Lin S.D. Water and Wastewater Calculations Manual

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4 Quantity of Wastewater

The quantity of wastewater produced varies in different communities

and countries, depending on a number of factors such as water uses,

climate, lifestyle, economics, etc. Metcalf and Eddy, Inc. (1991) lists typ-

ical municipal water uses and wastewater flow rates in the United

States including domestic, commercial, institutional, and recreational

facilities and various institutions.

A typical wastewater flow rate from a residential home in the United

States might average 70 gal (265 L) per capita per day (gal/(c ⭈ d) or gpcpd).

Approximately 60% to 85% of the per capita consumption of water becomes

wastewater. Commonly used quantities of wastewater flow rates that form

miscellaneous types of facilities are listed in Table 6.1 (Illinois EPA, 1997).

Municipal wastewater is derived largely from the water supply. Some

water uses, such as for street washing, lawn watering, fire fighting,

and leakage from water supply pipelines, most likely do not reach the

sewers. The volume of wastewater is added to by infiltration and inflows.

Wastewater flow rates for commercial developments normally range

from 800 to 1500 gal/(acre ⭈ d) (7.5 to 14 m

/(ha ⭈ d)), while those for indus-

tries are 1000 to 1500 gal/(acre ⭈ d) (9.4 to 14 m

/(ha ⭈ d)) for light indus-

trial developments and 1500 to 3000 gal/(acre ⭈ d) (14 to 28 m

/(ha ⭈ d))

for medium industrial developments.

Water entering a sewer system from ground through defective con-

nections, pipes, pipe joints, or manhole wells is called infiltration. The

amount of flow into a sewer from groundwater, infiltration, may range

from 100 to 10,000 gal/(d ⭈ in ⭈ mile) (0.093 to 9.26 m

/(d ⭈ cm ⭈ km)) or

more (Metcalf and Eddy, Inc. 1991). Construction specifications com-

monly permit a maximum infiltration rate of 500 gal/(d ⭈ in ⭈ mile)

(0.463 m

/(d ⭈ cm ⭈ km)) of pipe diameter. The quantity of infiltration may

be equal to 3 to 5% of the peak hourly domestic wastewater flow, or

approximately 10% of the average daily flow. With better pipe joint

material and tight control of construction methods, infiltration allowance

can be as low as 200 gal/(d ⭈ mile ⭈ in) of pipe diameter (Hammer, 1986).

Inflow to a sewer includes steady inflow and direct inflow. Steady

inflow is water drained from springs and swampy areas, discharged

from foundation, cellar drains, and cooling facilities, etc. Direct inflow

is from stormwater runoff direct to the sanitary sewer.

Example 1: Convert to the SI units for the construction allowable infiltra-

tion rate of 500 gal/(d ⭈ mile) per in of pipe diameter.

solution:

500 gal/sd

mile

ind 5

500 gal/d 3 0.003785 m

/gal

1 mile 3 1.609 km/mile 3 1 in 3 2.54 cm/in

5 0.463 m

/sd

kmd per cm of pipe diameter

Wastewater Engineering 545

546 Chapter 6

TABLE 6.1 Typical Wastewater Flow Rates for Miscellaneous Facilities

Gallons per

person per day

(unless otherwise

Type of establishment noted)

Airports (per passenger) 5

Bathhouses and swimming pools 10

Camps:

Campground with central comfort station 35

With flush toilets, no showers 25

Construction camps (semipermanent) 50

Day camps (no meals served) 15

Resort camps (night and day) with limited plumbing 50

Luxury camps 100

Cottages and small dwellings with seasonal occupancy 75

Country clubs (per resident member) 100

Country clubs (per nonresident member present) 25

Dwellings:

Boarding houses 50

(additional for nonresident boarders) 10

Rooming houses 40

Factories (gallons per person, per shift, exclusive of 35

industrial wastes)

Hospitals (per bed space) 250

Hotels with laundry (two persons per room) per room 150

Institutions other than hospitals including nursing 125

homes (per bed space)

Laundries—self service (gallons per wash) 30

Motels (per bed) with laundry 50

Picnic parks (toilet wastes only per park user) 5

Picnic parks with bathhouses, showers and flush 10

toilets (per park user)

