area surrounding the water plant, dust filters can be incorporated into the
exhaust system.
Dust collectors and exhaust fans are sometimes incorporated into the
hoppers of the larger dry feeders.
Wet scrubbers are a means for removing dust from exhausted air. The air
flows through a chamber in which there is a continuous water spray. The air is
thus "scrubbed" clean and the erstwhile dust particles are dissolved or carried
down the drain by water.
ALARMS
To prevent underfeeding or even loss of feed, alarm systems can be included
in either solution or dry feed systems. The alarm will serve to alert the operator
to the fact that the level of solution in the day tank is low, or that it is time to
add another bag of dry chemical to a hopper. An alarm can also notify the
operator that the water supply to a saturator or to a dissolving tank has either
stopped or diminished.
The alarms are based on level switches, flow switches, pressure switches, etc.
VACUUM BREAKERS
Any time there is a water connection to a chemical solution there is the
possibility of a cross-connection. This can be in the supply line to a saturator
or dissolving tank, or in the discharge from either of these or any solution
feeder.
25
-------
The simplest method for preventing a potential siphonage situation is to
provide an air-gap in the line. When pressure is high enough to make an
air-gap impractical, (as in the supply line to a dissolving chamber) a device
known as a vacuum-breaker must be used. A vacuum-breaker is essentially a
valve which is kept closed by water pressure, but when the water pressure fails,
the valve opens to atmosphere allowing air to be drawn into the system rather
than potentially hazardous solutions.
For utmost safety, the vacuum breaker should be installed as close to the
chemical solution as possible, and especially must be elevated above the lip of
the tank and on the discharge side of the last control valve.
In those States where mechanical vacuum breakers are not permitted,
potential siphonage hazards can be eliminated by other means. See the section
on cross-connections.
HOPPERS
Most dry feeders come equipped with a hopper, but for larger installations
additional hopper capacity is often desired. Quite often this entails extension of
the existing hopper up to the floor above, not only increasing hopper capacity
but making it easier to fill. If at all feasible, the chemical storage should be on
the floor above the feeder. If the extension hopper reaches to about 12"
above the floor of the storage room the edge of the hopper can serve as a
fulcrum point as an aid to upending drums or barrels.
In small plants, it is desirable to have the chemical hopper large enough to
hold slightly more than the entire shipping container of chemical. Thus the
hopper will not be completely empty before there is enough room in it for
the contents of a'fresh bag or drum. By loading an entire container in this
manner, there will be less handling of chemical and a saving of time. There will
be less dusting, with improved working conditions. There will be less chance of
spillage, with consequent saving of chemical. By not allowing the hopper to
become completely empty, there will be less chance of arching and flooding,
and also less chance for an interruption in feed.
Hoppers, whether those supplied with the feeder or those constructed at
the plant, may require vibrators to insure uniform feed. A rotary valve can be
installed between the hopper and the feeder to prevent flooding, and a gate can
be installed in the extension hopper to limit the amount of material falling to
the feeder hopper at any given time.
WEIGHT RECORDERS
Whenever a platform scale is used to measure the amount of dry chemical
or solution fed during a given period, a recorder can be attached so that a
record of the weight of chemical fed can be obtained. Many volumetric dry
feeders have such recorders available as an accessory (along with the scales) so
that the loss-of-weight feature makes the feeder somewhat equivalent to a
gravimetric dry feeder.
26
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CONTROLLERS
The feed rate of a dry feeder or metering pump can be automatically
adjusted to be proportional to water flow, a feature which is almost indis-
pensable when the water flow rate is extremely variable. When there is only one
service pump, operating at a fixed rate, the feeders can be tied electrically to
the pump operation and the fluoride feed will remain in the predetermined
proportion. When two pumps are used, two separate feeders can be used, or one
feeder can be adjusted manually each time the second pump cuts in or out.
For more than two pumps, or where there is no fixed delivery rate, manual
adjustment becomes impractical.
Controllers are based on the use of some type of primary flow measuring
device, such as an orifice plate, venturi meter, or magnetic meter. The con-
troller adapts the indication of flow rate to an electrical, pneumatic or hydraulic
impulse which in turn activates an adjusting mechanism on the feeder.
To go one step further, an automatic continuous analyzer can be used to
monitor the treated water, and the analysis information used in place of flow
information. The signal from the analyzer recorder is used to adjust the feeder
to produce a pre-determined fluoride concentration in a closed-loop arrange-
ment. This system, although admittedly expensive and not entirely foolproof,
has the advantages of compensating not only for changes in water flow rate,
but also for changes in raw-water fluoride levels, chemical purity, etc.
EDUCTORS
An eductor is a device which uses water pressure and flow for transporting
other fluids. In fluoridation, it finds application in transporting the solution
from the dissolving chamber of a dry feeder and injecting that solution into a
pressure main. Water at a pressure substantially higher than that of the dis-
charge head is required to operate an eductor.
While the solution from the dissolving chamber is diluted by the action of
the eductor, there will be no effect on the fluoride concentration in the
treated water since the water to operate the eductor is taken from the flow
which is being treated.
To prevent the introduction of air into the main, the eductor should draw
fluoride solution from a solution well which is supplied continuously with
water through a float-controlled valve.
PUMPS
Pumps find application in situations similar to those where an eductor is
used, and are also used for transferring fluosilicic acid from storage to day
tank and for transferring other fluoride solutions. Centrifugal pumps are most
commonly used, since they can be throttled without harm and can run
continuously.
The choice of materials of construction of pumps is as critical as it is for
solution feeders. Pump heads and impellers, and also pipe lines, must be
resistant to the material being handled or there will be leaks and pump failures
27
-------
to contend with. The avoidance of slurry transfer is important, also, since
slurries tend to be abrasive and can damage pump heads, impellers and packing.
TIMERS
An interval timer is basically a clock mechanism, usually electric, which
closes or opens an electric switch upon receipt of a signal, holds the switch in
position for the pre-set time interval and then reverses the switch position.
Timers are frequently used in conjunction with water meter contactors to
operate solution feeders. When the contact is made in the water meter, the
timer is energized and the feeder will run for the period selected. The
momentary contact made by the meter contactor, while sufficient to produce
a stroke of a solenoid-operated feeder, is too short to operate an electric-
motor operated feeder. Thus, the timer serves to extend the impulse received
from the contactor.
Another application of a timer is for those installations where the minimum
reliable feeder setting is still too high for the water flow. In these cases, the
timer can be set to provide a proportion of the full-time feed rate. For example,
by setting the timer to operate the feeder at 75% of each 1-minute period, the
feed rate will only be 75% of that obtained without the use of the timer.
A word of caution: Using a proportional timer at low percentages, and
particularly for long interval settings, can result in cyclic fluoride levels. If
there is insufficient detention time in clear wells or pipelines before water
reaches the consumers, the on-off action of the feeder will be apparent in
alternately too-high and too-low fluoride readings. The remedy is, other than
using a smaller feeder, to make the proportioned time interval as short as
possible.
HOPPER AGITATORS
To promote the smooth and even flow of dry material in a hopper, some
sort of agitation is helpful. The hopper agitator may take the form of a device
which imparts vibration magnetically or mechanically to the outside of the
hopper, or may be a rotating member inside the hopper.
Many dry feeders incorporate a hopper agitator into their construction.
FLOW-SPLITTERS
When the effluent from the dissolving chamber of a dry feeder is to be
used for more than one point of fluoride application, the effluent can be
divided with a flow-splitter. This device is nothing more than a movable
baffle in the dissolving chamber which can be adjusted to vary the proportion
of solution flowing out of two outlets.
An example of an application involving a flow-splitter would be when two
water sources, with no common point for fluoride injection, are to be
fluoridated with a single dry feeder. By adjusting the splitter so that the right
proportion of fluoride is fed to each source, the cost of an additional feeder is
saved.
It should be noted that a flow-splitter is grossly inaccurate and should not
be used except when no other alternative is available.
28
-------
(5) VARIABLE SPEED
CONTROL UNIT
„ 110 VOLT
FLOW (8) POWER "
METER SUPPLY
MODEL 304
o
HYDROFLUOSILICIC ACID
STORAGE TANK
SIGHT GAGE
(ft) FOOT VALVE
(?) BAG LOADING HOPPER — ft_
(5) LOSS OF WEIGHT if—.
RECORDING SCALE ~-«|
J-l
POWER SUPPLY -~"^n
\\
CHEMICAL BAG
I
/ QHELIX FEEDER MOD
•^ POWER SUPPLY
^; PACIN
®~M! C"H"A N"I C~A i~ M~I X"E R~
. ... (IF REQUIRED)
iiiiMi® °ISSOLVE" NO
J' T
-------
DUST COLLECTOR
HOPPER COVER
HOPPER SCREEN
FLOOR FLANGE
HOPPER RETAINER
VIBRATOR (OPTIONAL)
HELIX FEEDER MODEL 25-04
WEIGH SCALE —
FEEDER BASE
APPROX. 10 FT
53
DRAIN
Figure 7. Typical Arrangement of Dry Feed Hoppers and Dust Collectors
Figures 6 and 7, and Figures 13 through 17 illustrate the use and arrange-
ment of various pieces of auxiliary equipment in fluoridation installations.
30
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Chapter IV
Preparation Of Fluoride Solutions
MANUAL TECHNIQUE
The most basic fluoridation system, involving the least expenditure for
equipment, is one involving the proportional feeding of a fluoride solution into
a water flow in a pipeline. The only requirements are the preparation of a
fluoride solution of known concentration, and then pumping that solution into
the water so that the desired fluoride concentration in the water is achieved.
Of the most commonly used fluoride compounds, sodium fluoride and fluosil-
icic acid are of sufficient solubility for preparation of solutions of fixed
strength which then can be fed with a metering pump.
Fluosilicic acid, already a solution as purchased, can be fed directly from
the shipping container, from an intermediate storage tank (day tank), or can
be diluted and fed as a solution considerably weaker than the 22 - 30% acid
solution originally purchased. Dilution is necessary in small water plants,
where the water flow to be treated is so low that the smallest proportioning
pump cannot be adjusted to a low enough feed-rate setting for undiluted acid,
or where the feeder cannot be expected to operate reliably at the extremely low
setting required. As pointed out in the section on Feeders, extremes in feed-
rate range should be avoided, and even the smallest feeder will not deliver a
fraction of a drop per stroke with any degree of reliability.
Example:
Data known:
Water flow rate - 100 gpm
Cone, of fluosilicic acid - 25%
Concentration of F wanted - 1.0 ppm
Specific gravity of acid - 1.22
Problem:
Find volume of acid to be fed per minute, and the volume of acid to be
delivered per stroke of a feeder which operates at 46 strokes per minute.
Calculations:
R! X Cl = R2 X C2, where: R, = water flow rate in gal/min
Cj = desired F level in ppm
R2 = feed rate
C2 = solution strength as F
31
-------
then: 100 gal/min X 3785 ml/gal X 1.0 ppm = (x) X 25% X 79%
, , 100 gal/min X 3785 ml/gal X 1.0 ppm
W 25% X 79% X 1.22 (specific gravity)
By expressing percentages as decimals, and ppm as the fraction
1/1,000,000, units will cancel to give:
, .. , , ,; . , 1.6 ml/min _ „,.,. ., ,
(x) = 1.6 ml/mm, and — r~n—:— = 0.035 ml/stroke
v ' 46 strokes/mm
Obviously, the above volume per stroke is too small (a drop is usually
considered to be 0.05 ml) to be handled by even the smallest feeder, so
dilution of the acid is mandatory for reliable feed. If the acid were diluted at
the ratio of one gallon to nine gallons of water (resulting in acid of 2.5%
strength), the volume of acid to be fed per minute would be:
1001 gal/min X 3785 ml/gal Y1/inannmn ^
0.025 X 0.79 X 1.02 (sp. gr.) X i/1'000'000 0 P?m>
= 19 ml/min
19 ml/min n ., ., .
., c '—. . =0.41 ml/stroke
46 Strokes/mm
The above volume is well within the capability of a small solution feeder,
for example, one which has a delivery range of 0.1 to 1.0 ml/stroke. Note that,
while decreasing the stroke frequency would permit larger volumes to be
handled per stroke, such practice also has its limitations, since infrequent
stroking results in cyclic variations in fluoride levels in the water.
The dilution of fluosilicic acid should be made by carefully pouring the
measured volume (or weight) of acid into a measured volume of water. (While
adding water to acid would not be particularly hazardous in this case, because
of the low strength of commercial acid, it is always good practice to add acid
to water rather than the reverse.) All containers used for handling even dilute
acid must be made of a material resistant to the corrosive effects of the acid
(plastic or hard rubber are suitable) and the solution should be thoroughly
mixed with a plastic or wooden paddle. Avoid splashing or spilling, wear pro-
tective gloves and clothing, and mop up all spills with a solution of soda ash or
other alkaline material followed by several clear water rinses. The solution tank
should be kept on a platform scales, and records kept of the weights or
volumes of acid and water used in each batch of solution prepared, and particu-
larly of the weight of solution fed.
Precaution: Dilution of fluosilicic acid can result in the formation of a
precipitate (silica) when the dilution is in the range of ten to twenty parts of
water to one of acid. Such precipitation, which can result in clogged feeders,
valves and orifices, can be avoided by using fortified acid (acid to which a small
amount of hydrofluoric acid has been added by the supplier) or by using acid
manufactured from hydrofluoric acid rather than from phosphate rock.
Sodium Fluoride, being soluble up to 4% (4 pounds in 100 pounds of water)
can be fed as a solution by dissolving a weighed amount of the dry material in a
32
-------
measured volume of water. By usual means, a 4% solution is difficult to attain,
so for practical purposes, solution strength should be limited to about 2%.
Example:
Data known:
Water flow rate - 100 gpm
Sodium fluoride solution - 8 Ibs of 99% pure NaF dissolved in 50 gal. of
water
Problem:
Find volume of solution to be fed per minute to produce a concentration
of 1.0 ppm F in the water.
Calculations:
RI X Ci = R2 XC2; R2 = RI XCt where R! = water flow rate in
C2 gal/min
Ci = desired F level in ppm
R2 = feed rate
C2 = solution strength as F
100 gal/min X 1.0 ppm _nmo ,/ .
