TOPIC:DO;BOD;COD

Rajashree NaikM.Sc 1st Yr

Second Semister2014MSES012

DOThe dissolved oxygen content of water is

influenced by Higher temperature,• biological impurities, • Ammonia,• Nitrates, • ferrous iron,• Oxygen is poorly soluble in water. Its solubility is

about 14.6mg/l for pure water at 0°C Under normal atmospheric pressure and it drops

to 7 mg/l at 35°C.

DO

TITRIMETRIC ELECTROMETRIC

Winkler Method

-DO oxidising ability

Azide Modification-nitrate

Alum Flocculati

on Modificati

on-suspended solid

Permanganate Modification-iron

REAGENTS

PROCEDURE• Temp.should be 20deg.C• 300ml sample collected in BOD bottle• 2ml MnSO4+2ml alkali potassium iodide azide• Brownish-orange color ppt.• 2ml H2SO4 added to dissolve the floc precipitated• Stored at cool $ dark place for 8hrs• 201ml above prepared soln. + 3-4 drops starch

indicator taken in conical flask• Blue color appeared• Titrated with Na2S2O3

• Blue disappeares

Chemical reaction Alkaline condition:• MnSO4 + 2 KOH → Mn (OH)2 + K2SO4

• 2 Mn(OH)2 + O2→ 2 MnO (OH)2(Brown Ppt)

• Acidic condition(1ml H2SO4)MnO (OH )2 + 2KI + H2O→ Mn(OH)2 + I2+ 2KOH

• Titration• I2 + 2S2O3

2-→ S4O62- + 2I-

CALULATION

• Dissolved oxygen = Vol. of Sodium Thiosulphate*0.025M*8000 Vol.of sample taken0.025M= molarity of sod.thiosulphate8000--˃mili equi. Weight of O2

(8*1000)ml/ltr

DISCUSSION

• For Healthy water DO=8ppm• Desirable for aquatic life DO=4-5mg/l

BOD The BOD test is an estimate of the “food” available in

the sample. The more “food” present in the waste, the more

Dissolved Oxygen (DO) will be required. The BOD test measures the strength of the wastewater

by measuring the amount of oxygen used by the bacteria

“Total Biochemical Oxygen Demand” (TBOD), or often just simply “Biochemical Oxygen Demand” (BOD5).

Carbonaceous Biochemical Oxygen Demand”, or CBOD(excluding oxygen demand by nitrogenous compound)

BOD

• PRINCIPLE • The sample is filled in an airtight bottle

and incubated at specific temperature for 5 days.

• The dissolved oxygen (DO) content of the sample is determined before and after five days of incubation at 20°C and the BOD is calculated from the difference between initial and final DO.

Preparation of reagent1.Manganese Sulphate Solution:• 364gMnSO4.H2O+distl. H2O--˃1000ml2.Alkaline Iodide Sodium Azide Solution:• 700gKOH+150gKI+distl.H2O1000ml• (10gNaN3+40ml distl.H2O) add to above soln.3.Sodium Thiosulphate Stock Solution:2g Starch+100ml hot distl H204.Sulphuric acid5.Calcium chloride soln.:27.5g CaCl2(anhydrous)+distl.H20100ml

6.Magnesium Sulphate soln.:22.5g MgSO4+distl.H2O100ml7.Ferric chloride Solution:0.15gFeCl3+distl.H2O100ml8.Phosphate Buffer solution:8.5g KH2PO4 + distl.H2O + 21.75gK2HPO4 +

33.4gNa2HPO4.7H20 + 1.7g NH4Cl1000ml(PH-7.2)9.Dilution H2O:5lt organic free distl. H2O is aerated for 12hrsStabilised at 20deg.celcius5ml of above made nutrients soln.are addedIts prepared before 3-5days

Seeding Seeding: The process of adding live bacteria to a

sample to oxidise organic matter• Not required for:Municipal wastes,biologically treated

effluents,surface water samplesSome industrial wastes having high

temperature,etreme PH,toxic substances need seeding

SOURCES:settled sewageSoil cultureReceiving wastes from disposal

• PH of the sample should be maintained at 6.5-7.5(For acidicNaOH is added;for basicH2SO4)

