MISSION: WATER 66 XYLEM SPOTLIGHT Understanding BOD Measurement For many of us, when we wash our hands or flush a toilet, our water is taken via sewer lines to wastewater treatment plants. The water at these plants must then be cleaned before it can be reintroduced into the environment. But how do we know if it’s safe to return the reclaimed (treated) water to our waterways? One way is BOD monitoring and testing – done by almost every water reclamation facility in the world. WHAT IS THE BOD TEST? BOD, or Biochemical Oxygen Demand, is a test that indicates the amount of dissolved oxygen (DO) consumed by organisms in the water over a period of time, typically 5 days. Often a subscript is appended to the BOD acronym to indicate the length of time (in days) of the test, such as in the 5-day BOD5 test. In reality, it takes about 20 days to get a full picture of the rate of oxygen depletion from a given wastewater sample, but the initial 5 days of the BOD5 test paints a useful picture for operators to predict where the depletion of oxygen is in that 20-day process. All BOD tests are performed with a special type of dissolved oxygen (DO) probe, commonly called a BOD Probe. Interestingly, the sensing portion of the DO probe is no different than a field DO probe; they both require the same calibration and maintenance. Unlike a field probe, a BOD probe has a wide, funnel-like base that acts as a stopper when inserted into a BOD Bottle. Also unlike most field probes, the BOD probe has a built-in stirrer that keeps the samples well-mixed during measurement. Even optical BOD sensors, which are not stir-dependent, have stirrers to improve measurement time and to ensure EPA compliance with the BOD test method. PERFORMING A BOD TEST To perform a BOD test, an initial dissolved oxygen measurement is taken in the sample on Day 1. The sample is then incubated at 20o C for 5 days before the dissolved oxygen is measured again. The difference in DO measurements between Day 1 and Day 5 represents the amount of oxygen consumed over 5 days. While this seems simple enough, things get more complex when we consider where in the treatment process the sample is collected. Influent water coming directly from homes and businesses is far more concentrated with waste and microbes than the effluent, which is the water returned to the environment after treatment. If we were to test the BOD of influent directly, dissolved oxygen would be depleted far too quickly to measure it over a 5 day span. As a result, several different dilutions of the influent, aeration and effluent samples are made and tested. Oftentimes wastewater treatment facilities will have a good idea from historical context as to how much they should dilute the samples or if the samples need seeded. A series of sample dilutions incubated together is often called a ‘batch’. A batch will typically include samples that help verify the test setup and accuracy, including the Dilution Blank, GGA check and Seed control. Technical Tips Thoughts from the front line of customer support. BEN SUTTER YSI Applications & Technical Support Specialist To learn more about BOD measurements, visit YSI.com/BOD A laboratory setup with a YSI ProODO handheld instrument, Optical BOD probe, and BOD Analyst Pro desktop software.