Lin S.D. Water and Wastewater Calculations Manual

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Step 3. Solve with declining growth method

Assuming

⫽ 200,000

Using Eq. (5.4b)

⫽ 0.02032

From Eq. (5.4c)

3 Water Requirements

The uses of water include domestic, commercial and industrial, public

services such as ﬁre ﬁghting and public buildings, and unaccounted

pipeline system losses and leakage. The average usage in the United

States for the above four categories are 220, 260, 30, and 90L per capita

per day (L/(c ⋅ d)), respectively (Tchobanoglous and Schroeder 1985).

These correspond to 58, 69, 8, and 24 gal/(c ⋅ d), respectively. Total munic-

ipal water use averages 600 L/(c ⋅ d) or 160 gal/(c ⋅ d) in the United States.

The maximum daily water use ranges from about 120% to 400% of the

average daily use with a mean of about 180%. Maximum hourly use is

about 150% to 12,000% of the annual average daily ﬂow; and 250% to

270% are typically used in design.

3.1 Fire demand

Fire demand of water is often the determining factor in the design of

mains. Distribution is a short-term, small quantity but with a large

ﬂow rate. According to uniform ﬁre code, the minimum ﬁre ﬂow require-

ment for a one- and two-family dwelling shall be 1000 gal per min (gpm).

For the water demand for ﬁre ﬁghting based on downtown business

districts and high-value area for communities of 200,000 people or less,

the National Board of Fire Underwriters (1974) recommended the fol-

lowing ﬁre ﬂow rate and population relationship:

(SI units) (5.8a)

(US customary units) (5.8b)Q 5 1020!p

s1 2 0.01!pd

Q 5 3.86!p s1 2 0.01!pd

P 5 P

1 sP

2 P

ds1 2 e

5 129,000 1 s200,000 2 129,000ds1 2 e

20.02032 3 8.75

5 129,000 1 71,000 3 0.163

5 140,600

200,000 2 129,000

200,000 2 113,000

Public Water Supply 315

where Q ⫽ discharge, m

/min or gal/min (gpm)

P ⫽ population in thousands

The required ﬂow rate for ﬁre ﬁghting must be available in addition

to the coincident maximum daily ﬂow rate. The duration during the

required ﬁre ﬂow must be available for 4 to 10 h. National Board of Fire

Underwriters recommends providing for a 10-h ﬁre in towns exceeding

2500 in population.

The Insurance Services Ofﬁce Guide (International Fire Service

Training Association, 1993) for determination of required ﬁre ﬂow rec-

ommends the formula

(US customary units) (5.9a)

(SI units) (5.9b)

where F ⫽ required ﬁre ﬂow, gpm or m

C ⫽ coefﬁcient related to the type of construction

A ⫽ total ﬂoor area, ft

or m

F 5 320 C 2A

F 5 18 C 2A

316 Chapter 5

C value Construction Maximum ﬂow, gpm (m

/d)

1.5 wood frame 8000 (43,600)

1.0 ordinary 8000 (43,600)

0.9 heavy timber type building

0.8 noncombustible 6000 (32,700)

0.6 ﬁre-resistant 6000 (32,700)

Example 1: A 4-story building of heavy timber type building of 715 m

(7700 ft

) of ground area. Calculate the water ﬁre requirement.

solution: Using Eq. (5.9b)

⫽ 15,400 (m

/d)

or ⫽ 2800 (gpm)

Example 2: A 5-story building of ordinary construction of 7700 ft

(715 m

)

of ground area communicating with a 3-story building of ordinary construc-

tion of 9500 ft

(880 m

) ground area. Compute the required ﬁre ﬂow.

5 320 3 0.9 24 3 715

F 5 320 C 2A

solution: Using Eq. (5.9a)

Total area A ⫽ 5 ⫻ 7700 ⫹ 3 ⫻ 9500 ⫽ 67,000 ft

⫽ 4,660 (gpm)

4,750 gpm rounded to nearest 250 gpm for design purpose

or ⫽ 25,900 (m

/d)

Example 3: Assuming a high-value residential area of 100 ha (247 acres) has

a housing density of 10 houses/ha with 4 persons per household, determine

the peak water demand, including ﬁre, in this residential area.

solution:

Step 1. Estimate population P

P ⫽ (4 capita/house) (10 houses/ha) (100 ha)