8300 ppm (from Table 3) ~ °'012 gal/mm
0.012 gal/min X 3785 ml/gal = 46 ml/min
At the relatively low concentrations used in sodium fluoride solutions, the
specific gravity is so close to 1.0 that volume measurements instead of weight
may be used in calculations of feed rate.
Although sodium fluoride solutions are not particularly corrosive, a plastic
container is preferred for preparation of solutions. The mixture may be stirred
with a paddle until the sodium fluoride is completely dissolved, or a small
mechanical mixer may be.used. Table 3 gives quantities of sodium fluoride
and water to be used for various solution strengths.
AUTOMATIC DEVICES
Because sodium fluoride has a maximum solubility of around 4% (18,000
ppm as F), regardless of substantial variations in water temperature, devices for
automatically preparing saturated solutions can be used. The use of these
devices eliminates the need for weighing sodium fluoride, measuring solution
water volume, and for stirring to insure dissolution.
The principle of a saturator, as these devices are called, is that a saturated
solution will result if water is allowed to trickle through a bed containing a large
excess of sodium fluoride. There are two general types of saturators - upflow
and downflow. In the latter type, the solid sodium fluoride is isolated from
the prepared solution by a plastic cone or a pipe manifold. A filtration barrier
is provided by layers of sand and gravel to prevent particles of undissolved
sodium fluoride from infiltrating the solution area under the cone or within the
pipe manifold (Figs. 8, 9, & 10). In the upflow type, no barrier is used, since
the water comes up through the bed of sodium fluoride and the specific
33
-------
Table 3. PREPARATION OF SOLUTIONS OF SODIUM FLUORIDE
Pounds of
NaF
(98% pure)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
GALLONS OF WATER
10
1.2
5200
2.3
10300
20
0.6
2600
1.2
5200
1.75
7750
2.3
10300
30
0.4
1750
0.8
3500
1.2
5200
1.55
7000
2.0
8600
40
0.3
1300
0.6
2600
0.9
3900
1.2
5200
1.5
6500
1.8
7800
2.0
9000
50
0.5
2100
0.7
3100
1.0
4200
1.2
5200
1.4
6200
1.65
7300
1.9.
8300
2.1
9300
60
0.4
1750
0.6
2600
0.8
3500
1.0
4350
1.2
5200
1.4
6000
1.6
6900
1.75
7800
1.95
8600
2.15
9450
70
0.3
1500
0.5
2250
0.7
3000
0.85
3700
1.0
4300
1.2
5200
1.35
6000
1.5
6700
1.7
7400
1.85
8200
2.0
8900
80
0.45
1950
0.6
2000
0.75
3300
0.9
3900
1.0
4600
1.15
5100
1.3
5850
1.5
6500
1.6
7200
1.8
7800
1.9
8400
2.1
9100
90
0.4
1750
0.5
2300
0.7
2900
0.8
3500
0.9
4000
1.05
4650
1.2
5200
1.3
5800
1.45
6400
1.6
6900
1.7
7500
1.8
8100
1.95
8600
2.0
9000
100
0.4
1600
0.5
2100
0.6
2600
'0.7
3150
0.8
3650
0.95
4200
1.1
4700
1.2
5200
1.3
5750
1.4
6250
1.5
6750
1.65
7300
1.8
7800
1.9
8300
2.0
8800
The above table gives strengths of solutions prepared by dissolving the given weights of
sodium fluoride in various volumes of water. The upper figures represent approximate
solution strength in percent sodium fluoride, while the lower figures represent approxi-
mate solution concentration in ppm F. Example: If 2 Ibs of NaF are dissolved in 50
gallons of water, what is the solution strength? Reading across (horizontally) from 2 Ibs
to the 50 gal. column gives 0.5% NaF and 2100 ppm F.
The absence of figures in a column indicates that the preparation of the particular
solution would be impractical or impossible.
34
-------
LINE BEING TREATED
SATURATED
SOLUTION -
CHEM-0 FEEDER
W ATER METER
/
WALL MTD SHELF
OVERFLOW
GRANULAR NoF
MIN OF 10 FOR
47. SOLUTION
4 TO 6 OF 1/8
TO 1/4 SAND
{TORPEDO SAND)
2 COARSE
GRAVEL (1 TO 2
LEVEL CONTROL
•*-"FLOAT VALVE
SPRAY
/NOZZLE
FOOT VALVE
DRAIN
SATURATED
SOLUTION NoF
MULTIPORT
VALVE
DL
VE
L
L
ZEOLITE
SOFTENER
n. n
-E
— i
WATER SUPPLY
BRINE TUBING
BRINE
TANK
DRAIN
[/
Figure 8. BIF Downflow Saturator
-------
ANTI-SIPHON
VAl
DIAPHRAGM
PUMP
WATER .
SOFTENER
SATURATED
FLUORIDE
TO POINT OF
APPLICATION
DRAIN-'
PLUG
Figure 9. W & T Downflow Saturator
gravity of the solid material keeps it from rising into the area of clear solution
above. In the downflow type, the feeder takes suction from within the cone or
manifold, while in the upflow type, the feeder suction intake floats on the
solution in order to avoid withdrawal of undissolved sodium fluoride.
When a downflow-type saturator is in operation, water is admitted at the
top of the saturator tank (there is an air gap to avoid the possibility of a cross-
connection) and the level is regulated with a float-operated controller. The
water then trickles down through the bed of sodium fluoride, the solution is
clarified in the sand and gravel filter bed and ends up as a clear, saturated
solution at the bottom of the tank where it is withdrawn by the feeder. The
feeder adds the prepared solution, at the desired rate, to the water system. The
only operator attention required is to see that an adequate quantity of sodium
fluoride is kept in the saturator and that the saturator is kept in a reasonably
clean condition (See Table 4).
36
-------
HOW CONTROL-
SVPHON BREAKER.
SOLENOID VALVE.
WALL
OUTLET
US VAC.
ANTI-SIPHON VALVE
PCPC 5000 SERIES
FLUORIDE
!•.--'-
50 GAL. POLY -
ETHYLENE TANK
WATER "V**™0 SOLUTION
SOFTENER
-*f*
•iff-
PCPC LIQUID"*
LEVEL SWITCH
3/4" NPT
OVERFLOW
&:::.
-V3/4'-AitTVoAirSOPTIONAl'
Figure 10. Precision Up-Flow Saturator
-------
Table 4. RECOMMENDED MAXIMUM FEED RATES FOR DOWNFLOW
SODIUM FLUORIDE SATURATOR
Bed Depth
6"
6%"
7"
7&"
8"
8*4"
9"
9&"
10"
60°F
950 gpm
200 ml/min
1065 gpm
225 ml/min
11 90 gpm
250 ml/min
1300 gpm
275 ml/min
1425 gpm
300 ml/min
1550 gpm
325 ml/min
1660 gpm
350 ml/min
1780 gpm
375 ml/min
1900 gpm
400 ml/min
Water Temperature
50°F
830 gpm
175 ml/min
950 gpm
200 ml/min
1065 gpm
225 ml/min
11 90 gpm
250 ml/min
1300 gpm
275 ml/min
1425 gpm
300 ml/min
1550 gpm
325 ml/min
1 660 gpm
350 ml/min
1780 gpm
375 ml/min
40°F
710 gpm
150 ml/min
830 gpm
175 ml/min
950 gpm
200 ml/min
1065 gpm
225 ml/min
11 90 gpm
250 ml/min
1300 gpm
275 ml/min
1425 gpm
300 ml/min
1550 gpm
325 ml/min
1 660 gpm
350 ml/min
In the above table, the lower figures represent the maximum feed rate (or
withdrawal rate) of saturated sodium fluoride solution for each given bed
depth of sodium fluoride at each of three saturator water supply temperatures.
The upper figures represent the water flow rate which can be fluoridated to
a level of 1.0 ppm F for each respective solution feed rate, assuming there is
less than 0.1 ppm natural fluoride content in the water. Higher flow rates
can be fluoridated if there is a higher natural fluoride content or if the desired
level is less than 1.0 ppm F.
To prepare a downflow saturator for use:
1. With the manifold (W & T) or cone (BIF) in place, carefully place by
hand a 2-3" layer of coarse, clean gravel (1-2" size) in the saturator
tank, around the manifold or cone and over the manifold or over the
lower edge of the cone. Then place another 2 - 3" layer of finer gravel
(ft - 1" size) over the coarse gravel.
38
-------
2. Place a 4-6" layer of clean, sharp filter sand over the gravel. (Do not use
beach sand, clayey sand or ordinary soil.) Level the sand surface. (A 12"
bed of 1/8" to 1/4" filter gravel can be substituted for the sand and
coarse gravel layers.)
3. Add 200 Ibs of coarse crystalline sodium fluoride (Olin 20 - 60 mesh,
Allied coarse crystal, or similar. Do not use powdered NaF or fine
crystal.) Add water to keep down the dust and to assist in leveling the
fluoride surface.
4. Check to see if the float has room to operate. If necessary make a
depression in the fluoride surface to provide clearance for the float and
float-rod.
5. If you have not already done so, connect a cold-water supply line to the
water intake of the saturator. The line should contain a small water
meter (1/2") for use in calculating feed rate, and there should be a shut-
off valve between the meter and the saturator.
6. Turn on the water supply and adjust the float position if necessary. The
low-water-level should be no less than 2" above the fluoride surface, and
the high-water level should be just below the overflow outlet.
7. Insert the feeder suction line into the pipe leading to the inner cone or
manifold as the case may be. Adjust the length of suction line so that the
foot-valve and strainer are 2 - 3" above the bottom of the saturator
tank. The saturator is now ready for use.
8. By looking through the translucent wall of the saturator tank you should
be able to distinguish the layers of fluoride, sand and gravel. When the
thickness of the fluoride layer decreases to 6", add another 100#
quantity of fluoride. It would be wise to add the fluoride when the
water level is at its lowest level, or if necessary, to shut off the water
temporarily until there is enough room for the fluoride without causing
water to come out of the overflow opening.
9. If the saturator is being used at a high rate, that is, if more than 1000
gpm of water are being treated, fluoride should be added daily in
sufficient amounts to keep the fluoride layer at a thickness of at least
10". The same daily additions of fluoride should be made if the make-up
water temperature is below 60°F, even if less than 1000 gpm of water are
being treated.
10. Before more fluoride is added to the saturator, the surface of the fluoride
layer in the saturator should be scraped free of accumulated dirt,
insoluble material or the slimy film of fine particles that sometimes
forms. Such routine maintenance permits better percolation of water
through the fluoride layer and extends the length of time between
clean-outs.
11. At regular intervals, depending on severity of use, the saturator will have
to be cleaned out. A typical schedule calls for a clean-out every three
months when the quantity of water being treated is in the 100 gpm area
39
-------
and the accumulation of dirt in the saturator is moderate. The clean-out
procedure is as follows:
a. Continue using the saturator until the level of sodium fluoride is as low
as practicable. Shut off the water supply to permit the level of water
to drop down to the fluoride layer. This step minimizes the wastage
of chemical and decreases the amount of material which will have to
be removed.
b. Scoop out the remaining sodium fluoride, the sand and gravel. If the
sand and gravel are to be re-used, place them in separate buckets. If
fresh sand and gravel are available, bury the dirty material. The old
sodium fluoride can be flushed down a drain or buried.
c. Remove the inner cone or manifold assembly and clean the inside of
the saturator tank. Replace the cleaned cone or manifold.
d. If the old sand and gravel are to be re-used, wash them repeatedly
with water until all traces of fluoride and dirt are removed. Then
reconstruct the filter bed of sand and gravel as before with either the
cleaned or fresh material.
e. Add sodium fluoride as before and return the saturator to normal
operation. (Don't forget to turn on the water supply again.)
12. If the water supplied to the saturator is hard (more than 75 ppm hard-
ness), a household-type water softener in the line will minimize the
amount of insoluble material accumulating in the saturator and thus
increase the interval between clean-outs.
13. Use the readings of the water meter on the saturator supply line to
calculate the amount of fluoride fed. A 4% solution of sodium fluoride is
equivalent to 18,000 ppm F. Thus:
Gallons of water supplied to saturator -, , _ ,_. „
Tr-n T—; ——j X 18,000 = ppm F
Gallons of water pumped rr
(See Page 43 for example problem.)
In an upflow saturator, undissolved sodium fluoride forms its own bed,
below which water is forced upward under pressure. A spider type water
distributor located at the bottom of the tank contains hundreds of very small
slits. Water, forced under pressure through these slits, flows upwards through
the sodium fluoride bed at a controlled rate to assure the desired 4% solution.
The water pressure requirements are 20 psi minimum to 125 psi maximum,
and the flow is regulated at 4 gpm. Since introduction of water to the bottom
of the saturator constitutes a definite cross-connection, a mechanical syphon-
breaker is incorporated into the water line. Where such devices are not
permitted, the saturator can be factory-modified to include an air-gap and a
water feed-pump.
To prepare an upflow saturator for use:
1. With the distributor tubes in place, and the floating suction device
removed, add 200 to 300 pounds of sodium fluoride directly to the tank.
(Any type of sodium fluoride can be used, from coarse crystal to fine
40
-------
powder, but fine crystal will produce less dust than powder and will
dissolve better than coarse material.)
2. Connect the solenoid water valve to an electric outlet and turn on the
water supply. The water level should be slightly below the overflow; if it
is not, the liquid level switch should be adjusted.
3. Replace the intake float and connect it to the feeder intake line. The
saturator is now ready for use.
4. By looking through the translucent wall of the saturator tank you should
be able to see the level of undissolved sodium fluoride. Whenever the
level is low enough, add another 100# quantity of fluoride.
5. The water distributor slits are supposed to be essentially self-cleaning, and
the accumulation of insolubles and precipitates does not constitute as
serious a problem as it does in a downflow saturator. However, periodic
cleaning is still required. Frequency of cleaning is dictated by the severity
of use and the rate of accumulation of debris.
