Lin S.D. Water and Wastewater Calculations Manual

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could potentially cause disease in individuals with low-immunity or com-

promised immune systems. Factors related to increased survival of bac-

teria in chlorinated water include attachment to surfaces, encapsulation,

aggregation, low-nutrient growth conditions, and strain variation.

Research has been conducted using the simulators to determine the

effect of water quality changes on bioﬁlms in the distribution systems.

Results have indicated that bioﬁlms are resistant to disinfection regard-

less of the agent used. Other work has demonstrated that lowering system

pH can reduce bioﬁlm densities; however, such an effect is transient if

system pHs are returned to normal operating ranges (US EPA, 2001). It

is reported that, recently, the Denver City Water developed an uncommon

procedure—ozone disinfection of water mains for common ﬁeld operation.

Cost of control. Chapter 14 (US EPA, 2001) discusses the conditions

and causes of the formation of DBPs, and compares the various treat-

ment techniques and associated costs for both controlling DBPs and

ensuring microbial safety. Making direct comparisons among the vari-

ous alternatives is difﬁcult. For example, moving the point-of-

disinfection (chlorination) would seem to be the lowest cost option. NF,

although the most expensive technology for precursor removal, has the

advantage of removing other contaminants such as total dissolved solids

and various inorganics. Therefore, it might be used for achieving other

treatment goals in addition to removing DBP precursors. For example,

NF effectively removed microorganisms, thus serving as an alternative

for chemical disinfection. Although the cost of enhanced coagulation

was not evaluated, it could be very effective if a utility is only slightly

out of compliance. However, in addition to increased coagulation costs,

an additional cost may be associated with sludge handling. Clearly,

modifying the disinfection process is the lowest cost option for control-

ling DBPs, but as noted, there are by-products and problems associated

with the use of some of the alternate disinfectants. For example, chlo-

ramination is not as good a disinfectant as chlorine, and ozone may

enhance regrowth of some organisms.

Retroﬁtting may be fairly easy with chloramination. For example, to

switch from chlorine to chloramines may only require the addition of

ammonia feed equipment. However, the use of ozone will require con-

struction of expensive ozone contactors. The use of chlorine dioxide will

probably require the use of a reducing agent such as ferrous chloride,

which was not included in this costing analysis. The technologies dis-

cussed would normally be applied incrementally to a utility’s existing

treatment. Therefore, the base cost associated with an assumed conven-

tional treatment system and the incremental costs associated with vari-

ous DBP control alternatives have been summarized in Table 5.14 (US

EPA, 2001).

Public Water Supply 505

Regulations to control contaminants in drinking water in the United

States are expected to become more and more stringent. Forthcoming

DBP regulations will affect virtually every community water system in

the United States. There are various ways to control DBPs, one of which

is to use an alternative to chlorine to control halogenated by-products.

However, when this is done, one has to consider the consequences. The

unit processes considered are those that are effective for precursor

removal or for the use of alternate disinfectants. More efﬁcient treat-

ment will be required to meet future regulations. Also, water treatment

managers will have to become more knowledgeable about various treat-

ment options that are cost-effective in order for them to meet present

and future regulations. Although essentially exempt in the past, small

water systems will be required to comply with future regulations. Singer

(2006) claimed that the ultimate solution to DBPs will be more expen-

sive and will necessitate the use of more aggressive management and

control strategies.

19 Water Fluoridation

Fluoride occurs naturally in water at low concentrations. Water ﬂuori-

dation is the intentional addition of ﬂuoride to drinking water. During

the past 5 decades, water ﬂuoridation has been proven to be both wise

506 Chapter 5

TABLE 5.14 Incremental Costs of Disinfection By-Product Control in c/m

(c/1000 gal)

Design ﬂow in m

/d (average ﬂow)

[design ﬂow in MGD (average ﬂow)]

378,500

387.5 (189) 3785 (1890) 37,850 (26,495) (264,950)

Treatment process [0.10 (0.05)] [1.0 (0.50)] [10,0 (7.0)] [100 (70)]

Conventional treatment 142 (539) 47.5 (180) 12.6 (48) 8.51 (32)

Conventional treatment

and nanoﬁltration 205 (778) 88.7 (336) 40.4 (882) 32.1 (122)

