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.