Do not test Chlorinated/dechlorinated samples without seeding the dilution water

Samples with toxic need special study

Supersaturated DO samples(>9mg/lt)should be kept at 20deg.celsius

Procedure• 4 300ml glass stoppered BOD bottles are taken(2 for samples ;2 for blanks)10ml sample + DILUTION H202 BOD bottlesOnly DILUTION water in other 2 BOD bottles for

blank1 sample bottle $ 1 blank kept in incubator for 5

days at 20deg.celciusOther 2 analysed immediately

• 2ml MnSO4+alkali potassium iodide azidesample BOD bottle

• Flocs are formed by ppt.;disolved by adding 2ml H2SO4

• Immediately titrated with sodium thiosulphate with 1ml starch(blue) becomes colour disappears

• Same repeated after 5days

Calculation

• For non seeded sample

• BOD=(IDO-FDO) /(Vs/VB)

• Where IDO=initial DO• FDO=final DO• VS=vol.of sample• VB=vol. of bottle

• For seeded sample• BOD=(IDO-FDO)-(Cavg*SV) (VS/VB) Cavg=Correction factorSV=vol. of seed added• C=ISDO-IFDO VNISDO=Initial seed DOFSDO=Final sees DO

COD

• COD a useful measure of water quality is expressed in milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution for oxidation of polutants

• Ratio of BOD to COD greater than or equal to 0.8 indicates that wastewater highly polluted

• BOD value is always lower than COD value. For domestic and some industrial wastewater, COD value is about 2.5 times BOD value.

COD determines the amount of oxygen required for chemical oxidation of organic matter using a strong chemical oxidant, such as, potassium dichromate under reflux conditions. This test is widely used to determine:

• Degree of pollution in water bodies and their self-purification capacity,

• • Efficiency of treatment plants, • • Pollution loads, and • • Provides rough idea of Biochemical oxygen demand (BOD)

which can be used to determine sample volume for BOD estimation.

COD• PRINCIPLE • The organic matter present in sample gets oxidized

completely by potassium dichromate (K2Cr2O7) in the presence of sulphuric acid (H2SO4), silver sulphate (AgSO4) and mercury sulphate (HgSO4) to produce CO2 and H2O. The sample is refluxed with a known amount of potassium dichromate (K2Cr2O7) in the sulphuric acid medium and the excess potassium dichromate (K2Cr2O7) is determined by titration against ferrous ammonium sulphate, using ferroin as an indicator. The dichromate consumed by the sample is equivalent to the amount of O2 required to oxidize the organic matter.

•

Preparation of Reagents• A)Std.K2Cr2O7 Reagent for Digestion:• 4.913g of K2Cr2O7 (heated at 103deg.C for 2-4hrs)• 33.3g HgSO4• 167ml of onc.H2SO4• All mixed,dissolved in a beaker $ cooled to room temp.• Then dilluted to 1000ml in 1000ml beaker

• B)Sulphuric Acid Reagent:• 5.5g AgSO4 + 500ml conc.H2SO4--˃left for 24hrsto dissolve completely

C)Std. FAS Soln.0.01M(39.2g FAS + Distl. H20)---˃1000ml

Procedure• Conical Flasks washed with 20% H2SO4• 2.5ml sample +1.5ml K2CrO7 + 3.5ml H2SO4• Digested at 150deg C for 2hrs• Cooled to room temp.• 1-2drops of Ferroin added• Refluxed• Titrated with FAS• Titrated for Blank

Observation $ Calculation

• Color Change-Blue green--˃Reddish brown• COD(mg/l) = (A-B)*M *8000 Vol.sampleA=vol. FAS used for blank(ml)B=vol.FAS used for sample(ml)M=Molarity of FAS8000--˃mili equi. Weight of O2

(8*1000)ml/ltr

• INFERENCE • Chemical oxygen demand does not

differentiate between biologically available and inert organic matter, and it is a measure of the total quantity of oxygen required to oxidize all organic material into carbon dioxide and water. COD

• values are always greater than BOD values. For domestic and some industrial wastewater COD is about 2.5 times BOD.