6. Because of the thicker bed of sodium fluoride attainable in an upflow
saturator, higher withdrawal rates are possible. With 300# of sodium
fluoride in the saturator tank, more than 1000 ml/min of saturated
solution can be fed, a rate sufficient to treat about 5000 gpm of water
to a fluoride level of 1.0 ppm.
7. Precautions regarding hard water apply similarly to both types of
saturators.
8. Method for calculating the amount of fluoride fed is the same for both
types of saturators. The fixed water inlet rate of 4 gpm should register
satisfactorily ori a 1A" meter.
The preparation of solutions of fluoride compounds, whether performed
manually or automatically, requires a certain amount of care. The sources of
possible error in the dilution of fluosilicic acid are:
1. Incorrect volume or weight of acid.
2. Incorrect volume or weight of water.
3. Incomplete mixing.
4. Miscalculation of acid strength as received.
5. Miscalculation of fluoride concentration in dilution.
In the manual preparation of sodium fluoride solutions, the sources of
possible error are:
1. Incorrect weight of sodium fluoride.
2. Incorrect weight or volume of water.
3. Incomplete dissolution.
4. Incomplete mixing.
5. Attempting concentration above 2%.
6. Miscalculation of fluoride concentration in the sodium fluoride.
41
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7. Miscalculation of fluoride concentration of the solution.
The proper function of a device for automatically preparing solutions
(saturator) can be adversely affected by the following:
1. Insufficient quantity of sodium fluoride.
2. Water too cold for feed rate used.
3. Withdrawal rate too high.
4. Sifting of sodium fluoride through sand and gravel (downflow type).
5. Lifting of sodium fluoride (upflow type).
6. Wrong crystal size (downflow type).
7. Failure to scrape surface (downflow type).
8. Short-circuiting (downflow type).
9. Insufficient water supply (stoppage or valve closing).
CALCULATIONS INVOLVING SOLUTIONS
As in the examples above, the equation, RI X Ct = R2 X C2 (water flow
rate times fluoride level desired equals solution feed rate times fluoride con-
centration of the solution), should be used to calculate the theoretical mean
fluoride concentration for a given time interval. Obviously, accurate figures
for weights or volumes of materials used are necessary for accurate calculations.
The use of appropriate units (which can be cancelled arithmetically) will
produce a calculated result in the desired units and will verify the proper
insertion of figures into the equation. For instance, with R! in gallons, Ct in
ppm, and C2 in pr^m, R2 will be in gallons. A calculated result in any other
units would indicate an error.
Some examples of typical calculations are given below.
Example 1: A water plant pumps 300,000 gallons of water in one day. Sodium
fluoride solution, prepared by dissolving 8# of 98% NaF in 50 gallons of
water, is the fluoride source. The solution tank is mounted on scales, and
the scales indicate that 250# of solution was fed. What is the calculated
fluoride level in the treated water?
RV" I"1 O V f~* • f D Y" f
1 A V^i — K2 A L-2 ,l-i — t\2 A ^-2
calculations: — =
R! in Ibs, R2 in Ibs, C2 in ppm)
, 250 Ibs X 8300 ppm (from Table 3) . 0_
Di = °-—v ' = 0.83 ppm
300,000 gal X 8.34 Ib/gal
Example 2: If 250,000 gallons of water are pumped, and 150 Ibs of a fluosilicic
acid solution containing 25% acid diluted at the rate of 1 gallon to 9 gallons
of water are fed, what is the theoretical fluoride level?
R2 XC2
Calculations: Ct =—5
*M
(R! in Ibs, R2 in Ibs, C2 in %)
42
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_ 150 Ibs X 0.025 (acid strength) X 0.79 (available F)
1 ~ 250,000 gal X 8.34 Ib/gal
= 0.000142 % or 1.42 ppm
Example 3: In a saturator installation, the saturator supply meter shows that
12 gallons of water were admitted, while the master meter indicated that
200,000 gallons of water were pumped. What is the calculated fluoride
concentration?
R2 X C2
Calculations: C i = - 5 -
(R2 in gallons, R! in gallons, C2 in ppm)
_ 12 gal X 18,000 ppm (F cone, of saturated solution of NaF)
1 ~ 200,000 gal.
= 1 .08 ppm
If there is an appreciable natural fluoride level in the untreated water, that
amount should be added to the calculated level based on fluoride added.
Similarly, when calculating feed rates, the natural fluoride concentration should
be subtracted from the desired level in order to determine the quantity to be
added.
In order to verify the delivery rate of a solution feeder, for example when
starting up a solution feed fluoride installation or when making adjustments
to a feed rate that is too high or too low, some simple procedures are available.
Simply measuring the output from the discharge outlet of the feeder is unsatis-
factory, since even the output of so-called positive-displacement feeders varies
with pressure. One acceptable way is to measure a volume of the liquid being
pumped, preferably in a graduated cylinder. Then carefully insert the suction
tube of the feeder into the cylinder (without losing prime or spilling any
solution). Feed for a timed interval, withdraw the suction tube and note the
volume of solution remaining. The difference will represent the volume fed
during the measured interval. Another way is to equip the solution tank with a
calibrated sight glass. By closing the valve between the sight glass and tank,
while the feeder is operating normally, solution will be withdrawn from the
sight glass only, and the volume fed over a timed interval can be calculated.
This system has the advantage of freedom from interruption of fluoride
addition. After the measurement, opening the valve will be all that is necessary
to do to resume normal feed (Fig. 1 1).
Solution strength can also be verified, either by chemical analysis or by
specific gravity measurement. The extreme dilution required for chemical
analysis introduces possibility for considerable error, and specific gravity
measurements are at best only an approximation, but either of these procedures
will suffice to detect gross errors in solution preparation. The care with which
the dilutions and measurements are made will determine the degree to which
the results can be relied upon.
43
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PROPORTIONING
PUMP
I
SIPHON^^"^
ALVE i
SUCTION
i
GAGE
:
J
a )
y y
li-i^,fel>j^r:vr^*-V\V.i»;:l^^
Figure 11. Checking Delivery Rate of a Solution Feeder
When analytical methods are used, the electrode procedure is most suitable,
since its range far exceeds that of colorimetric methods. By standardizing the
electrode in the area in which it is to be used, concentrations up to 1,000 ppm
F can be determined with acceptable accuracy. Obviously, preparation of
standards must be done with painstaking care.
Specific gravity of technical fluosilicic acid seldom agrees closely with the
published figures, which are based on relatively pure acid, but in the greater
dilutions the figures are at least acceptable. (Assay procedures are available
for the acid as purchased. See the AWWA specifications.) Specific gravity
measurements must be made at the temperature specified in the tables (Table
5), and glassware used should be exposed as briefly as possible and rinsed
thoroughly after use to preclude etching. (See the AWWA Standards for com-
plete instructions.)
44
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Table 5. SPECIFIC GRAVITY OF FLUOSILICIC ACID SOLUTIONS
AT 17.5°C.
Specific
Gravity
1.0040
1.0080
1.0120
1.0161
1.0201
1.0242
1.0283
1.0324
1.0366
1.0407
1.0449
1.0491
1.0533
1.0576
1.0618
1.0661
1.0704
1.0747
1.0791
1.0834
Weight of H2SiF6 in Solution Expressed in:
Pounds per gallon
0.042
0.084
0.127
0.170
0.213
0.256
0.300
0.345
0.389
0.434
0.480
0.525
0.571
0.618
0.665
0.712
0.759
0.807
0.856
0.904
Percent H2SiF6
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
Degrees Baume1
0.6
1.2
1.7
2.3
2.9
3.4
4.0
4.6
5.1
5.7
6.2
6.8
7.3
7.9
8.4
9.0
9.5
10.1
10.6
11.2
Note:
These figures apply only to pure H2SiF6 in distilled water, but they
may be used to obtain approximate values for solution strengths.
45
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Chapter V
Selecting The Optimal Fluoridation System
Among the considerations in choosing the right fluoridation system for a
particular situation are: population served (or water usage rate), chemical
availability, cost, type of operating personnel available, and just plain personal
preference on the part of the person or persons making the decision. While
there is no one specific type of fluoridation system which is solely applicable.
to a specific situation, there are some general limitations imposed by the size
and type of water facility. For example, a large metropolitan water plant
would hardly be likely to consider a fluoridation installation involving the
manual preparation of batches of solution, any more than would a small
facility consisting of a number of unattended wells consider the appropriate
number of gravimetric dry feeder installations.
Figure 12 is a check-list, based essentially on water pumping rates, which
can be used as a rough guide in differentiating between the various types of
fluoridation installation. Obviously, the population divisions are flexible, at
least within limits, so considerable overlap can be expected. Other consid-
erations, such as the water plant lay-out and personal preference can be
expected to influence a choice which does not necessarily fall within the
selected limits.
For the smallest water plant, some type of solution feed is almost always
selected. There is almost no lower limit to the ranges of the smallest metering
pumps, and even if the feed rate for solution were impractically low, a more
dilute solution would solve the problem. Conversely, there is almost no upper
limit to the capacity of gravimetric dry feeders, or to pumps, for that matter,
so the largest water plants can select either dry feed with sodium silicofluoride
or direct feed of fluosilicic acid. For all those water plants in between, there
will be a choice, modified only by individual circumstances. Assistance can be
obtained from SHD Engineers, manufacturer's representatives, or consultants.
LOCATION OF FEEDER
Before a type of feeder can be selected, sufficient and appropriate space
for its installation must be provided. If there is an existing water plant, where
other water treatment chemicals are being fed, usually space for an additional
feeder is no problem. If there is no treatment plant, as is often the case with
well supplies, then there may be a well house, or perhaps even some type of
47
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Figure 12. Fluoridation Check-List
Chemical And System
Water Flow Rate
Population Served By
System Or Each Well Of
Multiple-Well System
Chemical Cost, FOB
Manufacturer
Chemical Cost/lb
Fluoride Ion
Equipment Cost/Unit
Equipment Required
Feed Accuracy
Chemical Specifications
And Availability
Handling Requirements
Feeding Point
Other Requirements
Hazards
Sodium Fluoride
Manual Solution
Preparation
Less Than 500 gpm
Less Than 5000
22 - 25 i /lb
50 -Sit
$100 -$500
Solution Feeder,
Mixing Tank, Scales,
Mixer
Depends On Solution
Preparation And
Feeder
Crystalline NaF,
Dust-Free, In Bags
Or Drums. Generally
Available.
Weighing, Mixing,
Measuring
Injection Into
Filter Effluent Line
Or Main
Solution Water May
Require Softening
Dust, Spillage,
Solution Preparation
Error
Sodium Fluoride
Automatic Solution
Preparation
Less Than 2000 gpm
Less Than 10,000
20 - 22t/lb
46 - 50
-------
Fluosilicic Acid
23 - 30%
More Than 500 gpm
More Than 10,000
$51-$58/ton
(23% Basis)
14 - 16tf
$500 And Up
Solution Feeder, Day
Tank, Scales, Transfer
Pump
Depends On Feeder
Sodium Silicofluoride Or Sodium Fluoride
Dry Feed
More Than 100 gpm More Than 2 MGD
More Than 10,000 More Than 50,000
Sodium
Silicon.
9 - \0il Ib
Sodium
Fluoride
18 - 20t lib
4l-46i
$1,000 And Up
Volumetric Dry Feeder,
Scales, Hopper,
Dissolving Chamber
Usually Within 3%
Sodium Sodium
Silicon. Fluoride
8-9tf/lb 18-20^/lb
$3000 And Up
Gravimetric Dry Feeder,
Hopper, Dissolving Chamber
Usually Within 1%
Bulk Acid In Tank Cars
Or Trucks. Available
On Contract
Powder In Bags, Drums Or Bulk. Generally Available.
All Handling By Pump
Injection Into Filter
Effluent Line Or Main
Acid-Proof Storage
Tank, Piping, Etc.
Corrosion, Fumes,
Leakage
Bag Loaders Or Bulk Handling Equipment Required
Gravity Feed From Dissolving Chamber Into Open Flume
Or Clear-Well, Pressure Feed Into Filter Effluent Line
Or Main
Dry Storage Area, Dust Collectors, Dissolving-Chamber
Mixers, Hopper Agitators, Eductors, Etc.
Dust, Spillage, Arching And Flooding In Feeder And Hopper
49
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shelter near an elevated storage tank. The feeder must be placed in a dry,
sheltered area, near to the point of fluoride injection, and preferably in a
place which has storage space for chemicals. There must be electrical power
available, in most cases, and a water line for solution preparation unless
undiluted fluosilicic acid will be used. The location must be accessible for
chemical replenishment and maintenance. Other than these basic requirements,
consideration should be given to the desirability for isolation of chemical
storage from other materials, for adequate ventilation and for general con-
venience.
FLUORIDE INJECTION POINT
The first and most important consideration in selecting the fluoride injection
point is that it must be a point through which all the water to be treated
passes. In a water plant, this can be in a channel where the other water treat-
ment chemicals are added, it can be in a main coming from the filters, or it can
be in the clear well. In a well-pump system, it can be in the discharge line of a
pump, if there is only one, or if there is more than one pump it can be in the
line leading to the elevated tank or other storage facility. If there is a com-
bination of facilities, say a treatment plant for surface water plus supplemental
wells, it must be a point where all water from all sources passes. If there is no
such common point, it means that separate fluoride feeding installations will
have to be made for each water facility. If at all possible, a point through which
all the water passes at a fixed rate or rates should be selected. For example, the
water from one or more well pumps is being discharged into a main leading to
an elevated storage tank. Water from the storage tank passes through a point
which might seem to be acceptable. In the latter case, flow can vary from that
of maximum demand on the system to that of minimum demand, making the
adjustment of the fluoride feed rate more difficult than it would be if it were
based on the fixed delivery rate of the pump or pumps. Thus, a better choice
would be to inject fluoride into the main leading to the storage tank, where
the flow rates are those of the pump deliveries.
Another consideration in selecting a fluoride injection point is the question
of fluoride losses in filters. Whenever possible, fluoride should be added after
filtration to avoid the substantial losses which can occur, particularly with
heavy alum doses or when magnesium is present and the lime-soda-ash softening
process is being used. On rare occasions it may be more practical to add
fluoride before filtering, such as in the case where the clear well is inaccessible
or so far away from the plant that moving chemicals would be uneconomical.