Restaurants (toilet and kitchen wastes per patron) 10

Restaurants (kitchen wastes per meal served) 3

Restaurants (additional for bars and cocktail lounges) 2

Schools:

Boarding 100

Day, without gyms, cafeterias, or showers 15

Day, with gyms, cafeterias, and showers 25

Day, with cafeterias, but without gyms or showers 20

Service stations (per vehicle served) 5

Swimming pools and bathhouses 10

Theaters:

Movie (per auditorium set) 5

Drive-in (per car space) 10

Travel trailer parks without individual water 50

and sewer hook-ups (per space)

Travel trailer parks with individual water 100

and sewer hook-ups (per space)

Workers:

Offices, schools and business establishments 15

(per shift)

SOURCE: Illinois EPA (1997)

Wastewater Engineering 547

Example 2: The following data are given:

Sewered population ⫽ 55,000

Average domestic wastewater flow ⫽ 100 gal/(c

⭈ d)

Assumed infiltration flow rate ⫽ 500 gal/(d

⭈ mile) per in of pipe diameter

Sanitary sewer systems for the city:

4-in house sewers ⫽ 66.6 miles

6-in building sewers ⫽ 13.2 miles

8-in street laterals ⫽ 35.2 miles

12-in submains ⫽ 9.8 miles

18-in mains ⫽ 7.4 miles

Estimate the infiltration flow rate and its percentage of the average daily and

peak hourly domestic wastewater flows.

solution:

Step 1. Calculate the average daily flow (Q) and peak hourly flow (Q

)

Assuming Q

⫽ 3Q

Q ⫽ 100 gal/(c ⭈ d) ⫻ 55,000 persons

⫽ 5500,000 gal/d

⫽ 5500,000 gal/d ⫻ 3

⫽ 16,500,000 gal/d

Step 2. Compute total infiltration flow, I

I ⫽ infiltration rate ⫻ length ⫻ diameter

⫽ 500 gal/(d ⭈ mile ⭈ in)

⫻ (66.6 ⫻ 4 ⫹ 13.2 ⫻ 6 ⫹ 35.2 ⫻ 8 ⫹ 9.8 ⫻ 12 ⫹ 7.4 ⫻ 18) mile

⭈ in

⫽ 439,000 gal/d

Step 3. Compute percentages of infiltration to daily average and peak

hourly flows

I/Q ⫽ (439,000 gal/d)/(5,500,000 gal/d) ⫻ 100%

⫽ 8.0%

I/Q

⫽ (439,000 gal/d)/(16,500,000 gal/d) ⫻ 100%

⫽ 2.66%

4.1 Design ﬂow rates

The average daily flow (volume per unit time), maximum daily flow, peak

hourly flow, minimum hourly and daily flows, and design peak flow are

generally used as the basis of design for sewers, lift stations, sewage

(wastewater) treatment plants, treatment units, and other wastewater

handling facilities. Definitions and purposes of flow are given as follows.

The design average flow is the average of the daily volumes to be

received for a continuous 12-month period of the design year. The aver-

age flow may be used to estimate pumping and chemical costs, sludge

generation, and organic-loading rates.

The maximum daily flow is the largest volume of flow to be received

during a continuous 24-hour period. It is employed in the calculation of

retention time for equalization basin and chlorine contact time.

The peak hourly flow is the largest volume received during a 1h

period, based on annual data. It is used for the design of collection and

interceptor sewers, wet wells, wastewater pumping stations, waste-

water flow measurements, grit chambers, settling basins, chlorine con-

tact tanks, and pipings. The design peak flow is the instantaneous

maximum flow rate to be received. The peak hourly flow is commonly

assumed as three times the average daily flow.

The minimum daily flow is the smallest volume of flow received during

a 24-h period. The minimum daily flow is important in the sizing of con-

duits where solids might be deposited at low flow rates.

The minimum hourly flow is the smallest hourly flow rate occurring

over a 24-h period, based on annual data. It is important to the sizing

of wastewater flowmeters, chemical-feed systems, and pumping systems.

Example: Estimate the average and maximum hourly flow for a community

of 10,000 persons.

