BODCOD.ppt
- 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. 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
- 3. 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
- 4. Options for measuring OD Biochemical Oxygen Demand BOD Chemical Oxygen Demand COD Total Organic Carbon TOC
- 5. 0 5 10 15 20 25 30 35 40 45 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 demand BODu = L 5 Day oxygen demand BOD5
- 6. 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
- 7. 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
- 8. Nitrification controlled for by – 5 day test (nitrifiers grow slowly) – inhibitors eg Alkylthiourea (ATU), TCMP Oxygen Demand 25 higher oxygen demand exerted due to nitrification 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 Time (days) (mg/l) Carbonaceous oxygen demand 5 Day oxygen demand BOD5
- 9. 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
- 10. 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 Cr2O7 2- – quantity of Cr2O7 2- remaining determined by titration with ferrous ammonium sulphate COD > BODu > BOD5 Chemical Oxygen Demand
- 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.