When other chemicals are being fed, the question of chemical compatibility
must be considered. If any of these other chemicals contain calcium, the
fluoride injection point should be as far away as possible in order to minimize
loss of fluoride by precipitation. For example, if lime (for pH control) is being
added to the main leading from the filters, fluoride can be added to the same
main but at another point, or it can be added to the clear well. If the lime is
50
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being added to the clear well, the fluoride should be added to the opposite
side. Another factor in selection of injection point is the desirability of having
the feeder as close to this point as possible.
TYPE OF FEEDER AND CHEMICAL
For the smaller water plants, the amount of chemical used is small enough
so that the cost per pound is not a major factor. Thus, sodium fluoride or
fluosilicic acid, even though relatively expensive in small lots, can both be used.
The decision of whether to use a manually prepared sodium fluoride solution,
a saturator, diluted or undiluted fluosilicic acid depends on the quantities to be
fed, the skill of the operator, the availability and desirability of acid, and
personal preference.
CHEMICAL PUMP WIRED IN
CONJUNCTION WITH WEU PUMP
FOOT VALVE ASSEMBLY
'/i/nimrm/.'n/
Figure 13. Acid Feed Installation
51
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Perhaps the simplest fluoridation installation is one based on the use of
fluosilicic acid, provided the acid can be fed undiluted. The acid is supplied in
carboys or drums, which need only be mounted on a platform scale to serve
as the feeding source. A solution feeder, mounted on a shelf above the carboy,
draws acid and injects it into a main in proportion to the water flow. An anti-
siphon valve is usually part of the feeder, and such extras as a loss-of-weight
recorder can be added (Fig. 13).
If the supply is too small to be able to utilize undiluted fluosilicic acid,
a sodium fluoride solution or diluted acid feeding system is almost as simple.
The equipment would be essentially the same as the acid feeding system plus
that needed for dilution or solution preparation.
A sodium fluoride solution-feed installation may be arranged as in Fig. 14,
or the mixing tank and transfer pump eliminated by using the day tank for
ALARM
WATER SUPPLY
TRANSFER PUMP
MIXER
--PLACE ON PLATFORM SCALE
Figure 14. Solution Feed Installation
52
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solution preparation. Similarly, the diluted acid feed system illustrated in Fig.
15 can be simplified by manual addition of the acid to the day tank, particu-
larly if the acid is purchased in small carboys which permit easy handling.
Except for the fact that the solution strength may be relatively high (4%),
the use of a sodium fluoride saturator provides the basis for an extremely
simple and virtually fool-proof solution-feed installation. Once the solution
feeder has been adjusted, the only operator attention required is the occasional
replenishing of chemical (without weighing) and cleaning of the saturator
(Figures 8, 9, and 10).
Sodium silicofluoride dry feed is limited to water plants large enough to
accommodate a volumetric or gravimetric feeder. Volumetric dry feeders are
capable of feeding at very low rates, and some of the smallest disc types, such
as those used for pilot plant installations, are able to handle water rates as low
as 25 gpm. If an open channel for feeding by gravity (from the dissolving tank)
is not available, an eductor or centrifugal feed pump can be used for injecting
fluoride solution into a main (Fig. 16).
WATER SUPPLY
TRANSFER P
—
307. ACID
J
II
'{"
!:
1
(' II [AIR GAP
.45^ „ --r
_ _ |l
:.: ft
^
OVERFLOW
MIXER
DAY TANK
(PLACE ON PIATFORM SCAIEI
E-^. •
Figure 15. Diluted Acid Feed System
53
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VOLUMETRIC FEEDER
f»
LOSS-OF-WEIGHT RECORDER
Figure 16. Dry Feed Installation with Volumetric Feeder
The roll-type and screw-type volumetric dry feeders are capable of handling
flow rates from less than 100 gpm to several MGD. Gravimetric dry feeders are
applicable to flows from about 2 MGD up to the largest water usage encount-
ered (Fig. 17). Of course, any of the dry feeders can be used for sodium
fluoride as well as sodium silicofluoride.
CHEMICAL AVAILABILITY
Fluoride chemicals are plentiful, but having the right one in the right place
at the right time may not be as simple as it might seem. The availability of
compounds derived from phosphate fertilizer production, fluosilicic acid and
sodium silicofluoride, is tied in with fertilizer sales, so if there is a decline in
54
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BEL! TYPE
GRAVIMETRIC FEEDER
SOLUTION TANK
Figure 17. Dry Feed Installation with Gravimetric Feeder
such sales there is a decline in recovery of the fluoride compounds. Such
occurrences are rare, and can be circumvented in large measure by purchasing
these chemicals on a contract basis. Most suppliers will preferentially deliver to
contract customers when there is a shortage. Sodium fluoride is derived from
fluorspar, so it not tied in with fertilizer production.
In any case, it would be wise to investigate the availability of fluoride
chemicals through a local supplier before making a choice. Ordinarily, stocks
of chemicals on hand at local distributors' warehouses are sufficient for the
smaller water plants even if there is a temporary shortage. It is the large user
of fluosilicic acid who must be assured of an ample supply before committing
himself to an installation designed around the use of the acid for fluoridation.
55
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Planned usage of a saturator installation hinges on the availability of coarse
crystalline sodium fluoride, at least for the down-flow types. There are only a
limited number of manufacturers who produce this grade, so not all distrib-
utors will routinely carry it in stock. Planning for two different chemicals will
allow continued operation in the event of a shortage of one of them.
CHEMICAL STORAGE AND HANDLING
There are a number of criteria governing the selection of a storage site for
fluoridation chemicals. The dry chemicals must be kept dry, they must be
convenient to the hopper in which they will be loaded, they should preferably
be isolated from other water treatment chemicals to preclude accidental
intermixing; the storage area must be clean and well ventilated, and should be
equipped with running water and a floor drain for ease in cleaning up spills.
Fluosilicic acid presents particular problems in storage, for the vapors are
corrosive and will even etch glass. Containers must be kept tightly closed, or
vented to the outdoors when the container is being fed from. Large quantities
of acid can be stored in underground or enclosed tanks equipped with outside
vents.
Sufficient area for storage of bagged chemicals should be provided, since
piling bags too high can cause compacting and "massing" of crystals, resulting
in lumps and irregular feeding characteristics. Similar conditions can result from
long periods of storage, so an over-supply of chemicals should be avoided.
For dry-feed chemicals, particularly if considerable quantities are to be
handled, it is preferable to have a storage area on the floor above the feeding
equipment. This arrangement allows dumping bags or drums directly into
extension hoppers without extensive lifting.
Bags, fiber drums and steel drums should be stored on pallets so that air can
circulate beneath them. Dry chemicals will cake if allowed to get damp, and
steel drums will rust.
The disposal of empty fluoride containers has always been a problem. The
temptation to re-use fiber drums is difficult to overcome, since the drums are
convenient and sturdy. Paper bags are dusty and could cause a hazard if they
are burned, and empty acid drums could contain enough acid to cause con-
tamination or even injury. The best approach is to rinse all empty containers
with plenty of water - even the paper bags are strong enough to withstand
repeated rinses. After all traces of fluoride are removed, the bags can be buried
or burned (if an incinerator is available). Even supposedly well-rinsed drums
should never be used for purposes where traces of fluoride could present a
hazard.
If possible, the storage area should be kept locked and not used for any
other purpose. Particularly, workers should be warned against eating in a
fluoride storage area.
A smooth, impervious floor will be easier to keep clean. A concrete floor
laid directly on the ground tends to be damp and should be avoided. An
exhaust fan, dust collector and/or wet scrubber should be used to keep the air
56
-------
relatively dust-free. Respirators, goggles, gloves, aprons and other protective
clothing should be available for workers.
The quantity of chemical to be kept in storage depends on individual
circumstances. Unless a delay or shortage is anticipated, no more than is
necessary should be kept on hand. Dry chemicals tend to cake and get lumpy
on long storage, and large quantities of acid present a hazard. The exception to
the latter is when it is economical to purchase acid in tank-truck lots, in
which case an enclosed or underground storage tank with adequate venting
and precautions against accidents must be provided. (Note: The storage tank
may be located outdoors if no other space is available. Fluosilicic acid, being
mostly water, is subject to freezing. The freezing point of a 22% solution is
about 4°F.)
For relative space requirements for storage of chemicals, see Table 1.
CROSS-CONNECTION CONSIDERATIONS
As mentioned in the section on feeders, water inlets to solution tanks,
dissolving chambers, etc., often constitute a cross-connection when the inlet
is below the level of solution. Whenever possible, the inlet line should have an
air gap as a positive safety precaution. In cases where pressure is too high to
make an air gap practical, a vacuum breaker must be installed at an elevated
location between all other restrictive devices (such as valves) and the point of
entry into the solution container.
Checks for possible cross-connections should be made not only when the
fluoridation installation is in the design stage, but also when later modifications
occur.
AUTOMATIC PROPORTIONING (PACING)
When water flows at a fixed rate, the fluoridation installation is relatively
simple since the fluoride feed rate, once adjusted to the proper ratio need not
be changed unless there is a change in pump delivery, feeder delivery or
chemical purity. However, when the flow is variable, such as when there are
multiple pumps or when pump delivery varies with demand, a variable fluoride
feed rate also becomes necessary. In some cases, such as when there are two or
three pumps, each with a fixed delivery rate, it is possible to use separate
fluoride feeders, each tied electrically to the individual pump operation, and
each adjusted to feed fluoride in the correct ratio to the water delivered by that
pump. In other cases, such as when the changes in water flow are predictable
and infrequent, it may be possible to manually adjust the feeder to a pre-
determined setting which corresponds to the new water flow rate. If neither of
the above apply, or are impractical or inconvenient, some means for auto-
matically adjusting the fluoride feed rate to the water flow rate must be pro-
vided. This automatic adjustment is called "pacing."
For smaller water plants, particularly those using a solution feed system,
a water meter contactor, with or without an interval timer, can be used to pace
the feeder. Essentially, a water meter contactor is a switch which is geared to
57
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the water meter movement so that it will make contact at specified gallonage
intervals. Thus, the impulse from the water meter contactor, when related to
the feeder motor, will energize the feeder in proportion to the meter's
response to flow. In cases where the duration of impulse is too short (momen-
tary switch contact) to activate the feeder motor, an interval timer, connected
between the contactor and motor, can be set for appropriate time durations.
To preclude extensive cycling and resultant wide variations in fluoride con-
centration, the contacts should be as close together as possible, and if a timer
is used, the durations of timed feeder operation should be as long as possible,
just short of possible overlap at the maximum flow rate. (Some small solution
feeders, usually solenoid operated, are particularly adapted to meter-contactor
operation.)
Larger water plants, already equipped with flow meters- and/or recorders,
can utilize the signal from these devices to adjust feeder delivery electrically,
pneumatically, or hydraulically. If this type of pacing is contemplated, feeders
which have adjusting mechanisms compatible with the type of flow-meter
signal available must be specified.
If the plant does not have flow meters which are readily adaptable to pacing
the fluoride feeders, or no flow meters at all, various types of meters can be
installed specifically for the purpose. For example, an orifice plate assembly
with a D/P cell transmitter will furnish an electrical signal which can be used
for control of feeders.
Another device providing an electrical signal is a magnetic flowmeter.
A signal converter will adapt the signal from one of these to a form which can
be used for adjusting some types of feeders. Other types of flowmeters require
the use of recorder-controllers which modify a pneumatic or hydraulic pressure
line flow so that the pressure thus modified will adjust the delivery rate of
either a dry or solution feeder equipped for the purpose. On solution feeders,
the adjustment is made to the stroke length; on dry feeders, the adjustment can
be made through a variable-speed drive, a poise-positioner, etc. The pneumatic
or hydraulic signals adjust variable-speed drives mechanically, while electrical
signals accomplish the same purpose through a silicon-controlled rectifier (SCR).
Another type of paced fluoride feed can be accomplished without the use
of remote flow-meter signals. In this case, a solution feeder is operated
directly (hydraulically) by a special water meter which diverts water under
pressure to the feeder's driving piston in proportion to the rate of water flow
in the main. (See also controllers)
As stated earlier, while there is no one specific type of fluoridation instal-
lation which is solely applicable to a specific situation, there is usually one
which is preferable. The best installation is one that has the best combination
of the following factors:
1. Minimum cost of equipment, consistent with good performance.
2. Simple mechanical feeding device, either liquid or dry.
3. A simple piping and injection system.
4. Minimum handling of chemicals.
58
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5. Minimum labor operations.
6. Minimum maintenance.
7. Means for maintaining reliable and permanent records.
8. Minimum chemical cost, consistent with water plant size.
INSTALLATION
The illustrations (Figs 13 to 17) show the general arrangement of equip-
ment in typical fluoridation installations. Some factors to consider are prox-
imity to fluoride application point, proximity to chemical storage, availability
of solution water and accessibility for operation and maintenance.
SOLUTION FEEDER INSTALLATIONS
A solution feeder should be placed above the solution container if at all
possible to lessen the chances for siphoning. The suction line should be as
short and straight as possible, and there should be a foot-valve and strainer at
its terminus. If the suction line tends to curl or float in the solution, it may be
necessary to weight it. Weights usually furnished with solution feeders are made
of porcelain, but any heavy material which is resistant to the solution may be
used.
The discharge line from the feeder should also be as short and straight as
possible, but circumstances may sometimes require that a long discharge line
be used. If such is the case, try to avoid sharp curves or loops in the line which
could provide sites for precipitation buildup and subsequent blockage. If the
solution is being injected into a pipeline, the injection fitting should be installed
preferably at the bottom or underside of the pipe. Injecting solution into the
top of a pipe should be avoided since air collects there and can work its way
into the injection check valve or the discharge line and cause air-binding. There
should be a check valve and an anti-siphon valve at the injection point.
Most solution feeders come equipped with, or have available as an accessory,
an anti-siphon discharge valve. This may be mounted directly on the feeder
head, or at the injection point as above. In addition, particularly if solution is
to be fed into an open channel or a low-pressure pipeline, a "loaded" discharge
valve should be used. This is a spring-loaded check or diaphragm valve which
will not open until the feeder discharge pressure exceeds a certain fixed value.
