Download Oxygen Demand

Oxygen Demand
• Objective
– To know the different expressions of Oxygen Demand
and their chemical basis,
– their use in Environmental Engineering,
– and the methods of laboratory determination.
• Reference
• Sawyer C.N. et al Chemistry for
Environmental Engineers
Oxygen Demand
Concept derives from 19th Century
 pollution of rivers with faeces, household
wastes
 ie organic wastes
 Stinking rivers, incapable of supporting fish
 Rivers devoid of oxygen
 Polluting potential expressed in terms of

–
oxygen demand
Oxygen Demand is a measure of
organic carbon

Organic pollutants complex carbon molecule

Enormous range of organics

Impossible to identify and quantify organics

Oxygen consumed as bacteria consume organics


Eg Aerobic Metabolism
Quantity of oxygen consumed is a measure of the
concentration of organics in water
Options for measuring OD

Biochemical Oxygen Demand


Chemical Oxygen Demand


BOD
COD
Total Organic Carbon

TOC
Biochemical Oxygen Demand

"Amount of oxygen required by bacteria to break down
decomposable organic matter under aerobic conditions"
45
40
35
30
Oxygen
25
Demand
20
(mg/l)
15
10
5
0
Ultimate oxygen demand
BODu = L
5 Day oxygen demand
BOD5
Carbonaceous oxygen demand
0
5
10
15
Time (days)
20
25
BOD
Bioassay
 Amount of oxygen used up under defined
conditions
 Temp 20 C
 Virtually all degradable matter oxidised in 20
days - BODu or ultimate BOD
 5 day BOD used

BOD5 is 70-80% of the BODu
 BOD wastewater (mg/l)
– <200 weak
– 300 typical
– 1000 strong
–
Complications of BOD


Conditions critical
– rigorous attention to and control of conditions
 nutrients and seed must be present
 toxins must be absent
Theoretical BOD > BOD ultimate
– 1. not all biodegradable carbon oxidised
 E.g. BOD glucose = 85% of theoretical BOD
u
C6H12O6 + 6O2
–

6CO2 + 6H2O
2. not all carbon biodegradable under conditions
 sometimes called humus
 eg lignin
Ammonia also exerts an oxygen demand
– Nitrification

Nitrification controlled for by
– 5 day test (nitrifiers grow slowly)
– inhibitors eg Alkylthiourea (ATU), TCMP
higher oxygen demand
exerted due to nitrification
5 Day oxygen demand
BOD5
45
40
35
Oxygen
30
Demand
25
20
(mg/l)
15
10
5
0
Carbonaceous oxygen demand
0
5
10
15
Time (days)
20
25
Measuring dissolved oxygen

Winklers Method (titration)
– Mn2+ oxidised to MnO2
-
Mn2+ + 2OH + 1/2 O2
–
At low pH MnO2 oxidises Iodide
-
MnO2 + 2I + 4H+
–

MnO2 + H2O
Mn2+ + I2 + 2H2O
Care required with sampling and interference
DO probe
– Quick simple, reliable
– Must calibrate carefully (temp sensitive)
– Probe is an electrode system covered by a PTFE coat
Chemical Oxygen Demand




Organic matter oxidised chemically (2-3 hours)
Virtually all organic matter oxidised,
– non-biodegradable material degraded, COD > BODu
– COD correlated BOD
– E.g. raw wastewater
Many oxidizing agents
– Potassium dichromate (K2Cr2O7) used at a high temp and v.
acidic conditions
– Silver catalyst required
– Mercuric ion required to control Cl effects
– Aromatics and pyridine not oxidised
2-
Samples oxidised with in presence of excess Cr2O7
2– quantity of Cr2O7 remaining determined by titration with
ferrous ammonium sulphate
COD > BODu > BOD5
TOC

Sample heated in
–
–
oxygen
gold catalyst

All carbon converted to CO2

CO2 measured by infrared spectroscopy

Quick, but kit expensive, increasingly reliable
Oxygen Demand
• Summary
– The differences between COD and BOD
– How they are measured
– What they are used for in Environmental Engineering.