Energy From Solid And Liquid Wastes - Ii

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 Residential

Facilities or locations where waste is

Types of wastes

Generated Low-, medium-, and high-rise

Food waste, paper,grass

apartments, single/multiple family

clippings, bush and tree

houses etc.

trimmings,diapers,wood,glass bottles, canes,plastic

Stores, restaurants, office buildings,

wrapping, etc. Paper, wood, foodwaste,

hotels, motels, repair shops, public

plastic, wrappings, glass and

kitchens, etc. Schools, hospitals, prisons,

metal continers etc . Paper, wood, food waste,

governmental centres

plastic, canes and bottles.etc.

Municipal

street cleaning, parks, other

Street cleaning, grass

services

recreational areas, etc.

clippings, plant and wood

Treatment plant

trimmings, general litter, etc. Water and waste water treatment plants Sewage sludge, sludge from

Commercial

Institutional

sites

drinking water treatment, etc

Industrial

. Food waste, metal waste,

Industrial production

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

Large single-family houses

120

0.20

Typical single-family houses

80

0.17

60 to 75

0.17

50

0.17

100 to 150

0.20

Mobile home parks

50

0.17

Tourist camps or trailer parks

35

35

Hotels and motels

50

0.10

Boarding schools

75

0.17

Day schools with cafeterias

20

0.06

Day schools without cafeterias

15

0.04

30

0.10

7 to 10

0.04

4

0.03

15

0.05

Domestic wastewater from residential Areas

Multiple-family dwellings (apartments) Small dwellings or cottages

Domestic wastewater from camps and Motels Luxury resorts

Schools

Restaurants Each employee Each patron Each meal served

Transportation terminals Each employee

Each passenger

5

0.02

150 to 300

0.30

Offices

15

0.05

Drive-in theaters, per stall

5

0.02

3 to 5

0.02

15 to 30

0.05

Hospitals

Movie theaters, per seat Factories, exclusive of industrial and Cafeteria wastes

. 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 Total solids

Raw 800

After Settling 680

Biologically Treated 530

Total volatile solids

400

340

220

Suspended solids

240

120

30

Volatile suspended solids

180

100

20

Biochemical oxygen demand

200

130

30

Inorganic nitrogen as N

22

22

24

Total nitrogen as N

35

30

26

Soluble phosphorus as P

4

4

4

Total phosphorus as P

7

6

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)xQ(mil gal) x 8.34

(2.1)

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

(2. 2)

Or

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/m3 x 300 m3 1000 g/kg

= 600 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, byproduct 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 metalplating 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/l

1,000

1,400

1,500

4,300

COD, mg/l

1,900

2,100

3,300

5,400

Total solids, mg/l

1,600

3,300

8,000

53,000

Suspended solids mg/l

300

1,000

2,000

1,200

Nitrogen, mg N/l

50

150

30

0

Phosphorus, mg P/l

12

16

0

0

pH

7

7

5

7

Temperature, 0C

29

28

----

----

Grease, mg/l

----

500

----

----

Chloride, mg/l

----

----

----

27,000

Phenols, mg/l

----

----

----

140

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

BOD (mg/l) (lb/day)

Suspended Solids (mg/l) (lb/day)

Cod

Grease (mg/l) 460

1,300

13,000

960

9,600

(mg/l) 2,500

0

220

880

140

560

440

----

Rubber Products

478,000

200

310

250

390

300

----

Ice cream

189,000

910

1,050

260

300

1,830

----

Cheese

138,000

3,160

2,900

970

890

5,600

----

Metal plating

110,000

8

7

27

24

36

----

Carpet mill

108,000

140

120

60

51

490

----

Candy

103,000

1,560

1,270

260

210

2,960

200

Motor scooters

97,700

30

23

26

20

70

----

Potato chips

93,500

600

450

680

510

1,260

----

Flour

90,400

330

230

330

250

570

----

Milk processing

83,100

1,400

760

310

170

3,290

----

Industrial Laundry

65,100

700

290

450

190

2,400

520

Pharmaceuticals

50,000

270

91

150

50

390

160

Chicken Hatchery

40,700

200

59

310

90

450

----

Luncheon meats

35,300

270

47

60

10

420

----

Soft drinks

20,900

480

64

480

64

1,000

----

Milk bottling

16,000

230

24

110

12

420

----

Totals

12,700

Meat Processing Soybean oil Extraction

(Gpd) 1,200,00

2,930,00 0

21,600

13,400

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___ 120 gal/person/day

= 540 persons

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 m3/d x 1300 mg/1________

=

=

11.7 kg

500 thousands of kg/d x 1000 g/kg BOD equivalent population 4500 m3/d x 1300 mg/1

=

=

65,000 persons

90 g BOD /person .d

Hydraulic equivalent population =

4500 m3/d x 1000 1/m3

=

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: Seweredpopulation

= 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

Average domestic flow

x 100

2,400,000 = 9.8 percent

Peak hourly domestic flow = 24,000 x 240 Infiltration

= 5,760,000 gpd = 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 SOURCE Domestic

PER DAY

BOD IN POUNDS PER DAY

7500 x 120 = 900,000

0.20 x 7500 =1500

30,000

= 500

Potato Creamery

= 120,000

0.120 x 1000 x 8.34= 1000

Total

BOD Concentration

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

------

1,050,000

3050

= 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