Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are the primary techniques used to determine the amount of organic matter present in a given water sample. Although these two approaches are somewhat distinct from one another, they are often combined to determine the amount of nonbiodegradable organic materials present in wastewater.
In this article, we will compare and contrast COD and BOD, as well as discuss why it is necessary to monitor these parameters.
See also: Water treatment plant process: All you need to know
BOD and COD: What is BOD or biological oxygen demand?
The term “BOD” refers to the “biological oxygen demand” that is present in water. Organic matter, the number of microorganisms, and the amount of oxygen that is dissolved in water all have major connections to one another. For their metabolism, aerobic microbes need the presence of oxygen.
Therefore, they extract energy from organic materials using dissolved oxygen. As a result, the biological oxygen demand refers to the quantity of dissolved oxygen that aerobic organisms need to decompose organic molecules and receive the energy necessary for their metabolism.
This value will be affected by the number of nutrients and the rate of enzymatic processes, hence testing at a certain temperature and for a specific amount of time is required. As an added bonus, the BOD of polluted water is often greater than that of clean water. Sewage from homes, oil spills, and agricultural and animal waste may all lead to elevated BOD levels.
BOD test procedure
BOD isn’t a particularly reliable test but is used to evaluate water’s organic quality. The following are the steps that make up this test.
- It is crucial to check the biological activity of the water sample to obtain accurate BOD readings. For this to occur, the water sample must meet the following criteria:
- The presence of chlorine is not acceptable in the water. An application of a chemical for dichlorination, such as sodium sulphate, may be necessary if chlorine is found in the water.
- The water sample must have a pH between 6.5 and 7.5. An acid or perhaps a base may be added to a water sample to adjust the pH if it is too low or too high.
- A diverse population of microorganisms should be present in the water. A seed mixture of bacteria may be introduced together with the necessary nutrition if the microbiota is either unknown or insufficient.
- Sample water is diluted and placed in BOD bottles, which are specially engineered to prevent air bubbles from forming during filling (ideally, 300 MLS). The sample may be diluted with distilled water, and varying amounts of the water are used to get different concentrations in the various bottles. To serve as a control, however, one container should contain nothing but distilled water.
- After the five days have passed, grab the DO metre and take a reading to determine the total DO concentration. This number should have come down to roughly 4.0 mg/L in a perfect scenario.
- You may calculate the BOD value of the water by taking the initial DO and subtracting it from the ultimate DO. The difference between the two values will provide you with the BOD value.
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BOD and COD: What is COD or chemical oxygen demand?
Total organic compound concentration in water may be estimated using chemical oxygen demand (COD). Since oxygen is required to convert all organic carbon to CO2 and H2O, COD measures how much oxygen is present. In addition to its role in the breakdown of organic materials, chemical oxygen demand also plays a role in oxidising inorganic compounds (ammonia and nitrite).
As an added bonus, a chemical reagent is used in COD testing to oxidise the contaminants. So it doesn’t need microbes to decompose the organic matter in the sample through aerobic respiration. Moreover, COD is usually high in contrast to BOD since it measures a full breakdown of pollutants. In contrast, BOD is a measurement of a biochemical breakdown, which is not necessarily as thorough as a chemical breakdown.
COD test procedure
The BOD test normally takes 5 days, while the COD test only takes 3 hours. Therefore, the COD test is much quicker. Additionally, the COD test is not susceptible to influence from harmful compounds. The following are the steps involved:
- Standard solutions of potassium hydrogen phthalate will need to be prepared before the COD test can be completed. Most wastewater will fall into the high range. Thus, criteria of 100, 250, 500, and 1,000 mg/L are suggested.
- The next step is to switch on a COD reactor unit and a measuring cylinder and wait for them to reach equilibrium.
- After that, containers with a low-range or high-range concentration of COD are chosen for the test based on the predicted findings. Both ranges are an option to consider when it is unclear what outcomes are to be anticipated.
- There is a “blank” container and three or four containers that have been labelled with the normal level. The second round of testing on the wastewater sample will employ the other two containers. It is recommended to replicate 10% of the specimens when conducting several wastewater tests.
- The appropriate containers are then filled with 2 ml from every standard. A total of 2 ml of wastewater sample should be added to the matching container after it has reached full strength. The 2 ml fraction is introduced to the container when further dilution is required.
- After thoroughly mixing the contents of each container, put them in a reactor unit for two hours. After cooling for fifteen minutes, take them from the unit.
- Follow the maker’s directions for setting and calibrating the colorimeter, then load the vials into the device and get a reading for the COD concentration in each.
BOD and COD: Similarities between them
- The concentrations of COD and BOD may be expressed as mg/L or ppm.
- Both of these measurements determine how much oxygen is required to oxidise the contaminants that are present in the water.
- Additionally, the results of both tests show the degree to which water is polluted.
- In addition, the BOD and COD values of wastewater are essential for determining the total quantity of garbage in the water.
BOD and COD: Differences between them
The following table provides an overview of the primary distinctions that exist between BOD and COD.
| Parameters | COD | BOD |
| Definition | COD is a measurement that determines how much oxygen is required to break down both organic and inorganic materials in wastewater. | The biological oxygen demand, or BOD, is a measurement used to determine how much oxygen is required to break down organic matter by microbes present in wastewater. |
| Oxidation | By chemical process | By biological process |
| Oxidant used | Potassium Dichromate | Aerobic microorganisms |
| Test conditions | Acidic conditions | Aerobic conditions |
| Decomposition matter | Organic and Inorganic matter | Only organic matter |
| Required Time | It takes roughly two to three hours and is a speedy approach. | It takes five days (for the usual BOD test), and it is a process that moves very slowly. |
| Required Temperature | 150°C | 20°C |
| Values | Higher than BOD | Lower than COD |
| Permissible Limits | 250 – 500 ppm | 30 mg/l |
| Preferable waste treated | Industrial and domestic waste. | Mostly used water from lakes, rivers, oceans and human waste. |
| Limitation | Compounds such as acetate, toluene, benzene, etc., cannot be oxidised with the oxidising agent. | This method can only quantify biodegradable materials and cannot be used for industrial waste. |
FAQs:
Why are COD levels higher than BOD in wastewater treatment?
The chemical oxidation of organic molecules is more efficient than the biological oxidation of the same substances because COD is often higher than BOD. Since BOD testing cannot identify hazardous substances to live organisms, COD tests may be extremely helpful for analysing industrial effluent.
Why is COD preferred over BOD?
The BOD test has a minimum time requirement of five days, and as a result, it cannot be used effectively in the management of treatment operations. The chemical oxygen demand test is an alternative method for determining the level of organic matter in either natural or treated water. Although the BOD test takes 5 days, the COD test only takes 3.
How to minimise COD levels in wastewater?
Several strategies have been effective in the past when it comes to controlling COD in wastewater treatment. The two most prevalent techniques for eliminating COD from wastewater are as follows: Separation of wastewater Microbial action
What is the optimum DO concentration for aquatic organisms?
The dissolved oxygen content of 5 parts per million is considered to be optimal for aquatic life.