Conventional treatment plus 203 (772)– 70.8 (268)– 28.3 (101)– 15.7 (150)–

GAC (C

⫽ 100 mg/L)* 216 (818) 82.1 (311) 32.7 (124) 19.2 (73)

Conventional treatment plus 206 (781)– 73.1 (276)– 29.4 (111)– 16.2 (61)–

GAC (C

⫽ 50 mg/L) 226 (856) 91.2 (345) 36.0 (136) 22.5 (85)

Conventional treatment plus

-chlorine/chloramines 147 (556) 47.8 (181) 12.6 (48) 8.5 (32)

-ozone/chlorine 163 (619) 49.7 (188) 14.0 (53) 9.3 (35)

-ozone/chloramines 168 (636) 50.0 (189) 14.0 (53) 9.3 (35)

-chlorine dioxide/chlorine 167 (633) 48.9 (185) 12.6 (48) 8.5 (32)

-chlorine dioxide/chloramine 172 (651) 49.1 (186) 12.9 (49) 8.5 (32)

*GAC: granular activated carbon, C

: efﬂuent concentration

SOURCE: US EPA, 2001

and a most cost-effective method to prevent dental decay. The ratio of

beneﬁts (reductions in dental bills) to cost of water ﬂuoridation is 50:1

(US Public Health Service, 1984). In order to prevent decay, the optimum

ﬂuoride concentration has been established at 1mg/L (American Dental

Association, 1980). The beneﬁts of ﬂuoridation last a lifetime. While it

is true that children reap the greatest beneﬁts from ﬂuoridation, adults

beneﬁt also. Although there are beneﬁts, the opponents of water ﬂuor-

idation exist. The charges and the facts are discussed in the manual (US

PHS, 1984).

Fluoride in drinking water is regulated under section 1412 of the Safe

Drinking Water Act (SDWA). In 1986, US EPA promulgated an enforce-

able standard, a Maximum Contamination Level (MCL) of 4 mg/L for

ﬂuoride. This level is considered as protective of crippling skeletal ﬂu-

orosis, an adverse health effect. A review of human data led the EPA

(1990) to conclude that there was no evidence that ﬂuoride in water pre-

sented a cancer risk in humans. A nonenforceable Secondary Maximum

Contamination Level (SMCL) of 2 mg/L was set to protect against objec-

tionable dental ﬂuorisis, a cosmetic effect.

19.1 Fluoride chemicals

Fluoride is a pale yellow noxious gaseous halogen. It is the thirteenth

most abundant element in the earth’s crust and is not found in a free

state in nature. The three most commonly used ﬂuoride compounds in

the water system are hydroﬂuosilicic acid (H

SiF

), sodium ﬂuoride

(NaF), and sodium silicoﬂuoride (Na

SiF

). Fluoride chemicals, like

chlorine, caustic soda, and many other chemicals used in water treat-

ment can cause a safety hazard. The operators should observe the safe

handling of the chemical.

The three commonly used ﬂuoride chemicals have virtually 100 %

dissociation (Reeves, 1986, 1990):

(5.186)

(5.187)

The SiF

radical will be dissociated in two ways: hydrolysis mostly dis-

sociates very slowly

(5.188)

and/or

(5.189)SiF

4 2F

1 SiF

SiF

1 2H

O 4 4H

1 6F

1 SiO

SiF

4 2Na

1 SiF

NaF 4 Na

1 F

Public Water Supply 507

Silicon tetraﬂuoride (SiO

) is a gas which will easily volatilize out of

water when present in high concentrations. It also reacts quickly with

water to form silicic acid and silica (SiO

(5.190)

and

(5.191)

then

(5.192)

Hydroﬂuosilicic acid has a dissociation very similar to sodium silico-

ﬂuoride:

(5.193)

then it follows Eqs. (5.190), (5.191), and (5.192).

Hydroﬂuoric acid (HF) is very volatile and will attack glass and elec-

trical parts and will tend to evaporate in high concentrations.

19.2 Optimal ﬂuoride concentrations

The manual (Reeves, 1986) presents the recommended optimal ﬂuoride

levels for ﬂuoridated water supply systems based on the average and the

maximum daily air temperature in the area of the involved school and

community. For community water system in the United States the rec-

ommended ﬂuoride levels are from 0.7 mg/L in the south to 1.2 mg/L to

the north of the country. The recommended ﬂuoride concentrations for

schools are from four to ﬁve times greater than that for communities.

