Lecture No: 2

2.1. Sources and types of solid wastes

Solid wastes .are generated at a variety of different sources such as industries, private

households, public institutions and small businesses such as restaurants. Some of the most

important sources of solid wastes are listed in Table 2.1.

Table 2.1. Some sources of solid wastes within an urban community

Source Facilities or locations where waste is

Generated

Types of wastes

Residential Low-, medium-, and high-rise

apartments, single/multiple family

houses etc.

Food waste, paper,grass

clippings, bush and tree

trimmings,diapers,wood,glass

bottles, canes,plastic

wrapping, etc.

Commercial Stores, restaurants, office buildings,

hotels, motels, repair shops, public

kitchens, etc.

Paper, wood, foodwaste,

plastic, wrappings, glass and

metal continers etc .

Institutional Schools, hospitals, prisons,

governmental centres

Paper, wood, food waste,

plastic, canes and bottles.etc.

Municipal

services

street cleaning, parks, other

recreational areas, etc.

Street cleaning, grass

clippings, plant and wood

trimmings, general litter, etc.

Treatment plant

sites

Water and waste water treatment

plants

Sewage sludge, sludge from

drinking water treatment, etc

.

Industrial Industrial production Food waste, metal waste,

wood waste, plastics, etc.

The sources listed in Table 2.1 represent a broader suite of sources of wastes that may

occur in most areas. In a strict sense, however, generation of solid wastes is a result of the

activities taking place in the community, and, therefore, the level of industrialization, type of

society, culture, etc. have influence on the production rate and type of the wastes generated.

The number and types of sources can therefore vary significantly between communities,

regions, and countries.

In highly developed countries where consumption of preprocessed foods is more

common the foodstuff producing industry is likely more important as a source of wastes

compared to residential households, whereas the opposite may be the case in developing

countries where more foods are prepared at home. Another example is densely populated

areas without Opt spaces such as many of the cities in Southeast Asia and in the US. In such

cities the area occupied by parks and gardens are likely limited or nonexistent and

generation of related biodegradable wastes (garden and park wastes) is therefore likely

insignificant.

Solid wastes are usually divided into different types depending on their source. Major

types of wastes are residential waste, commercial waste, sewage treatment sludge, and

industrial process waste. Each type of waste can again be divided into different material

fractions depending on the actual material contained in the residual. Types and fractions or

components of solid wastes will be discussed in more detail in the following sections.

2.2. Wastewater Flows and Characteristics

Domestic or sanitary wastewater refers to liquid discharge from residences, business

buildings, and institutions. Industrial wastewater is discharge from manufacturing plants.

Municipal wastewater is the general term applied to the liquid collected in sanitary sewers

and treated in a municipal plant. In addition, interceptor sewers direct dry weather flow from

combined sewers to treatment, and unwanted infiltration and inflow enters the collector pipes.

A schematic of the system is given in Fig. 2.1.

Fig 2.1 Sources of Municipal wastewater in relation to collector sewers and treatment

Storm runoff water in most communities is collected in a separate storm sewer

system, with no known domestic or industrial connections, and is conveyed to the nearest

watercourse for discharge without treatment. Rain water washes contaminants from roofs,

streets, and other areas. Although the pollutional load of the first flush may be significant, the

total amount from separated storm-water systems is relatively minor compared with other

wastewater discharges. Several large cities have a combined sewer system where both storm

water and sanitary wastewaters are collected in the same piping. Dry weather flow in the

combined sewers is intercepted and conveyed to the treatment plant for processing, but

during storms, flow in excess of plant capacity is by-passed directly to the receiving

watercourse. This can constitute significant pollution and a health hazard in cases where the

receiving body is used for a drinking water supply. One solution is to replace the combined

sewers with separate pipes, but the cost in large cities would be prohibitive, although this

technique can be applied where only a few combined sewers exist in a municipal system.

