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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.

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Oxygen Demand

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  1. 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

  2. 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

  3. 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 Oxygen Demand is a measure of organic carbon

  4. Options for measuring OD • Biochemical Oxygen Demand • BOD • Chemical Oxygen Demand • COD • Total Organic Carbon • TOC

  5. Biochemical Oxygen Demand • "Amount of oxygen required by bacteria to break down decomposable organic matter under aerobic conditions" 45 Ultimate oxygen demand BODu = L 40 5 Day oxygen demand BOD5 35 30 Oxygen 25 Demand 20 (mg/l) 15 10 5 Carbonaceous oxygen demand 0 0 5 10 15 20 25 Time (days)

  6. 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 BOD

  7. 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 Complications of BOD

  8. 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 Carbonaceous oxygen demand 5 0 25 0 5 10 15 20 Time (days)

  9. 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 Measuring dissolved oxygen

  10. 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 • Samples oxidised with in presence of excess Cr2O72- • quantity of Cr2O72- remaining determined by titration with ferrous ammonium sulphate COD > BODu > BOD5

  11. TOC • Sample heated in • oxygen • gold catalyst • All carbon converted to CO2 • CO2 measured by infrared spectroscopy • Quick, but kit expensive, increasingly reliable

  12. Oxygen Demand • Summary • The differences between COD and BOD • How they are measured • What they are used for in Environmental Engineering.

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