The amount of oxygen required by aerobic microorganisms to decompose the organic matter in a sample of water, such as that polluted by sewage is called biological oxygen demand.. It is used as a measure of the degree of water pollution. BIOLOGICAL OXYGEN DEMAND Biological Oxygen Demand (BOD) is one of the most common measures of pollutant organic material in water. BOD indicates the amount of putrescible organic matter present in water. Therefore, a low BOD is an indicator of good quality water, while a high BOD indicates polluted water. Dissolved oxygen (DO) is consumed by bacteria when large amounts of organic matter from sewage or other discharges are present in the water. DO is the actual amount of oxygen available in dissolved form in the water. When the DO drops below a certain level, the life forms in that water are unable to continue at a normal rate. The decrease in the oxygen supply in the water has a negative effect on the fish and other aquatic life. Fish kills and an invasion and growth of certain types of weeds can cause dramatic changes in a stream or other body of water. Energy is derived from the oxidation process. BOD specifies the strength of sewage. In sewage treatment, to say that the BOD has been reduced from 500 to 50 indicates that there has been a 90 percent reduction. The BOD test serves an important function in stream pollution-control activities. It is a bioassay procedure that measures the amount of oxygen consumed by living organisms while they are utilizing the organic matter present in waste, under conditions similar in nature. The other traditional tests or indicators for water quality are chemical oxygen demand (COD) and pH. For results of the BOD test to be accurate, much care must be taken in the actual process. For example, additional air cannot be introduced. Temperature must be 20°C, which is the usual temperature of bodies of water in nature. A five-day BOD test is used in environmental monitoring. This test is utilized as a means of stating what level of contamination from pollutants is entering a body of water. In other words, this test measures the oxygen requirements of the bacteria and other organisms as they feed upon and bring about the decomposition of organic matter. Time and temperature, as well as plant life in the water, will have an effect on the test. High BOD burdens or loads are added to wastewater by food processing plants, dairy plants, canneries, distilleries and similar operations, and they are discharged into streams and other bodies of water. BOD - biochemical (biological) oxygen demand is a test used to measure the concentration of biodegradable organic matter present in a sample of water. It can be used to infer the general quality of the water and its degree of pollution by biodegradable organic matter. It is used in water quality management and assessment, Ecology and environmental science. BOD is not an accurate quantitative test and should be considered as providing an indicator of the quality of a water body.

Typical BOD values

Most pristine rivers will have a 5 day BOD of less than 1 mg/l. Moderately polluted rivers may have BODs in the range 2 mg/l to 8 mg/l. Efficiently treated municipal sewage treated by a three stage process would have a BOD value of about 20 mg/l. Untreated sewage is variable but might average out at around 600 mg/l in Europe and as low as 200 mg/l in the USA or where there is severe groundwater or surface water infiltration. The generally lower values in the USA derive from the much greater water use per capita than in other parts of the world. Slurry from dairy farms is around 8000 mg/l and silage liquor around 60000 mg/l.

BOD Level (in ppm)	Water Quality
1 - 2	Very Good
3 - 5	Moderate
6 - 9	Fairly Polluted
10+	Very Polluted

The BOD test

BOD measures the rate of uptake of oxygen by micro-organisms in the sample of water at a fixed temperature (20°C) and over a given period of time( usually 5 days) in the dark. To ensure that all other conditions are equal, a very small amount of micro-organism seed is added to each sample being tested. This seed is typically generated by diluting activated sludge with de-ionised water. The test generally takes place over an elapsed period of 5 days but other BOD tests are also used.

Method

The BOD test is carried out by diluting the sample with de-ionised water saturated with oxygen, inoculating it with a fixed aliquot of seed, measuring the dissolved oxygen and sealing the sample (to prevent further oxygen dissolving in). The sample is kept at 20 °C in the dark (to prevent photosynthesis and thereby the addition of oxygen) for five days and the dissolved oxygen is measured again. The difference between the final D.O and initial D.O is the B.O.D. The apparent BOD for the control is subtracted from the control result to provide the corrected value. The loss of dissolved oxygen in the sample, once corrections have been made for the degree of dilution, is called the BOD5. In the UK allylthiourea is also added at the start of the test to prevent oxidation of ammonia. Results from such tests are represented as BOT5 (ATU) and referred to as Carbonaceous BOD (CBOD) in the U.S.. Less frequently used is the Ultimate BOD (UBOD) test, in which DO is repeatedly measured by DO meter in the same specialized bottles until it has reached equilibrium. BOD is similar in function to chemical oxygen demand (COD), in that both measure the amount of organic compounds in water. However, COD is less specific since it measures everything that can be chemically oxidized rather than just levels of biology active organic matter. BOD is used as a gauge of the effectiveness of wastewater treatment plants. Various commercial devices are available for its determination. BOD can be calculated by: Undiluted; Initial D.O - Final D.O = BOD Diluted; ((Initial D.O - Final D.O)- BOD of Seed) x Dilution Factor

