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