2.3. Domestic wastewater

The volume of wastewater from a community varies from 50 to 250 gal per capita per

day (gpcd) depending on sewer uses. A common value for domestic wastewater flow is 120

gpcd (450/person .d), which assumes that the residential dwellings have modern water-using

appliances, such as automatic washing machines. The organic matter contributed per person

per day in domestic wastewater is approximately 0.24 1b (110 g) of suspended solids and

0.20 1b (90 g) of BOD in communities where a substantial portion of the household kitchen

wastes is discharged to the sewer system through garbage grinders. In selection of data for

design, the quantity and organic strength of wastewater should be based on actual

measurements taken throughout the year to account for variations resulting from seasonal

climatic changes and other factors. The average values during the peak month may be used

for design. Excluding unusual infiltration and inflow, the average daily sanitary wastewater

flow during the maximum month of the year is commonly 20 to 30 percent greater than the

average annual daily flow. Excluding seasonal industrial wastes, the average daily BOD load

from sanitary wastewater during the maximum month is greater than the annual average by

30 percent or more in small plants (less than 0.5 mgd) and less than 20 percent in large plants

(greater than 50 mgd).

Estimated wastewater flows for residential dwelling and other establishments are

listed in Table 2.2.

Mobile homes and hotels generate less wastewater than residences, since they have

fewer appliances. The quantity and strength of wastewater from schools, offices, factories,

and other commercial establishments depend on hours of operation and available eating

facilities. Although cafeterias do not provide a great deal of flow, the wastewater strength is

increased materially by food preparation and cleanup.

Table 2.2 Approximate Wastewater Flows for Various Kinds of Establishments

Type Gallons Per Person Per

Per Day

Pounds Of Bod Per Person Per Day

Domestic wastewater from residentialAreas Large single-family houses Typical single-family houses Multiple-family dwellings (apartments) Small dwellings or cottages

Domestic wastewater from camps andMotels Luxury resorts Mobile home parks Tourist camps or trailer parks Hotels and motels

Schools Boarding schools Day schools with cafeterias Day schools without cafeterias

Restaurants Each employee Each patron Each meal served

Transportation terminals Each employee Each passenger

HospitalsOfficesDrive-in theaters, per stallMovie theaters, per seat Factories, exclusive of industrial and Cafeteria wastes

12080

60 to 75

50

100 to 150503550

752015

307 to 10

4

155

150 to 300155

3 to 5

15 to 30

0.200.170.17

0.17

0.200.1735

0.10

0.170.060.04

0.100.040.03

0.050.02

0.300.050.020.02

0.05

.

The common value for sanitary wastewater of 120 gpcd includes residential and

commercial wastewaters plus reasonable infiltration, but excludes industrial discharges.

Characteristics of this wastewater prior to treatment, after settling, and following

conventional biological processing are given in Table 2.3.

Total solids, residue on evaporation, include both dissolved salts and organic matter;

the latter is represented by the volatile fraction. BOD is a measure of the wastewater strength.

Sedimentation of a typical domestic wastewater diminishes BOD approximately 35 percent

and suspended solids 50 percent. Processing, including secondary biological treatment,

reduces the suspended solids and BOD content more than 85 percent, volatile solids 50

percent, total nitrogen about 25 percent, and phosphorus only 20 percent.

Table 2.3. Approximate Composition of Average Sanitary Wastewater (mg/1) Based on

120 gpcd (450 1/person .d)

Parameter Raw After Settling Biologically Treated

Total solids

Total volatile solids

Suspended solids

Volatile suspended solids

Biochemical oxygen demand

Inorganic nitrogen as N

Total nitrogen as N

Soluble phosphorus as P

Total phosphorus as P

800

400

240

180

200

22

35

4

7

680

340

120

100

130

22

30

4

6

530

220

30

20

30

24

26

4

5

The surplus of nutrients in the treated effluent indicates that sanitary wastewater has

nitrogen and phosphorus in excess of biological needs. The generally accepted BOD/N/P

weight ratio required for biological treatment is 100/5/1 (100 mg/1 BOD to 5 mg/1 nitrogen

to 1 mg/1 phosphorus). Raw sanitary wastewater has a ratio of 100/17/3 and after settling

100/23/5, and thus contains abundant nitrogen and phosphorus for microbial growth. (The

exact BOD/N/P ratio needed for biological treatment depends on the process method and

availability of the N and P for growth; 100/6/1.5 is often related to unsettled sanitary

wastewater, while 100/3/0.7 is used where the nitrogen and phosphorus are in soluble forms.)

