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What is BOD & how is it significant in water quality or potability testing? This article describes Biological Oxygen Demand Testing or BOD5 wastewater testing, explains what the test involves, why it is used, and what it can tell us about private septic systems as well as the condition of public wastewater treatment facilities. Understanding BOD or BOD5 tests of wastewater help diagnose drinking water quality problems in both private wells and community or public water supplies.To be clear, a BOD test is performed on wastewater to establish its level of treatment. It is not performed on drinking water. We include citations of standards & codes for wastewater testing. Graph at page top excerpted from the USGS Delzer and McKenzie publication described in this article.
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Daniel Friedman, Isabel Sanchez Luna
The standard 5-day BOD5 test for biological oxygen demand in wastewater is used to evaluate the effectiveness of wastewater treatment by a public or private sewer or septic system. If a system is not adequately treating its wastewater to remove biological pollutants the implication is that biological contaminants - a health hazard - are being discharged into the environment and thus potentially into local waterways and into drinking water supplies.
Our lab photo (left) illustrates a positive detection of total coliform in the left test tube and no coliform detected in the two samples at right. The test was performed using Lauril triptosa at dilution 1x10-4 as the culture media. Photo (left) I. Sanchez Luna.
Although there are numerous qualifications and potential test interferences that must be considered, the bottom line for a BOD5 test is that the acceptable level of wastewater BOD5 discharged to the environment must be at less than 0.2 mg/L (milligrams per liter), usually written as
Delzer and McKenzie, writing for the USGS offer a succinct, clearly-worded technical explanations of the function of the biological oxygen demand test used to assay wastewater. The following excerpted quotations are from their USGS document 
Our lab photo (left) illustrates a positive detection of total coliform in all three test samples. Photo (left) I. Sanchez Luna.
Watch out: the use of BOD to evaluate the effectiveness of wastewater treatment is focused on biological pollutants. It does not directly address many other contaminants that can be expected to appear in wastewater such as certain chemicals or heavy metals.
The BOD Wastewater Test Procedure
A sample of wastewater is collected in a clean container and delivered to a qualified testing laboratory (including labs provided by the USGS in the USA) within 24-hours of collection. If the sample is not to be delivered immediately to a test facility it should be stored in a refrigerated or cooled container but kept above freezing. Ideal storage is at 1-4 degC.
Ideally the wastewater sample is delivered immediately to the test lab and the BOD test is performed immediately. If the sample cannot be delivered to the test laboratory within two hours it must be refrigerated and delivered to the test laboratory within 24-hours. The sample quantity depends on the number and type of tests required; for municipal wastewater treatment systems generally a liter of sample is sufficient. For private lab testing of private septic systems, a smaller quantity is often acceptable. Check with your test lab. The manual gives this procedure for collecting a "grab sample" from a wastewater stream or from a nearby waterway:
An example of private use of this test method is a check on a pond or waterway located close to a private septic system suspected of leaching effluent directly into the waterway. Also see SEPTIC DYE TEST PROCEDURE for additional site tests for septic effluent leakage out of the system.
Watch out: Close the wastewater sample and protect it from sunlight. Do not freeze the wastewater sample, and get it to the lab within 24-hours. Keeping a sample longer than 24-hours results in an invalid test.
Watch out: amateur collection of wastewaters for BOD testing risks an inaccurate result, as the field manual explains:
Technical note: for private septic systems whose design includes a final disinfection stage the BOD procedure can produce erroneous results unless extra steps are taken by the test laboratory. Therefore it is important in private septic system treatment to inform the test laboratory of the type and condition of the septic system being tested.
In the U.S. the test laboratory follows detailed sample preparation and assay procedures specified in the NFM, National Field Manual for the Collection of Water-Quality Data; other countries use a very similar if not identical standard. Typically a lab will prepare three different dilutions of the test sample to address the full anticipated range of possible biological oxygen demand. The sample(s) is/are prepared and then incubated at 20 deg C for five days.
At the end of the test interval the dissolved oxygen level (DO level) is measured to determine the BOD. By comparing the beginning oxygen level (DO at start of the test) with the ending oxygen level (DO at the end of the test), the BOD can be calculated. 
BOD5 Test Conclusion & Standard Goal for Acceptable Level of Wastewater Treatment
In sum, at the end of the test interval the laboratory examines the quantity of dissolved oxygen consumed by biological entities in the sample. If the wastewater has been adequately treated the level of biological contaminants in the sample will be low enough that the BOD5 will be < 0.2 mg/L of liquid.
The Field Manual explains several possible causes of abnormal BOD5 readings such as poorly-cleaned lab glassware, de ionized water in the sample - factors that can increase the BOD5 reading, as well as interferences or improper lab ware that can cause abnormally low BOD5 readings.
Wastewater Treatment Level Terms & Definitions
Definition of BOD - Biological Oxygen Demand
The following definitions are excerpted from class notes provided by Nanyang Technological University.
Definition of DO - Dissolved Oxygen
Definition of CO or COD - Chemical Oxygen Demand
Definition of TOC - Total Organic Carbon
Typical BOD5 Levels Found in Types of Wastewater or Sewage
Typical Methods for Measuring BOD in Wastewater
Additional techniques for measuring BOD include
Two Stages of Decomposition of Biological Materials in Wastewater
The USGS graph repeated here illustrates the following [continuing to excerpt from the USGS document cited]:
Where does the "Five-Day" requirement for BOD5 Originate
Testing for an acceptable level of wastewater treatment needs a standard for comparison. The particular choice of five-days as the length of time used to measure oxygen demand is based on an interesting history cited by Delzer and McKenzie as a technical note:
Frequently Asked Questions (FAQs) about Biological Oxygen Demand - BOD5 Wastewater Testing
Questions & answers or comments about the significance of oxygen levels in wastewater and the definition of & standards for biological oxygen demand BOD5 used in Water Testing.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.