Transcript Oxygen Demand

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 Demand
C6H12O6 + 6O2 = 6CO2 + 6H2O
Oxygen Demand
6moles*32g/mole Oxygen/1mole*180g/mole
= 1.06 O2 / Glucose
If glucose concentration is 360 mg/L THOD is
1.06 * 360 mg/L = 384 mg/L Oxygen
BOD: A Bioassay
Briefly, the BOD test employs a
bacterial seed to catalyze the
oxidation of 300 mL of full-strength or
diluted wastewater. The strength of
the un-diluted wastewater is then
determined from the dilution factor
and the difference between the initial
D.O. and the final D.O.
BODt  DOi  DO f
BOD
Bottle
BOD with Dilution
When BOD>8mg/L
- DOf
DO
i
BOD t =
 Vs 


 Vb 
Where
BODt =
DOi =
[mg/L]
DOf =
Vb
=
Vs
=
biochemical oxygen demand at t days, [mg/L]
initial dissolved oxygen in the sample bottle,
final dissolved oxygen in the sample bottle, [mg/L]
sample bottle volume, usually 300 or 250 mL, [mL]
sample volume, [mL]
L or BOD remaining
BOD - loss of biodegradable
organic matter (oxygen demand)
BOD
Bottle
Lo
Lt
Lo-Lt = BODt
Time
BOD
Bottle
BOD
Bottle
BOD
Bottle
BOD
Bottle
BOD
y
NBOD
or
BOD
(mg/L)
CBOD
Time
L=oxidizable carbonaceous material remaining to be oxidized
BODt  yt  Lo  Lt
BOD Modeling
"L" is modelled as a simple 1st order decay:
Which leads to:
L  Lo e
And combining with:
We get:
dL
  k1 L
dt
 k1t
BODt  yt  Lo  Lt
BODt  yt  Lo (1  e  k1t )
NBOD Nitrogeneous BOD (NBOD)
NH3  15
. O2 
 NO2  H2 O  H 
Nitrosomonas
1
Nitrobacter

NO  O2  NO3
2

2
2 moles oxygen/1 mole of ammonia
4.57 grams oxygen/gram ammonia-nitrogen
Inorganic NH3, NO3- NO2Organic Proteins, Amino acids
TKN = Ammonia-N + Organic Nitrogen
COD: A chemical test
The chemical oxygen demand
(COD) of a waste is measured in
terms of the amount of potassium
dichromate (K2Cr2O7) reduced by the
sample during 2 hr of reflux in a
medium of boiling, 50% H2SO4 and in
the presence of a Ag2SO4 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 Analysis
Total Solids
TS
Filtration
filtrate
TDS
TSS
retained matter
Total Dissolved Solids
Total Suspended Solids
ignition
FSS
VSS
Fixed S.S.
Volatile S.S.
Filtration for Solids Analysis
1. Weigh new
filter and insert
5. Measure
Change in
Weight
2. Pour
Sample
3. Start
suction pump
4. Remove filter
and re-weigh
6. Divide this by the
Volume filtered and you get TSS Suction Flask & Filter Holder