Water Supply Subsystem
Two major sources to supply community and industrial needs:
• Surface water (e.g. streams, lakes, and rivers)
• Groundwater (pumped from wells)
2

Water Supply Subsystem
Municipal water demand:
• Domestic: water furnished to houses, hotels, etc.
• Commercial and industrial: e.g. factories, offices, and stores.
• Public use: water furnished to public buildings
(e.g. city buildings, schools, flushing streets, fire protection.
3

4

Example
Estimate the per capital daily water withdrawal for public supply in the United States in 2005 (in
Lpcd). Use the following population data and water supply data:
5

Example
The average per-capita household water use in the U. S. is about 400 liters per day.
6

Water Usage
Water quality management is concerned with the control of pollution from human activity so that the water is not degraded to the point that is not suitable for intended uses (e.g. drinking, recreation, agriculture).
7

Water Usage
Water that has been withdrawn, used for some purpose, and then returned will be polluted in one way or another from:
• Agricultural return water: pesticides, fertilizers, and salts
• Municipal return water: human sewage, pharmaceuticals, and surfactants.
• Power plant: discharges water that is elevated in temperature.
• Industry: chemical pollutants and organic wastes.
8

Water Usage
Pollutants also enter water from:
• Natural sources: e.g. Arsenic from natural mineral deposits.
• Human sources via non-aqueous routes: e.g. mercury in water is deposited from the air (coal combustion).
9

Water Pollutants
To know how much waste can be tolerated by a water body
(lakes, rivers, ponds, and streams), you must know the type of pollutants discharged
10

Water Pollutants Sources
• Point sources: collected by a network of pipes or channels and conveyed to a single point of discharge into the receiving water.
• Non-point sources: multiple discharge points. The polluted water flows over the surface of the land or along drainage channels to the nearest water body.
11

Water Pollutants
12

Oxygen-Demanding Wastes
• One of the most important measures of the quality of a water is the amount of dissolved oxygen (DO) present.
• Oxygen-demanding wastes: are substances that oxidize in the receiving water body with the consumption of DO. The waste material is normally biodegradable organic matter (e.g. municipal wastewater), certain inorganic compounds, and naturally occurring organic matter (e.g. leaves, animal droppings)
13

Oxygen-Demanding Wastes
Importance of DO: Higher forms of aquatic life must have DO to live. The critical level of DO varies with species.
• Trout and salmon: 8 mg/L
• Bluegill and bass: 5 mg/L
14

Water Pollutants
15

Nutrients
• Nutrients are chemicals, such as nitrogen, phosphorous, carbon, sulfur, calcium, iron, manganese, boron, and cobalt, that are essential to the growth of living things. They are considered as pollutants when their concentrations are sufficient to allow excessive growth of some organisms particularly algae.
• Nitrogen: sources include municipal wastewater discharge, runoff from animal feedlots, chemical fertilizers, and nitrogen deposition from the atmosphere, especially in the vicinity of coal-fired power plants.
16

Nutrients
• Phosphorous: sources include agricultural runoff in fertilized areas, discharge from animal feedlots, and domestic sewage
(human feces + detergents). Most of the phosphorous is from non-point sources.
• Nutrient enrichment can lead to blooms of algae, which eventually die and decompose. The process of nutrient enrichment, called Eutrophication , is especially important in lakes.
17

Water Pollutants
18

Pathogens
• Pathogen = An organism which causes disease
• Typical pathogens: virus, bacteria, protozoa, worms.
• Waterborne diseases: spread by ingestion of contaminated water.
• Water-based diseases: involve water contact but don’t require ingestion.
• Water-related diseases (e.g. malaria) : involve a host that depends on water for its habitat (e.g. mosquitoes). Human contact with water is not required
19

Water Pollutants
20

Suspended Solids
• Organic and inorganic particles that are carried by the wastewater into a receiving water are termed suspended solids (SS).
• Colloidal particles that do not settle readily, cause the turbidity found in many surface waters.
21