The optimal ﬂuoride concentration can be calculated by the following

equation:

F (mg/L) ⫽ 0.34/E (5.194)

where E is the estimated average daily water consumption for children

through age 10, in ounces of water per pound of body weight. The value

of E can be computed from

E ⫽ 0.038 ⫹ 0.0062 T (5.195)

where T is the annual average of maximum daily air temperature in

degrees Fahrenheit.

SiF

4 2HF 1 SiF

HF 4 H

1 F

SiF

1 2H

O 4 4HF 1 SiO

SiF

1 3H

O 4 4HF 1 H

SiO

508 Chapter 5

Example 1: The annual average of maximum daily air temperature of a

city is 60.5°F (15.8°C). What should be the recommended optimal ﬂuoride

concentration?

solution:

Step 1. Determine E by Eq. (5.195)

Step 2. Calculate recommended F by Eq. (5.194)

F ⫽ 0.34/E ⫽ 0.34/0.413 ⫽ 0.82 (mg/L)

Example 2: Calculate the dosage of ﬂuoride to the water plant in Example 1,

if the naturally occurring ﬂuoride concentration is 0.03 mg/L.

solution: The dosage can be obtained by subtracting the natural ﬂuoride

level in water from the desired concentration:

Dosage ⫽ 0.82 mg/L ⫺ 0.03 mg/L ⫽ 0.79 mg/L

As for other chemicals used in water treatment, the ﬂuoride chemicals are

not 100% pure. The purity of a chemical is available from the manufactur-

ers. The information on the molecular weight, purity ( p), and the available

ﬂuoride ion (AFI) concentration for the three commonly used ﬂuoride chem-

icals are listed in Table 5.15. The AFI is determined by the weight of ﬂuoride

portion divided by the molecular weight.

Example 3: What is the percent available ﬂuoride in the commercial

hydroﬂuosilicic acid (purity, p ⫽ 23%)?

solution: From Table 5.15

% available F ⫽ p ⫻ AFI ⫽ 23 ⫻ 0.792

⫽ 18.2 (5.196)

19.3 Fluoride feed rate (dry)

Fluoride can be fed into treated water by (1) dry feeders with a mixing

tank (for Na

SiF

), (2) direct solution feeder for H

SiF

, (3) saturated

E 5 0.038 1 0.0062T 5 0.038 1 0.0062 3 60.5 5 0.413

Public Water Supply 509

TABLE 5.15 Purity and Available Fluoride Ion (AFI) Concentrations for the

Three Commonly Used Chemicals for Water Fluoridation

Chemical Molecular weight Purity, % AFI

Sodium ﬂuoride, NaF 42 98 0.452

Sodium silicoﬂuoride, Na

SiF

188 98.5 0.606

Hydroﬂuosilicic acid, H

SiF

144 23 0.792

solution of NF, and (4) unsaturated solution of NF or Na

SiF

. The feed

rate, FR, can be calculated as follows:

(5.197a)

(5.197b)

Example 1: Calculate the ﬂuoride feed rate in lb/d and g/min, if 1.0 mg/L

of ﬂuoride is required using sodium silicoﬂuoride in a 10 MGD (6944 gpm,

0.438 m

/s) water plant assuming zero natural ﬂuoride content.

solution: Using Eq. (5.197a) and data from Table 5.15, feed rate FR is

⫽ 139.7 lb/d

or ⫽ 139.7 lb/d ⫻ 453.6 g/lb ⫻ (1 d/1440 min)

⫽ 44.0 g/min

Example 2: A water plant has a ﬂow of 1600 gpm (0.1m

/s). The ﬂuoride con-

centration of the ﬁnished water requires 0.9 mg/L using sodium ﬂuoride in a

dry feeder. Determine the feed rate if 0.1 mg/L natural ﬂuoride is in the water.

solution: Using Eq. (5.197), and data from Table 5.15

⫽ 0.024 lb/min

or ⫽ 10.9 g/min

Example 3: If a water plant treats 3500 gpm (5 MGD, 0.219 m

/s) of water

with a natural ﬂuoride level of 0.1 mg/L. Find the hydroﬂuosilicic acid feed

rate (mL/min) with the desired ﬂuoride level of 1.1 mg/L.

solution:

Step 1. Calculate FR in gal/min

FR 5

s1.1 2 0.1d mg/L 3 3500 gpm

1,000,000 3 0.23 3 0.792

5 0.0192 gpm

F 5

s0.9 2 0.1dmg/L 3 1600

gpm 3 8.34 lb/gal

1,000,000 3 0.98 3 0.452

FR 5

1.0 mg/L 3 10 MGD 3 8.34 lb/gal

0.985 3 0.606

FR slb/mind 5

smg/Ld 3 gpm 3 8.34 lb/gal

1,000,000 3 p 3 AFI

FR slb/dd 5

dosage smg/Ld 3 plant flow sMGDd 3 8.34 lb/gal

p 3 AFI

510 Chapter 5

Step 2. Convert gpm into mL/min

FR ⫽ 0.0192 gpm

⫽ 0.0192 gal/min ⫻ 3785 mL/gal

⫽ 72.7 mL/min

19.4 Fluoride feed rate for saturator

In practice 40 g of sodium ﬂuoride will dissolve in 1 L of water. It gives

40,000 mg/L (4% solution) of NF. The AFI is 0.45. Thus the concentra-

tion of ﬂuoride in the saturator is 18,000 mg/L (40,000 mg/L ⫻ 0.45). The

sodium ﬂuoride feed rate can be calculated as

(5.198)

Example 1: In a 1 MGD plant with 0.2 mg/L natural ﬂuoride, what would

the ﬂuoride (NF saturator) feed rate be to maintain 1.2 mg/L of ﬂuoride in

the water?

solution: By Eq. (5.198)

Note: Approximately 56 gal of saturated NF solution to treat 1 Mgal of water

at 1.0 mg/L dosage.

Example 2: Convert the NF feed rate in Example 1 to mL/min.

solution:

FR ⫽ 55.6 gal/d ⫻ 3785 mL/gal ⫻ (1d/1440 min)

⫽ 146 mL/min

or ⫽ 2.5 mL/s

Note: Approximately 2.5 mL/s (or 150 mL/min) of saturated sodium ﬂuoride

solution to 1 MGD ﬂow to obtain 1.0 mg/L of ﬂuoride concentration.

19.5 Fluoride dosage

Fluoride dosage can be calculated from Eqs. (5.196) and (5.198) by rear-

ranging those equations as follows:

(5.199)Dosage smg/Ld 5

FRslb/dd 3 p 3 AFI

plant flowsMGDd 3 8.34 lb/sMgal

mg/Ld

FR 5

s1.2 2 0.2d mg/L 3 1,000,000 gpd

18,000 mg/L

5 55.6 gpd

FRsgpm or L/mind 5

dosagesmg/Ld 3 plant flowsgpm or L/mind

18,000 mg/L

Public Water Supply 511

and for the saturator:

(5.200)

For a solution concentration, C, of unsaturated NF solution

(5.201)

Example 1: A water plant (1 MGD) feeds 22 lb/d of sodium ﬂuoride into the

water. Calculate the ﬂuoride dosage.

solution: Using Eq. (5.199) with Table 5.15:

Example 2: A water plant adds 5 gal of sodium ﬂuoride from its saturator

to treat 100,000 gal of water. Find the dosage of the solution.

solution: Using Eq. (5.200)

Example 3: A water system uses 400 gpd of a 2.8% solution of sodium ﬂuo-

ride in treating 500,000 gpd of water. Calculate the ﬂuoride dosage.

solution:

Step 1. Find ﬂuoride concentration in NF solution using Eq. (5.201)

Step 2. Calculate ﬂuoride dosage by Eq. (5.200)

Example 4: A water system adds 6.5 lb/d per day of sodium silicoﬂuoride to

ﬂuoridate 0.5 MGD of water. What is the ﬂuoride dosage?

Dosage 5

400 gpd 3 12,600 mg/L

500,000 gpd

5 10.08 mg/L

C 5

18,000 mg/L 3 2.8%

5 12,600 mg/L

Dosage 5

5 gal 3 18,000 mg/L

100,000 gal

5 0.9 mg/L

Dosage 5

22 lb/d 3 0.98 3 0.452

1,000,000 gal/d 3 8.34lb/sMgal

mg/Ld

5 1.17 mg/L

C 5

18,000 mg/L 3 solution strengths%d

Dosagesmg/Ld 5

FRsgpmd 3 18,000 mg/L

plant flowsgpmd

512 Chapter 5

solution: Using Eq. (5.199), p ⫽ 0.985, and AFI ⫽ 0.606

Example 5: A water plant uses 2.0 lb of sodium silicoﬂuoride to ﬂuoridate

160,000 gal water. What is the ﬂuoride dosage?

