BIOLOGICAL OXYGEN DEMAND INTRODUCTION:
The bio- chemrcal oxygen demand (BOD) determination is an emprical test in which standardised laboratory procedures are used to determine the relative oxygen requirements of waste waters, effluents and polluted waters. The test has its widest application in measuring waste loadings on treatment plant and in evaluating the BOD removal efficiency of such treatment system. The test measures the oxygen utilsed during a specified incubation period for the bio-chemical degradation of organic material(carbonaceous demand) and the oxygen used to oxidise inorganrc materials such as sulphides and ferrous ions lT may also measure the oxygen used to oxidise to oxidise reduced forms of nitrogen(nitrogenous demand) unless their oxidation is prevented by an inhibitor, The seeding and dilution procedures provide an estimate of the BoD at pH 6 5 - 7.5. SCOPE & APPLICATION:
The BOD test is employed for determining the relative oxygen requirements of the drinking and industrial waste water. Application of the test to organic waste discharge allows calculatron of
the effect of the discharges on oxygen development of engineering criteria for the design of waste water treatment plant. PRINCIPLET
The BOD is determined by measuring the loss in dissolved oxygen of the sample after incubating it for 3 days at 27 degree celsius. The BOD is difference of dissolved oxygencontent of the sample of before incubation and after incubation for 3 days at 27 degree celsius. The dissolved oxygen of the sample oxidises manganous hydroxide to manganic hydroxide, which in turn oxidises iodide to free iodine in an acid medium. The iodine liberated is determined by titration with hypo solution, INTERFERENCES:
a number of interference to BOD test, including reducing reagent, nitrate ion, ferrous ion and organic matter. The method given here is most suitable for waters containing not more than 0.1 mg/dm3 of nitrite nitrogen and not more than 1 mg/dms of ferrous ion, Other reducing or oxidsing materials should be absent. lf '1 ml of KCI solution is added before acidifying the sample and there is little delay in titration, the method is also applicable in presence of 100 - 200 mg/dm3 of ferric ion. There are
REAGENTS:
1,
2. 3. 4.
Manganous Sulphate ; Dissolve 480 gm. of manganous sulphate (MnSO+) in polished water, filter and dilute to 1000 ml. The solution sholud liberate not more than a trace of iodine when added to an acidified solution of potassium iodide.
lodine-Azide:Dissolve500gmofNaOH(700gmofKOH) and'l35gmofsodiumiodide(or 1 50 gm.of potassium iodide) in polished water and dilute to 1000 ml The reagent should not give a colour with starch solution when diluted and acidified Dissolve 10 gm of sodium azide in 40 ml. ol polished water and add to 950 ml. of the first solution with constant stirring. Sulphuric acid : (sp. gr. 1 84) Standard sodium thiosulphate: Exactly 0 25 N, freshly standardised against potassium
7.
dichromate. One ml. of this solution is equivalent to 0.2 mg, of oxygen as "O". Starch lndicatorsolution: Prepare a pastewith 1 gm of starch in 10 ml. of polished waterand add to 90 ml of boiling polished water, Stir briefly, cool and decant the clean supernatant liqud . Prepare a fresh solution daily Add 1 ml. of chloroform to prevent moulding. Phosphate Buffer solution: Dissolve 8.5 gm KHzPO+ ,21 15 gm KzHPO+, 33 4 gm. Na2HPO47H2O and 1.7 gm. NH+CI in about 500 ml. of polished water and dilute to 1000 ml, The pH should be1.2 without further adjustment. Magnesium sulphate solution: Dissolve 22.5 gm. MgSO+ THzO in polished water and dilute to
8.
Ferric Chloride solution: Dissolve 0.25 gm. FeCls,6HzO in polished water and dilute
5 6.
E
'1000 ml.
to 1000 ml
Oct . 2011