cvl100 environmental science - indian institute of...

August 11, 2016 Arun Kumar ([email protected])

1

(Aug 9th, 2016)

by Dr. Arun Kumar ([email protected])

CVL100 Environmental Science

Lecture 4: Oxygen demanding substances

Objective: To learn about oxygen demandingsubstances

This week

• Tuesday

• Wed

• Friday

• All three days

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Parameters

• Oxygen demanding wastes

• Pathogens (parameters: indicators: fecal coliforms, total coliforms, coliphage; direct measurement of pathogens)

• Inorganic ions (arsenic ions; chromium ions, nitrates; phosphates)

• Excess nutrients (ammonium ions; phosphates)

Drain discharging wastewater to stream

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upstream

downstream

After mixing (Qmix;Cmix)

Drain(Qdrain;Cdrain)stream (Qstream;Cstream)

Qmix=Qstream+Qdrain

Cmix=(CstreamQstream+CdrainQdrain)/Qmix where C : parameter

• 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?

Dissolved oxygen (DO)

Oxygen Demanding Wastes-measurement/estimation

• Estimated stoichiometrically by theoretical oxygen demand (ThOD)

• Measured by oxygen demand potential– biochemical oxygen demand (BOD)– Nitrogenous oxygen demand (NBOD)

– chemical oxygen demand (COD)

• When organic substances are broken down in water, oxygen is consumed

organic C + O2 → CO2

• For example:

CH3COOH + 2O2 => 2CO2 + 2H2O

C6H15O6N + 6O2 => 6 CO2 + 6 H2O + NH3

Oxygen-Demanding Wastes

Oxygen Demanding Wastes

Oxygen-Demanding Wastes

• High oxygen levels necessary for healthy stream ecology.

• For example:

– trout require 5-8 mg/L dissolved oxygen (DO)

– carp require 3 mg/L DO

Oxygen Demanding Wastes

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.

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

BOD

The amount of oxygen required by microorganisms to oxidize organic wastes aerobically is called biochemical oxygen demand (BOD)

Organic matter + O2 CO2+ H2O + New Cells + Stable Products

microorganisms

BOD

• It is expressed in mg of oxygen required per liter of wastewater (mg/L)

• The five-day BOD (BOD5) is the total amount of oxygen consumed by microorganisms during the first five days of biodegradation

Unseeded BOD test• Put a sample of wastewater into a 300 mL bottle

• Incubate for 5 days at 20oC, in the dark, and with stopper on.

• Measure concentration of dissolved oxygen (DO) in the beginning, and 5 days later

• Need at least 2 mg DO change in 5 days

• Usually need to dilute with water (oxygen saturated)

P = dilution factor = volume of wastewater/total volume

DOi = initial dissolved oxygen

DOf = final dissolved oxygen

P

DODOBOD fi

5

−=

• Take sample of waste, dilute with oxygen saturated water, add nutrients and microorganisms (seed)

• Measure dissolved oxygen (DO) levels over 5 days

• Temperature 20°C, In dark (prevents algae from growing)

• Final DO concentration must be > 2 mg/L

• Need at least 2 mg/L change in DO over 5 days

Seeded BOD Test

P

P))(1B(B-)DO(DOBOD fifi

5

−−−=

DOi and DOf = initial and final (5 d) DO conc. of diluted sample

Bi and Bf = initial and final (5 d) DO conc. of seeded diluted water (blank)

P = dilution factor = volume of wastewater/total volume

Example 1

• A BOD test was conducted in the laboratory using wastewater being dumped into Lake Spartan. The samples are prepared by adding 3 mL of wastewater to the 300 mL BOD bottles. The bottles are filled to capacity with seeded dilution water.

Time(days)

Dilutedsample

DO (mg/L)

Blank SeededSample DO

(mg/L)

0 7.95 8.15

1 3.75 8.10

2 3.45 8.05

3 2.75 8.00

4 2.15 7.95

5 1.80 7.90

Example 1

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6

time (days)

BO

D (

mg

/L)

Example 1

0

1

2

3

4

5

6

0 10 20 30

Time (days)

Conc.

(mg/L

)

BOD exerted (BODt) (organic matter oxidized)

BOD remaining (Lt) (organic matter remaining)

Modeling BOD, Lt and L0

Time (days)

0 5 10 15 20 25 300

2

4

6

8

Co

nc (

mg

/L)

(BOD remaining)

L0(ultimate carbonaceous

oxygen demand)

L0- Lt

Lt

= BODt


ktot eL L −

=

t0t L L BOD −=

ktoot eL L BOD −

−=

)e(1LBOD ktot

−−=

• Replace Lt using:

• To give:

• Simplified to:


At any time, Lo = BODt + Lt (ultimate BOD equals the amount of DO

demand used up and the amount of DO that could be used up eventually)

Chemical Oxygen Demand

• Chemical oxygen demand - similar to BOD but is determined by using a strong oxidizing agent to break down chemical (rather than bacteria)

• Still determines the equivalent amount of oxygen that would be consumed

• Value usually about 1.25 times BOD

COD measurement

• Potassium dichromate is a strong oxidizing agent and it can be used to prepare solution of exact normality.

