Post on 06-Feb-2018
BOD / CBOD
FROM
A TO Z
Amy Starkey
Stark County Sanitary Engineers
What
is
BOD????
What is BOD? It is a measure of the amount of oxygen consumed
by bacteria during the decomposition of organic
materials.
Organic materials from the wastewater treatment facility
act as a food source for the bacteria.
Directly related to Dissolved Oxygen
The bacteria require oxygen in the form of dissolved
oxygen to decompose or eat the food source. Through a
calculation, the amount of DO depletion between the initial
day and final day of the analysis determines the BOD.
Thus, BOD directly affects the amount of Dissolved
Oxygen
The greater the BOD = more rapid oxygen depletion = less
oxygen available to aquatic life.
What is the Difference Between
BOD and CBOD
BOD
represents the oxidation
of carbons and
nitrogenous compounds
present in the water
CBOD
measures the oxidation of
carbons present in water
TCMP
prevents the oxidation of reduced forms of nitrogen such
as ammonia, and organic nitrogen which exert a
nitrogenous demand.
Should add at the beginning of the test because
nitrification will begin almost immediately if the right
organisms are present (Baird and Smith, 2002).
2-chloro-6-(trichloromethyl)pyridine
BOD
verses
COD
BOD verses COD
BOD
represents the oxidation
of carbons and
nitrogenous compounds
present in the water
Analysis completion is
done in 5-days
COD
Is the measure of the
total amount of oxygen
required to oxidize all
organic material into
carbon dioxide and water
analysis only takes a few
hours
BOD verses COD
COD results are always higher than BOD
results.
Useful in determining an unknown BOD range
for a sample but it can NOT replace the BOD
test.
Methods
Approved Methods
Standard Methods 18th, 19th, and 20th
editions (5210B, 5-Day BOD Test)
Two Ways in Determining DO
Iodometric Method (Winkler DO Method)
Membrane Electrode Method
Winkler Method
Azide Modification Method Preferred for most wastewaters
Removes interferences caused by nitrite which is common in
wastewaters.
Permanganate Modification
Used when ferrous iron is present
Azide plus Potassium Fluoride Modification
Used when 5 mg or more of ferric iron salts/L are present
Alum Flocculation Modification
Used when there is interference caused by suspended solids
Copper Sulfate-Sulfamic Acid Flocculation
Modification
Used for biological flocs such as activated sludge mixtures which
also have a high oxygen utilization rate
Sample
Collection,
Holding Time,
and
Storage
Grab Samples
Ideally samples should be analyzed within
6-hrs of collection, however if this is not
possible, then analyze samples within 48
hours of collection (40 CFR part 136).
Store samples at < 6C.
Composite Sampling
Samples should be kept at or below < 6C during
compositing (limited to 24-hour period).
start the measurement of holding time from the end of
the compositing period.
For example if the compositing was started at 8:30 am
on Tuesday and ended at 8:30 am on Wednesday, then
the 48-hour holding time would start from the end of the
compositing period which would be 8:30am on
Wednesday.
Store samples at < 6C
Quality
Control &
Procedure
Requirements
BOD Quality Controls
Blank Control Checks
GGA Control Checks
Glucose Glutamic Acid
Seed Control Checks
Blank Control Checks
Straight dilution water
Used to determine cleanliness of bottles
as well as the source water.
It must have a DO uptake NO greater than 0.2 mg/L
GGA Control Checks
Used to check the quality of the seeding
material.
Low results reflect poor seeding material.
The ideal GGA range for a BOD sample is
198 + 30 mg/L.
GGA Control Checks
GGA Needs to be pH adjusted
Initially the pH is around 4
Adjust between 6.5 -7.5, like any other
samples
Seed Controls
Must have a DO uptake attributable to the
seed added to each bottle between
0.6mg/L 1.0 mg/L.
Most domestic wastewater, unchlorinated
or undisinfected effluents will contain a
sufficient population of microorganisms.
Used to calculate the BOD results of
samples which are seeded
BOD / CBOD Requirements
pH of samples should be between a pH of 6.5-7.5
Sample temperature should be adjusted to 20 + 1C before making dilutions
After 5 days of incubation the final DO of samples must result in a DO depletion of at least 2.0 mg/L with a DO residual of no less than 1.0 mg/L. This is why it is recommended to make several dilutions of a sample.
Example: The initial DO of a sample is 8.2 and after 5 days of incubation the final DO is 7.8. Then the final DO does not meet the required DO depletion of at least 2.0mg/L so a BOD result can not be calculated from this sample.
Example #2: The initial DO of a sample is 8.2 and after 5 days of incubation the final DO is 0.20 mg/L. Here the final DO does not meet the required DO residual of at least 1.0 mg/L, so again the BOD result can not be calculated
Dilution
Water
Dilution Water Sources
Distilled
Tap
De-mineralized
Natural Waters
Dilution Water Source
Must be free of heavy metals, and toxic
substances which inhibit micro-bacterial
growth.
Must also be able to maintain no more
than a 0.20 mg/L DO depletion during the
5-day incubation period.
Reagents Added to Dilution Water
Phosphate Buffer Solution
Magnesium Sulfate Solution (MgSO4)
Calcium Chloride Solution (CaCl2)
Ferric Chloride Solution (FeCl3)
Purpose of Adding Trace Metals,
Nutrients and Buffering Dilution
Water
Bacteria growth requires nutrients and
trace metals.
It is buffered to ensure the pH of the
incubated samples remain in a range
suitable for bacteria growth.
Why Dilute Samples Before
Incubation?
Because the BOD concentration in most
wastewaters exceeds the concentration of
DO available in an air-saturated sample.
Seeding
Why Seed?
To add a population of microorganisms capable
of oxidizing the biodegradable organic matter.
Most domestic wastewater, unchlorinated or
undisinfected effluents will contain a sufficient
population of microorganisms.
Samples That may Require Seeding
Chlorinated samples
High temperature wastes
Wastes with extreme pH values
Selecting a Seed Source
Select a material to be used for seeding which
will have a BOD of at least 180 mg/L.
Example of seed sources according to Standard
Methods 20th Edition
Raw domestic Sewage prepared as stated above
Small quantities of digester supernatant, return
activated sludge
Commerically available seed material (Polyseed)
Seeding
Must have a DO uptake/depletion of 2
mg/L after the 5-day incubation period,
and also result in at least 1 mg/L residual
DO (final DO).
Over Mixing the Ployseed
Never let the vortex
touch the stir bar
Micro-organisms in the
seed will be too tired to
get the job done in your
samples and may see
low results in the seed
factor.
Proper way to mix the Polyseed
Mix on a speed of about 5,
or so that the vortex is not
touching the stir bar and
splashing out.
Mix for an hour
Let bran settle out and
transfer to another beaker to
allow to mix for up to 5 hours
on a speed setting between
1-2
Seed Calculations
The DO uptake attributable to the seed (the seed factor or SF) added to each bottle is between 0.6mg/L 1.0 mg/L. The SF is calculated by using the equation below:
SF = (B1 B2) x (f)
Seed Calculations
SF = (B1 B2) x (f)
Where :
B1 = Initial Seed Control DO (before incubation)
B2 = Final Seed Control DO (after 5-day
incubation)
f = ratio of seed in diluted sample to seed in
seed control , or better see as
f = (volume, mls of seed in diluted sample)
(volume, mls of seed in seed control)
BOD5, mg/L = (D1 D2) (SF)
P
Where:
D1