⫽ 4000 capita

Step 2. Estimate average daily ﬂow Q

⫽ residential ⫹ public service ⫹ unaccounted

⫽ (220 ⫹ 30 ⫹ 90)

⫽ 340 (L/(c ⋅ d))

Step 3. Estimate maximum daily ﬂow Q

for the whole area

Using the basis of Q

is 180% of Q

⫽ (340 L/(c ⋅ d))(1.8) (4000 c)

⫽ 2,448,000 L/d

2400 m

Step 4. Estimate the ﬁre demand, using Eq. (5,8a)

Step 5. Estimate total water demand Q

Note: In this area, ﬁre demand is a control factor. It is measured to compare

Q and peak daily demand.

Q 5 Q

1 Q

5 2400 m

/d 1 4540 m

5 6940 m

5 3.86!p

s1 2 0.01!pd m

/min

5 3.86!4s1 2 0.01!4d

5 7.57 m

/min

5 7.57 m

/min 3 60 min/h 3 10 h/d

5 4540 m

F 5 18 3 1.0 267,000

Public Water Supply 317

Step 6. Check with maximum hourly demand Q

The Q

is assumed to be 250% of average daily demand.

⫽ 2400 m

/d ⫻ 2.5

⫽ 6000 m

Step 7. Compare Q versus Q

Q ⫽ 6940 m

/d > Q

⫽ 6000 m

Use Q ⫽ 6940 m

/d for the main pipe to this residential area.

Example 4: Estimate the municipal water demands for a city of 225,000

persons.

solution:

Step 1. Estimate the average daily demand Q

avg

⫽ 600 L/(c⋅d) ⫻ 225,000 c

⫽ 135,000,000 L/d

⫽ 1.35 ⫻ 10

⫽ 25.7 MGD

Step 2. Estimate the maximum daily demand Q

(f ⫽ 1.8)

⫽ 1.35 ⫻ 10

/d ⫻ 1.8

⫽ 2.43 ⫻ 10

⫽ 64.2 MGD

Step 3. Calculate the ﬁre demand Q

, using Eq. (5.8a)

For 10-h duration of daily rate

⫽ 49.215 m

/min ⫻ 60 min/h ⫻ 10 h/d ⫽ 0.30 ⫻ 10

Step 4. Sum of Q

and Q

(ﬁre occurs coincident to peak ﬂow)

⫹ Q

⫽ (2.43 ⫹ 0.30) ⫻ 10

⫽ 2.73 ⫻ 10

⫽ 72.1 MGD

5 3.86!p

s1 2 0.01!pd m

/min

5 3.86!225 s1 2 0.01!225d sm

/mind

5 49.215 m

/min

318 Chapter 5

Step 5. Estimate the maximum hourly demand Q

(f ⫽ 2.7)

⫽ f Q

avg

⫽ 2.7 ⫻ 1.35 ⫻ 10

⫽ 3.645 ⫻ 10

⫽ 96.3 MGD

Step 6. Compare Steps 4 and 5

⫽ 3.645 ⫻ 10

/d > Q

⫹ Q

⫽ 2.73 ⫻ 10

Use Q

⫽ 3.65 ⫻ 10

/d (96.4 MGD) to design the plant’s storage capacity.

Fire ﬂow tests. Fire ﬂow tests involve discharging water at a known

ﬂow rate from one or more hydrants and examining the corresponding

pressure drop from the main through another nearby hydrant. The

discharge from a hydrant nozzle can be determined as follows

(Hammer, 1975):

Q ⫽ 29.8Cd

(5.10)

where Q ⫽ hydrant discharge, gpm

C ⫽ coefﬁcient, normally 0.9

d ⫽ diameter of outlet, in

p ⫽ pitot gage reading, psi

The computed discharge at a speciﬁed residual pressure can be com-

puted as

(5.11a)

or (5.11b)

where Q

⫽ computed ﬂow rate at the speciﬁed residual

pressure, gpm

⫽ total discharge during ﬁre ﬂow test, gpm

⫽ pressure drop from beginning to special pressure, psi

⫽ pressure drop during ﬁre ﬂow test, psi

Example: All four hydrant numbers 2, 4, 6, and 8, as shown in the fol-

lowing figure, have the same nozzle size of 2.5 in and discharge at the same

rate. The pitot tube pressure at each hydrant during the test is 25 psi. At

this discharge the residual pressure at hydrant #5 dropped from the original

⌬

5 Q

⌬H

0.54

5 a

⌬H

0.54

Public Water Supply 319

solution:

Step 1. Determine total discharge during the test

Using Eq. (5.10)

Step 2. Compute Q

at 30 psi by using Eq. (5.11b)

3.2 Leakage test

After a new main is installed, a leakage test is usually required. The test-

ing is generally carried out for a pipe length not exceeding 300 m (1000 ft).