Water Pollutants
22

Salts
• Water accumulates a variety of dissolved solids, or salts, as it passes through soils and rocks on its way to the sea.
• These salts include cations (sodium, calcium, magnesium, and potassium) and anions (chloride, sulfate, and bicarbonate).
• Salinity is the list of the concentrations of the primary cations and anions.
23

Salts
Total dissolved solids (TDS) is a measure of salinity.
• Fresh water: TDS < 1,500 mg/L
• Saline water : TDS > 5,000 mg/L
• Seawater: TDS ≈ 30,000-34,000 mg/L
• The concentrations of dissolved solids is an important indicator of the usefulness of water for various applications.
• Drinking water: recommended maximum TDS ≈ 500 mg/L.
24

Water Pollutants
25

Toxic Metals
• Most metals are toxic. The most important heavy metals in terms of their environmental impacts are mercury, lead, cadmium, and arsenic.
• Metals are totally nondegradable.
26

Water Pollutants
27

Toxic Organic Compounds
• Toxic Organic Compounds: Pesticides. (insecticides, herbicides, rodenticides, and fungicides)
• Pesticides are used to cover a range of chemicals that kill organisms that human consider undesirable.
• Toxic Volatile Organic Chemicals (VOC): are among the most commonly found contaminants in groundwater. They are often used as solvents in industrial processes and a number of them are either known or suspected carcinogens.
28

Water Pollutants
29

Endocrine-Disrupting Chemicals
They alter the normal physiological function of the endocrine system in humans and wildlife either by being or acting like a natural hormone, blocking the action of a natural hormone, or increasing or reducing the production of natural hormones (i.e. interferes with the regulation of reproductive and developmental process in mammals, birds, fish)
30

Water Pollutants
31

Heat
• Large steam-electric power
• Nuclear plant
• If the heat is released into local river or lake, the resulting rise in temperature can adversely affect life in the vicinity of the thermal plume.
• As water temperature increases, the a mount of DO that the water can hold decreases.
32

• Objective: To control the discharge of pollutants so that the quality of water is not degraded to an unacceptable extent below the natural background level.
• Oxygen-demanding wastes and nutrients have a profound impact on almost all types of rivers.
• Therefore, it is important to determine the amount of O
2 to degrade the waste required
33

Oxygen-Demanding Wastes
The are several measures of oxygen demand:
1. Theoretical oxygen demand (ThOD)
2. Chemical oxygen demand (COD)
3. Biochemical oxygen demand (BOD)
34

ThOD
If the chemical composition of the substance is known then the amount of O
2 required to completely oxidize a particular organic substance may be calculated from stoichiometry. This amount of oxygen is known as the Theoretical Oxygen Demand (ThOD)
Example :
C
6
H
12
O
6
+ 6O
2
6CO
2
+ 6H
2
O
Calculate the ThOD of 108.75 mg/L of glucose
35

ThOD
36

COD
• The Chemical Oxygen Demand (COD) is a measured quantity that does not depend on knowledge of the chemical composition of the substance in water.
• In a COD test, a strong chemical oxidizing agent is mixed with a water sample and then boiled. The difference between the amount of oxidizing agent at the beginning of the test and that remaining at the end of the test is used to calculate COD.
37

BOD
• Amount of oxygen consumed by microorganisms to oxidize the waste aerobically .
• The BOD test is an indirect measure of organic measure because we actually measure only the change in dissolved oxygen concentration caused by the microorganisms as they degrade the organic matter.
• BOD test is the most widely used method for measuring organic matter because of the direct conceptual relationship between BOD and oxygen depletion in receiving waters.
38

COD vs BOD
• COD is a test of the amount of oxygen required to oxidise organic matter in a sewage sample by chemical oxidation with a powerful oxidising agent such Potassium Dichromate.
• COD is closely related to BOD , the difference being that BOD is a test of the level of organic matter that can be biologically oxidised while COD is a test of the amount of organic matter that can be chemically oxidised.
39

