Biological oxygen demand and dissolved relationship

biological oxygen demand and dissolved relationship

BOD indicates the amount of organic matter present in the water. Therefore, a low Figures Correlation between the temperature and dissolved oxygen. To Analyze The Relationship between BOD,. Nitrogen And Phosphorus Contents at. Constant Dissolved Oxygen Concentration In. Municipal. The DO (dissolved oxygen) has a direct relationship with the BOD (biochemical oxygen demand) in that it is determined to test the level of the BOD in water.

The DO levels in and below riffle areas, waterfalls, or dam spillways are typically higher than those in pools and slower-moving stretches.

If you wanted to measure the effect of a dam, it would be important to sample for DO behind the dam, immediately below the spillway, and upstream of the dam. Since DO levels are critical to fish, a good place to sample is in the pools that fish tend to favor or in the spawning areas they use. An hourly time profile of DO levels at a sampling site is a valuable set of data because it shows the change in DO levels from the low point just before sunrise to the high point sometime in the midday.

However, this might not be practical for a volunteer monitoring program. It is important to note the time of your DO sampling to help judge when in the daily cycle the data were collected. Percent saturation is the amount of oxygen in a liter of water relative to the total amount of oxygen that the water can hold at that temperature.

DO samples are collected using a special BOD bottle: You can fill the bottle directly in the stream if the stream is wadable or boatable, or you can use a sampler that is dropped from a bridge or boat into water deep enough to submerse the sampler.

Samplers can be made or purchased. Dissolved oxygen is measured primarily either by using some variation of the Winkler method or by using a meter and probe. Winkler Method The Winkler method involves filling a sample bottle completely with water no air is left to bias the test. The dissolved oxygen is then "fixed" using a series of reagents that form an acid compound that is titrated.

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.

Biological Oxygen Demand

The advantage of the pillows is that they have a longer shelf life and are much less prone to contamination or spillage. 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.

Biological Oxygen Demand |

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. 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.

biological oxygen demand and dissolved relationship

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.

Biological Oxygen Demand

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.

  • 5.2 Dissolved Oxygen and Biochemical Oxygen Demand

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. If you see an air bubble, pour out the sample and try again. Remove the stopper and add the fixing reagents to the sample. 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. Take 2 samples of water and record the DO level ppm of one immediately using the method described in the dissolved oxygen test.

biological oxygen demand and dissolved relationship

Place the second water sample in an incubator in complete darkness at 20 oC 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 oC or 68 oF. After 5 days, take another dissolved oxygen reading ppm using the dissolved oxygen test kit. Record your final BOD result in ppm. There will not be much organic waste present in the water supply. A water supply with a BOD level of ppm is considered moderately clean.

In water with a BOD level of ppm, the water is considered somewhat polluted because there is usually organic matter present and bacteria are decomposing this waste. At BOD levels of ppm or greater, the water supply is considered very polluted with organic waste. 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 that are more tolerant of lower dissolved oxygen i. Organisms that need higher oxygen levels i.