Biochemical Oxygen Demand (BOD)

Christina Dietzen

By: Nathan Hathaway and Allison Arling
Published: October 24, 2019


One of the principal concerns in America’s waterways is oxygen deprivation due to wastewater discharge from industrial facilities. If wastewater is not adequately treated before discharge, the remaining organic constituents can exert an oxygen demand on the receiving waterway. This oxygen demand can have widespread and damaging impacts on the local aquatic organisms, ultimately leading to the potential of fish kills, if oxygen levels drop too low. Due to the potential negative impacts of poorly treated wastewater, the Clean Water Act was put into place as a means of holding these industrial facilities accountable for the cleanliness of their wastewater. The National Pollutant Discharge Elimination System (NPDES) is how the EPA regulates these point source discharges. Each industrial facility is given an NPDES permit, which requires routine testing of the most environmentally impactful parameters for that given industry or siteThe five-day Biochemical Oxygen Demand test or BOD5 is often one of the most common tests that the NPDES permit mandates.

The BOD5 test measures the amount of oxygen bacteria use while degrading organic materials over five days and at a fixed temperature. The test requires a sample, nutrient water, and in some cases, a seed source. Nutrient water is made by adding phosphate buffer, magnesium sulfate, calcium chloride, and ferric chloride to either deionized or distilled water. This nutrient fortified water ensures the bacteria have all of the essential macro and micronutrients need for BOD degradation. Utilizing nutrient water and specific amounts of wastewater, a series of different dilutions is setup.

The goal of using multiple dilutions is to maximize the potential for a successful test result. Before addition to the sample bottles, dilution water is aerated to achieve the closest dissolved oxygen (DO) content possible to saturation. Once the bottles are filled, an initial DO concentration is measured, and the bottles are sealed to ensure no additional oxygen is introduced. The DO measurement can be taken using either a luminescent dissolved oxygen probe or a Clark cell electrode. After initial readings are recorded and bottles are sealed, the samples are placed into a 20 C incubator and left for five days (+/-6 hours). At the conclusion of the 5-day period, bottles are removed from the incubator, and the final DO measurements are recorded.

BOD5 is defined simply as the amount of oxygen consumed in 5-days. Essentially this means the difference between the initial and final DO concentrations, multiplied by the dilution factor. Since the BOD test is only measuring oxygen consumption over 5-days, it is considered an indirect measurement of food or organic material. However, rather than having to test for every conceivable organic material in the wastewater, it characterizes the effect it will have on a receiving waterway. Knowing that untreated wastewater will cause a large oxygen depletion in a stream or river is much more meaningful and readily understandable than simply identifying the chemical concentration of analytes. For this reason, the BOD test has been adopted and in use for over 100 years as a means of determining water quality, and there is no sign of change to come. EBS performs thousands of BOD tests each year in an effort to help our clients better understand the impact of their wastewater, as well as the efficiency of their treatment system. Using data generated in the laboratory, we can provide clients guidance on ways to better optimize treatment systems and ensure the highest water quality possible prior to discharge.

Learn about the microorganisms in your wastewater treatment system.