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 alChemistry for Environmental Engineers

Oxygen Demand n Concept derives from 19th Century n pollution of rivers with faeces, household wastes n ie organic wastes n Stinking rivers, incapable of supporting fish n Rivers devoid of oxygen n Polluting potential expressed in terms of – oxygen demand

Oxygen Demand is a measure of organic carbon n Organic pollutants complex carbon molecule n Enormous range of organics n Impossible to identify and quantify organics n Oxygen consumed as bacteria consume organics n Eg Aerobic Metabolism n Quantity of oxygen consumed is a measure of the concentration of organics in water

Options for measuring OD n Biochemical Oxygen Demand l BOD n Chemical Oxygen Demand l COD n Total Organic Carbon l TOC

Time (days) Oxygen Demand (mg/l) Carbonaceous oxygen demand Biochemical Oxygen Demand n "Amount of oxygen required by bacteria to break down decomposable organic matter under aerobic conditions" Ultimate oxygen demand BODu = L 5 Day oxygen demand BOD 5

BOD n Bioassay n Amount of oxygen used up under defined conditions n Temp 20 C n Virtually all degradable matter oxidised in 20 days - BOD u or ultimate BOD n 5 day BOD used – BOD 5 is 70-80% of the BOD u n BOD wastewater (mg/l) – <200 weak – 300 typical – 1000 strong

Complications of BOD n Conditions critical – rigorous attention to and control of conditions l nutrients and seed must be present l toxins must be absent n Theoretical BOD > BOD ultimate – 1. not all biodegradable carbon oxidised l E.g. BOD u glucose = 85% of theoretical BOD C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O – 2. not all carbon biodegradable under conditions l sometimes called humus l eg lignin n Ammonia also exerts an oxygen demand – Nitrification

n 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 Time (days) (mg/l) Carbonaceous oxygen demand 5 Day oxygen demand BOD 5

Measuring dissolved oxygen n Winklers Method (titration) – Mn 2+ oxidised to MnO 2 Mn OH - + 1/2 O 2 MnO 2 + H 2 O – At low pH MnO 2 oxidises Iodide MnO 2 + 2I - + 4H + Mn I 2 + 2H 2 O – Care required with sampling and interference n DO probe – Quick simple, reliable – Must calibrate carefully (temp sensitive) – Probe is an electrode system covered by a PTFE coat

n Organic matter oxidised chemically (2-3 hours) n Virtually all organic matter oxidised, – non-biodegradable material degraded, COD > BODu – COD correlated BOD – E.g. raw wastewater n Many oxidizing agents – Potassium dichromate (K 2 Cr 2 O 7 ) used at a high temp and v. acidic conditions – Silver catalyst required – Mercuric ion required to control Cl - effects – Aromatics and pyridine not oxidised n Samples oxidised with in presence of excess Cr 2 O 7 2- – quantity of Cr 2 O 7 2- remaining determined by titration with ferrous ammonium sulphate COD > BOD u > BOD 5 Chemical Oxygen Demand

TOC n Sample heated in – oxygen – gold catalyst n All carbon converted to CO 2 n CO 2 measured by infrared spectroscopy n 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.