COD vs BOD
• COD is normally higher than BOD because more organic compounds can be chemically oxidised than biologically oxidised. This includes chemicals toxic to biological life, which can make COD tests very useful when testing industrial sewage as they will not be captured by BOD testing.
• COD does have a big advantage over BOD in that the test only takes approximately three hours, as opposed to the five days required for BOD testing.
40

Carbonaceous BOD (CBOD)
When a water sample containing degradable organic matter is placed in a closed container and inoculated with bacteria, the oxygen consumption typically follows the pattern shown in the figure below.
41

CBOD
The amount of organic matter remaining in the container will decrease with time.
42

CBOD
• The translation of the figure into a mathematical form is straightforward. To do so, it is assumed that the rate of decomposition of organic waste is directly proportional to the concentration of degradable organic matter remaining at any time t, (L t
). Assuming a first-order reaction, we can write: where k= BOD rate constant, d -1
• The solution to the above equation after rearranging and integrating where L
0
= oxygen demand of organic compound at time t= 0
43

CBOD
L
0 is often referred to as the ultimate BOD, that is, the total amount of oxygen required by microorganisms to oxidize the waste completely to carbon dioxide and water.
44

CBOD
• Rather than L t
, we are interested in the amount of oxygen utilized in the consumption of the organics (BOD t
) . From the figure, BOD t is the difference between L
0 and L t
• The above equation is called the BOD rate equation and is often written in base 10:
45

CBOD: Example
46

CBOD
• The reaction rate constant k indicates how rapidly oxygen will be depleted in a receiving water. As k increases, the rate at which dissolved oxygen is used increases.
47

CBOD
The numerical value of the rate constant is dependent on the following:
1. The nature of the waste: simple sugars and starches degrade easily while cellulose does not.
2. Ability of the available microorganisms to degrade the waste
3. Temperature: rate of biodegradation increases with increasing T
48

CBOD
Effect of Temperature: rate of biodegradation increases with increasing T where
T = temperature of interest, ⁰C k
T
= BOD rate constant at the temperature of interest, d -1 k
20
= BOD rate constant determined at 20⁰C, d -1
θ= temperature coefficient.
θ = 1.135 for temperature between 4 and 20⁰C
θ = 1.056 for temperatures between 20 and 30⁰C
49

CBOD: Example
50

Note on BOD
• The 5 days BOD (BOD
5
) has been chose as the standard value for most wastewater analysis.
• Ultimate BOD is a better indicator of total waste strength.
51

NBOD
• Oxygen consumption due to nitrogen oxidation is called nitrogenous BOD (NBOD).
• Many organic compounds contain proteins, which contains nitrogen, and the nitrogen is released to the surrounding water as ammonia (NH
3
). At normal pH values, this ammonia is in the form of ammonium cation(NH
4
+ ).
• The ammonia released by organic compounds, plus that from other sources such as industrial wastewater and agricultural runoff
(fertilizers), is oxidized to nitrate (NO
3
) by a special group of microorganisms called nitrifying bacteria. The process is called
nitrification.
52

NBOD
• The theoretical NBOD can be calculated as follows:
• The actual NBOD is slightly less than the theoretical value due to incorporation of some of the nitrogen into new bacterial cells.
53

Example
54

NBOD vs CBOD
• The rate at which NBOD is exerted depends heavily on the number of nitrifying organisms present. In untreated sewage, there are few of these organisms, while in a well-treated effluent, the concentration is high.
• Lag: time it takes for nitrifying bacteria to reach a sufficient population.
• No Lag: higher population of nitrifying organisms reduces the lag time.
55

NBOD vs CBOD
• Once nitrification begins, NBOD can be described by the equations used for CBOD with a BOD rate constant comparable to that for CBOD with well-treated effluent (K =
0.04 to 0.10 d -1 ).
• When measurements of only CBOD is required, chemical inhibitors are added to stop the nitrification process.
56

DO Sag Curve
• One of the major tools of water quality management in rivers is the ability to assess the capability of a stream to absorb a waste load. This is done by determining the profile of DO concentration downstream from a waste discharge. This profile is called the DO sag curve.
57

DO Sag Curve
• The biota of the stream are often a reflection of the DO conditions in the stream
58