Step 1. Estimate wastewater daily flow rate

Assume average water consumption ⫽ 200 L/(c

⭈ d)

Assume 80% of water consumption goes to the sewer

Average wastewater flow ⫽ 200 L/(c ⭈ d) ⫻ 0.80 ⫻ 10,000 persons

⫻ 0.001 m

⫽ 1600 m

Step 2. Compute average hourly flow rate

Average hourly flow rate ⫽ 1600 m

/d ⫻ 1 d/24 h

⫽ 66.67 m

Step 3. Estimate the maximum hourly flow rate

Assume the maximum hourly flow rate is three times the average hourly

flow rate, thus

Maximum hourly flow rate ⫽ 66.67 m

/h ⫻ 3

⫽ 200 m

548 Chapter 6

5 Urban Stormwater Management

In the 1980s, stormwater detention or retention basin became one of the

most popular and widely used best management practices (BMPs) for

quality enhancement of stormwater. In the United States, Congress

mandated local governments to research ways to reduce the impact of

separate storm sewer systems and CSO discharges on all receiving water

bodies. In Europe, most communities use combined sewer systems, with

some separate storm sewer systems in newer suburban communities. The

CSO problem has received considerable attention in Europe.

Both quality and quantity of stormwater should be considered when

protecting water quality and reducing property damage and traffic delay

by urban flooding. Stormwater detention is an important measure for

both quality and quantity control. Temporarily storing or detaining

stormwater is a very effective method. Infiltration practices are the

most effective in removing stormwater pollutants (Livingston, 1995).

When stormwater is retained long enough, its quality will be enhanced.

Several publications (US EPA 1974c, 1983a, (NIPC) 1992, WEF and

ASCE 1992, Wanielista and Yousef, 1993, Urbonas and Stahre, 1993,

Pitt and Voorhees, 1995, Shoemaker et al., 1995, Terstriep and Lee,

1995, Truong and Phua, 1995) describe stormwater management plans

and design guidelines for control (e.g. detention or storage facilities) in

detail. Storage facilities include local disposal, inlet control at source

(rooftops, parking lots, commercial or industrial yards, and other sur-

faces), on-site detention (swales or ditches, dry basins or ponds, wet

ponds, concrete basins, underground pipe packages or clusters), in-line

detention (concrete basins, excess volume in the sewer system, pipe

packages, tunnels, underground caverns, surface ponds), off-line stor-

age (direct to storage), storage at treatment plant, and constructed

wetland.

Two of the largest projects for urban stormwater management are

given. Since the early 1950s, Toronto, Canada, has expanded with urban

development. The city is located at the lower end of a watershed. In 1954,

Hurricane Hazel brought a heavy storm (6 in. in 1 hour). Subsequently,

overflow from the Don River flooded the city. Huge damage and losses

occurred. Thereafter, an urban management commission was formed

and two large flood control reservoirs were constructed. Natural tech-

niques were applied to river basin management.

After Hurricane Hazel in 1954, the Government of Ontario created

“Conservation Authorities” to manage water quality in major drainage

basins in the province. Their initial focus was on flood control. It grad-

ually expanded to what we now call integrated watershed management.

In the late 1960s, the Government of Canada initiated federal-provincial

comprehensive river basin planning exercises in various basins across

the country. The results of these comprehensive river basin, or “aquatic

Wastewater Engineering 549

ecosystem” planning exercises were mixed, but much was learned about

technical matters, public participation, and combining technical, eco-

nomic, environmental, and social issues.

In Chicago, USA, before the 1930s combined sewers were built, as in

other older municipalities. About 100 storms per year caused a combi-

nation of raw wastewater and stormwater to discharge into Chicagoland

waterways. Approximately 500,000 homes had chronic flooding problems.

Based on the need for pollution and flood controls, the mass $3.66 billion

Tunnel and Reservoir Plan (TARP) was formed. Also known as Chicago’s

Deep Tunnel, this project of intergovernmental efforts has undergone

more than 25 years of planning and construction. The two-phased plan

was approved by the water district commissions in 1972. Phase 1 aimed

to control pollution and included 109 miles (175.4 km) of tunnels and

three dewatering pumping stations. Phase 2 was designed for flood con-

trol and resulted in construction of three reservoirs with a total capacity

of 16 billion gal (60.6 billion L) (Carder, 1997).

