The organic strength of wastewater is measured in three ways: 1) as 5-day biochemical oxygen demand (BOD5), 2) chemical oxygen demand (COD), and/or 3) total organic carbon. 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. In my experience, I see TOC being used much less often (rarely) than BOD or COD.
The BOD5 test measures the oxygen consumed by microorganisms as they oxidize (consume or eat) the soluble organic matter in the wastewater. But the BOD5 test is a somewhat unreliable means of determining the amount of organic matter present in water. The test measures only the approximate amount of oxygen that will be required (absorbed or consumed) 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 the actual amount of organic matter present would suggest.
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.
Most industrial wastewater treatment plants use chemical oxygen demand (COD) rather than the BOD5 test. Due to the length of time required to complete the BOD5 test (five days), BOD results provide historical data only and do not facilitate rapid water quality assessment for optimal process control. The often highly variable chemical composition and strength of industrial wastewater requires a much more rapid method for measuring the organic concentration, hence the use of the two hour COD test or, in some plants, the 30 minute TOC analysis. Municipal wastewater plants operate with much greater consistency (less variation) in the strength of the influent organic loading which allows the municipality to wait five days to determine the organic concentration entering the plant.
The TOC test can take several minutes to several hours to complete, and information obtained from a TOC analysis is less useful than information obtained from the BOD5 or the COD analysis. The TOC test does not differentiate between compounds with the same number of carbon atoms in different stages of oxidation and will thus produce different oxygen demand results. Because BOD5 and COD tests directly measure the amount of oxygen required to stabilize a waste sample, results reflect the original oxidation state of the chemical pollutants. The relationship between BOD5, COD, and TOC is shown below.
COD test results can also be used to estimate the BOD5 results for a given sample. An empirical relationship exists between BOD5, COD, and TOC. However, the specific relationship must be established for each sample location in a wastewater treatment plant. That is, the relationship between the BOD5, COD, and TOC from a given sample location is site-specific. Once the correlation has been established, the COD test is useful for monitoring and process control. Without BOD5 data correlated to COD data, the ratios in the table below can be used to estimate (roughly approximate, really) the relationship between COD, BOD5, and/or TOC. Use the table with caution though. These ratio values are only a guide and may vary significantly from what is actually taking place at your wastewater plant.
COD/BOD ratios can be highly variable. The more variable the ratio values, as in, the higher the COD/BOD ratio, the greater the percentage of slowly biodegradable and non-biodegradable material in the sample. And that means the BOD5 test will give a lower value than is truly representative of the oxygen demand in the sample.
The graph below shows the variability in three years of influent COD and BOD from a refinery in North America. The histogram has been fit with a distribution using the Monte Carlo simulation program @Risk. The fitted distribution is a gamma distribution which has two parameters: 1) a shape parameter, alpha and 2) a scale parameter, beta. The fitted distribution tells us that 92.2% of the COD/BOD ratio values are between 2.51 and 6.36. As you can see, these actual COD/BOD ratio values are significantly higher than the typical 2.1 value from the table above.
You might think a highly variable COD/BOD ratio is to be expected with industrial waste streams. And I would say that I agree with you. But take a look at the bar graph below which shows influent COD/BOD ratio data for a municipal plant, spanning a two year period, that does have a small industrial contribution. 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.
A statistical summary of this municipal COD/BOD data is tabulated below. The mean of 2.3 is 9.5% higher than the mean of 2.1 from the table above. The two values are close enough. But the maximum value of 3.1 from the actual data is 47.6% higher than the 2.1 COD/BOD value from the table above. And the minimum value of 1.7 is 19.0% less than the 2.1 value. What's my point you're very likely asking? It's a simple one. You need to be very careful in assuming the COD/BOD ratio is 2.1. It's always variable and it's always site-specific. So in the absence of actual data, when you have no choice but to pick a value, be careful in how you use the typical COD/BOD ratio of 2.1.