Another important wastewater characteristic is that not all of the organic matter is

biodegradable. Although a substantial portion of the carbohydrates, fats, and proteins are

converted to carbon dioxide by microbial action, a waste sludge equivalent to 20 to 40

percent of the applied BOD is generated in biological treatment.

Loadings on treatment units are often expressed in terms of pounds of BOD per day or

pounds of solids per day, as well as quantity of flow per day. The relationship between the

parameters of concentration and flow is based on the following conversion factors: 1.0 mg/1,

which is the same as 1.0 part per million parts by weight, equals 8.34 1b/mil gal, since 1 gal

of water weighs 8.34 1b: and used less frequently, the value 62.4 1b/mil cu ft, since 1 cu ft. of

water weighs 62.4 1b. These relationships are defined by the following equations:

Pounds of C = concentration of C (mg/l)x Q(mil gal) x 8.34 (2.1)

Or

Pounds of C = concentration of C (mg/l) x Q(mil cu ft) x 62.4 (2. 2)

Where C = BOD, SS, or other constituent, milligrams per liter

Q = volume of wastewater, million gallons or million cubic feet

8.34 = lb/mil gal

mg/l

62.4 = lb/mil cu ft

mg/l

Calculations in Example 2.1 show that 120 gal of the sanitary wastewater as described

in Table 2.3 contain 0.20 lb of BOD and 0.24 lb of suspended solids; Examples 9-2 and 9-3

illustrate applications of Eqs. 2.1 and 2.2.

Example 2.1

Sanitary wastewater from a residential community is 120 gpcd containing 200 mg/l BOD and

240 mg/l suspended solids. Compute the pounds of BOD per capita and pounds of SS per

capita.

Solution

Using Eq.2.1

1b BOD = 200mg/1 x 0.000,120 mil gal x 8.34 = 0.201b mil gal x mg/1

1b SS = 240 mg/1 x 0.000,120 mil gal x 8.34 = 0.241b Mil Gal X Mg/1

Example 2.2

Industrial wastewaters (Table 2.5) have a total flow of 2,930,000 gpd, BOD of 21,600 1b/day,

and suspended solids of 13,400 1b/day. Calculate the BOD and suspended solids

concentrations.

Solution

From the relationship in Eq. 2.1,

BOD concentration = 21.600 1b/day

2.93 mil gal/day x 8.34

= 880 mg/1

SS concentration = 13,400 1b/day = 550 mg/1

2.93 mgd x 8.34

Example 2.3

An aeration basin with a volume of 300m3 contains a mixed liquor (aerating activated sludge)

with a suspended solids concentration of 2000 mg/1 (g/m3). How many kilograms of mixed

liquor suspended solids are in the tank?

Solution

MLSS = 2000 g/m 3 x 300 m 3 = 600 kg

1000 g/kg

2.4. Industrial Wastewaters

Industries within municipal limits ordinarily discharge their wastewater to the city’s

sewer system after pretreatment. In joint processing of wastewater, the municipality accepts

responsibility of final treatment and disposal. The majority of manufacturing wastes are more

amenable to biological treatment after dilution with domestic wastewater; however, large

volumes of high-strength wastes must be considered in sizing of a municipal treatment plant.

Uncontaminated cooling water is directed to the storm sewer.