The pipe is ﬁlled with water and pressurized (50% above normal oper-

ation pressure) for at least 30 min. The leakage limit recommended by

the American Water Work Association is determined as:

for SI units (5.12a)

for English units (5.12b)L 5 sND !P

d/1850

L 5 sND !pd/326

5 Q

⌬H

0.54

5 3353 3 a

100 2 30

100 2 65

0.54

5 4875 sgpmd

5 4 3 29.8 C d

5 4 3 29.8 3 0.9 3 s2.5d

3 !25

5 3353 sgpmd

320 Chapter 5

2 3

4 5

100 to 65 psi. Compute the flow rate at a residual pressure of 30 psi based

on the test.

where L ⫽ allowable leakage, L/(mm diameter ⭈ km ⭈ h) or gal/

(in ⭈ mi ⭈ h)

N ⫽ number of joints in the test line

D ⫽ normal diameter of the pipe, mm or in

P ⫽ average test pressure

Leakage allowed in a new main is generally speciﬁed in the design. It

ranges from 5.6 to 23.2 L/mm diameter per km per day (60 to 250 gal/in

diameter/miles/d). Nevertheless, recently, some water companies are not

allowing for any leakage in a new main due to the use of better sealers.

4 Regulations for Water Quality

Water quality is a term used to describe the physical, chemical, and bio-

logical characteristics of water with respect to its suitability for a par-

ticular use. In the United States, all federal drinking water standards

are set under the Safe Drinking Water Act (SDWA).

4.1 Safe drinking water act

The Safe Drinking Water Act was originally passed by the Congress in

1974. The SDWA gives the US Environmental Protection Agency (EPA)

the authority to set drinking water standards to protect public health

by regulating public drinking water supply. It was amended in 1986

(interim) and 1996.

There are two categories of federal drinking water standards: National

Primary Drinking Water Regulations (Table 5.1) and National Secondary

Drinking Water Regulations (Table 5.1a). The Maximum Contaminant

Level Goals (MCLGs) for chemicals suspected or known to cause cancer

in humans are set to zero. If a chemical is carcinogenic and a safe dose

is determined, the MCLG is set at a level above zero that is safe. For

microbial concentrations (Giardia, Legionella, TC, FC, E. coli, and

ciruses) the MCLGs are set at zero.

The SDWA regulates every public water supply in the United States

(160,000 plants) with some variances and does not apply to water systems

that serve fewer than 25 individuals. The SDWA established a multiple

barrier protection for drinking water. It starts watershed management

to protect drinking water as well as its water sources (lakes, reservoirs,

rivers, streams, springs, and groundwater). It also requires assessing

and protecting collection systems, properly treated by qualiﬁed operators,

the integrity of the distribution systems, monitoring for the regulated and

unregulated contaminants, providing information available to the public

on the drinking water quality, and submitting the required operational

reports to the state EPA. The state EPA is responsible for enforcement,

technical assistances, granting variances, and even ﬁnancial assistance

Public Water Supply 321

TABLE 5.1 National Primary Drinking Water Regulations

MCL Potential health Sources of

MCLG

or TT

effects from contaminant in

Contaminant (mg/L)

(mg/L)

ingestion of water drinking water

Microorganisms

Cryptosporidium zero TT

Gastrointestinal illness (e.g. Human and fecal

diarrhea, vomiting, cramps) animal waste

Giardia lamblia zero TT

Gastrointestinal illness (e.g. Human and

diarrhea, vomiting, cramps) animal fecal

waste

Heterotrophic n/a TT

HPC has no health effects; HPC measures a

plate count it is an analytic method used range of bacteria

to measure the variety of that are naturally

bacteria that are common present in the

in water. The lower the environment

concentration of bacteria in

drinking water, the better

maintained the water

system is

Legionella zero TT

Legionnaire’s disease, a type Found naturally

of pneumonia in water;

multiplies in

heating systems

Total coliforms zero 5.0%

Not a health threat in itself; Coliforms are

(including fecal it is used to indicate whether naturally

coliform and other potentially harmful present in the

E. Coli) bacteria may be present

environment; as

well as feces;

fecal coliforms

and E. coli only

come from human

and animal fecal

waste

Turbidity n/a TT

Turbidity is a measure of the Soil runoff

cloudiness of water. It is used

to indicate water quality and

ﬁltration effectiveness (e.g.

whether disease-causing

organisms are present).