solution:

Example 6: A water plant feeds 100 lb of hydrofluosilicic acid of 23%

purity during 5 days to ﬂuoridate 0.437 MGD of water. Calculate the ﬂuoride

dosage.

solution:

(obtain AFI⫽ 0.792

from Table 5.15)

Example 7: A water system uses 1200 lb of 25% hydroﬂuosilicic acid in

treating 26 Mgal of water. The natural ﬂuoride level in the water is 0.1 mg/L.

What is the ﬁnal ﬂuoride concentration in the ﬁnished water.

solution:

20 Health Risks

Human health risk is the probability that a given exposure or a series

of exposures may have or will damage the health of individuals exposed.

This section discusses the possible risk of drinking water, and assessing

5 1.2 mg/L

Final floride 5 s1.1 1 0.1d mg/L

5 1.1 mg/L

Plant dosage 5

1200 lb 3 0.25 3 0.792

26 3 10

gal 3 8.34 lb/sMgal

mg/Ld

5 1.0 mg/L

20 lb/d 3 0.23 3 0.792

0.437 MGD 3 8.34 lb/sMgal

mg/Ld

Dosage 5

FR 3 p 3 AFI

sMGDd 3 8.34 lb/sMgal

mg/Ld

FR 5 100lb/5 d 5 20 lb/d

Dosage 5

2.0

lb 3 0.985 3 0.606 3 0.606 3 10

mg/L

0.16 Mgal/d 3 8.34 lb/sMgal

mg/Ld

5 0.9 mg/L

Dosage 5

6.5 lb/d 3 0.985 3 0.606

0.5 MGD 3 8.34 lb/sMgal

mg/Ld

5 0.93 mg/L

Public Water Supply 513

and managing risks by the public water suppliers through the rules and

guidelines of the US EPA. This section speciﬁcally covers assessing

and managing risks, radionuclides, and the value of health advisories

concerning drinking water.

20.1 Risk

Risk is the potential for realization of unwanted adverse consequences or

events. In general terms, human health risk is the probability of injury,

disease, or death under a given chemical or biological exposure or under

series of exposures. Risk may be expressed in quantitative term (zero to

one). In many cases, it can only be described as, high, low, or trivial.

We do not live in a risk-free world, but in a chemical world. There are

more than 65,000 chemicals produced, and they are increasing in

number every year. Through use and abuse, many of those chemical

products will end up in our environment—water, air, and land. These

chemicals include organics and inorganics that are used in industries

(including water treatment plants), pharmaceuticals, agricultures

(insecticides), home, personal cosmic purposes, etc. Even terrorism may

threaten our drinking water with chemical contamination.

Certain areas on the earth’s crest and rocks contain high levels of some

naturally occurring chemical elements, such as lead, mercury, ﬂuoride,

and sulfur. In addition, radionuclides such as natural radium, Ra

226

228

; radon, Rn; uranium, U, etc.; and man-made radioactive sub-

stances occur throughout the world. Many contaminants may end up in

source waters. Trace amounts of these contaminants might be present

in the drinking water.

All human activities involve some degree of risk. Table 5.16 lists the

risks of some common activities. In addition, pathogen contamination in

food and drinking liquids poses risks. Waterborne disease outbreaks

(Giardia, Cryptosporidium, acute gastroenteritis, E. Coli, etc.) have

occurred in the past and can threaten at anytime (Hass, 2002; Lin, 2002).

Sand ﬁltration, disinfection, and application of drinking water stan-

dards reduce waterborne diseases to protect public health. It was dis-

covered that DBPs result from the reaction of chlorine with natural

organic matter (NOM) in source water. The risk of carcinogenic DBPs

is a dilemma for water utilities. Alternative treatment measures and

microbial control have to be properly managed (see Section 19).

The term “safe,” in its common usage, means “without risk.” In tech-

nical terms, however, this common usage is misleading because science

cannot ascertain the conditions under which a given chemical exposure

is likely to be absolutely without a risk of any type.

Human health risk is the likelihood (or probability) that a given chem-

ical exposure or series of exposures may damage the health of exposed

individuals. Chemical risk assessment involves the complex analysis of

514 Chapter 5