A sewer code, user fees, and separate contracts between an industry and city can

provide adequate control and sound financial planning while they accommodate industry by

joint treatment. Pre-treatment at the industrial site must be considered for wastewaters having

strengths or characteristics significantly different from sanitary wastewater. Consideration

should be given to modifications in industrial processes, segregation of wastes, flow

equalization, and waste strength reduction. Process changes, equipment modifications, by-

product recovery, and in plant wastewater reuse can result in cost savings for both water

supply and wastewater treatment.

Modern industrial plant design dictates segregation of separate waste streams for

individual pretreatment, controlled mixing, or separate disposal. The latter applies to both

uncontaminated cooling water that can be discharged directly to surface watercourses and

toxic wastes that cannot be adequately processed by the municipal plant and must be

processed or disposed of by the industry. Manufacturing plants using a diversity of operations

may be required to equalize wastewaters by holding them in a basin for stabilization prior to

their discharge to the sewer. Unequalized flows may have dramatic fluctuations in quality

that could upset the efficiency of a biological treatment system.

Certain industrial discharges, such as dairy wastes, can be more easily reduced in

strength by treatment in their concentrated form at the industrial site. Others , like metal-

plating wastes, require pretreatment for the removal of toxic metal ions. If reuse of the

municipal wastewater is planned, rather stringent controls on industrial discharges are

needed, since many of the sunstances in manufacturing wastes are only partially removed by

conventional treatment and will interfere with water reuse.

The characteristics of four selected industrial wastewaters are listed in Table 2.4 for

comparison.

Table 2.4 Average Characteristics of Selected Industrial Wastewaters

Milk Processing Meat Packing Synthetic Textile

Chlorophenolic Manufacture

BOD, mg/lCOD, mg/lTotal solids, mg/lSuspended solids mg/lNitrogen, mg N/lPhosphorus, mg P/lpHTemperature, 0CGrease, mg/lChloride, mg/l Phenols, mg/l

1,0001,9001,6003005012729------------

1,4002,1003,3001,00015016728500--------

1,5003,3008,0002,000

3005

----------------

4,3005,40053,0001,200

007

--------

27,000140

With sanitary wastewater in Table 2.3, BOD concentrations range from 5 to 20 times

greater than for domestic wastewater. Total solids are also greater but vary in character from

colloidal and dissolved organics in food processing wastewaters to predominantly inorganic

salts, such as the chlorophenolic waste. Suspended solids concentration relative to BOD is

important when considering conventional primary sedimentation and secondary biological

treatment. Settling of the synthetic textile wastewater with a suspended solids to BOD ratio of

2000 mg/1 to 1500 mg/1 would be as effective as clarifying a sanitary wastewater with a ratio

of 240/200, but settling a milk-processing wastewater with a suspended solids to BOD ratio

of 300 mg/1 to 1000 mg/1 would remove very little organic matter. In addition to high

strength and settleability, particular consideration must be given to nutrient content, grease,

and toxicity. Food-processing wastes generally contain sufficient nitrogen and phosphorus for

biological treatment, but discharge from chemical and materials industries is deficient in

growth nutrients. Handling animal fats, plant oils, and petroleum products may result in a

wastewater too high in grease content for admission to a municipal system without

pretreatment. The chlorophenolic waste in Table 2.4 could not be discharged to sewer

without extensive reduction in phenol; the limit applied by sewer ordinances is in the range of

0.5 to 1.0 mg/1.

Metal finishing wastes are pretreated to remove oil, cyanide, chromium, and other heavy

metals such that the pretreated discharge has fewer contaminants than domestic wastewater.

Each municipality should have an inventory of industrial wastewaters discharged to the

sanitary sewer system as is illustrated in Table 2.5 in this city the major wastewater

contributors are food-processing industries. The manufacturing wastewaters from rubber

products, metal working, and carpet weaving have strengths comparable to, or less than,

domestic wastewater.

