Dissolved Oxygen and Biochemical Oxygen Demand | Monitoring & Assessment | US EPA
COD (Chemical Oxygen Demand) is the most popular alternative test to BOD for sir plz tell me abt the relation between tem? The relation of DO and BOD. Biochemical Oxygen Demand (BOD) refers to the amount of oxygen that When BOD levels are high, dissolved oxygen (DO) levels decrease because the. Biological Oxygen Demand (BOD) is a measure of the oxygen used by When BOD levels are high, dissolved oxygen (DO) levels decrease because the.
Titration involves the drop-by-drop addition of a reagent that neutralizes the acid compound and causes a change in the color of the solution. The point at which the color changes is the "endpoint" and is equivalent to the amount of oxygen dissolved in the sample. The sample is usually fixed and titrated in the field at the sample site. It is possible, however, to prepare the sample in the field and deliver it to a lab for titration. Dissolved oxygen field kits using the Winkler method are relatively inexpensive, especially compared to a meter and probe.
Replacement reagents are inexpensive, and you can buy them already measured out for each test in plastic pillows. You can also buy the reagents in larger quantities, in bottles, and measure them out with a volumetric scoop. The advantage of the pillows is that they have a longer shelf life and are much less prone to contamination or spillage.
5.2 Dissolved Oxygen and Biochemical Oxygen Demand
The advantage of buying larger quantities in bottles is that the cost per test is considerably less. The major factor in the expense of the kits is the method of titration they use eyedropper, syringe-type titrator, or digital titrator. Eyedropper and syringe-type titration is less precise than digital titration because a larger drop of titrant is allowed to pass through the dropper opening and, on a micro-scale, the drop size and thus the volume of titrant can vary from drop to drop.
A digital titrator or a buret which is a long glass tube with a tapered tip like a pipet permits much more precision and uniformity in the amount of titrant that is allowed to pass.
If your program requires a high degree of accuracy and precision in DO results, use a digital titrator. A kit that uses an eye dropper-type or syringe- type titrator is suitable for most other purposes.
The lower cost of this type of DO field kit might be attractive if you are relying on several teams of volunteers to sample multiple sites at the same time. Meter and Probe A dissolved oxygen meter is an electronic device that converts signals from a probe that is placed in the water into units of DO in milligrams per liter.
Most meters and probes also measure temperature. The probe is filled with a salt solution and has a selectively permeable membrane that allows DO to pass from the stream water into the salt solution.
- BIOLOGICAL OXYGEN DEMAND
- Biological Oxygen Demand
The DO that has diffused into the salt solution changes the electric potential of the salt solution and this change is sent by electric cable to the meter, which converts the signal to milligrams per liter on a scale that the volunteer can read. DO meters are expensive compared to field kits that use the titration method. You can also measure the DO levels at a certain point on a continuous basis. The results are read directly as milligrams per liter, unlike the titration methods, in which the final titration result might have to be converted by an equation to milligrams per liter.
However, DO meters are more fragile than field kits, and repairs to a damaged meter can be costly. This means that only one team of samplers can sample DO and they will have to do all the sites.
With field kits, on the other hand, several teams can sample simultaneously. Laboratory Testing of Dissolved Oxygen If you use a meter and probe, you must do the testing in the field; dissolved oxygen levels in a sample bottle change quickly due to the decomposition of organic material by microorganisms or the production of oxygen by algae and other plants in the sample.
This will lower your DO reading. If you are using a variation of the Winkler method, it is possible to "fix" the sample in the field and then deliver it to a lab for titration. This might be preferable if you are sampling under adverse conditions or if you want to reduce the time spent collecting samples.
It is also a little easier to titrate samples in the lab, and more quality control is possible because the same person can do all the titrations. How to collect and analyze samples The procedures for collecting and analyzing samples for dissolved oxygen consist of the following tasks: In addition to the standard sampling equipment and apparel, when sampling for dissolved oxygen, include the following equipment: Confirm that the meter has been calibrated according to the manufacturer's instructions.
Operating manual for the meter and probe Extra membranes and electrolyte solution for the probe Extra batteries for the meter Extension pole Data sheet for dissolved oxygen to record results TASK 2 Confirm that you are at the proper location The directions for sampling should provide specific information about the exact point in the stream from which you are to sample; e.
The most common sizes are milliliters mL and 60 mL. Be sure that you are using the correct volume for the titration method that will be used to determine the amount of DO.
There is usually a white label area on the bottle, and this may already be numbered. If so, be sure to record that number on the field data sheet. If your bottle is not already numbered, place a label on the bottle not on the cap because a cap can be inadvertently placed on a different bottle and use a waterproof marker to write in the site number.
If you are collecting duplicate samples, label the duplicate bottle with the correct code, which should be determined prior to sampling by the lab supplying the bottles. Use the following procedure for collecting a sample for titration by the Winkler method: Remember that the water sample must be collected in such a way that you can cap the bottle while it is still submerged. That means that you must be able to reach into the water with both arms and the water must be deeper than the sample bottle.
Carefully wade into the stream. Stand so that you are facing one of the banks. Collect the sample so that you are not standing upstream of the bottle. Remove the cap of the BOD bottle.
Slowly lower the bottle into the water, pointing it downstream, until the lower lip of the opening is just submerged. Allow the water to fill the bottle very gradually, avoiding any turbulence which would add oxygen to the sample. When the water level in the bottle has stabilized it won't be full because the bottle is tiltedslowly turn the bottle upright and fill it completely.
Keep the bottle under water and allow it to overflow for 2 or 3 minutes to ensure that no air bubbles are trapped. Cap the bottle while it is still submerged. Lift it out of the water and look around the "collar" of the bottle just below the bottom of the stopper.
What is BOD/DO relationship
If you see an air bubble, pour out the sample and try again. Remove the stopper and add the fixing reagents to the sample. Immediately insert the stopper so air is not trapped in the bottle and invert several times to mix. This solution is caustic. Rinse your hands if you get any solution on them. An orange-brown flocculent precipitate will form if oxygen is present.
Wait a few minutes until the floc in the solution has settled. Again invert the bottle several times and wait until the floc has settled. This ensures complete reaction of the sample and reagents.
Biological Oxygen Demand
The sample is now fixed, and atmospheric oxygen can no longer affect it. If you are taking the sample to the lab for titration, no further action is necessary.
You can store the sample in a cooler for up to 8 hours before titrating it in a lab. If you are titrating the sample in the field, see Task 4: Cap underwater when full. Using a DO Meter If you are using a dissolved oxygen meter, be sure that it is calibrated immediately prior to use. 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. 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.