Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD), an Intro

In the wastewater treatment process, the presence and absence of oxygen plays a significant role in determining the treatment’s performance. Oxygen can stimulate some chemical and biochemical reactions between the chemical compounds and biological organisms in the wastewater. Therefore, the presence, absence, and the change of the oxygen concentration in the wastewater can be indicator of activity of chemical and biochemical reactions occurring in the treatment process. Specifically, dissolved oxygen (DO) is a parameter that can be measured and evaluated for this purpose.

In aerobic wastewater treatment, sufficient oxygen is essential to support the microorganisms’ life by converting the organic matter and removing it from the wastewater. There are several parameters that can be used to determine organic matter concentration in the wastewater, namely Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and Total Organic Carbon (TOC). TOC is a direct measurement of the organic content, whereas BOD and COD are indirect method for the quantification of organic matter by measuring the amount of consumed oxygen. This laboratory focuses on the BOD and COD tests.

The main difference between the BOD and COD test is that the means by which the material is oxidized. The oxidation process in the BOD test is carried out by microorganisms (only measures biodegradeable material), meanwhile the COD test is purely chemical oxidation reaction through a very strong oxidant, such as dichromate. The COD test only takes 2 or 3 hours whereas the the various BOD tests take many days. This quicker response of the COD test is very useful in terms of informing oprational decisions for controlling the performance of the wastewater plant if a problem occurs. For this reason, the COD test is used more often in process control. Another benefit of using the COD measurement is that the nitrification process, which is also related to the biochemical oxidation of organic matter in the wastewater, doesn’t have a significant effect on the result of this test (but must be inhibited or accounted for in the BOD test). It means that COD test only gives the indication of carbonaceous matter’s oxidation without involving nitrogeous oxygen demand.[1] In addition, the presence of heavy metals in the waste can also interfere the result of BOD measurement because it can inhibit the metabolic process or even kill the bacteria that process the oganic material.

In the COD test, inert material also be oxidized in addition to the material that can be oxidized through the BOD test. Therefore, the COD measurement is almost always higher than the value from BOD₅ test. In the practical operation, there is a certain ratio between COD and BOD₅ to give a good estimation for conversion between those values for a particular facility, and also give the estimation of the wastewater’s biodegradability. For example, for raw domestic wastewater, the ratio of COD/BOD₅ typically varies between 1.7 and 2.4.[2]

BOD measurement can be categorized into 2 types, BOD₅ and ultimate BOD (BODu). BOD₅ is associated with the biodegradable fraction of carbonaceous organic compounds that measures oxygen consumption within 5 days by the microorganisms in the incubation condition at temperature 20^o C. However, complete stabilisation of the chemical and biochemical reactions with the organic matter can take longer. Therefore another measure, ultimate BOD, expresses the total concentration of oxygen that is used up until the process is stoichiometrically complete. The oxygen concentration will be lower at the end of each test than at the start because some oxygen was consumed by the bacteria for the oxidation of the organic matter. Both BOD₅ and BODu carry meaning on the existence of a bio-stabilization process for the organic matter.

For performance evaluation, the standard BOD₅ limit mandated by the Federal government is 30 mg/L. In the Pullman Wastewater Treatment Plant, the average BOD₅ value of wastewater influent that enters the treatment process is about 255 mg/L with a maximum value is 570 mg/L. The expected average and maximum value of BOD₅ after treatment are about 14.1 mg/L and 40.5 mg/L respectively.[3] Additionally, the permits data reports of the Washington State’s Department of Ecology shows that the COD value that has been determined as the limitation for wastewater industries in the Washington State is about 120 mg/L.[4]

[1] Sperling, Marcos Van, “Wastewater Characteristics, Treatment, and Disposal”, IWA Publishing, 2007, ebook

[2] Ibid.

[3] Fact Sheet for NPDES Permit WA0044652 City of Pullman Waste Water Treatment Facility, ebook

[4] https://fortress.wa.gov/ecy/wqreports/public/f?p=128:5:800489660646801::NO::::YES, accessed October 15^th 2014