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BOD Vs COD What Is BOD What Is COD

• BOD refers to the biochemical demand for oxygen and COD is the chemical demand for oxygen.

relation between bod and cod

The ratio is therefore a measure for the degree of reduction of the carbon compounds. BOD is a measurement of organics that can be oxidized by microorganisms while COD is a measurement of organics that can be oxidized by chemicals that why COD is higher than BOD in most of the cases. Once COD and BOD data has been gathered over time, the average BOD result is divided by the average COD result to determine the ratio, or conversion factor.

What is the relation between BOD and COD?

PH is a scale used to specify how acidic or basic a water-based solution is. At 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic. Simply put, cut down on your chemical usage outdoors whenever possible, and you should make a difference in your COD present in your stormwater. Organic pollutants at a site could be things like chemicals, petroleum, solvents, cleaning agents, etc. Not what you immediately think of when you see the word organic, but that’s what it means.

To measure oxygen demand, the BOD method relies on enzymes produced by bacteria to catalyze the oxidation of organic matter during a five-day incubation period. In contrast, COD methods use chemical oxidants to oxidize organic matter. BOD simulates the actual treatment plant process by measuring the organic material that can be oxidized with the oxygen in the sample when catalyzed by bacterial enzymes.

Also, it can be categorized in several ways such as biological, physical and chemical. They are namely pH, turbidity, microorganisms, dissolved-oxygen content, and dissolved-nutrients. The main parameter, which can affect the water quality, is the composition of water.

relation between bod and cod

The new staff members were a pretty sharp bunch, and I think they’ll have no trouble staying in compliance at all. There will not be much organic waste present in the water supply. A water supply with a BOD level of 3-5 ppm is considered moderately clean. Table-1 Showing some representative measured values of TOC, BOD and COD. Intially, the pH of the treated effluent sample was brought to 7.0, which was then tested for BOD , COD and TOC.

throughts on “Relationship between BOD, COD, TOC and ThOD”

Three years ago, he was employed by Hach Company as an R&D chemist assigned to the wastewater field where he is currently investigating possibilities for simplification and improvement in oxygen demand testing. For testing, the Lewiston lab uses the closed-reflux micro method using Hach COD digestion vials. The pre-dosed reagents are contained in 16mm glass tubes, or vials, that fit directly into all commercially available COD reactors and spectrophotometers. The prepared COD reagents minimize handling of corrosive and toxic chemicals, and eliminate pipetting and measuring hazardous reagents. The Lewiston facility is participating in Hach’s SIRR plan, a program designed to lower inventory costs and reduce time-consuming paperwork at the plant relative to its reagent use.

relation between bod and cod

All organic compounds may convert into carbon dioxide, water and ammonia. It is a significant parameter to determinate water characteristic. The COD value depends on oxidizing agent, pH, temperature, and period of time.

Another difference between BOD and COD is that it is possible to complete the COD test within a few hours in the lab. Moreover, since COD measures the amount of oxygen necessary to oxidize total pollutants, COD value is always greater than the BOD value. Also, in COD, a complete breakdown of pollutants takes place while it does not occur in BOD.


If the BOD/COD ratio for untreated wastewater is 0.5 or greater, the waste is considered to be easily treatable by biological means. If the ratio is below about 0.3, either the waste may have some toxic components or acclimated microoorganisms may be required in its stabilization. COD testing is simple and less sensitive in lower concentrations, whereas TOC requires instrumental analysis but much more precise. Therefore, it is advisable to use TOC for following biodegradation reactions of compounds at low concentration (below COD mg/L).

COD may be high due to presence of inorganic reducing substances and in that case TOC may be less. Wayt said COD testing is a very straightforward procedure. Before it began its long-term validation study, the Lewiston plant had never conducted COD analysis before.

Although COD is comparable to BOD, COD actually measures chemically oxidizable matter. Therefore, the biological oxygen demand is the amount of dissolved oxygen required by aerobic organisms to breakdown organic materials, to obtain energy for their metabolism. This value should be tested under a given temperature for a given period, and it will depend on the nutrient concentration and enzymatic reactions too. Besides, BOD value in polluted water is normally higher than the fresh water.

