Biological oxygen demand

July 15, 2017 Chemistry

Introduction:

Biological Oxygen Demand is the step of uptake rate of O by micro-organisms in oxidization province. BOD is one of the most common measuring methods of mensurating pollutant organic stuff in H2O. The dissolved O ( DO ) degree of a H2O sample is measured for five yearss which is stated as BOD5. The DO degree of the sample is measured on the same twenty-four hours of aggregation of sample and the sample is incubated for five yearss in dark at a temperature of 20-oc and DO degree is measured once more. BOD5 is the difference between the DO degrees of initial and concluding value on 5th twenty-four hours of incubation.

In 1908 The Royal Commission on River Pollution and The Royal Commission of Pollution Disposal has selected BOD5 as the unequivocal method to mensurate organic pollution of rivers. The trial period five yearss appropriate as it the clip taken for the river H2O to go from beginning to finish.

The sum of Nitrates and Phosphates which are works foods in H2O can lend the addition in BOD degrees. The workss and algae grow rapidly which in bend decomposed by bacteriums lending the organic waste in H2O ensuing increased BOD degrees. The temperature of the H2O has besides consequence on BOD degrees. The BOD degrees increases with addition in temperature of H2O.

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Dissolved Oxygen ( DO ) is determined by Winkler ‘s Method developed by Lajos Winkler in 1988. The sum of dissolved O is considered as step of the biological activity of the H2O.

Materials Required:

Apparatus and Lab ware:

  1. Seriess of 250-300 milliliter BOD incubation bottles with land glass stoppers, and caps
  2. Incubator set at 20 grades Celsius and keeping no visible radiation.
  3. Two big carboys ( capacity depending on sample sum )
  4. Seriess of category A pipets ( 0.2 ml-10 milliliter )
  5. Aeration device
  6. pH metre
  7. Erlenmeyer flasks for readying of reagents and titration.
  8. Seed
  9. 50 ml category A burette
  10. Seriess of volumetric flasks
  11. Seriess of beakers
  12. Double distilled and De-ionised H2O

Reagents:

  1. Phosphate buffer solution
  2. Dissolve 8.5g K dihydrogen phosphate ( KH2PO4 ) , 21.75g dipotassium H phosphate ( K2HPO4 ) , 33.4g disodium H phosphate ( NA2HPO4.7H2O ) , 1.7g ammonium chloride ( NH4Cl ) in H2O ( distilled ) to makeup 1 liter of solution.

  3. Magnesium sulfate solution
  4. Dissolve 22.5g MgSO4.7H2O in distilled H2O to give 1 liter of solution.

  5. Calcium chloride solution
  6. Dissolve 27.5g anhydrous CaCl2 in distilled H2O to give 1 liter of solution.

  7. Ferric chloride solution
  8. Dissolve 0.25g FeCl3 6H2O in distilled H2O to give 1 liter of solution.

Reagents for Winkler Method:

  1. 0.125M Sodium thiosulphate solution:
  2. Dissolve 6.25g of Na2S2O3.5H2O in 1litre distilled H2O.

  3. Alkali-Iodide – azide solution:
  4. Dissolve 500g NaOH and 140g of NaI to 1 liter of distilled H2O with dissolved10g of NaN3 in 40ml distilled H2O.

  5. Concentrated sulfuric acid
  6. Manganese sulphate solution:
  7. Dissolve 480g MnSO4. — — — — 4H2O in 1litre of distilled H2O.

  8. Starch Index:

Dissolve 2g of soluble murphy amylum in 100ml of distilled H2O heated and stir to blend it good.

Procedure:

  1. The sample is aerated ab initio such that the initial DO degree is a satured degree.
  2. The sample is diluted with several dilution factor based on the possible BOD degree.
  3. Fill the incubation bottles with sample and dilution H2O to doing up the volume and infix the stoppers.
  4. Precaution is taken such that no air is trapped inside the incubation bottles.
  5. Mix the samples in the bottles by inverting the them several times.
  6. The Dissolved O is determined by the Winkler method.
  7. The bottles are incubated at 200C for 5 yearss in dark and are used for mensurating DO degree by Winkler Method.

Winkler Method:

  1. Add 2ml of Manganese sulphate solution and 2ml of Alkali Iodide – Azide solution to H2O sample.
  2. Care should be taken while adding ; the pipette tip should be below the surface degree of the sample in the bottle.
  3. Replace the stopper carefully without air bubbles by tending the bottle.
  4. Invert the bottle several times such that the contents of bottle are exhaustively assorted.
  5. Therefore leave the sample to settle down the precipitate. Mix and let subsiding until a clear supernatant is formed.
  6. Add 2ml of concentrated Sulphuric acid along the walls of the bottle.
  7. Precaution is taken in managing the concentrated acid by have oning protective baseball mitts.
  8. Allow the precipitate to fade out by inverting mix the contents exhaustively. The sample turns into bright gold colour.
  9. Transfer 200ml of sample to Erlenmeyer flask for titration with 0.0125M Na thiosulphate solution.
  10. Titration is carried out with Na thiosulphate solution until a pale straw colour appears.
  11. Few beads of starch solution is added to bring forth bluish colour as indicant of presence of I.
  12. The titration is continued until bluish colour disappears. The sum of Na thiosulphate used for titration is recorded.