The entire plan consisted of four separate systems—Mainstream,

Calumet, Des Plaines, and O’Hare. TARP has a total of 130 miles (209 km)

of tunnels, 243 vertical drop shafts, 460 near-surface collecting struc-

tures, three pumping stations, and 126,000 acre ⭈ ft (155,400,000 m

) of

storage in three reservoirs. The reservoir project began in the middle of

the 1990s (F. W. Dodge Profile, 1994; Kirk, 1997).

The tunnels were excavated through the dolomite rock strata, using

tunnel boring machines. The rough, excavated diameter of the tunnel

is about 33 ft (10 m). Tunnel depths range from 150 to 360 ft (45.7 to

110 m) below the ground surface and their diameter ranges from 9 to

33 ft (2.7 to 10 m). The construction of the Mainstream tunnel started

in 1976 and it went into service in 1985.

The TARP’s mainstream pumping station in Hodgkins, Illinois, is the

largest pumping station in the United States. It boasts six pumps: four

with a combined capacity of 710 Mgal/d (31.1 m

/s) and two with a com-

bined capacity of 316 Mgal/d (13.8 m

/s). The stored wastewater is

pumped to wastewater treatment plants. The TARP system received the

1986 Outstanding Civil Engineering Achievement award from the

American Society of Civil Engineers (Robison, 1986) and the 1989

Outstanding Achievement in Water Pollution Control award from the

Water Pollution Control Federation (current name, Water Environment

Federation). It is now one of the important tour sites in Chicago.

5.1 Urban drainage systems

Urban drainage systems or storm drainage systems consist of flood

runoff paths, called the major system. Major (total) drainage systems

are physical facilities that collect, store, convey, and treat runoff that

550 Chapter 6

exceeds the minor systems. It is composed of paths for runoff to flow to

a receiving stream. These facilities normally include detention and

retention facilities, street storm sewers, open channels, and special

structures such as inlets, manholes, and energy dissipators (Metcalf

and Eddy, Inc. 1970; WEF and ASCE, 1992).

The minor or primary system is the portion of the total drainage

system that collects, stores, and conveys frequently occurring runoff, and

provides relief from nuisance and inconvenience. The minor system

includes streets, sewers, and open channels, either natural or con-

structed. It has traditionally been carefully planned and constructed,

and usually represents the major portion of the urban drainage infra-

structure investment. The major drainage system is usually less con-

trolled than the minor system and will function regardless of whether

or not it has been deliberately designed and/or protected against

encroachment, including when the minor system is blocked or otherwise

inoperable (WEF and ASCE, 1992). The minor system is traditionally

planned and designed to safely convey runoff from storms with a spe-

cific recurrence interval, usually 5 to 10 years (Novotny et al., 1989). In

the United States, flood drainage systems are included in flood insur-

ance studies required by the Office of Insurance and Hazard Mitigation

of the Federal Emergency Management Agency.

6 Design of Storm Drainage Systems

Urban drainage control facilities have progressed from crude drain

ditches to the present complex systems. The systems start from surface

runoff management control facilities, such as vegetation covers, deten-

tion or storage facilities, sedimentation basin, filtration, to curbs, gutters,

inlets, manholes, and underground conduits.

Managing surface runoff in urban areas is a complex and costly task.

Hydrology, climate, and physical characteristics of the drainage area

should be considered for design purposes. Hydrologic conditions of

precipitate–runoff relationship (such as magnitude, frequency, and dura-

tion of the various runoffs, maximum events) and local conditions (soil

types and moisture, evapotranspiration, size, shape, slope, land-use, etc.)

of the drainage (watershed) should be determined.

Clark and Viessman (1966) presented an example of hydraulic design

of urban storm drainage systems to carry 10-year storm runoffs from

eight inlets. A step-by-step solution of the problem was given.

7 Precipitation and Runoff

Precipitation includes rainfall, snow, hail, and sleet. It is one part of the

hydrologic cycle. Precipitation is the primary source of water in springs,

streams and rivers, lakes and reservoirs, groundwater, and well waters.