Table 2.5

Results from a Municipal Industrial Wastewater Survey Listing Discharges to the Sanitary

Sewer in a City with a Population of 145,000

Flow

(Gpd)

BOD Suspended Solids Cod

(mg/l)

Grease

(mg/l)(mg/l) (lb/day) (mg/l) (lb/day)

Meat Processing

Soybean oil Extraction

Rubber Products

Ice cream

Cheese

Metal plating

Carpet mill

Candy

Motor scooters

Potato chips

Flour

Milk processing

Industrial Laundry

Pharmaceuticals

Chicken Hatchery

Luncheon meats

Soft drinks

Milk bottling

Totals

1,200,000

478,000

189,000

138,000

110,000

108,000

103,000

97,700

93,500

90,400

83,100

65,100

50,000

40,700

35,300

20,900

16,000

12,700

2,930,000

1,300

220

200

910

3,160

8

140

1,560

30

600

330

1,400

700

270

200

270

480

230

13,000

880

310

1,050

2,900

7

120

1,270

23

450

230

760

290

91

59

47

64

24

21,600

960

140

250

260

970

27

60

260

26

680

330

310

450

150

310

60

480

110

9,600

560

390

300

890

24

51

210

20

510

250

170

190

50

90

10

64

12

13,400

2,500

440

300

1,830

5,600

36

490

2,960

70

1,260

570

3,290

2,400

390

450

420

1,000

420

460

----

----

----

----

----

----

200

----

----

----

----

520

160

----

----

----

----

Industrial wastewaters expressed in terms of quantity of flow and pounds of BOD are

relatively meaningless to the general public. Therefore, the quantity and strength can be

related to the number of persons that would be required to contribute an equivalent quantity

of wastewater. Hydraulic and BOD population equivalents, based on average sanitary

wastewater, are 120 gpcd and 0.201b BOD per person per day, respectively. In addition to

equivalent populations, it is desirable to express the quantity of wastewater produced per unit

of raw material processed or finished product manufactured. Examples 2.4 and 2.5 illustrate

wastewater production and equivalent population calculations.

Example 2.4

A dairy processing about 250,000 1b of milk daily produces an average of 65,100 gpd of

wastewater with a BOD of 1400 mg/1. the principal operations are bottling of milk and

making ice cream, with limited production of cottage cheese. Compute the flow and BOD per

1000 1b of milk received, and the equivalent populations of the daily wastewater discharge.

Solution

Flow per 1000 1b of milk

= 1000 1b______ x 65,100 gpd = 260 gal

250,000 1b/day

BOD per 1000 1b of milk

= 0.0651 mil gal/day x 1400 mg/1 x 8.34

250 thousands of 1b/day

= 3.0 1b

BOD equivalent population

= 0.0651 mil gal/day x 1400 mg/1 x 8.34

0.20 1b BOD/person/day

= 3800 persons

Hydraulic equivalent population

= 65,000 gal/day___ = 540 persons

120 gal/person/day

Example 2.5

A meat processing plant slaughters an average 500,000 kg of live beef per day. The majority

is shipped as dressed halves with some production of packaged meats. Blood is recovered for

a salable by-product, paunch manure (undigested stomach contents) is removed by screening

and hauled to land burial, and process, wastewater is settled and skimmed to recover heavy

solids and some grease for inedible rendering with other meat trimmings. After this

pretreatment, the waste discharged to the municipal sewer is 4500 m3/d containing 1300 mg/1

BOD. Calculate the BOD waste per 1000 kg LWK (live weight kill) and the equivalent

populations of the daily wastewater flow.

Solution

BOD per 1000 kg LWK

= __4500 m 3 /d x 1300 mg/1________ = 11.7 kg

500 thousands of kg/d x 1000 g/kg

BOD equivalent population

= 4500 m 3 /d x 1300 mg/1 = 65,000 persons

90 g BOD /person .d

Hydraulic equivalent population

= 4500 m 3 /d x 1000 1/m 3 = 10,000 persons

450 1/person .d

2.5. Infiltration and Inflow

Infiltration is groundwater entering sewers and building connections through defective joints

and broken or cracked pipe and manholes. Inflow is water discharged into sewer pipes or

service connections from such sources as foundation drains, roof leaders, cellar and yard area

drains, cooling water from air conditioners, and other clean-water discharges from

commercial and industrial establishments. In comparison to storm sewers, sanitary lines are

small, being sized to handle only domestic and industrial wastewaters plus reasonable

infiltration. Excessive infiltration and inflow can create several serious problems including

surcharging of sewer lines with back-up of sanitary wastewaters into house basements,

flooding of street and road areas, overloading of treatment facilities, and by passing of

pumping stations and treatment works.

