Biological Oxygen Demand (BOD) tests measure oxygen use by aerobic microorganisms growing on samples. In a sealed bottle, you measure oxygen consumption by a seed over usually 5 days under strict temperature control and darkness (to control for photosynthesis by algae).
Things that can confuse test results
Tests used to check for variation in seed
Options on BOD Seed
Goal of all procedures is to reduce variation from test to test. One way to help with this is to use a consistent seed. While MLSS is often used, it can vary in composition due to influent changes and seasonal variation. Additionally, MLSS often contains AOB/NOB cultures – which may require a nitrification inhibitor to get true BOD5 results.
What about primary effluent as the seed? This can contain high levels of organic compounds and a blend of true anaerobic cultures and other enteric organisms that can be highly variable.
Commercial seed – oldest and most used being PolySeed which was approved by EPA in 1982. Advantages related to the PolySeed:
Things that can confuse test results
- Insoluble organics – COD and TOC tests pick this up. But insoluble organics do not usually cause BOD test problems.
- Recalcitrant compounds – bacteria don’t grow on these. This means that the test misses these compounds and they do not show up in the test.
- .In bottle nitrification by AOB/NOB cultures – ammonia and nitrite oxidation consume high levels of DO. So any nitrification can compromise test accuracy.
- Seed variation – using MLSS or primary solids can have variation from operations and environmental factors.
Tests used to check for variation in seed
- Seed Control Factor (SCF)
- Too low and seed may not have enough microbes.
- Too high and seed may have too much background organic
- Target is 0.6 – 1.0 mg/L
- GGA – Glucose Glutamic Acid standard
- Well known soluble organic compound for standard. Used by most heterotrophic organisms.
- Uniform QA/QC check - we know what the end result should be
- Target is 198 + 30.5 mg/L
Options on BOD Seed
Goal of all procedures is to reduce variation from test to test. One way to help with this is to use a consistent seed. While MLSS is often used, it can vary in composition due to influent changes and seasonal variation. Additionally, MLSS often contains AOB/NOB cultures – which may require a nitrification inhibitor to get true BOD5 results.
What about primary effluent as the seed? This can contain high levels of organic compounds and a blend of true anaerobic cultures and other enteric organisms that can be highly variable.
Commercial seed – oldest and most used being PolySeed which was approved by EPA in 1982. Advantages related to the PolySeed:
- Contains a fixed blend of microorganisms with each batch tested for SCF and GGA standards.
- Does not contain AOB/NOB cultures but there is a PolySeed NX with nitrification inhibitor for use if you suspect your sample has high levels of AOB/NOB.
- By having a shelf-stable, fixed blend of seed, you have reduced variation in the test procedure.
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