History of the use of BOD

The Royal Commission on River Pollution which was established in 1865 and the formation of the Royal Commission on Sewage Disposal in 1898 led to the selection in 1908 of BOD5 as the definitive test for organic pollution of rivers. Five days was chosen as an appropriate test period because this is supposedly the longest time that river water takes to travel from source to estuary in the UK. In 1912 the commission also set a standard of 20 ppm BOD5 as the maximum concentration of permitted in sewage works discharging to rivers provided that there was at least an 8:1 dilution available at dry weather flow. This was contained in the famous 20:30 (BOD: Suspended Solids) + full nitrification standard which was used as a yardstick in the UK up to the 1970s for sewage works effluent quality.

Biological Oxygen Demand

Materials

2 Bottle with closeable lids Dissolved Oxygen test kit Data Collection Student Worksheet (optional)

Background Information
Microorganisms such as bacteria are responsible for decomposing organic waste. When organic matter such as dead plants, leaves, grass clippings, manure, sewage, or even food waste is present in a water supply, the bacteria will begin the process of breaking down this waste. When this happens, much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen they need to live. Biological Oxygen Demand (BOD) is a measure of the oxygen used by microorganisms to decompose this waste. If there is a large quantity of organic waste in the water supply, there will also be a lot of bacteria present working to decompose this waste. In this case, the demand for oxygen will be high (due to all the bacteria) so the BOD level will be high. As the waste is consumed or dispersed through the water, BOD levels will begin to decline. Nitrates and phosphates in a body of water can contribute to high BOD levels. Nitrates and phosphates are plant nutrients and can cause plant life and algae to grow quickly. When plants grow quickly, they also die quickly. This contributes to the organic waste in the water, which is then decomposed by bacteria. This results in a high BOD level. The temperature of the water can also contribute to high BOD levels. For example, warmer water usually will have a higher BOD level than colder water. As water temperature increases, the rate of photosynthesis by algae and other plant life in the water also increases. When this happens, plants grow faster and also die faster. When the plants die, they fall to the bottom where they are decomposed by bacteria. The bacteria require oxygen for this process so the BOD is high at this location. Therefore, increased water temperatures will speed up bacterial decomposition and result in higher BOD levels. When BOD levels are high, dissolved oxygen (DO) levels decrease because the oxygen that is available in the water is being consumed by the bacteria. Since less dissolved oxygen is available in the water, fish and other aquatic organisms may not survive. Test Procedure
The BOD test takes 5 days to complete and is performed using a dissolved oxygen test kit. The BOD level is determined by comparing the DO level of a water sample taken immediately with the DO level of a water sample that has been incubated in a dark location for 5 days. The difference between the two DO levels represents the amount of oxygen required for the decomposition of any organic material in the sample and is a good approximation of the BOD level.

1. Take 2 samples of water
   
2. Record the DO level (ppm) of one immediately using the method described in the dissolved oxygen test.
   
3. Place the second water sample in an incubator in complete darkness at 20 °C for 5 days. If you don't have an incubator, wrap the water sample bottle in aluminum foil or black electrical tape and store in a dark place at room temperature (20 °C or 68 °F).
   
4. After 5 days, take another dissolved oxygen reading (ppm) using the dissolved oxygen test kit.
   
5. Subtract the Day 5 reading from the Day 1 reading to determine the BOD level. Record your final BOD result in ppm.

What to Expect

BOD Level (in ppm)	Water Quality
1 - 2	Very Good There will not be much organic waste present in the water supply.
3 - 5	Fair: Moderately Clean
6 - 9	Poor: Somewhat Polluted Usually indicates organic matter is present and bacteria are decomposing this waste.
100 or greater	Very Poor: Very Polluted Contains organic waste.

NOTE: Generally, when BOD levels are high, there is a decline in DO levels. This is because the demand for oxygen by the bacteria is high and they are taking that oxygen from the oxygen dissolved in the water. If there is no organic waste present in the water, there won't be as many bacteria present to decompose it and thus the BOD will tend to be lower and the DO level will tend to be higher. At high BOD levels, organisms such as macro invertebrates that are more tolerant of lower dissolved oxygen (i.e. leeches and sludge worms) may appear and become numerous. Organisms that need higher oxygen levels (i.e. caddishly larvae and mayfly nymphs) will NOT survive.