OXYGEN DEMAND

OD

Concept derives from river support aquatic life at 20oC

Pollution of rivers with sewage

Incapable of supporting aquatic species such as fish

Rivers devoid oxygen

Pollution potential expressed in terms of oxygen demand

Options of measuring OD

Biochemical Oxygen Demand (BOD)

Chemical Oxygen Demand (COD


BIOCHEMICAL OXYGEN DEMAND
Biochemical oxygen demand (BOD) is a measure of the quantity of oxygen used by microorganisms (e.g., aerobic bacteria) in the oxidation of organic matter

Natural sources of organic matter include plant decay and leaf fall

Urban runoff carries pet wastes from streets and sidewalks; nutrients from lawn fertilizers; leaves, grass clippings, and paper from residential areas, which increase oxygen demand.

Oxygen consumed in the decomposition process robs other aquatic organisms of the oxygen they need to live

Organisms that are more tolerant of lower dissolved oxygen levels may replace a diversity of more sensitive organisms.

BIOCHEMICAL OXYGEN DEMAND (BOD)
Principle:

The method consists of filling with sample, to overflowing, an airtight bottle of the specified size and incubating it at the specified temperature for 5 days

Dissolved oxygen is measured initially and after incubation, and the BOD is computed from the difference between initial and final DO

Because the initial DO is determined shortly after the dilution is made, all oxygen uptake occurring after this measurement is included in the BOD measurement


In the presence of free oxygen, aerobic bacteria use the organic matter found in wastewater as “food”

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 as they stabilize the organic matter under controlled conditions of time and temperature.


Significance of BOD test

The BOD test is used to determine the relative oxygen requirements of wastewaters, effluents, and polluted waters

The test measures the oxygen utilized during a specified incubation period for the biochemical degradation of organic material.

It is also used to determine treatment plant efficiency in terms of BOD removal

measure waste loads to treatment plants

determine the effects of discharges on receiving waters

A major disadvantage of the BOD test is the amount of time (5 days) required to obtain the results.

When a measurement is made of all oxygen consuming materials in a sample, the result is termed “Total Biochemical Oxygen Demand” (TBOD), or often just simply “Biochemical Oxygen Demand” (BOD)


test is performed over a five day period, it is often referred to as a “Five Day BOD”, or a BOD5.

effluent contains large numbers of nitrifying organisms

organisms can exert an oxygen demand as they convert nitrogenous compounds (ammonia and organic nitrogen) to more stable forms (nitrites and nitrates)

measure just the oxygen demand exerted by organic (carbonaceous) compounds, excluding the oxygen demand exerted by the nitrogenous compounds

termed “Carbonaceous Biochemical Oxygen Demand”, or CBOD

the nitrifying organisms can be inhibited from using oxygen by the addition of a nitrification inhibitor to the samples


Sample Preservation

Samples may change greatly during handling and storage

Testing should be started as quickly as possible

To reduce the changes in those samples which must be held, keep the samples at or below 4°C

Do not allow samples to freeze

Samples may be kept for no more than 48 hours before beginning the BOD test.


Winkler Method

sample is pipetted into a 300ml BOD bottle containing aerated dilution water.

The DO content is determined and recorded and the bottle is incubated in the dark for five days at 20°C

At the end of five days, the final DO content is determined

the difference between the final DO reading and the initial DO reading is calculated

The decrease in DO is corrected for sample dilution, and represents the biochemical oxygen demand of the sample.

5 day period oxidation is 60-80%

20 day period - oxidation of carbonaceous matter is 95-99%


When dilution is not seeded,

BOD5 mg/l = (D1-D2)/P


When dilution is seeded,

BOD5 mg/l = (D1 - D2) - (B1 – B2)f/P


Where

D1 =Initial DO before incubation
D2 = Final DO after incubation
B1 = initial DO seed blank before incubation
B2 = final DO seeded blank after incubation
P = decimal volumetric fraction of sample used
f = ratio of seed diluted sample to seeded blank


Interferences

BOD test is dependent on biological activity

major interferences will be those substances which inhibit the growth of the microorganisms

chlorine, caustic alkalinity or acidity, mineral acids, and heavy metals (such as copper, zinc, chromium, and lead)

Excessive nitrites can interfere with the BOD determination

Growth of algae in the presence of light can cause problems
increasing the DO of the sample before testing
must be removed by deaeration


Nitrification

Noncarbonaceous matter – ammonia

two bacteria – capable of oxidizing ammonia to nitrite and subsequently to nitrate

Ammonia > Nitrite (Nitrosomonas)
Nitrite > Nitrate (Nitrobacter)

NH3 + O2 → NO2− + 3H
NO2− + H2O → NO3− + 2H


Oxygen demand - oxidation of ammonia to nitrate is called nitrogemous biochemical oxygen demand (NBOD)

Nitrification occurs – measured value will be higher than the true value



Pretreating Samples

Nitrification

Add 1 ml of nitrification inhibitors allylthiourea (ATU to each liter of dilution water

alkalinity or acidity

prevent bacteria from growing during the course of the BOD test

to prevent this, samples which have pH values higher than pH 8.0 or lower than pH 6.0 must be neutralized to pH 7.0 before the test is performed.

Chlorine

is such a strong oxidizing agent, it will inhibit the growth of living bacteria in the BOD test

samples containing residual chlorine must be pretreated to remove chlorine before the test is run by adding sodium sulfite to the sample

Seeding

BOD test relies on the presence of healthy organisms

If the samples tested contain materials which could kill or injure the microorganisms (such as chlorine, high or low pH, toxic materials)

then the condition must be corrected and healthy active organisms added

this process is known as seeding

preferred seed is effluent or mixed liquor from a biological treatment system processing the waste



Chemical oxygen demand
(COD)

Measure the oxygen equivalent of the organic matter in wastewater that can be oxidized chemically using dichromate in an acid solution

used in biological and non-biological oxidation of materials in water

COD = 2 X BOD

Advantages of COD test
Organic substances which are difficult to oxidize biologically – lignin – can be oxidized chemically
Certain organic substances may be toxic to microorganisms used in BOD test
COD test – can be completed in 2.5 hours