A common setting is about 15 psi.
When mounting a solution feeder on a shelf or platform above the solution
container, it is advisable to off-set it sufficiently to permit access to the
container (or saturator) for filling and cleaning.
DRY FEEDER INSTALLATIONS
When installing a dry feeder, placement should be such that solution from
the dissolving chamber can fall directly into the chemical feed channel if
possible. If other considerations dictate that the feeder be placed some distance
from the point of application, the drain line should be as direct as possible,
59
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with adequate slope and of sufficient size to preclude precipitation build-ups
and subsequent stoppages.
Obviously, the dry feeder installation must be on a firm, level foundation if
the scales are expected to perform satisfactorily. If there is a small hopper on
the feeder, it must be readily accessible for filling, and if an extension hopper
is used, it should extend vertically upward to the filling area without angles
which could trap material. For the water supply line to a volumetric feeder,
there must be a section of flexible hose between the dissolving chamber and
the water pipe to permit free movement of the feeder and scale platform.
The water supply line to a dry feeder must be equipped with an air-gap
or mechanical vacuum-breaker, or some other type of anti-siphon device. The
air gap is the simplest and most positive protection against the dangers of a
cross-connection. If water pressure is too high to permit the use of an air gap,
one of the other devices may be used, but in any case the vacuum-breaker
must be placed between the point of entry to the feeder dissolving chamber
and any restrictive device in the pipeline, and must be installed in an elevated
location.
VALVES AND METERS
Shut-off valves are sometimes installed where solution is fed under pressure.
Their use permits repairs to the feeder or discharge line, but they also are a
hazard. Attempting to feed solution when the valve is closed can result in
ruptured diaphragms or discharge lines, or even severely damaged feeders. If
there is to be a shut-off valve at the injection point, always make sure it is kept
in the fully-open position.
The use of a sodium fluoride saturator requires that a water meter be
placed in the water supply line. A shut-off valve in this line is a necessity to
permit dismantling and cleaning the saturator. The valve must be placed
between the water meter and the saturator inlet so that the meter chamber will
remain filled with water and thus will register properly.
No water supply can be properly fluoridated unless the water flow rate in the
main is known, and calculated fluoride levels cannot be made unless a record
of total flow is available. Thus, a master water meter on the main is a pre-
requisite for fluoridation. While flow requirements may dictate the type of
water meter to install where none now exists, some thought should be given to
utilizing a meter signal for pacing a fluoride feeder.
INSTALLATION PLANS
Planning a fluoridation installation usually must be done with the con-
currence or approval of State Health Departments. However, before deciding
on a specific design, the sections on chemicals, feeders and auxiliary equip-
ment should be scanned for possible alternatives or inclusions. The installation
design can then be submitted for approval. Some examples of preliminary
planning of a fluoridation installation follow.
60
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Example 1. A small water supply consists of an unattended well pump, pump-
ing into an elevated storage tank. The pump operation is controlled by an
altitude switch on the elevated tank. There is no other water treatment, and
the only building is a small shed at the well site. Water pumping rate is about
250 gpm.
Plan: Provided the shed at the well site is large enough and of reasonably
sound construction, a saturator and solution feeder can be placed inside. The
solution feeder can be tied electrically to the operation of the well pump so
that feeding cycles will coincide with the on-off operation of the well pump.
The fluoride injection will be directly into the pump discharge line. If there is
enough room in the shed, and the building is weather-tight and can be locked,
a few bags of sodium fluoride can also be stored inside (off the floor). A water
meter ought to be installed in the main, if there is none at present, and a hypo-
chlorinator added as soon as possible.
Example 2. A water treatment plant is presently feeding chlorine, alum,
soda ash and carbon to a surface supply. There is an open channel in the plant
where post-filtration chemicals are added. The service pumping rate is 3000
gpm, and water is pumped directly from the clear well to the mains.
Plan: Since bulk fluosilicic acid is currently available nearby, the economics
of acid use appear favorable. The acid storage tank can be placed outdoors,
with a day tank and feed pump located at the site of post-filtration chemical
feed. An alternative system, for feeding sodium silicofluoride, will be provided.
This will consist of a gravimetric dry feeder with the dissolving chamber dis-
charge falling into the post-filtration feed channel. Chemical storage will be on
the floor above the feeder, where there is a separate area provided for fluoride
chemicals. An extension hopper from the feeder to the storage floor facilitates
loading. Either feed system will be controlled by electrical signals from a
rate-of-flow transmitter on the main.
Example 3. A city's water supply consists of eight wells, all of which pump
directly into the mains. There is an elevated storage tank, but it rides on the
water system and there is no point in the system where all the water can be
treated. Pumping rates vary from 200 to 500 gpm, with the operation of each
well pump controlled by pressure switches on the mains.
Plan: Since there is no central point where all the water can be fluoridated
with a single feed system, each well pump will have to be considered as an
individual water supply. The most economical approach would be to use a
solution feeder and fluosilicic acid at each pump location. The acid would be
purchased in carboys and platform scales will be provided at each location.
Operation of feeders will be controlled by well pump operation. Meters and
hypochlorinators will also be installed at each location.
61
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Chapter VI
Control And Surveillance
ANALYTICAL PROCEDURES
The ability to detect and measure accurately trace amounts of fluoride ion
was the key to the discovery of both the benefits and hazards of fluoride in
water. Now that fluoride is being deliberately added to water, it is perhaps
even more essential that the quantities present be measured accurately. Not
only must the fluoride concentration of the untreated water be determined, so
that one will know how much to add, but also the fluoride concentration of
the treated water must be determined as confirmation that the correct amount
of chemical is being added.
Because the quantity of fluoride ion being measured is so small (usually
about 1 part fluoride ion to 1 million parts water, or 1 milligram in a liter of
water), the analytical method must be adapted to the measurement of these
quantities and the accuracy of measurement must be well within the limits
established as the maximum permissible variation from the optimum concen-
tration. Thus, if a concentration of 0.9 to 1.1 (l.ChtO.l) ppm is considered
optimal for a certain area, the analysis must be accurate to 0.1 ppm or better
in order that one is assured the actual concentration succeeds or fails to fall
within these limits.
It should be remembered that all fluoride analysis concerns the determin-
ation of the quantity of fluoride ion in solution, irrespective of the source of
that ion. It may come from fluoride compounds occurring naturally in the
water, or from fluoride or silicofluoride compounds added to the water.
There is no method for distinguishing one fluoride ion from another, nor is
there any method for determining to which metallic ion the fluoride ion was
originally attached.
THE STANDARD METHODS
The generally-accepted analytical methods are outlined in detail in the latest
edition of "Standard Methods for the Examination of Water and Wastewater."
The Thirteenth Edition (1971) lists three methods: the Alizarin Visual (Scott-
Sanchis), the Photometric (SPADNS) and the Electrode Methods. There are
additional methods available, mostly based on colorimetric principles similar to
those used in the first two methods above, and there are modifications of the
Standard Methods which adapt them to the use of simplified equipment and
63
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procedures, that is, the co-called "test kits." "Standard Methods" also
describes a preliminary distillation procedure, a step necessary when substances
present in the water interfere with the accurate determination of the fluoride
ion. As with the analytical methods themselves, other preliminary procedures
are available, but, just as with the unlisted analytical methods, these procedures
are regarded as inferior to the one listed in "Standard Methods."
ALIZARIN VISUAL METHOD
The basis for the alizarin visual method is the zirconium-alizarin reaction,
in which a red lake (a deep color) is produced by the combination of alizarin
and zirconium. Any fluoride present in the water sample or standard solution
removes zirconium from the reaction, thus decreasing the intensity of color
present. In water samples which are high in fluoride, the only color apparent is
the yellow color of the unreacted dye. (Alizarin is sensitive to pH and its color
will vary from purple in alkaline solutions to yellow in acid.) Conversely, in
low fluoride samples, the color approaches the deep red of the zirconium
alizarin lake. Intermediate fluoride concentrations give colors which are
intermediate between these two. The reaction is not an immediate one, but
progresses with time. After one hour, the reaction rate is extremely slow, and
for this reason, the time interval between adding reagent and making the
measurement has been selected as 60 minutes.
The color comparisons are made in 100-ml Nessler tubes - tall glass cylindri-
cal tubes with flat bottoms. Usually, these are held in a rack with a reflector
below the tubes so that light is reflected up through the longitudinal axis of
the tubes to the eye of the observer. In practice, a number of tubes containing
standard fluoride solutions at fixed concentration intervals are prepared in
addition to the tubes containing the unknown samples. Fluoride reagent is
added to both standards and samples, and after the one-hour color development
time, the unknown samples are compared visually with the standards and a
determination made by matching the colors. Sometimes, this involves inter-
polation. For example, if the color of the unknown sample appears to be mid-
way between that of an 0.8 ppm standard and a 1.0 ppm standard, the
unknown sample is determined to have a fluoride concentration of 0.9 ppm.
Obviously, the smaller the interval between standards, the greater the likelihood
of attaining a close match between color of sample and that of one of the
standards.
SPADNS METHOD
The photometric method is based on a reaction similar to the one used for
the alizarin visual determination - a dye lake is formed with zirconium and
SPADNS dye, and any fluoride present has the effect of decreasing the intensity
of color. However, the colors produced by different concentrations of fluoride
are all shades of red, making it almost impossible to detect these differences
by eye and making the use of a photometer imperative.
64
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A photometer is an instrument for detecting differences in color, and
consists of a light source, a means for producing monochromatic light, and a
photocell for measuring the intensity of the light transmitted through the
sample. There are two general types of photometer; the filter photometer
which produces monochromatic light by the use of a colored glass filter, and
the spectrophotometer which achieves the same end by the use of a prism or
diffraction grating.
The operating procedure for the use of the photometer in the SPADNS
method for fluoride analysis depends on the particular instrument used, but
in general the determination consists of adding a measured volume of reagent
to a measured volume of sample, placing a portion of the mixture in a cell or
cuvette, placing the cell in the instrument and taking the absorbance reading.
The absorbance reading is then converted to fluoride concentration by consult-
ing a curve prepared by plotting the absorbance of known standard .solutions
against their concentrations. Unlike the visual method, the photometric method
involves a reaction which is instantaneous, permitting completion of the analysis
within seconds after the reagent is added.
ELECTRODE METHOD
The electrode method is based on the use of a specific-ion electrode,
resembling somewhat the electrodes used for pH determinations. In the case
of the fluoride electrode, the key element is a laser-type doped single crystal
through which only fluoride ions can move. When the electrode is immersed
in a solution containing fluoride ions, an electrical potential is set up between
the solution and a standard fluoride solution within the electrode. This
potential, expressed in millivolts, can be either positive or negative depending
on whether the fluoride concentrations of the sample is lower or higher than
that of the internal solution.
Since only fluoride ions can move through the crystal, the electrode is
essentially specific for fluoride. A buffer is added to the sample to eliminate
potential interferences and to insure uniform electrode response despite
variations in total ionic concentrations in the sample.
In addition to the fluoride electrode itself, the equipment needed for
fluoride determinations includes a reference electrode and a meter for reading
the millivolt potential. As with the photometric method, the fluoride electrode
method requires that a standard curve be prepared using solutions of known
fluoride concentration.
Except for the electrode method, none of the fluoride analytical methods are
entirely specific for fluoride, and can be subject to error caused by other
substances in the water. To avoid the effect of these interferring substances,
the water sample can be distilled from a solution of sulfuric acid. The acid
converts the fluoride to a volatile compound which is distilled over by the
application of heat, leavir" u"hind most other mineral constituents of the
water.
65
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TEST KITS
Test kits based on the standard methods include color comparators, portable
colorimeters, and portable ion-meters. The color comparators, based on the
alizarin visual method, use a set of permanently colored standards with which
the reagent-treated sample is compared. These standards take the form of
colored glass or colored liquids in glass vials, and eliminate the necessity of
preparing standards each time a water sample is to be analyzed. The portable
colorimeters, based on the SPADNS method, are small photometers which are
pre-calibrated so that the fluoride concentration in a reagent-treated sample
can be read directly from a scale or chart without the necessity of preparing
standards or a standard curve. The portable electrode meter is similar, in that
the scale of the meter is essentially pre-calibrated so that fluoride concentra-
tions are read directly instead of being converted from millivolt readings.
SAMPLING
Surveillance of fluoridation, based on these analytical methods, is absolutely
necessary for control of the process. While state health departments may require
only occasional tests, a minimum sampling of once per day is recommended.
Sampling should include both the plant tap and some point in the distribution
system, and if the water source is a surface supply, a raw water sample as well.
When fluoridation is just starting, sampling should be more frequent, and
should include a representative number of points in the distribution system.
The fluoride ion, being a stable and persistent substance, provides an excellent
indicator of flow patterns, and distribution system analyses will show how
long it takes for fluoridated water to reach the ends of mains and whether or
not untreated water is entering the system. One possibility of the latter
occurring is when a reservoir or standpipe is merely "riding" on the system,
contributing unfluoridated water during periods of low or no pumping.
Eventually, the water in such storage facilities is replaced with fluoridated
water, but in some instances, the exchange is so slight that a considerable
amount of time elapses after fluoridation begins before the full fluoride level
is reached. Other possibilities include infiltration and cross-connections.
CALCULATIONS AND RECORD-KEEPING
Besides analytical determinations of fluoride concentration in the water,
daily records of weight of chemical fed and volume of water treated will
provide a record of the theoretical fluoride concentration for that day and will
furnish an additional check on the adequacy of the treatment process. Most
states require that records be kept for both analytically determined and
calculated fluoride concentrations.
The calculations are simple, using the same formula previously used for
dosage calculations, etc.:
66
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RI X Cl = R2 X C2, where R! is the rate or quantity of water being
treated in gpm or gallons,
C i is the fluoride concentration in the water
in ppm,
R2 is the rate or quantity of fluoride compound
added in Ibs/min or Ibs,
C2 is the fluoride concentration in the
compound used, in percent or Ibs/100 Ibs.
