oxygen demand
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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. - PowerPoint PPT PresentationTRANSCRIPT
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 BOD
Chemical Oxygen Demand COD
Total Organic Carbon TOC
05
1015202530354045
0 5 10 15 20 25
Time (days)
Oxygen Demand
(mg/l)
Carbonaceous oxygen demand
Biochemical Oxygen Demand
"Amount of oxygen required by bacteria to break down decomposable organic matter under aerobic conditions"
Ultimate oxygen demandBODu = L5 Day oxygen demand
BOD5
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. BODu glucose = 85% of theoretical BOD 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
Oxygen Demand
25
higher oxygen demandexerted due to nitrification
05
1015202530354045
0 5 10 15 20Time (days)
(mg/l)
Carbonaceous oxygen demand
5 Day oxygen demandBOD5
Measuring dissolved oxygen
Winklers Method (titration)– Mn2+ oxidised to MnO2
Mn2+ + 2OH- + 1/2 O2 MnO2 + H2O
– At low pH MnO2 oxidises Iodide
MnO2 + 2I- + 4H+ 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
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
Samples oxidised with in presence of excess Cr2O72-
– quantity of Cr2O72- remaining determined by titration with ferrous
ammonium sulphateCOD > BODu > BOD5
Chemical Oxygen Demand
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.