Wastewater Engineering 551

The US National Weather Service maintains observation stations

throughout the United States. The US Geological Survey operates

and maintains a national network of stream-flow gage stations

throughout the country’s major streams. State agencies, such as the

Illinois State Water Survey and the Illinois Geological Survey, also

have some stream flow data similar to the national records. In other

countries, similar governmental agencies also maintain long-term

precipitation, watershed runoff, stream flow data, and groundwater

information.

Upon a catchment area (or watershed), some of the precipitation runs

off immediately to streams, lakes or lower land areas, while some evap-

orates from land to air or penetrates the ground (infiltration). Snow

remains where it falls, with some evaporation. After snow melts, the

water may run off as surface and groundwater.

Rainwater or melting snow that travels as overland flow across the

ground surface to the nearest channel is called surface runoff. Some

water may infiltrate into the soil and flow laterally in the surface soil

to a lower land water body as interflow. A third portion of the water may

percolate downward through the soil until it reaches the ground-water.

Overland flows and interflow are usually grouped together as direct

runoff.

7.1 Rainfall intensity

The rainfall intensity is dependent on the recurrence interval and

the time of concentration. The intensity can be determined from the

cumulative rainfall diagram. From records of rainfall one can

format the relationship between intensity, and frequency as well as

duration. Their relationship is expressed as (Fair et al., 1966,

Wanielista, 1990)

(6.8)

where i ⫽ rainfall intensity, in/h or cm/h

t ⫽ frequency of occurrences, yr

d ⫽ duration of storm, min

a, b,m,n ⫽ constants varying from place to place

By fixing a frequency of occurrence, the intensity–duration of n-year

rainstorm events can be drawn. For many year storm events, the

intensity–duration–frequency curves (Fig. 6.1) can be formatted for

future use. The storm design requires some relationship between

expected rainfall intensity and duration.

i 5

sb 1 dd

552 Chapter 6

Let A ⫽ at

and n ⫽ 1. Eliminating t in Eq. (6.8) the intensity–duration

relationship (Talbot parabola formula) is

(6.9)

where i ⫽ rainfall intensity, mm/h or in/h

d ⫽ the duration, min or h

A, B ⫽ constants

Rearrange the Talbot equation for a straight line

Use the following two equations to solve constants A and B:

From the data of rainfall intensity and duration, calculate ⌺d, ⌺(1/i),

⌺(1/i

), ⌺(d/i;) then A and B will be solved. For example, for a city of

5-year storm, i(mm/h) ⫽ 5680/[42 ⫹ d (min)]. Similarly, a curve such as

that in Fig. 6.1 can be determined (42 ⫹ d (min)).

d 5 A

2 nB

5 A

2 nB

d 5

2 B

i 5

B 1 d

Wastewater Engineering 553

Figure 6.1 Intensity–duration–frequency curves.

7.2 Time of concentration

The time for rainwater to flow overland from the most remote area of

the drainage area to an inlet is called the inlet time, t

. The time for water

to flow from this inlet through a branch sewer to the storm sewer inlet

is called the time of sewer flow, t

. The time of concentration for a par-

ticular inlet to a storm sewer, t, is the sum of the inlet time and time of

sewer flow, i.e.

t ⫽ t

⫹ t

(6.10)

The inlet time can be estimated by

⫽ C(L/S i

)

1/3

(6.11)

where t

⫽ time of overland flow, min

L ⫽ distance of overland flow, ft

S ⫽ slope of land, ft/ft

i ⫽ rainfall intensity, in/h

C ⫽ coefficient

⫽ 0.5 for paved areas

⫽ 1.0 for bare earth

⫽ 2.5 for turf

The time of concentration is difficult to estimate since the different

types of roof, size of housing, population density, land coverage, topog-

raphy, and slope of land are very complicated. In general, the time of con-

centration may be between 5 and 10 min for both the mains and the

submains.

Example: The subdrain sewer line to a storm sewer is 1.620 km (1 mile). The

average flow rate is 1 m/s. Inlet time from the surface inlet through the over-

land is 7 min. Find the time of concentration to the storm sewer.

solution:

Step 1. Determine the time of sewer flow, t

Step 2. Find t

t 5 t

1 t

5 7 min 1 27 min

5 34 min

1620 m

1 m/s

5 1620 s 5 27 min

554 Chapter 6