So, the calculated fluoride concentration (not including any fluoride already
present in the water) is:
R2 XC2
Cl=~~R7~
By converting gallons to pounds, and cancelling units, the results will be in
pounds per million pounds, or ppm.
Examples:
1. A plant is using sodium fluoride solution, 2% strength (7 Ibs dissolved
in 40 gal. of water. See Table 3), and fed 100 gallons in one day. During that
day, 1 million gallons of water were pumped. What is the calculated fluoride
concentration (disregarding any natural fluoride content)?
_ R2 X C2 _ 100 gal. X 7 lbs/40 gallons X 0.45 X 0.98
1 ~ R! 8.331bs/galX 1,000,000 gal
(0.45 is the fraction of sodium fluoride which
is fluoride ion, 0.98 is the commercial purity)
d = 0.92 Ibs/million Ibs, or 0.92 ppm
2. When 50 Ibs of 23% fluosilicic acid are added to 1 million gallons of
water how much fluoride has been added (in ppm)?
_ SO Ibs X 0.23 X 0.79
1 ~ 1,000,000 gal X8.33 Ibs/gal
(0.23 is the acid strength, 0.79 is the fraction of H2SiF6 which is
fluoride ion.)
= 1.09 Ibs/million Ibs, or 1.09 ppm
3. A dry feeder record indicated a loss in weight of 26 pounds of sodium
silicofluoride in one day. During that time, 1.8 million gallons of water were
treated. The commercial purity of the sodium silicofluoride was 98%. Calculate
the concentration of fluoride added.
= 26 Ibs X 0.607 X 0.98
1 ~ 1,800,000 gal X 8.33 Ibs/gal
(0.607 is the fraction of Na2SiF6 which is fluoride ion, 0.98 is the commer-
cial purity.)
= 1.03 ppm
67
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In calculating the pounds of fluoride ion added, both the fraction of
fluoride ion in the pure compound and the purity of the chemical must be
taken into account, as in the above examples. Note that these calculations
give the amount of fluoride added. If there is a measurable concentration
of fluoride in the water before this addition, it must be taken into account in
order to find the total theoretical fluoride concentration. For example, if the
raw water in (1.) above had 0.5 ppm F before treatment, and 0.2 ppm after
flocculation and filtering, the total F concentration would be 0.2 plus 0.92
or 1.12 ppm. Note again that raw water fluoride can be reduced by filtration,
and only that natural fluoride present at the point of fluoride compound
addition may be included in the calculations.
Calculations can be simplified by using charts or tables, such as Tables
3 and 6.
Examples:
1. Given the same conditions as in No. 1. above, calculate the fluoride
concentration using Table 3.
100 gal. X 9000 ppm no
c'= 1,000,000 =0-9PPm
(Note that using tables will give approximations only.)
2. Using information from Table 6, calculate Example (2.) above. If 45.7
Ibs of 23% acid will add 1.0 ppm F to 1 million gallons of water, 50 Ibs. would
add: 50/45.7 or 1.09 ppm.
3. Using information from Table 6, calculate Example (3.) above.
Calculation: The table gives pounds of chemical for 1 million gallons. For
1.8 million gallons, the quantity to produce 1.0 ppm would be: 1.8 X 14 Ibs.
Since 26 pounds were actually used, the fluoride concentration would be:
1.8 X 14Slbs. X L°PPm= !-03 ppm
Another device often used to simplify calculations is the nomograph, or
alignment chart (Figs. 18 & 19). Nomographs are available for specific com-
pounds, or for several different materials as in the illustrations. They may take
into account the purity of the chemical, or may not. In any case, they are not
likely to be very accurate, but are useful for at least getting rough estimates of
chemical requirements or, by working backwards, for estimating the theoretical
fluoride concentration achieved.
MONITORS
A monitor is a device for automatically providing a continuous record of
analyses: in this case, for providing a continuous record of fluoride concentra-
tion. The advantage of a continuous record over a spot-check, such as a daily
fluoride analysis, is that the continuous record will show the fluoride concen-
tration at any given time rather than only at the one time the daily sample
was taken. This type of record could prove helpful in answering complaints
regarding under- or over-feeding, as well as in detecting variations in fluoride
68
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Table 6. FLUORIDE CALCULATION FACTORS
Compound
Sodium
Fluoride
Sodium
Silicofluo-
ride
Fluosilicic
Acid
Available F
(Pounds F per Pound of
Compound at 100% purity)
0.4525
0.607
0.792
Commercial
Purity
95%
96%
97%
98%
99%
95%
96%
97%
98%
98.5%
99%
20%
21%
22%
23%
24%
25%
26%
27%
28%
29%
30%
Pounds Compound
per Million Gallons
of Water to Add
1 .0 ppm F
19.4
19.2
19.0
18.8
18.6
14.5
14.3
14.1
14.0
13.9
13.8
52.5
50.0
47.8
45.7
43.8
42.0
40.5
38.9
37.4
36.3
35.2
The figures in the column at the extreme right can be used for calculating the
amount of fluoride compound to add for quantities of water other than one
million gallons. For example, the amount of 97% sodium fluoride needed for
100,000 gallons would be 100,000/1,000,000 or 0.1 the amount indicated.
For two million gallons, the amount of fluoride compound needed would be
twice as much, etc.
Similarly, if other than 1.0 ppm F is to be added, because of the presence of
natural fluoride or because of an optimum concentration other than 1.0 ppm,
multiplying the figures in the right-hand column by the appropriate factor will
give the number of pounds to use. For example, if there is 0.3 ppm F
occurring naturally, and the optimum level is 0.8 ppm, only 0.5 ppm would
have to be added, or 0.5 as much as indicated.
(Level desired, 0.8 ppm, minus natural fluoride, 0.3 ppm, eqvals 0.5 ppm, the
amount to be added.)
69
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INSTRUCTIONS
1. Substract the natural level from the desired
level to get the fluoride treatment figure
(ppm fluoride ion added).
2. Draw a line from the flow figure straight
through the fluoride treatment figure to
the fluoride ion line. The intersection will
show the pounds of fluoride ion per day
required.
3. This weight of fluoride ion may be obtained
from any of the chemicals listed. Starting
at the pounds of fluoride ion per day required.
draw a horizontal line to the right and read
the weight of dry chemical or volume of
liquid chemical required.
4. Thus in the example, a flow of 600 gpm
requiring 0. 9 ppm fluoride ion added
requires 6. 45 pounds of fluoride ion per
day. This can be obtained from 14. 8 Ib
per day of sodium fluoride, 11. 0 per day
of sodium silicofluoride, 2. 55 gal per day
of 30% fluosilicic acid, or 44. 2 gal per day
of saturated sodium fluoride solution.
Figure 18. Fluoridation Nomograph
70
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concentration for unexplained reasons. If the monitor is equipped with an alarm
system, it can alert the operator to feeder malfunctions or other problems
affecting fluoride level.
Most monitors are actually continuous or semi-continuous analyzers,
performing colorimetric or electrical analyses on a flowing sample stream
or on discrete water samples taken at close time intervals. Some monitors use
the SPADNS procedure, metering out reagent in proportion to sample flow
and then passing the treated sample through a photometer. A recorder converts
absorbance readings of the photometer to concentration figures which then
appear on a recorder chart. Other monitors use the electrode procedure,
either with or without the addition of buffer. When no buffer is used, the
fluoride and reference electrodes are immersed in the water sample stream and
an appropriate recorder converts millivolts of potential to parts per million of
fluoride. Obviously, the absence of buffer may render this type of monitor
unreliable when there is a change in water quality. When buffer is used, the
monitor record is definitely more reliable. A monitor using buffer is necessarily
more complicated, since metering pumps must be used for both water sample
and buffer to insure maintenance of the correct ratio of volumes of each. Still
another monitor uses a different type of electrode and buffer, and is based on
an analytical procedure adapted specifically to flowing sample streams.
There are other types of monitors which are not analyzers, for example,
the one based on conductivity measurements. The differences in conductivity
between untreated water and water after fluoride has been added can be
interpreted to give the amount of fluoride added, but here again we have a
system which may be unreliable when a change in water quality occurs,
especially if that change involves a change in the raw water fluoride content.
Some water plants employ monitors on both the raw water and the treated
water, the former being used to alert the plant operator to changes in raw
water fluoride level. If the treated-water monitor is equipped as a controller,
it can be used to automatically adjust the fluoride feeder to deliver fluoride
compound so that the final fluoride concentration is maintained within pre-set
limits.
It should be pointed out that a monitor, even one used as a controller, does
not relieve water plant personnel from the necessity for making the prescribed
fluoride analyses at the prescribed intervals. If for no other reason, the regular
spot-checks will serve to verify the proper functioning and accuracy of the
monitor.
TROUBLE-SHOOTING
The ideal situation occurs when the fluoride feed rate is accurately
calculated, the water flow rate does not vary, and once set, the fluoride
feeder operates merrily day after day, providing exactly the desired concen-
tration of fluoride in the water without the slightest variation and with no
attention from the operator other than the occasional refilling of a hopper.
There may be such situations, but even if they exist, the water plant personnel
ought to know about the potential difficulties and how to overcome them.
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LOW FLUORIDE READINGS
When the fluoride concentration determined by analysis is consistently
lower than that determined by calculation, there are a number of possible
explanations. Presuming that the calculations are correct, and are based on
accurate weight and flow figures, the first logical suspect is the analysis. If alum
is used for flocculation, traces of aluminum in the finished water can interfere
markedly with colorimetric analysis, always influencing the readings negatively.
A high iron content can do the same thing if the SPADNS method is used. In
rare cases, chloride and alkalinity can interfere also, but the concentrations of
these would have to be extremely high. The analysis of a distilled sample, or
conversion to the electrode analytical method can confirm or rule out this
potential error.
If the analysis can be verified as accurate, the'next suspect would be
chemical purity. Fluosilicic acid has the most variable purity, and can be
anywhere from over 30 down to 20% pure. Ordinarily, the manufacturer
specifies the purity of a given batch, but if there is some doubt, the acid
should be analyzed according to directions given in AWWA Specifications.
Sodium fluoride and sodium silicofluoride usually exhibit less variation in
purity, but occasionally a relatively impure lot is found. The AWWA specifi-
cations give procedures for determining purity of these materials also.
If sodium fluosilicate is being used, the possibility of incomplete solution
should be explored next. An examination of the feeder dissolving chamber and
the area near where the solution is being fed should reveal deposits of
undissolved chemical if detention time is insufficient, solution water flow rate
wrong, or baffling of the dissolving chamber ineffective.
In the case where manually prepared solutions are used, incomplete mixing
can be at fault, as can be the measurement of both chemical and water.
If none of the above possibilities prove to be the source of error, and all
measurements and calculations have been checked out, it may be advisable to
check out the water system to find out if unfluoridated water is entering at
some point and is thus diluting the water being fluoridated at the plant.
HIGH FLUORIDE READINGS
When fluoride concentration determined by analysis is consistently higher
than that determined by calculation, analysis, and chemical purity can also be
the sources of error. The presence of Calgon or other polyphosphate is the
usual cause of analytical error in the positive direction. Failure to eliminate
chlorine from the water sample can also lead to high results in colorimetric
analysis.
Failure to take into account the fluoride content of the raw water may
result in adding more fluoride than is needed, and surface supplies, which
can. show considerable variability, should be analyzed daily in order that the
correct dose can be calculated. If the water supply comes from wells, the
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variability is much less, but in the case of a higher-than-calculated fluoride
concentration the possibility of a contribution from a high-fluoride well
should be investigated.
VARYING FLUORIDE READINGS
The most difficult type of problem to solve is the situation when the
fluoride concentration is variable even though calculations show that the
fluoride feed rate is in the desired proportion to water flow rate. One
possibility which can be readily eliminated is the fluoride feeder. A check on
the delivery rate, with weight measurements at short intervals, will reveal
whether or not the feeder delivery rate is constant.
Almost all of the factors which can produce consistently low or consistently
high fluoride analyses can also produce variable errors, if the analytical
interference, the chemical purity, the raw-water fluoride or the completeness
of chemical solution are variable conditions. In the latter case, undissolved
sodium silicofluoride can eventually go into solution after a quantity of
undissolved material accumulates at some point, and a solution feeder can begin
drawing from a concentrated stratum after feeding from a dilute stratum in an
improperly mixed solution tank.
One of the causes of varying fluoride content in a treated water system
is the intrusion, on an intermittent basis, of un-fluoridated water into the
system. This can be from an outside water source, such as a well or a connection
to another system, or can be from a storage reservoir which is part of the
system. The lattej situation occurs usually when fluoridation is just beginning,
and no attempt has been made to fluoridate the reservoir separately. What
happens then is that during periods when no water is being pumped, or the
pumping rate is less than the demand, water flows into the system from the
reservoir, and since this water has not yet been fluoridated, low-fluoride
readings will result, particularly at the sampling points nearest the reservoir.
Eventually, what with flow pattern reversals as the pumps operate inter-
mittently, the reservoir contents will become displaced by fluoridated water,
but there have been cases, involving a large reservoir at the end of the water
system, when it has taken years before there was a complete turn-over of the
reservoir contents. The obvious solution to this type of problem is, of course,
to fluoridate the reservoir separately at the time fluoridation of the system
begins, if such is possible.
A similar situation occurs when an elevated tank or other storage facility
merely "rides" on the system, and its contents rarely enter the system or at
best there is only slight intermixing. Sampling points near the tank will have
varying fluoride concentrations - normal when there is pumping, and low
when water is being drawn from the tank. The solution here is to allow the
tank contents to drain into the system before fluoridation begins and then not
refill the tank until the entire system is up to the optimum fluoride level.
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Cyclic fluoride levels can result when the feeder is operated intermittently,
such as when capacity is reduced by the use of a cycle timer, and there is
insufficient storage capacity between the feeder and the consumers. Detention
time in mains or storage facility between feed point and first consumer is an
important factor in providing homogeneous fluoridated water.
OTHER PROBLEMS
There are undoubtedly many other possible causes for fluoride levels
which aren't exactly what they are supposed to be, but one thing is certain -
fluoride doesn't "disappear" in the pipelines, nor is it likely that fluoride will
concentrate at points or become leached out of incrustations in the mains.