DO Sag Curve
• The DO concentration decreases near the point of discharge of waste as oxygen-demanding material are oxidized. As we move further downstream, less and less organic material remains and the oxygen is replenished from the atmosphere
59

DO Sag Curve
To develop a mathematical expression for the DO sag curve the following factors need to be considered:
1. Sources of oxygen : reaeration from the atmosphere and photosynthesis of aquatic plants
2. Factors affecting oxygen depletion
• CBOD and NBOD
• BOD already in the river upstream of the waste discharge
60

DO Sag Curve
3. DO in the waste discharge is usually less than that in the river.
– Thus DO in the river is lowered as soon as the waste is discharged (Initial DO reduction)
4. Non-point source pollution
5. Respiration of organisms living in the sediments
6. Respiration of aquatic plants
61

Mass Balance Approach
Simplified mass balances help us understand and solve the DO sag curve problem.
A. Two conservative (no chemical rxn) mass balances may be used to account for the initial mixing of the waste stream and the river
– Mass balance for DO
– Mass balance for CBOD
DO and CBOD change as the result of mixing of the waste stream and the river.
B. Once these are accounted for, the DO sag curve may be viewed as a nonconservative mass balance.
62

63

64

Example
65

Example
66

Example
67

Oxygen Deficit
• The DO sag equation has been developed using oxygen deficit rather than dissolved oxygen concentration, to make it easier to solve the integral equation that results from the mathematical description of the mass balance.
• The oxygen deficit is given by the following equation:
68

Oxygen Deficit
• Initial deficit: The initial deficit is calculated as the difference between saturated DO and the concentration of the DO after mixing
69

DO Sag Curve
70

DOs Values For Fresh Water
71

Example
Calculate the initial deficit of the Bald Eagle Creek after mixing with the wastewater from the town of State College (see previous Example). The stream temperature is 10⁰C and the wastewater is 10⁰C.
72

DO Sag Curve
• Figure a is a comprehensive mass balance diagram of DO in a small reach (stretch) of river that accounts for all the inputs and outputs.
73

DO Sag Curve
• Figure b is a simplified mass balance diagram developed by
Streeter-Phelps. The DO sag equation is also known as the
Streeter-Phelps equation.
74

DO Sag Equation
• The mass balance equation for figure b is:
75

DO Sag Equation
76

DO Sag Equation
77

DO Sag Equation
78

DO Sag Equation
79

DO Sag Equation
80

DO Sag Equation
81

DO Sag Equation
82

Example
83

Example
84

Example
85

DO Sag Curve
86

DO Sag Curve
87

DO Sag Curve
88

Example
89

Example
90

Example
91

Example
92

Example
93

Management Strategy
• A DO standard is set to protect the most sensitive species that exist or could exist in a particular river.
• For a known waste discharge and a known set of river characteristics, the DO sag equation can be solved to find the DO at the critical point.
• If the DO at the critical point is higher than the standard, the stream can adequately assimilate the waste.
94

Management Strategy
• If the DO at the critical point is less than the standard DO, then additional waste treatment is needed.
• Environmental engineers and scientists have control over just two parameters, L a and D a
. Improving treatment efficiency or adding additional treatment steps will reduce L a
.
• Adding oxygen to wastewater to bring it to saturation level before discharge. This will reduce D a
.
• The new values of L a and D a are used to determine if DO standard will be violated at the critical point
95

Management Strategy
• When using the DO sag curve to determine the adequacy of wastewater treatment, it is important to use river conditions that will cause the lowest DO concentration. These conditions are:
 Late summer when the river flows are low and temperatures are high.
 Low rivers flows results in higher values for L a
 K r is reduced more than k reduced velocities.
d and D a
.
by low river flow because of
 Higher temperatures increase k d more than k
 Higher temperature reduces DO saturation. r
.
96

Example
97

Example
98

Example
99

Example
100

Nitrogenous BOD
• The NBOD can be incorporated into the DO sag equation by adding an additional term to the equation
101

102

103

• Due:
Homework 2
104