Consequences:

Calculation of Initial Dissolved O ( DO0 ) by Winkler method:

1 milliliter of 0.0125M thiosulphate solution = 0.0125×10-3/4 mole of dissolved O

= ( 0.0125×10-3/4 ) X 32 g of dissolved O

= 0.1 milligrams dissolved O.

Sum of sample used for titration = 272ml

Sum of Sodium thiosulphate solution used for titration = 26.8ml

Initial Dissolved O ( DO0 ) = ( 2.68 x 1000 ) /272

DO0 = 9.853 mg/L

… ( Since 1ml = 0.1mg DO, 26.8ml = 2.68mg DO )

Calculation of Final Dissolved O ( DO1 ) by Winkler method:

Sum of sample used for titration = 200ml

Sum of Sodium thiosulphate solution used for titration = 10.2ml

Final Dissolved O ( DO1 ) = ( 1.02 x 1000 ) /200

DO1 = 5.1 mg/L

… . ( Since 1ml = 0.1mg DO, 10.2ml = 1.02mg DO )

Calculation of Biological Oxygen Demand ( BOD )

Biological Oxygen Demand ( BOD ) = ( DO0 – DO1 ) /F

Where, DO0= initial dissolved oxygen concentration ( mg/L )

DO1= dissolved solution degree after 7 yearss incubation ( mg/L )

F= dilution factor

Therefore, BOD ( mg/L ) = ( 9.853- 5.1 ) / 1/5

= 4.753 Tens 5

= 23.765 mg/L

The BOD trial was carried out for the primary and secondary effluent samples. The tabular column shows the BOD values obtained.

Discussion:

Effluent samples were collected from primary intervention wastewater works and secondary intervention wastewater works. BOD values of these wastewaters are determined by Winkler method. The samples are diluted with several dilution factor, for primary treated wastewater sample a dilution factor of 1:20 is used and for secondary treated outflowing sample a dilution factor of 1:5 is used.

The initial DO values are determined by Winkler method and the samples are incubated at 20oC in dark for 7days ( practically for 5days ) . The samples are added with Manganese sulfate and Alkali- Iodide azide solution which produces white precipitate which turns into Orange-brown precipitate by the dissolved O. By the add-on of concentrated sulfuric acid to solution the I from K iodide is liberated by manganese hydrated oxide and the solution appears in brown colour. On titration with Na thiosulphate the brown colour turns into pale straw colour. The sum of I is found by add-on of amylum to it which consequences in bluish colour. The titration is continued until the bluish colour disappears. The sum of Na thiosulphate drained to titrate is recorded and the value of Dissolved Oxygen ( DO ) is found. 1ml of Na thiosulphate ( 0.0125M ) is tantamount to 0.1mg dissolved O.

BOD values of primary wastewater are higher than the secondary wastewater as the organic affair in the primary wastewater is more either in suspended solids signifier or dissolved signifier. The BOD degrees of primary wastewater are found to be runing from 128-157mg/L, but the acceptable scope for primary outflowing intervention is 80-120mg/L ( Table:1 ) hence the primary wastewater is to be treated farther to let go of into environment. The BOD degrees of secondary wastewater is found to be runing from 17-23mg/L ( Table: 2 ) , the acceptable scope for release of secondary wastewater is less than 20mg/L. The initial DO value is found to be 9.853 mg/L and concluding DO value for secondary wastewater is 5.1mg/L. The BOD degree of the secondary wastewater is found to be 23.8. The overall

Decision:

  • BOD values of primary wastewater and secondary wastewater are determined.
  • Winkler method is used to find the DO values.
  • The BOD values of primary wastewater are higher than the secondary wastewater.
  • The organic affair in the primary samples is more than the secondary wastewaters.
  • The BOD degrees of secondary wastewater

Mentions:

  • Montgomery, H.A.C. , Thorn, N.S. and Cockburn, A. Determination of dissolved O by the Winkler method and the solubility of O in pure H2O and sea H2O. J. Applied Chemistry 1964, 14, 280.
  • “ Standard Methods for the Examination of Water and Wastewater, ” American Public Health Association, 18th Edition, 1992.
  • Warren Viessman, Jr, Mark J. Hammer. 1993. Water Supply and Pollution Control. Harper Collins College Publishers. 5th erectile dysfunction.
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