The quantity of infiltration water entering a sewer depends on the condition of pipe and

pipe joints, groundwater levels, and the permeability of the soil. Seepage into new lines is

controlled by proper design, selection of sewer pipe, close supervision of construction, and

limiting infiltration allowances. Construction specifications usually permit a maximum

infiltration rate of 500 gpd per mile of sewer length and inch of pipe diameter (46 1/d per

kilometer of length and millimeter of pipe diameter). The quantity of this seepage flow is

equal to 3 to 5 percent of the peak hourly domestic flow rate, or approximately 10 percent of

the average flow. With development of better pipe jointing materials and tighter control of

construction methods, infiltration allowances as low as 200 gpd/mile/in. (191/d.km.mm) of

pipe diameter are being specified. Correction of infiltration conditions in existing sewer

systems involves evaluation and interpretation of wastewater flow conditions in determining

the source and rate of excessive infiltration, followed by consideration of corrective

measures. Present techniques to reduce infiltration are grouting or sealing of soils

surrounding the sewer pipe, pipe relining, and sewer replacement; all of them are costly.

Inflow is the result of deliberately planned, or expediently devised, connections of

extraneous water sources to sanitary sewer systems. Although unwanted storm water or

drainage should be disposed of in storm sewers, the sanitary system is often a more

convenient conduit because of greater depth of burial and more convenient location. Excess

inflow can be prevented by establishing and enforcing a sewer use regulation that excludes

storm and surface waters from separate sanitary collectors. The ordinance should be explicit

in directing surface runoff from roofs and other areas, foundation drainage, unpolluted water

from air conditioning systems, industrial cooling operations, swimming pools, and the like to

storm lines leading to natural drainage outlets. A few ordinances allow cellar drainage into

sanitary sewers; however, this is no longer considered proper under present day conditions.

This permit was probably derived from the days when basements were built with stone walls

and unpaved floors. Where inflow problems already exist, surveys can be conducted to locate

connections and to institute corrective measures.

Example 2.6

Calculate the infiltration and compare this quantity to the average daily and peak hourly

domestic wastewater flows for the following:

Sewered population = 24,000 persons

Average domestic flow = 100 gpcd

Peak hourly domestic flow = 240 gpcd

Infiltration rate = 500 gpd/mile/in.of pipe diameter

Sanitary sewer system:

4-in. building sewers = 36 miles

8-in. street laterals = 24 miles

10-in. submains = 6 miles

12-in. trunk sewers = 6 miles

Solution

Infiltration (gpd)

= rate ( gal ) x dia (in.)

day x miles x in. x length (miles )

= 500(4 x 36 + 8 x 24 + 10 x 6 + 12 x 6)

= 234,000 gpd

Average domestic flow = 24,000 x 100

= 2,400 000 gpd

Infiltration = 234,000 x 100

Average domestic flow 2,400,000

= 9.8 percent

Peak hourly domestic flow

= 24,000 x 240 = 5,760,000 gpd

Infiltration = 234,000__ x 100

Peak hourly flow 5,760,000

= 4.1 percent

2.6. Municipal Wastewater

As shown in Figure 2.1, the flow in sanitary sewers is a composite of domestic and industrial

wastewaters, infiltration and inflow, and intercepted flow from combined sewers. Collector

sewers must have hydraulic capacities to handle maximum hourly flow including domestic

and infiltration, plus any additional discharge from industrial plants. New sewer systems are

usually designed on the basis of an average daily per capita flow of 100 gal (400 litres),

which includes normal infiltration. However pipes must be sized to carry peak flows that are

often assumed to be 400 gpcd (1500 1/person.d) for laterals and submains when flowing full,

250 gpcd (950 1/person.d) for main trunk, and outfall sewers; and in the case of interceptors,

collecting from combined sewer systems, 350 percent of the average dry weather flow. Peak

hourly discharges in main and trunk sewers are less than the maximum flows in laterals and

submains, since hydraulic peaks tend to level out as the wastewater flows through a pipe

network picking up an increasing number of connections.

