Lecture 5: River Water Quality
(Jan 16th, 2015)
by Dr. Arun Kumar (arunku@civil.iitd.ac.in)
Objective: To introduce river water quality
concepts and fundamentals
January 15, 2015 Arun Kumar 1
(arunku@civil.iitd.ac.in)
Previous class
• Discussion on parameters
– Pathogens
– Inorganic ions
– Heavy metals
– Organic compounds
– Suspended solids
– Excess nutrients
Implications of wastewater discharge
on river water quality
• What happens when wastewater is discharged in river?
• Which parameter to study first?
• How to track trend of important parameter with time and
distance along the movement of river?
• What other parameters to measure?
Dissolved oxygen (DO)
• Important for aquatic species-need some minimum level of
DO
• Important for aquatic plants
• Lack of DO can result in development of anaerobic
conditions which can be result in anaerobic breakdown;
generation of methane and carbon dioxide
Dissolved oxygen (DO)
• What are factors that affect the amount of dissolved oxygen
concentration in a river?
• What is the approximate dissolved oxygen concentration in
a healthy natural water body?
• Which are the steps in developing a DO sag curve?
• How is the lowest DO concentration point in the sag curve
called?
• If there was no change in the waste addition in a stream
throughout the year, will the DO be higher in winter or
summer?
Effect of Oxygen Demanding
Wastes on Rivers
• Amount of dissolved oxygen (DO) in water is the most
commonly used indicator of a river’s health.
• The solubility of oxygen depends on temperature,
pressure, and salinity and the dissolved oxygen
concentration in a healthy stream ranges from 7-9 mg/L.
• As DO drops below 4 or 5 mg/L the forms of life that can
survive begin to be reduced.
• In an extreme case, when anaerobic conditions exist,
most higher forms of life are killed.
(Source: Environmental Science: A Global Concern, 3rd ed. by W.P Cunningham and B.W. Saigo, WC Brown Publishers, © 1995)
Dissolved Oxygen Depletion
Factors Affecting Amount of DO
Available in Rivers
• Oxygen demanding wastes affect available DO
• Tributaries bring their own oxygen supply
• Photosynthesis adds DO during the day but the
same plants remove oxygen at night
• Respiration of organisms living in water as well as in
sediments remove oxygen
• In the summer rising temperatures reduce solubility
of oxygen
• In the winter oxygen solubility increases, but ice may
form blocking access to new atmospheric oxygen
Modeling DO in a River
• To model all the effects and their
interaction is a difficult task
• The simplest model focuses on two
processes:
– The removal of oxygen by microorganisms
during biodegradation (de-oxygenation)
– The replenishment of oxygen at the
interface between the river and the
atmosphere (re-aeration)
Dissolved Oxygen Sag Curve
Mass Balance Approach
• River described as “plug-flow reactor”
• Mass balance
• Oxygen is depleted by BOD exertion (de-
oxygenation)
• Oxygen is gained through re-aeration
Steps in Developing the DO Sag Curve
1. Determine the initial conditions
2. Determine the de-oxygenation rate from
BOD test and stream geometry
3. Determine the re-aeration rate from
stream geometry
4. Calculate the DO deficit as a function of
time
5. Calculate the time and deficit at the
critical point (worst conditions)
1. Determine Initial Conditions
Mass Balance for Initial Mixing
Qw = waste flow (m3/s)
DOw = DO in waste (mg/L)
Lw = BOD in waste (mg/L)
Qr = river flow (m3/s) Qmix = combined flow (m3/s)
DOr = DO in river (mg/L) DO = mixed DO (mg/L)
Lr = BOD in river (mg/L) La = mixed BOD (mg/L)
1. Determine Initial Conditions
a. Initial dissolved oxygen concentration:
QwDOw + Qr DOr
DO =
Qw + Qr
b. Initial DO deficit:
Da = DOs − DO
where:
Da=initial DO deficit (mg/L)
DOs=saturation DO conc.(mg/L)
1. Determine Initial Conditions
Therefore, the initial deficit after mixing is
Qw DOw + Qr DOr
Da = DOs −
Qmix
where Da is the initial deficit (mg/L)
Note: DOs is a function of temperature,
atmospheric pressure, and salinity. Values
of DOs are found in tables.
1. Determine Initial Conditions
Solubility of Oxygen in Water
(DOs = DOsaturation)
DOs is a function of temperature, atmospheric pressure
and salinity
1. Determine Initial Conditions
c. Initial ultimate BOD concentration:
If, the BOD data for the waste or river are in
terms of BOD5, calculate L for each
BODt
L=
1 − e −kt
Therefore, initial ultimate BOD concentration
Qw Lw + Qr Lr
La =
Qw + Qr
2. Determine de-oxygenation rate
2. Determine De-oxygenation Rate
rate of de-oxygenation = kdLt
where: kd = de-oxygenation rate coefficient (day-1)
Lt = ultimate BOD remaining at time (of
travel down-stream) t
− kd t
If kd (stream) = k (BOD test) and Lt = L0 e
rate of de - oxygentation = k d L0e − kd t
3. Determine re-aeration rate
3. Determine Re-aeration Rate
rate of re-aeration = kr D
kr = re-aeration constant (time -1)
D = dissolved oxygen deficit (DOs-DO)
DOs = saturated value of oxygen
DO = actual dissolved oxygen at a given
location downstream
3. Determine re-aeration rate
• O’Connor-Dobbins correlation:
1/ 2
3.9u
kr = 3/ 2
h
where kr = re-aeration coefficient @ 20ºC (day-1)
u = average stream velocity (m/s)
h = average stream depth (m)
• Correct rate coefficient for stream temperature
k r = k r , 20 ΘT − 20
where Θ = 1.024
4. Calculate DO deficit as a function of time
4. DO as function of time (Streeter-Phelps
equation or oxygen sag curve)
• Rate of increase of DO deficit = rate of
deoxygenation – rate of reaeration
dD
= k d Lt − k r D
dt
• Solution is:
k d Lo − kd t
Dt =
kr − kd
( ) ( )
e − e − k r t + Da e − kr t
4. Calculate critical time and DO
5. Calculate Critical time and DO
Critical Point = point where steam conditions are at their worst
1 kr k r − k d
tc = ln 1 − Da
kr − kd kd k d La
k d La − kd tc
Dc =
kr − ka
e ( )
− e − kr tc + Da e − k r tc
D = dissolved oxygen deficit
Example 1
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