Higher turbidity levels are

often associated with higher

levels of disease-causing

microorganisms such as

viruses, parasites, and some

bacteria. These organisms can

cause symptoms such as

nausea, cramps, diarrhea,

and associated headaches

Viruses (enteric) zero TT

Gastrointestinal illness (e.g. Human and

diarrhea, vomiting, cramps) animal fecal

waste

322 Chapter 5

MCL Potential health Sources of

MCLG

or TT

effects from contaminant in

Contaminant (mg/L)

(mg/L)

ingestion of water drinking water

Disinfection Byproducts

Bromate zero 0.010 Increased risk of cancer By-product of

drinking water

disinfection

Chlorite 0.8 1.0 Anemia; infants and young By-product of

children: nervous system drinking water

effects disinfection

Haloacetic acids n/a

0.060 Increased risk of cancer By-product of

(HAA5) drinking water

disinfection

Total none

0.10 Liver, kidney, or central By-product of

Trihalomethanes — — nervous system problems; drinking water

(TTHMs) n/a

0.080 increased risk of cancer disinfection

Potential health Sources of

MRDLG

MRDL

effects from contaminant in

Contaminant (mg/L)

(mg/L)

ingestion of water drinking water

Disinfectants

Chloramines MRDLG=4

MRDL=4.0

Eye/nose irritation; Water additive

(as Cl

) stomach discomfort, used to control

anemia microbes

Chlorine (as MRDLG=4

MRDL=4.0

Eye/nose irritation; Water additive

) stomach discomfort used to control

microbes

Chlorine MRDLG=0.8

MRDL=0.8

Anemia; infants and Water additive

dioxide (as young children: used to control

ClO

) nervous system microbes

effects

MCL Potential health Sources of

MCLG

or TT

effects from contaminant in

Contaminant (mg/L)

(mg/L)

ingestion of water drinking water

Inorganic Chemicals

Antimony 0.006 0.006 Increase in blood cholesterol; Discharge from

decrease in blood sugar petroleum

reﬁneries; ﬁre

retardants;

ceramics;

electronics;

solder

Arsenic 0

0.010 Skin damage or problems Erosion of natural

as of with circulatory systems, deposits; runoff

01/23/06 and may have increased risk from orchards,

of getting cancer runoff from glass,

and electronics-

production

wastes

Public Water Supply 323

(Continued)

TABLE 5.1 (continued )

MCL Potential health Sources of

MCLG

or TT

effects from contaminant in

Contaminant (mg/L)

(mg/L)

ingestion of water drinking water

Asbestos 7 million 7 MFL Increased risk of developing Decay of asbestos

(ﬁber >10 ﬁbers per benign intestinal polyps cement in water

␮m) liter mains; erosion of

natural deposits

Barium 2 2 Increase in blood pressure Discharge of

drilling wastes;

discharge from

metal reﬁneries;

erosion of

natural deposits

Beryllium 0.004 0.004 Intestinal lesions Discharge from

metal reﬁneries

and coal-burning

factories;

discharge from

electrical,

aerospace, and

defense

industries

Cadmium 0.005 0.005 Kidney damage Corrosion of

galvanized pipes;

erosion of

natural deposits;

discharge from

metal reﬁneries;

runoff from

waste batteries

and paints

Chromium (total) 0.1 0.1 Allergic dermatitis Discharge from

steel and pulp

mills; erosion of

natural deposits

Copper 1.3 TT

; Short term exposure: Corrosion of

Action Gastrointestinal distress household

Level=1.3 plumbing

Long-term exposure: Liver systems; erosion

or kidney damage of natural deposits

People with Wilson’s disease

should consult their personal

doctor if the amount of

copper in their water

exceeds the action level

Cyanide (as 0.2 0.2 Nerve damage or thyroid Discharge from

free cyanide) problems steel/metal

factories;

discharge from

plastic and

fertilizer factories

324 Chapter 5