As a point of clarification it should be noted that all municipal wastewater treatment plants use five-day biochemical oxygen demand (BOD5) as a measure of the organic concentration into, and through, the wastewater plant. The BOD5 test measures the oxygen taken up by wastewater during the biological oxidation of the soluble organic matter in the wastewater. But the BOD5 test is a highly 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) or total organic carbon (TOC) 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 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 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 in Figure 1.
Figure 1: BOD, COD, and TOC Relationship
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 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 Table 1 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. To see examples of how well these ratios correlate with specific sample data, click here.
Table 1: Ratio of BOD to COD to TOC
The ratio comparisons shown in Table 2 are particularly insightful. Starting with a theoretical total oxygen demand, determined stoichiometrically, of 850 mg/L, we can see that the 5-day BOD test only estimates 32.9% of the total oxygen demand, far from (much lower than) the potential oxygen demand actually occurring in the bioreactor. The COD, measured using the potassium dichromate method, does a much better job of estimating the oxygen demand, at 600 mg/L or 70.6% of the theoretical total oxygen demand, but the COD test still does not capture the total oxygen demand in the bioreactor. Keep in mind, the most accurate method for determining the “true” or actual oxygen demand in the bioreactor at any point in time, is through the use of oxygen uptake rate testing, a simple 15-minute test.
Table 2: Ratio Comparison of BOD to COD to TOC