  • The main intention of this article is to give a clear idea about both of the concepts, similarities, differences, and practical usage of them.
  • COD may be high due to presence of inorganic reducing substances and in that case TOC may be less.
  • Normally water contains gases, inorganic ions, organic compounds, living organisms and some of the other trace chemical compounds.
  • The greater the BOD, the more rapidly oxygen is depleted in the stream.
  • This method does however rely on the use of chemicals, and requires high reoccurring cost.
  • Not only COD engages with the decomposition of organic matter, but also, it relates to the oxidation of inorganic chemicals .

Then allow the flask to cool about room temperature and wash the condenser with distilled water. Learn what to do if your stormwater sample results exceed limits or benchmarks in New York under a SPDES stormwater permit. When these pollutants get spilled, mixed, etc., and end up in stormwater, they’ll eventually break down, and add an additional strain on the oxygen demand in the water. When too much organic matter, either from a chemical or biological source, is added to a system, the natural balance gets out of sync, and things like fish kills occur, or extreme algae blooms.

The higher the chemical oxygen demand, the higher the amount of pollution in the water. BOD and COD are two parameters to measure water quality and the amount of organic pollution of water. BOD measures the amount of oxygen utilizing by aerobic microorganisms to breakdown organic matter in water.

This study is limited to wastewater from a refinery processing heavy crudes. BOD directly affects the amount of dissolved oxygen in rivers and streams. The greater the BOD, the more rapidly oxygen is depleted in the stream. This means less oxygen is available to higher forms of aquatic life. The COD / BOD5 ratio gives an indication of the source and origin of the organic pollution. Also, both measurements indicate the intensity of water pollution.

Principle of chemical oxygen demand

The term also refers to a chemical procedure for determining this amount. This is not a precise quantitative test, although it is widely used as an indication of the organic quality of water. The BOD value is most commonly expressed in milligrams of oxygen consumed per liter of sample during 5 days of incubation at 20 °C and is often used as a robust surrogate of the degree of organic pollution of water. Most industrial wastewater treatment plants use chemical oxygen demand rather than the BOD5 test. Due to the length of time required to complete the BOD5 test , BOD results provide historical data only and do not facilitate rapid water quality assessment for optimal process control. Quality of a given water sample depends on some variable factors.

The higher the COD value, the more serious the pollution of organic matter by water. This content in the inlet water can increase in case of non-domestic wastewater additions. These contain organic matter that is resistant to biological treatment (certain industrial effluents, landfill leachate, etc.). Moreover, COD testing uses chemical reagent to oxidize the pollutants. Hence, it does not rely on the use of microorganisms to break down the organic material in the sample by aerobic respiration.

There are only 11 data points so we need to be careful in drawing too many conclusions. Still, the mean of these 11 values is 2.3 with a sample standard deviation of 0.39. The BOD5 test is limited in some applications such as industrial wastewaters, which often contain heavy metal ions, cyanides, and other substances toxic to microorganisms. When microorganisms become poisoned by toxic substances, they are unable to oxidize waste, in which case the BOD5 test becomes an ineffective measure of organic pollution. There’s a fair amount of natural, organic matter which makes its way to waterbodies via stormwater runoff.

Why does BOD decrease?

The test measures only the approximate amount of oxygen that will be required by a wastewater when it is exposed to air or oxygen for an extended period of time. Toxic substances in the wastewater inhibit or even prevent bacterial growth and, therefore, oxidation of the organic matter. When this happens, the test result is lower than relation between bod and cod the actual amount of organic matter present would suggest. Typically, municipal wastewater treatment plants will use BOD5 as a measure of the organic concentration into, and through, the wastewater plant. Industrial wastewater systems will more often use COD to measure the organic concentration moving through the treatment plant.

I’m not talking about using less, I’m suggesting don’t use them in a place that’s exposed to stormwater. One way to reduce COD in water is to use coagulants and flocculants to bind sludge together. Once they are bound into big enough masses that they can be easily filtered out and deposited into a sedimentation tank for removal.

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