Chapter 8: Oxygen Demand

• It is a measure of the amount of “reduced” organic and inorganic matter in a water

• Relates to oxygen consumption in a river or lake as a result of a pollution discharge

• Measured in several ways– BOD - Biochemical Oxygen Demand

– COD - Chemical Oxygen Demand

– ThOD - Theoretical Oxygen Demand

Theoretical Oxygen DemandCC66HH1212OO66 + 6O + 6O22 = 6CO = 6CO22 + 6H + 6H22OO

Oxygen DemandOxygen Demand

6moles*32g/mole Oxygen/1mole*180g/mole 6moles*32g/mole Oxygen/1mole*180g/mole = 1.06 O= 1.06 O2 2 / Glucose/ Glucose

If glucose concentration is 360 mg/L THOD isIf glucose concentration is 360 mg/L THOD is

1.06 * 360 mg/L = 384 mg/L Oxygen1.06 * 360 mg/L = 384 mg/L Oxygen

BOD: A Bioassay

Briefly, the BOD test employs a Briefly, the BOD test employs a bacterial seed to catalyze the bacterial seed to catalyze the oxidation of 300 mL of full-oxidation of 300 mL of full-strength or diluted wastewater. strength or diluted wastewater. The strength of the un-diluted The strength of the un-diluted wastewater is then determined wastewater is then determined from the dilution factor and the from the dilution factor and the difference between the initial difference between the initial D.O. and the final D.O.D.O. and the final D.O. BOD

BottleBOD DO DOt i f

BOD with Dilution

ti f

s

b

BOD = DO - DO

V

V

WhereBODt = biochemical oxygen demand at t days, [mg/L]DOi = initial dissolved oxygen in the sample bottle, [mg/L]DOf = final dissolved oxygen in the sample bottle, [mg/L]Vb = sample bottle volume, usually 300 or 250 mL, [mL]Vs = sample volume, [mL]

When BOD>8mg/LWhen BOD>8mg/L

BOD - loss of biodegradable organic matter (oxygen demand)

Lo

Lt

L o

r B

OD

rem

aini

ng

Time

Lo-Lt = BODt

BODBottle

BODBottle

BODBottle

BODBottle

BODBottle

BOD

CBOD

NBOD

yor

BOD(mg/L)

Time

L=oxidizable carbonaceous material remaining to be oxidized

BOD y L Lt t o t

BOD Modeling

"L" is modelled as a simple 1st order decay: dL

dtk L 1

L L eok t 1Which leads to:

We get: BOD y L et t ok t ( )1 1

BOD y L Lt t o t And combining with:

NBOD Nitrogeneous BOD (NBOD)

NH O NO H O HNitrosomonas3 2 2 215 .

NO O NONitrobacter2 2 3

1

2

2 moles oxygen/1 mole of ammonia4.57 grams oxygen/gram ammonia-nitrogen

Inorganic NH3, NO3- NO2

-

Organic Proteins, Amino acids

TKN = Ammonia-N + Organic Nitrogen

COD: A chemical test

The chemical oxygen The chemical oxygen demand (COD) of a waste is demand (COD) of a waste is measured in terms of the measured in terms of the amount of potassium amount of potassium dichromate (Kdichromate (K22CrCr22OO77) reduced by ) reduced by the sample during 2 hr of reflux the sample during 2 hr of reflux in a medium of boiling, 50% in a medium of boiling, 50% HH22SOSO4 4 and in the presence of a and in the presence of a AgAg22SOSO44 catalyst. catalyst.

Solids: significance

• TDS: used as a measure of inorganic salt content in drinking waters and natural waters

• TSS: used to assess clarifier performance

• VSS: used to estimate bacterial populations in wastewater treatment systems

Solids AnalysisTotal Solids

Total Dissolved SolidsTDS

TS

Total Suspended SolidsTSS

FSS

VSS

Fixed S.S.

Volatile S.S.

Filtration

filtrate retained matter

ignition

Filtration for Solids Analysis

3. Start suction pump

2. PourSample1. Weigh new

filter and insert

4. Remove filterand re-weigh

6. Divide this by theVolume filtered and you get TSS

5. MeasureChange inWeight

Suction Flask & Filter HolderSuction Flask & Filter Holder