Presentation on theme: "Biochemical Oxygen Demand"— Presentation transcript:
1Biochemical Oxygen Demand General Considerations* BOD is defined as the amount of oxygen required by bacteria whilestabilizing decomposable organic matter under aerobic conditions at 20o C* Decomposable means that the organic matter can serve as food forthe bacteria, and energy is derived from the oxidation.* BOD test is used to determine the pollutional strength of domestic,and industrial wastes in terms of oxygen that they will require ifdischarged into natural watercourses in which aerobic conditions exist.* BOD Test is used by regulatory for evaluating sewage treatmentplants and purification capacity of receiving bodies of water.
2BOD Test Design* Conditions of test is similar to those that occur in nature at 20 C.* Samples are protected from the air to prevent re-aeration as D.O.diminishes.* Samples of strong wastes are diluted due to limited Oxygen solubility.* Test conditions must be suitable for living organisms to thrive.* No toxic substances but all necessary nutrients available, N,P,& traceelements.* Complete biological oxidation to completion, i.e., to CO2 and H2O.
3Why 5 Day BOD? * The BOD is considered complete after 20 days * 20 Days is too long to wait.* 5 days is a reasonable period for most of the BOD to be exerted.* 5 Day BOD of Domestic and many industrial wastes is about70 to 80 % of total BOD.
4Nature of BOD Reaction* First Order Reaction: Rate is proportional to the amount of oxidizableorganic matter remaining at any time* -dC/dt = kC, where C = concentration of oxidizable organic matter atstart of the time interval t, and k is the rate constant for the reaction* In BOD, it is customary to use L in place of C, where L represents theultimate demand* -dL/dt = kL by integration the expression is: Lt /L = e-kt = 10-kt* In translating 5 day BOD to total BOD, the following modification is done:y = L(1 – 10-kt)
7Method of Measuring BOD * Dilution procedure is used due to limited O2 solubility* Major Items of importance in the BOD test are:1. Freedom from Toxic Materials.2. Favorable pH and osmotic conditions3. Available nutrient elements4. Standard temperature of 20 degrees C5. Presence of significant population of mixed organisms of soil originDilution Water is comprised of :Phosphate buffer furnishes phosphorous and pH of 7.0Ferric chloride, magnesium sulfate and ammonium chloride supply therequirements for iron, sulfur, and nitrogen.Potassium, sodium, calcium, and magnesium salts give buffering andproper osmotic conditions.
8The Need for BlanksSince the dilution water should always be seeded to provide a uniformpopulation of organisms for biodegradation of organic matter. Therefore,A correction must be applied.DO value of the dilution water at zero day compared to DO of dilution waterAfter 5 days will determine the amount of organic matter in dilution water.
9Dilutions of Waste* BOD is not influenced by oxygen levels as low as 0.5 mg/L* Oxygen depletions <2.0 mg/L are not statistically reliable* Reliable BOD values are >2.0 mg/L D.O. depletion but with at least0.5 mg/L D.O. remaining* These restrictions usually means a range of 2 to 7 mg/L depletion.
10Incubation Bottles* Bottles have glass stoppers ground to a point to prevent trapped air.* Water seal to prevent air from entering the bottle during incubation.* Bottles be free of organic matter.* Cleaning with chromic acid or a good grade of detergent.* If detergent is used then bottles should be rinsed with hot water to killNitrifying organisms.* Four rinses with tap water and a final rinse with distilled water.
14L Values versus Theoretical oxygen demand Values Oxidation of glucose to carbon dioxide and water requires 192 g ofOxygen per mole or mg of oxygen per mg of glucoseC6H12O6 + 6CO2 + 6H2OExample: 300 mg/L of glucose = 320 mg/L Theoretical Oxygen DemandBut actual 20 day BOD yield L values = 250 to 285 mg/LReason for discrepancy between L values and theoretical oxygen demandIs: there remains a certain amount of organic matter quite resistant toFurther biological attack which is referred to as humus, the amount of theDiscrepancy.