• CnHaObNc+d Cr2O72- + (8d+c) H+ => nCO2

+[(a+8d-3c)/2]H2O+c NH4+ +2dCr3+ (1)

• Here d=(2n/3)+(a/6)-(b/3)-(c/2)

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Nitrogenous Oxygen Demand• So far we have dealt only with carbonaceous demand

(demand to oxidize carbon compounds)

• Many other compounds, such as proteins, consume oxygen

• Additional oxygen demand required when nitrogen compounds are oxidized

Nitrification (microorganisms convert ammonia to nitrite)

2 NH3 + 3O2 → 2 NO2- + 2H+ + 2H2O

2 NO2- + O2 → 2 NO3

-

Overall reaction:

NH3 + 2O2 → NO3- + H+ + H2O

NH3 + 2O2 → NO3- + H+ + H2O

• Theoretical NBOD can be determined using the

ratio:

used nitrogen of g

used oxygen g( )

( ) N g

O g 4.57

mole

g 14mole 1

mole

g 32moles 2

2=

=

• NBOD = oxygen needed to convert NH3 to NO3-

0

1

2

3

4

5

6

0 10 20 30

Time (days)

Co

nc. (m

g/L

)

BOD exerted (BODt) (organic matter oxidized)BOD exerted (BODt) (organic matter oxidized)

BOD remaining (Lt)

(organic matter remaining)

BOD remaining (Lt)

(organic matter remaining)

Questions to think

• Why do the COD analysis and BOD analysis give different results for the same waste?

• What could be inferred from the following samples concerning the relative ease of biodegradability: Sample A (5-d BOD/COD=24/30) and Sample B (5-d BOD/COD=10/50)?

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River Water Quality

Objective: To introduce river water quality concepts and fundamentals

• 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?

Implications of wastewater discharge on river water quality

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)

Mass Balance for Initial Mixing

Qr = river flow (m3/s)DOr = DO in river (mg/L)Lr = BOD in river (mg/L)

Qmix = combined flow (m3/s)DO = mixed DO (mg/L)La = mixed BOD (mg/L)

Qw = waste flow (m3/s)DOw = DO in waste (mg/L)Lw = BOD in waste (mg/L)

1. Determine Initial Conditions

a. Initial dissolved oxygen concentration:

b. Initial DO deficit:

where:

Da=initial DO deficit (mg/L)

DOs=saturation DO conc.(mg/L)

rw

rrww

QQ

DOQDOQDO

+

+=

DODOD sa −=


Therefore, the initial deficit after mixing is

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.

mix

rrww

saQ

DOQDOQDOD

+−=


Solubility of Oxygen in Water(DOs = DOsaturation)

DOs is a function of temperature, atmospheric pressure and salinity


c. Initial ultimate BOD concentration:

If, the BOD data for the waste or river are in

terms of BOD5, calculate L for each

Therefore, initial ultimate BOD concentration

rw

rrww

aQQ

LQLQL

+

+=

kt

t

e

BODL

−−

=1


2. Determine De-oxygenation Rate

where: kd = de-oxygenation rate coefficient (day-1)

Lt = ultimate BOD remaining at time (of

travel down-stream) t

If kd (stream) = k (BOD test) and tk

tdeLL

−= 0

tk

ddeLk

−= 0onoxygentati-de of rate

rate of de-oxygenation = kdLt

2. Determine de-oxygenation 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 oxygenDO = actual dissolved oxygen at a given

location downstream

3. Determine re-aeration rate

• O’Connor-Dobbins correlation:

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

where Θ = 1.024

2/3

2/19.3

h

ukr =

20

20,

−Θ=

T

rr kk

3. Determine re-aeration rate

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

• Solution is:

DkLkdt

dDrtd −=

( ) ( )tk

a

tktk

dr

od

trrd eDee

kk

LkD

−−−+−

−=

4. Calculate DO deficit as a function of time

5. Calculate Critical time and DO

−−

−=

ad

dr

a

d

r

dr

cLk

kkD

k

k

kkt 1ln

1

( ) crcrcd tk

a

tktk

ar

ad

c eDeekk

LkD

−−−+−

−=

Critical Point = point where steam conditions are at their worst

4. Calculate critical time and DO

D = dissolved oxygen deficit

Example 1

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Parameter Measurements

Objective: To introduce parameters to indicate pollution and methods to measure them

• Dissolved solids, or salts, may be present as any number of ions

– cations: Na+, K+, Mg2+, Ca2+

– anions: Cl-, SO42-, HCO3

-

• Typically measures as total dissolved solids (TDS)

• Water classification

– freshwater <1500 mg/L TDS

– brackish water 1500 – 5000 mg/L

– saline water >5000 mg/L

– sea water 30-34 g/L

Salts

Salts

• Sources

– industrial discharges

– deicing

– evaporative losses

– minerals

– sea water intrusion

• Effects

– natural fresh water population threatened

– limits use for drinking

– crop damage/soil poisoning (cannot use for irrigation)

Salts

Salts

Suspended Solids• Organic and inorganic particles in water are

termed suspended solids

• May be distinguished from colloids, particles that do not settle readily

Suspended Solids

• Problems

– sedimentation

– may exert oxygen demand

– primary transport mechanism for many metals, organics and pathogens

– aesthetic

– complicates drinking water treatment

• Sources

– storm water

– wastes

– erosion

Solids• Used in controlling biological process; drinking water

quality, etc.

