Water Analysis : To Estimate Biological Oxygen Demand (BOD) of The Given Sample

INTRODUCTION

Biological oxygen demand can be defined as the amount of oxygen (mg/L or mg/kg) used by the non-photosynthetic bacteria at 20^OC to metabolize biologically degradable organic compounds. To obtain stable reading, oxygen consumed is measured over a period of 5 days and it is called BOD₅. Upon prolonged incubation cells will completely mineralize the organic matter, so it is called ultimate biological oxygen demand. The BODµ approaches initial COD value. BOD/COD ratio once established can be used to monitor and operate treatment plant. BOD is measured in ppm or mg of O₂/lit. Decomposition of sewage by microbes results in oxidized end products, which decreases dissolved 0₂ content. Such sewage when mixed in natural water bodies proves harmful.

PRINCIPLE:

lodometric test is the most precise and reliable titration method for dissolved oxygen analysis. It is based on the addition of divalent manganese solution followed by strong alkali azide. Equivalent amount of dispersed divalent manganese hydroxide precipitates to hydroxide of higher valency state (Mn⁺⁴). In the presence of iodine ions in acidic solution, oxidized manganese reverts to divalent state with liberation of I₂ equivalent to original DO content. The liberated I₂ is titrated with standard solution of sodium thiosulfate.

REAGENTS:

·         Phosphate buffer solution (pH: 7.2)

: KH₂PO₄ = 8.5gms

  K₂HPO₄ = 20.75gms

  Na₂HPO₄ = 33.4gms

  NH₄Cl = 1.7gms

Dissolve to make final volume 100ml with D/W

·         MgSO₄ solution = 22.5gms in 100ml D/W

·         CaCl₂ Solution = 27.5gms in 100ml D/W

·         FeCl₃ solution = 0.25gms in 100ml D/W

·         0.025 M Na₂S₂0₃  

·          2 % starch

·         MnSO₄ solution = 36.4gms in 100ml D/W

·         Alkali iodide azide solution

: KOH = 70gms

  KI = 15gms

  NaN₃ = 1gm

Dissolve to make final volume 100ml with D/W

·         Concentrated sulfuric acid

PROCEDURE:

Preparation of dilution water

·          Filter the given sample to remove suspended particles

·          Take 4 BOD bottles containing 300ml of D/W

·          Label them as 1:100, 1:50 and 1:33 and prepare dilution by adding 3mm of sewage sample in 1:100 bottles, 6ml sample in 1:50 bottle and 9ml in 1:33 bottle.

·          Add 1ml phosphate buffer to maintain pH along with 1 ml of each MgSO₄, CaCl₂ and FeCl₃ to provide proper osmotic conditions and essential nutrients.

·        The water is diluted 1:100, 1:50 and 1:33 times and the bottles are incubated in dark or BOD incubator for 5 days to avoid utilization of dissolved oxygen by photosynthetic microorganisms. BOD is measured by azide modification method on the 0 day and then on the 5th day.

Finding the DO

·       To the sample collected in 250/300 ml bottle, add 1 ml MnSO₄ solution followed by 1 ml of alkali azide reagent.

·       Stopper the bottle to exclude air bubbles and mix by inverting the bottle.

·       When the precipitates settle sufficiently, add 2 ml of concentrated H₂SO₄ to dissolve the precipitates.

·        Re-stopper the bottle and mix.

·        Take 200 ml sample from it and add 1-2 drops of starch

·        Titrate with 0.025 M Na₂S₂O₃ solution till the first disappearance of blue color.

CALCULATION:

BOD₅ mg/L  = D₁ - D2      .

                P     

Where, D₁ = DO of diluted Sample immediately after preparation, mg/L

D₂= DO of diluted Sample after five day incubation at 20^OC, mg/L

P = Decimal volumetric fractions of sample used