File Name: difference between biological oxygen demand and chemical oxygen demand .zip
- Biochemical oxygen demand
- BOD Analysis
- Biological Oxygen Demand (BOD) and Water
- What are the differences between Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD)?
Biochemical oxygen demand
Jump to main content or area navigation. Contact Us. The stream system both produces and consumes oxygen. It gains oxygen from the atmosphere and from plants as a result of photosynthesis. Running water, because of its churning, dissolves more oxygen than still water, such as that in a reservoir behind a dam. Respiration by aquatic animals, decomposition, and various chemical reactions consume oxygen. Wastewater from sewage treatment plants often contains organic materials that are decomposed by microorganisms, which use oxygen in the process.
The amount of oxygen consumed by these organisms in breaking down the waste is known as the biochemical oxygen demand or BOD. A discussion of BOD and how to monitor it is included at the end of this section. Other sources of oxygen-consuming waste include stormwater runoff from farmland or urban streets, feedlots, and failing septic systems.
Oxygen is measured in its dissolved form as dissolved oxygen DO. If more oxygen is consumed than is produced, dissolved oxygen levels decline and some sensitive animals may move away, weaken, or die.
DO levels fluctuate seasonally and over a hour period. They vary with water temperature and altitude. Cold water holds more oxygen than warm water Table 5. Thermal discharges, such as water used to cool machinery in a manufacturing plant or a power plant, raise the temperature of water and lower its oxygen content. Aquatic animals are most vulnerable to lowered DO levels in the early morning on hot summer days when stream flows are low, water temperatures are high, and aquatic plants have not been producing oxygen since sunset.
In contrast to lakes, where DO levels are most likely to vary vertically in the water column, the DO in rivers and streams changes more horizontally along the course of the waterway. This is especially true in smaller, shallower streams. In larger, deeper rivers, some vertical stratification of dissolved oxygen might occur. 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: a glass bottle with a "turtleneck" and a ground glass stopper. 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. 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. 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. 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. 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.
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. The procedures for collecting and analyzing samples for dissolved oxygen consist of the following tasks:.
Refer to section 2. In addition to the standard sampling equipment and apparel, when sampling for dissolved oxygen, include the following equipment:. The directions for sampling should provide specific information about the exact point in the stream from which you are to sample; e.
Use a BOD bottle to collect the water sample. 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:. If you are using a dissolved oxygen meter, be sure that it is calibrated immediately prior to use.
Check the cable connection between the probe and the meter. Make sure that the probe is filled with electrolyte solution, that the membrane has no wrinkles, and that there are no bubbles trapped on the face of the membrane.
You can do a field check of the meter's accuracy by calibrating it in saturated air according to th e manufacturer's instructions. Or, you can measure a water sample that is saturated with oxygen, as follows. Once the meter is turned on, allow 15 minute equilibration before calibrating.
After calibration, do not turn the meter off until the sample is analyzed. Once you have verified that the meter is working properly, you are ready to measure the DO levels at the sampling site. You might need an extension pole this can be as simple as a piece of wood to get the probe to the proper sampling point. Simply secure the probe to the end of the extension pole. A golfer's ball retriever works well because it is collapsible and easy to transport. To use the probe, proceed as follows:.
Three types of titration apparatus can be used with the Winkler method: droppers, digital titrators, and burets. The dropper and digital titrator are suited for field use.
The buret is more conveniently used in the lab Fig. For titration with a dropper or syringe, which is relatively simple, follow the manufacturer's instructions. The following procedure is for using a digital titrator to determine the quantity of dissolved oxygen in a fixed sample:. Some water quality standards are expressed in terms of percent saturation.
To calculate percent saturation of the sample:. If you are using the Winkler method and delivering the samples to a lab for titration, double-check to make sure that you have recorded the necessary information for each site on the field data sheet, especially the bottle number and corresponding site nu mber and the times the samples were collected.
Deliver your samples and field data sheets to the lab. If you have already obtained the dissolved oxygen results in the field, send the data sheets to your sampling coordinator. Biochemical oxygen demand, or BOD, measures the amount of oxygen consumed by microorganisms in decomposing organic matter in stream water.
BOD also measures the chemical oxidation of inorganic matter i.
Food Analysis pp Cite as. Oxygen demand is a commonly used parameter to evaluate the potential effect of organic pollutants e. Because microorganisms utilize these organic materials, the dissolved oxygen is greatly depleted from the water, which can have a detrimental effect on fish and plant life. This chapter focuses on the two main methods used to measure the oxygen demand of water and wastewater: biochemical oxygen demand BOD and chemical oxygen demand COD. Described in the chapter are the principles, procedures, applications, and limitations of each method.
BOD analysis is performed to determine what effect dirty water, containing bacteria and organic materials, will have on animal and plant life when released into a stream or lake. The test measures the potential of the incoming water to deplete the oxygen of the receiving waters due to the bacterial activity. Low levels of organic waste and fewer bacteria present mean the BOD will be lower and the dissolved oxygen levels higher. Specifically, it measures the equivalent amount of oxygen required to chemically oxidize organic compounds in water thus removing the pollution. COD is an integral part of all water quality management programmes. Traditional COD measurement methods use potassium dichromate and mercury. We have compiled a handy bundle of all of the equipment needed to get you started with perform COD measurements here.
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Biological Oxygen Demand (BOD) and Water
Specifically designed for laboratories analyzing a medium or large number of samples per day, this system is available with the standard mL bottles or 60mL bottles.
What are the differences between Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD)?
BOD measures the amount of organic components that can be biologically degraded in water. The ratio of BOD to COD is the percent of the organic materials in water that can be degraded by natural micro-organisms in the environment. Chemican oxygen demand COD and biochemical oxygen demand BOD — a slightly more refined measure of water quality constituents than COD — measures the amount of oxygen that would be required to fully break down the organic components that can be biologically degraded in water.
The biochemical oxygen demand BOD of wastewater and surface water is the measurement of the amount of molecular oxygen in water required by micro-organisms in the biochemical oxidation of organic matter i. Examples of sources of organic matter that can be discharged into surface waters e. A concern is that all of these sources will result in micro-organisms using all the oxygen in these surface waters to biodegrade these organic wastes, such that anaerobic conditions result.
BOD reduction is used as a gauge of the effectiveness of wastewater treatment plants. BOD of wastewater effluents is used to indicate the short-term impact on the oxygen levels of the receiving water. BOD analysis is similar in function to chemical oxygen demand COD analysis, in that both measure the amount of organic compounds in water. However, COD analysis is less specific, since it measures everything that can be chemically oxidized, rather than just levels of biologically oxidized organic matter. Most natural waters contain small quantities of organic compounds. Aquatic microorganisms have evolved to use some of these compounds as food. Microorganisms living in oxygenated waters use dissolved oxygen to oxidatively degrade the organic compounds, releasing energy which is used for growth and reproduction.
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