Unlike chlorine, fluoride does not have the ability to dissipate, and even
though trace amounts are incorporated into tubercles in pipelines, the extreme
insolubility of these formations precludes subsequent dissolution. When there
is an unexplained difference between the calculated and observed fluoride
concentration, more often than not, the calculations are at fault. If they are
not, and none of the above possibilities apply or they can otherwise be
eliminated, common sense and a knowledge of the individual system should
enable the operator to locate and correct the cause of trouble.
It should be noted that over- or under-feeding for short periods, for example
a variation of 0.2 or 0.3 ppm for two or three days, actually is of no serious
consequence, but such variations should be investigated since they may be
indications of potential problems of a more serious nature.
Erratic feed rates or failure to feed are usually due to problems which are
discussed in the chapter entitled, "Maintenance." Saturator problems are
discussed in the chapter on solution preparation, and other potential troubles
are mentioned in the chapter on "Selecting the Optimal Fluoridation System."
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Chapter VII
Maintenance
To insure uninterrupted and unvarying fluoride feed, proper maintenance of
equipment is required. This includes maintaining not only the fluoride feeder,
but also all the appurtenances, feed lines and the laboratory testing equipment
as well.
CLEANING AND LUBRICATION
Like any other mechanical device, fluoride feeders must be kept clean and
lubricated if they are expected to perform their function efficiently. A
regular program of maintenance will also minimize costly break-downs and
insure long life for the equipment. Electric motors usually come with a pre-
scribed schedule for lubrication - the right type, amount and frequency of
lubrication are all important. Gear boxes must be kept filled to the prescribed
level with the proper lubricant, and all moving parts and unpainted metal
surfaces should be kept clean and rust-free. If there are grease fittings, the
proper grade, quantity and frequency of greasing should be observed.
SPARE PARTS
Fluoride feeders and related equipment, when purchased, usually are
accompanied by an instruction booklet and/or parts list. The instruction
book will contain information on maintenance and repairs, and the parts list
will enable the operator to select replacement parts when needed.
If these papers have been lost, a call or letter to the manufacturer of the
equipment or his representative will suffice for replacement. The equipment
man will also be happy to suggest a list of spare parts to be kept on hand.
Having parts available can greatly minimize the length of shut-downs due to
equipment failure. In the larger water plants, having an entire spare feeder
available may prove to be prudent.
INSPECTION AND RE-CALIBRATION
The best fluoride feeders will feed as intended only if the measuring
mechanism is kept clean and operative. Thus, the diaphragms or pistons of
solution feeders, the rolls, belts, discs or screws of dry feeders and associated
mechanisms of both types should be regularly inspected for signs of wear or
damage and repairs or replacements made before the machine actually breaks
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down. Even with all parts in the best mechanical condition, fluoride delivery
can be affected by leaks, spillages, build-ups of precipitates from solutions or
accumulations of dry chemicals on or around measuring mechanisms.
Occasional re-calibration of the feeder will reveal evidence of potential mal-
function, and is generally a good idea for insuring accurate feed rates.
LEAKS
Leaks in and around the discharge line of a solution feeder are more than
an annoyance — they can materially affect the quantity of solution delivered
and thus result in low fluoride levels. Leaks are always somewhat corrosive,
ranging from the salt effect of sodium fluoride solutions to the acid corrosion
of fluosilicic acid. Even the smallest leak can result in damage to the feeder,
appurtenances or surroundings is left unattended. Leaks of strong solutions
result in the formation of crystalline deposits which, if allowed to build up,
make subsequent cleaning difficult. A leak in the suction line of a solution
feeder, while not immediately apparent, will adversely affect delivery and can
eventually lead to air-binding and cessation of feed. Air-binding can also
be caused by injecting fluoride solution at the top of a main, where air can
collect.
Leaks in a dry feeder installation cause a dust problem, and if there is a
leak in the feeding mechanisms, for example around the rollers of a volumetric
feeder, there will be an error in feed rate. The dust represents, in addition to
an economic loss, a hazard to equipment and personnel.
Leaks in equipment not related to fluoride feed can present problems none-
theless. For example, a water leak can result in dampness and subsequent
caking of dry chemicals. A leak in a chlorine gas system can result in damage to
feeders and associated equipment, etc. Leaks in other dry feed equipment can
result in dust contamination of the fluoridation installation.
PRECIPITATES
Anytime strong solutions are used, the possibility of precipitation build-up
is present. In a solution feed system, precipitates in the feeder pumping
chamber or on the check-valves will affect delivery rate or even stop the
feeder entirely. Deposits in suction or feed lines can build up until flow stops,
and a coating of insoluble matter on a saturator bed can prevent water from
percolating through. If the deposits are the result of water hardness, softening
the make-up water will eliminate the pioblem. If softening is impractical,
frequent inspection and removal of th
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Tanks in which solutions are prepared invariably show precipitates of the
insoluble impurities from the chemical used or of insoluble compounds formed
by the reaction of the chemical with mineral constituents of the water. If a
separate tank is used for solution preparation, and the clear supernatant layer
transferred to a day tank, problems will be minimized, but not necessarily
completely eliminated.
A regular schedule for cleaning out a saturator should be established, the
time interval between cleanings depending on the amount of usage and the
accumulation of impurities in the saturator. The cleaning operation is described
in the chapter on solution preparation.
STORAGE AREA
Storage areas, while not necessarily a major factor in maintaining accurate
feed of fluoride, need also to be kept in an orderly and clean condition. Bags
of dry chemicals should be piled neatly on pallets, with empty bags rinsed out
and disposed of promptly. Whenever possible, whole bags should be emptied
into hoppers, since partially empty bags present a spillage hazard and are a
nuisance to store. Metal drums should be kept off the floor and kept tightly
closed. The storage area for fluoride chemicals should be well isolated from
areas used to store other chemicals to preclude mix-ups, and all extraneous
material, such as lubricating oil and cleaning equipment, should be kept out of
the area. Both the stored chemicals and the general area should be kept free
of dust, not only from the chemicals used in fluoridation but from other
chemicals stored or used nearby. In general, a neat and clean storage area is an
indicator of good water plant practice and maintenance.
LABORATORY
The laboratory or fluoride testing area calls for special precautions in
cleanliness. The merest hint of fluoride dust on glassware can result in gross
analytical errors. Dirty pipets or other measuring glassware can produce errors
in volume measurements, and proper drainage is hindered, resulting in carry-
over of solutions from one sample to the next. In colorimetric analysis, dirty
glassware prevents accurate color determination whether the method is visual
or photometric. Permanent standards for colorimetric test kits, since they are
difficult to clean, should be kept covered when not in use. Reagent bottles,
buffers and standard solutions should all be kept tightly closed when not in
use. Evaporation ruins not only the reagents, but acid fumes from them can
damage objects nearby. Analytical instruments should always be covered
when not in use, since fumes and dust can corrode electrical connections,
cloud mirrors, and result in expensive repairs or replacement.
The use of phosphate-based detergents for cleaning glassware presents a
hazard because of potential interference of phosphates in colorimetric analysis.
Since tap water contains fluoride, it is imperative that all glassware be
thoroughly rinsed in distilled water of good quality.
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The quality of distilled water is best verified by conductivity measure-
ments, but when equipment for this procedure is lacking, help can often be
obtained from State Health Departments. This help can take the form of
checking a water plant's distilled water or of furnishing a sample of good
quality distilled water for comparison. Comparing fluoride analyses of the
plant's distilled water vs. that furnished by the State will reveal contamination,
if any. (Many State Health Department laboratories will furnish a standard
fluoride solution for check purposes, also.)
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Chapter VIII
Safety And Hazards
In Handling Fluoride Chemicals
SAFETY EQUIPMENT AND CHEMICAL HANDLING
Admittedly, fluoride chemicals, in the large quantities likely to be present
in a water plant, present a health hazard to plant personnel. While fluorides in
water, at the recommended concentration of about 1.0 ppm, have been
exhaustively studied and have been firmly established as safe beyond question,
the fluoride levels to which a water plant operator can be exposed are at least
potentially much higher. Since the operator is presumably already drinking
water containing the optimum level of fluoride, any additional contact with
fluorides constitutes overexposure. Obviously, the best safety measure is the
prevention of unnecessary hazards, and this implies the proper handling of
fluoride chemicals and the use of adequate safety equipment.
INGESTION
Overexposure to fluoride chemicals can be the result of ingestion, inhalation,
or bodily contact with spills. The most likely source of oral exposure would
be the contamination of food or drink, either by accident (such as if the
fluoride chemical were mistaken for sugar or salt) or through carelessness (such
as if meals were eaten in areas where fluorides were stored or applied).
INHALATION
Perhaps the greatest chance for overexposure to fluoride chemicals comes
from the accidental inhalation of dust. While the use of masks and other
protective devices is commendable, everything possible should be done to
minimize the production of dust. Logically, if fluoride sacks are moved care-
lessly or if the bags are emptied too quickly, the concentration of fluoride
dust in the air will rise. Cautious handling will help a great deal in keeping the
concentration of fluoride dust at a safe level. As examples: bags should not be
dropped; bags should be opened with an even slit at the top to avoid tearing
down the side; and the contents of bags should be poured gently into hoppers.
The use of crystalline chemicals and installation of bag-loading hoppers are
other ways for reducing the production of dust in the air. Good ventilation is
absolutely necessary in work areas, even if there is no visible dust and masks are
worn.
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SAFETY PRECAUTIONS
The recommended safety equipment for handling fluoride chemicals includes
goggles, gloves, aprons, boots, dust masks, respirators, exhaust fans, dust
collectors, etc. While the use of this equipment will minimize an individual's
contact with the chemicals, additional safeguards should be applied. First,
meals and snacks should be eaten in non-work areas in order to avoid contact
with fluoride powders and dust. If there are no cafeterias or spare rooms, then
going outdoors should be considered whenever possible. If it is necessary to
eat in a work area, then person should be additionally cautious in avoiding
contamination of food. (Blue-tinted sodium fluoride and sodium silicofluoride
will prevent mistaking fluoride for food, and will also give clues as to the
dispersion of fluorides in the work area.)
Second, the use of properly labeled fluoride containers will minimize
the danger of mistaken identity, or the inherent danger of a container Which
has no label at all. It is best to keep fluoride chemicals in their original con-
tainers, but if this is not always practical, these chemicals should be kept in
labeled containers restricted to fluoride chemicals only. A further precaution,
required in some States, is to keep fluoride chemicals in a separate, locked,
storage area.
ACID HANDLING
Fluosilicic acid requires special precautions in handling. Spilling on the skin,
splashing in the eyes and inhaling vapors are all serious hazards. The absorption
of fluoride through the skin is negligible, but the acid is corrosive and it is this
characteristic which constitutes a hazard in skin contact. Careful handling,
quick rinsing of spills or splashes, the use of respirators, and especially ventila-
tion are the best safety measures.
FIRST AID
There is no record of any water plant operator ever being seriously injured
in the handling of fluoride chemicals, but knowledge of first-aid measures for
treatment of accidental fluoride over-exposure is a good thing to have.
FLUORIDE EXPOSURE SYMPTOMS
It is important to be able to detect if someone is suffering from over-
exposure to fluorides, so that treatment can be initiated early, thereby
increasing the patient's chances for complete recovery. In acute poisoning,
generally the first symptoms to appear are vomiting, stomach cramps and
diarrhea. If the poisoning is due to the igestion of large amounts of fluorides,
the vomitus may be white or colored, depending on whether the fluoride
contains dye. The appearance of the vomitus may be important in determining
whether fluoride is indeed the toxic agent. Illness could be due to something
else. Usually the patient becomes very weak, has difficulty in speaking, is
thirsty, and has disturbed color vision.
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The signs of acute poisoning by inhalation consist of sharp biting pains in
the nose followed by a nasal discharge or nosebleed. The most likely response
to an acid spill or splash is a tingling or burning sensation, or if the eyes are
involved, severe eye irritation.
TREATMENT
The importance of rapid treatment cannot be over-emphasized. Once acute
poisoning by fluoride is apparent, treatment should be initiated while awaiting
proper medical assistance. The treatment is as follows:
1. Remove the patient from exposure and keep him warm.
2. Administer 3 teaspoonsful of table salt in a glass of warm water.
3. Induce vomiting by irritating the back of the throat with a spoon.
4. Administer a glass of milk.
5. Repeat the salt and vomiting procedure several times.
Treatment for the individual suffering from nosebleed after inhalation of a
high concentration of fluoride consists of the following:
1. Remove the patient from exposure.
2. Place the patient's head back while placing absorbent material inside
the nasal passages. Change the material often.
3. Take the individual to a physician.
Treatment for an individual suffering from acid exposure consists of rapid
and thorough rinsing of the affected area with copious quantities of water.
Further treatment, if necessary, should be performed by a physician.
In summary, the first goal in fluoride handling safety is to prevent poisoning
resulting from overexposure; this means, use safety equipment and avoid
unnecessary contact. Next, be alert to the early signs of fluoride-related
illness in yourself and your co-workers. Should illness from fluoride occur,
react quickly with the proper treatment for the patient.
Remember, no water plant personnel have ever been seriously injured by
fluorides. Do you want to be the first?
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Chapter IX
Technical Problems
Attributed To Fluoridation
As stated earlier, feeding fluorides is essentially the same as feeding other
water treatment chemicals, and problems can be expected just as they can be
with other chemicals. However, due to the controversial nature of the
fiuoridation process, many of the problems attributed to fluoridation are more
imagined than real.
