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.

A Consumer's Guide to BOD₅ 5-day Biochemical Oxygen Demand used in wastewater testing

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 septic 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 usingLauril triptosaat dilution as the culture media.

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

< 0.2 mg/L

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[3]

Our lab photo (left) illustrates a positive detection of total coliform in all three test samples.

The test for biochemical oxygen demand (BOD) is a bioassay procedure that measures the oxygen consumed by bacteria from the decomposition of organic matter (Sawyer and McCarty, 1978).

The change in DO concentration is measured over a given period of time in water samples at a specified temperature. Procedures used to determine DO concentration are described in NFM 6.2.

It is important to be familiar with the correct procedures for determining DO concentrations before making BOD measurements. BOD is measured in a laboratory environment, generally at a local or USGS laboratory.

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 withintwo 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:

When collecting a hand-dipped sample:

1. Grasp the sample container near the base on the downstream side of the bottle.

2. Plunge the bottle opening downward below the water surface. Avoid contact with the streambed during this process.

3. Allow the sample container to fill with the opening pointed slightly upward into the current.

4. Cap the container securely and protect the sample from light during transport to the laboratory for analysis.

As you will see on the water test form provided by your test laboratory, you will also need to record the date, time, and location from which the sample was collected.

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

SeeSEPTIC 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:

Bacteria are commonly associated with suspended sediment, which can vary spatially and temporally along a stream cross section(Britton and Greeson, 1989). Like suspended sediment, the oxygen-demanding compounds may not be equally distributed along a cross section.

Where possible, use the equal-width-increment or equal-discharge-increment procedures described in NFM 4 to collect a BOD sample representative of the stream cross section.[3]

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 thedissolved 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.[1][2]

BOD₅ Test Conclusion & Standard Formula & Calculation 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 general equation for the determination of a BOD5 value is:

BOD₅ (mg/L) = D₁ - D₂ / P

D₁= initial DO (dissolved oxygen level) of the sample,

D₂= final DO of the sample at the end of the 5 day incubation period

P = decimal volumetric fraction of sample used [if the sample required dilution]

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.

"Biochemical oxygen demand (BOD) is defined as the amount of oxygen required by bacteria while stabilizing decomposable organic matter under aerobic conditions.

The BOD test is widely used to determine the pollution strength of domestic and industrial wastewaters in terms of the oxygen that they will require if discharged into natural watercourses in which aerobic conditions exist.

The test is one of the most important both in regulatory work and in studies designed to evaluate the purification capacity of receiving water bodies. Its disadvantage is the long time required by the test, generally taking 5 days." - Nanyang Technological University [4]

Definition of DO - Dissolved Oxygen

"All gases of the atmosphere are soluble in water to some degree. Oxygen is classified as poorly soluble, and its solubility is affected both by atmospheric pressure, and physical and chemical properties of water such as temperature, salinity, pollutants, etc. The solubility of atmospheric oxygen in fresh waters ranges from 14.6 mg/L at 0^oC to about 7 mg/L at 35^oC under 1 atm. of pressure.

Most of the critical conditions related to dissolved-oxygen deficiency, both in natural waters and biological wastewater treatment, occur during the warmer months when temperatures are high and solubility of oxygen is at a minimum. The low solubility of oxygen is a major factor limiting the purification capacity of natural waters.

In aerobic biological treatment processes, the limited solubility of oxygen is also of great importance, because it governs the rate at which oxygen will be absorbed by the medium and therefore the cost of aeration. Hence, DO analysis is a key test both in natural waters and water pollution control practice." - pedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES:ARTICLE INDEX to SEPTIC SYSTEMS

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Citations & References

In addition to any citations in the article above, a full list is available on request.