• Gravimetric analysis is based on the determination ofconstituents or categories of materials by measurementof their weight.

• Three analytic operations:filtration, evaporation, andcombustion.– Filtration is used to separate suspended or particulate (non-filterable)

fraction from dissolved or soluble (filterable) fractions.

– Evaporation separates water from material dissolved or suspended in it.

– Combustion differentiates between organic and inorganic matter.

Organic matter will be destroyed completely by burning at 550oC for 30

min.

•

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Ref: AWWA, WEF, APHA, 1998, Standard Methods for the Examination of Water and Wastewater (2540 D. Total Suspended Solids Dried at 103-105oC; 2540 E. Fixed and Volatile Solids Ignited at 550oC)

Total Suspended SolidsProcedure:

• Filter samples (50 ml); Oven dry at 103oC for 30 min ; At this stage all water will be evaporated and only suspended solids will be retained on filter.

• Total suspended solids

mg total suspended solids =1000*(A-B)/(sample volume in mL) (1)

Weight crucibles (mass=C) and then weight crucible and filter (total

mass=E); Initial filter weight=B; Weigh filters now (mass =A g).

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Volatile and Fixed Suspended Solids

• Put crucibles with filter paper in muffle furnace and Ignite at 550oC for 15 min. At this stage all volatile components of solids will be volatilized and only fixed inert solid materials will be left in crucible. Weigh crucible after proper cooling (mass=D g).

• Volatile suspended solids (F) :

mg volatile suspended solids/L =1000*(E-D)/(sample volume in mL) (2)

• Fixed suspended solids (G) :

mg volatile suspended solids/L =1000*(D-C)/(sample volume in mL)

•

•

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• Chlorinated solvents

– C1 and C2 aliphatics

– widely used in degreasing, dry cleaning, extraction

– somewhat soluble, volatile, difficult to degrade

C

Cl

Cl

Cl

Cl C

Cl

Cl

Cl

H C C

Cl

Cl

Cl

H

H

H

C C

H

H

Cl

H

carbon tetrachloride chloroform 1,1,1-trichloroethane vinyl chloride

Volatile Organic Compounds (VOCs)

3. Organics

Pesticides (hydrophobic organic

compounds)• Organochlorine Insecticides: were commonly used

in the past (e.g. DDT and chlordane).

• Organophosphate Pesticides: most are insecticides, some are very poisonous (they were used in World War II as nerve agents)

• Carbamate Pesticides: affect the nervous system

• Pyrethroid Pesticides: were developed as a synthetic version of the naturally occurring pesticide pyrethrin, which is found in chrysanthemums

3. Organics

naphthalene anthracene phenanathrene

pyrene benzo[a]pyrene

Polycyclic Aromatic Hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons (PAHs) are a group of over 100 different chemicals that are formed during the incomplete burning of coal, oil and gas, garbage, or other organic substances.

They are potent carcinogens, hydrophobic organiccompounds and they tend to associate with particles.

3. Organics

• Polychlorinated biphenyls are mixtures of individual chlorinated compounds (known as congeners).

• PCBs are either oily liquids or solids that are colorless to light yellow.

• PCBs have been used as coolants and lubricants in transformers, capacitors, and other electrical equipment.

• The manufacture of PCBs was stopped in the U.S. in 1977.

• They are hydrophobic and they bio-accumulate in the food chain.

Polychlorinated biphenyls (PCBs)

Biphenyl

Cl

Cl

Cl

2,3',4-trichlorobiphenyl

3. Organics

• Petroleum constituents:

– benzene and substituted benzenes

– prevalent in gasoline, diesel fuel, heating oil

– most easily transported, slow degradation, toxic

CH2CH3CH3 CH3

CH3

Benzene Toluene Ethylbenzene m-Xylene


3. Organics

• Oxygentated gasoline additives

– added to gasoline to improve air quality

– very soluble, resistant to degradation, toxic

– attempt to solve one problem caused another (spills)

CH3 C

CH3

CH3

O CH3

Methyl tertiary butyl ether (MTBE)


3. Organics


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Why do we measure in EnvEngg.

• Quality of what we

want to treat

• Quality of finished

product

• Performance of

process

• Environmental/health

impact

• Air– Emissions

– Ambient

• Water & Wastewater– Supply/raw

– During treatment

– Discharge/Distribution

• Land– Solid & Haz Waste

– Leachate impacts

– Air impacts

Courtesy: Dr. Charles Haas


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Water Test Kit

ThermometerTest strip in Water

Courtesy: Shamia Hoque


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Courtesy: Shamia Hoque

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