CORROSION
One of the first objections to the feeding of fluorides was that the
chemicals were thought to be extremely corrosive and would cause disinte-
gration of the pipelines. Part of this erroneous conception can be traced to the
confusion over fluoride versus fluorine, and the association of fluoride with
hydrofluoric acid. It is well known that fluorine is a violently reactive gas, and
especially that hydrofluoric acid is strong enough to eat glass. What is added
to the water is an essentially neutral salt of hydrofluoric acid, or fluosilicic
acid or one of its salts, not fluorine or hydrofluoric acid. Fluorine could not be
used, since it is too hazardous and too expensive, and hydrofluoric acid has
been used in only one fluoridation installation. But even if they were used, the
important thing to remember is that the amount being put in the water is
usually around one part per million as the fluoride ion, and one part per
million of the most corrosive substance would have little or no effect at this
dilution. Fluosilicic acid and its salts exhibit a low pH in solution, the
depression of pH being most apparent in poorly buffered waters, but there is
no indication that such waters have their corrosivity increased by the addition
of fluoride. There has been no reported instance of pipeline corrosion traceable
to the fluoride content of the water.
ENCRUSTATIONS
The analysis of tubercles and other pipeline encrustations has in some
cases revealed an extremely high fluoride concentration, even in places where
fluoridation was not practiced! This revelation has given rise to fears that
fluoride will be absorbed in the pipeline, then released at dangerously high
concentrations at some future time. Again, these fears are groundless. The
deposition of fluoride in pipeline precipitates is so gradual that the quantity
lost from the water cannot be detected by the best analytical means, and
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studies have shown that feed-back from such postulated sources as pipe
coatings, tuberculation, treatment sludges and storage-reservoir sediments does
not occur. On the contrary, these studies show that to achieve undesirable
fluoride ion concentrations from such additions would require suspensions of
the coating, sludge, or rust in quantities so great as to render the water non-
potable and unfit for use. Even hydrant flushings with water clearly unaccept-
able for normal use have not produced concentrations of fluoride above the
optimum recommended in the U.S.P.H.S. Drinking Water Standards. In
another study, the use of a cold-chisel was required to dislodge the fluoride-
containing tubercles and even this extreme means failed to produce an appre-
ciable increase in the fluoride concentration.
A related fear is that it is impossible to maintain a constant fluoride
"residual" in a long pipeline. Part of this fear is based on the deposition of
fluoride in tubercles or other encrustations, as mentioned above, and part is
based on experience with chlorine. But fluoride is an ion, not a gas or unstable
entity as are chlorine or the hypochlorite radical, and the term, "residual,"
does not apply here. Fluoride is not dissipated by organic materials, as is
chlorine, and there is no "demand" to be satisfied before the amount added
can be detected in the water. In other words, the fluoride added to the water
can be fully accounted for by analysis immediately or at any subsequent
time. A study has proved that neither the length of line, period of flow, age
of pipe, nor materials to which the water is exposed affect significantly the
average fluoride ion content in distribution systems.
FLUORIDE LOSSES
There is one situation which does result in the loss of fluoride, and that is
when the fluoride is added before coagulation and filtering, or before
softening by the lime-soda ash process. In either of these processes fluoride is
lost by precipitation or absorption on floe, and the amount of loss is pre-
dictable. (At a dosage rate of 100 ppm alum, fluoride loss will be about 30%.
Loss of fluoride in the softening process is directly proportional to the con-
centration of magnesium in the water or the lime.) In most cases such losses
can be avoided by merely changing the fluoride addition point to a pipeline
leading from the filters or to the clearwell itself.
COMPATIBILITY
The question of compatibility of fluoridation with chlorination or any
other water treatment process has been raised, but aside from the processes
mentioned above, there does not appear to be any significant conflict. One
possible exception to this occurs when calcium-containing compounds, such
as lime or calcium hypochlorite, are used. If the calcium-containing chemical
and fluoride are injected in close proximity, a precipitate of calcium
fluoride will form, the amount of precipitate depending on the concentrations
of the respective solutions. The obvious preventive measure is to space the
respective injection points as far apart as possible.
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TASTES AND ODORS
Objectionable tastes or odors of the water have been attributed to fluoride,
but one can surmise that this imaginary response can be attributed to a
mental association of fluoride with chlorine. Taste panels have verified that
the fluoride ion is both tasteless and odorless, even at much higher concen-
trations than those employed in water treatment.
INDUSTRIAL PROCESSES
When fluoridation first began, there were fears that fluoridated water
would adversely affect many industrial processes which use the water. Some
of the processes investigated included baking, brewing, ice-making, and the
manufacture of chemicals, porcelain enamel and frit, drugs, soap, porcelain
insulators, food, beverages, etc. The effects of fluoridated water on steam
generation and sewage treatment have also been investigated. In only two
cases has there been any problem, and one of these is still not fully understood.
The first case involved the manufacture of baby food, in which the long
cooking process could possibly result in the concentration of fluoride as the
water evaporated. The Food and Drug Administration has ruled that the facts
in each particular case will be controlling, or in other words, there will be an
objection only if there is a significant concentration of fluoride in a particular
case. In such a case, defluoridated water will have to be used. The other
problem involved the manufacture of ice, in which an increased number of
cracked ice blocks was found after fluoridation began. While this phenomenon
has never been explained, the cracking of ice blocks can be corrected by the
use of a commercial additive commonly used by ice manufacturers long before
fluoridation began.
ENVIRONMENTAL EFFECTS
Recently, as part of the increased interest in the environment, there have
been fears that the waste water from municipalities which practice fluoridation
will be a detriment to the body of water which receives it, and that the fluoride
concentration in such bodies of water will continually build up as a result.
Experts agree that fluoride, in the concentrations usually found in municipal
wastewater, is not in any way harmful to fish or other aquatic life. Actually,
since all bodies of water naturally contain some quantities of fluoride, there
is very little change in the fluoride concentration of the water in these bodies,
due to the diluting effect of rain and run-off. As for the oceans and seas,
these naturally contain about 1.4 ppm fluoride, so the addition of municipal
waste water results in a localized lessening of the fluoride concentration.
Fluoride will not concentrate in a river any more than it will in a pipeline, and
the blending of fluoridated wastewater with the normal flow again results in
dilution.
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Chapter X
References And Suggested Reading
The following list of books, articles, and manufacturers' bulletins contains
much helpful information on fluoridation practice in general and on specific
installations and pieces of equipment. The books are usually available in the
larger libraries, and engineering reference libraries will probably have the
technical journals in which the articles appeared. Manufacturers' bulletins can
be obtained from company representatives or from the company itself. (Com-
pany addresses are given the first time the company name appears in the list.)
1. Manual of Water Fluoridation Practice, by F.J. Maier. McGraw-Hill
Book Company, Inc., New York ($8.50)
2. Fluoridation of Water Supplies, Ref. 2-SIC 49-2, B.I.F., a unit of General
Signal Corporation, Providence, Rhode Island 02901. (Free copies on request)
3. Fluoridation Systems, Wallace & Tiernan Division, Belleville, New
Jersey 07109. (Free copies on request)
4. Fluoridation Systems Designed for Hydrofluosilicic Acid, Precision
Control Products Corporation, Waltham, Massachusetts 02154. (Free copies
on request)
5. Fluorides (Water Fluoridation with Sodium Fluoride), Allied Chemical
Corporation, Morristown, New Jersey 07960. (Free copies on request)
6. AWWA Standards for Sodium Fluoride, Sodium Silicofluoride and
Fluosilicic Acid. AWWA, January 1971 and June 1971. ($1.00 each for
Members, $2.00 each for Non-Members)
7. Standard Methods for the Examination of Water and Wastewater, (13th
Edition) APHA, New York. ($22.50 for Non-members, $16.50 for Members of
APHA, AWWA, or APCF)
8. Fluoridation Chemicals - The Supply Picture, by E. Bellack and R.J.
Baker, JAWWA, April 1970. (Free copies from the authors)
9. Fluoridation in Major Cities of the United States, by W.T. Ingram and
G.W. Moore, JAWWA, September 1959.
10. Upflow Saturator, Precision Control Products Corporation, Information
Letter No. 49.
11. B.I.F. Sodium Fluoride Saturator, BIF Specification Data Sheet 1210.
201-2.
89
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12. W & T Fluoride Saturator, W & T Brochure 60.510.
13. Water Meter Operated Feeder, W & T Brochure 80.110.
14. W & T Loss-of-Weight Recorder, W & T Brochure 370.120.
15. W & T Volumetric Dry Feeders, W & T Brochures 320.100 & 320.120.
16. W & T Gravimetric Dry Feeder, Brochure 310.100.
17. W & T Solution Feeders, W & T Brochures 80.120 and 80.135.
18. Precision Solution Feeders, Precision Control Bulletin 132.
19. The Art of Feeding Fluoride, BIF 2.20-3.
20. Water Fluoridation with Fluosilicic Acid, Technical Bulletin F-l-670,
Grace Agricultural Products Division, Baltimore, Maryland 21203.
21. Guidelines to Automatic Proportional Chemical Feeding, Precision
Bulletin 960-357.
22. Continuous Analysis and Control of Fluoride, by R.J. Walker and R.R.
Smith, JAWWA, April 1971.
23. Anafluor Continuous Fluoride Analyzer, Fischer & Porter Specification
17S4000 & 17S4400, Fischer & Porter Co., Warminster, Pa. 18974.
24. Chemonitor pF Fluoride Analyzer, Calgon Bulletin No. B-5-054.
Calgon Corp., Pittsburgh, Pa. 15230.
25. Practical Automation of Water Treatment Monitoring and Control
Systems, by H.M. Rivers & G.W. Sweitzer, Calgon Corporation.
26. Fluoride Automatic, Continuous Analyzer/Controller, Delta Scientific
Specification Sheet 8030, Delta Scientific Corp., Lindenhurst, N.Y. 11757.
27. Foxboro Model 32FMS - A Fluoride Measuring System for Potable
Water, General Specification Sheet GS 6-1A2 A, The Foxboro Co., Foxboro,
Mass. 02035.
28. A Selective Ion Electrode System for Fluoride Analysis, by R.H.
Babcock & K.A. Johnson, The Foxboro Company.
29. Automatic Fluoride Monitor, Advance Information Bulletin, E.I.L.
(Electronic Instruments Limited) England. (Cambridge Instrument Company,
Ossining, New York 10562).
30. Technicon CSM-6 Water Monitor, Technicon Instruments Corporation,
Tarrytown, New York 10591.
31. Beckman Fluoride Ion Analyzer, Beckman Bulletin 4102, Beckman
Instruments, Inc., Fullerton, Calif. 92634.
32. Water Analysis Instrumentation, Hach Series CR-2 Bulletin, Hach
Chemical Co., Ames, Iowa 50010.
33. AES Series 1400 Process Analyzers, Automated Environmental Systems,
Inc., Woodbury, L.I., New York 11797.
34. Safe Handling of Water Works Chemicals, by R.W. Ockershausen, JAWWA
Vol. 63, June 1971.
35. W & T Fluoride Analyzer, W & T Cat. File 810.030.
36. Automatic Control for Precision Chemical Metering Pumps, Precision
Bulletin 240.
90
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37. Omega Gravimetric Dry Feeder, BIF Ref. No. 31-12.201-1.
38. Omega Volumetric Dry Feeder, BIF Ref. No. 25-04.201-1.
39. BIF Diaphragm Pumps. BIF Ref. 1210-201-1.
40. BIF Rotodip Feeder. BIF Ref. No. 65.201-1.
41. Manual or Automatic Fluoridator, Fischer & Porter Specification
70D1100.
42. Cabinet-Mounted Fluo-chlorinator. Fischer & Porter Specification
70F3400.
43. Chemical Feed Pump. W & T Cat. File 940.100.
44. Water Fluoridation. Olin Chemicals, Stamford, Conn. 06904, Product
Application Bulletin CD-169-1070.
45. Sodium Fluoride; Chemtech, St. Louis, Mo. 63143, Product Data Sheet,
July 1970.
46. Polyethylene Tanks for Precision Chemical Metering Pumps, Precision
Bulletin 960386.
47. Fiberglas Tanks. Owens-Corning Fiberglas Corp., Toledo, Ohio 43601,
Pub. No. 1-PE-3578-F.
48. mRoy Controlled Volume Pumps. Milton Roy Co., St. Petersburg, Fla.
33733, Bulletin No. 15.001.
49. mRoy Packaged Chemical Feed Systems, Milton Roy Co., Bulletin
15.002.
50. Corrosive Chemical Feeding, BIF Ref. 1200.20-1.
51. Metering Pumps Series 1700, BIF Ref. 1700.20-1.
52. Dry Material Feeders, BIF Ref. 25.20-1.
53. Fluoridation Equipment for Potable Water Supplies, BIF Ref. No.
2-SIC 494.2.
54. Chemicals Used in Treatment of Water and Wastewater, BIF Ref. No.
1.21-15.
55. Application of Fluorides to Water, by W.T. Ingram, Water & Sewage
Works, May 1961.
91
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Appendix
The following symbols and abbreviations are employed throughout this manual.
Abbreviation
APHA
AWWA
°C
cm
cone
cuft
°F
ft
g
gal
gph
gpm
hr
in
JAWWA
1
Ib
m
mg
MGD
mg/1
min
ml
mm
Referent
American Public Health Association
American Water Works Association
degree (s) Centigrade
centimeter(s)
concentrated
cubic foot (feet)
degree (s) Fahrenheit
foot (feet)
gram(s)
gallon (s)
gallons per hour
gallons per minute
hour(s)
inch(es)
Journal of the American Water Works Association
liter(s)
pound (s)
meter(s)
milligram (s)
million (s) of gallons per day
milligrams per liter (sometimes used interchange-
ably with ppm)
minute (s)
milliliter(s)
millimeter(s)
93
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Abbreviations
Referent
mV
m/u
PH
ppm
psi
rpm
sec
spgr
SPM
sqcm
sqft
sq in
sqmm
IDS
WPCF
millivolt (s)
millimicron (s)
Hydrogen ion potential (a measure of the acidity
or alkalinity of a solution. A pH of 7.0 indi-
cates neutrality; below 7.0 indicates acidity
and above 7.0 indicates alkalinity)
parts per million (pounds of fluoride ion per
million pounds of water, etc.)
pounds per square inch (water pressure)
revolutions per minute
second (s)
specific gravity
strokes per minute (of a solution feed pump)
square centimeter(s)
square foot (feet)
square inch(es)
square millimeter(s)
total dissolved solids (in water)
Water Pollution Control Federation
US GOVERNMENT PRINTING OFFICE 1973- 759-907/1129
94
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