A typical discharge pattern from a separate sanitary sewer system is illustrated in Fig. 2.2

a. hourly flow rates range from a minimum to a maximum of 20 to 250 percent of the average

daily rate for small communities and from 50 to 200 percent for larger cities. The lowest

flows occur in early morning about 5 A.M., and peak discharge takes place near midday. The

BOD concentration in wastewater also varies with time of day in a path that follows the flow

variation (Fig. 2.2 b). Waste strength is greatest during the workday when household and

industrial activities are contributing a large amount of organic matter, and it is reduced during

the night when entering flow is less contaminated and slow velocities in pipes permit settling

of solids. If both flow and BOD concentration variations are known the time-BOD loading on

a treatment plant can be calculated and plotted as shown in Figure 2.2b. Knowledge of

influent hydraulic and BOD loadings is essential in evaluating the operation of a treatment

plant.

The quantity and characteristics of wastewater fluctuate with season of the year and

between weekdays and holidays. Summer discharges frequently exceed winter flows by 10 to

20 percent, and industrial contributions are reduced on Sundays. Hourly fluctuations in large

cities are modified in comparison with small towns because of the diversity of activities and

operations that take place throughout the 24-hr day. Large volumes of high strength industrial

waste contributions can distort typical flow and BOD patterns by accentuating the peak

hydraulic and BOD loadings during operational hours. Excessive infiltration and inflow,

while diluting wastewater strength, can have considerable impact on a treatment facility by

increasing both the average and peak flows during periods of high rainfall. All of these

factors must be considered in assessing the wastewater flow and strength variations for a

particular community.

Fig. 2.2. Wastewater flow and strength variations for a typical medium sized city

Example 2.7

The sanitary and industrial waste from a community consists of domestic wastewater from a

sewered population of 7500 persons; potato processing waste of 30,000gpd containing 550 1b

of BOD; and creamery wastewater flow of 120,000 gpd with a BOD concentration of 1000

mg/1. estimate the combined wastewater flow in gallons per day and BOD concentration in

milligrams per liter.

Solution

FLOW IN GALLONS BOD IN POUNDS

SOURCE PER DAY PER DAY

Domestic 7500 x 120 = 900,000 0.20 x 7500 =1500

Potato 30,000 = 500

Creamery = 120,000 0.120 x 1000

x 8.34= 1000

-------------- ------

Total 1,050,000 3050

BOD Concentration = 3050 1b/day________

1.05 mil gal/day x 8.34

= 348 mg/1

Example 2.8

A city with a sewered population of 145,000 has an average wastewater flow of 18.9 mgd

with an average BOD of 320 mg/1. an inventory of the industrial wastewaters entering the

sanitary sewer system is given in Table 2.5. (a) compute the equivalent populations for this

municipal wastewater flow that includes both sanitary and industrial wastewaters. (b)

Determine the per capita contribution of sanitary wastewater flow and BOD based on the

city’s population excluding the industrial wastewaters.

Solution

For the municipal wastewater,

Hydraulic equivalent population = 18,900,000 gpd

120 gpcd

= 158,000

BOD equivalent population = 18.9 mgd x 320 mg/1 x 8.34

0.20 1b/person/day

= 252,000

Per capita contributions excluding industrial wastewaters are

Sanitary flow = 18,900,000 – 2,930,000

145,000

= 110 gpcd

Sanitary BOD = 18.9 x 320 x 8.34 – 21600

145,000

=0.201b /person/day