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.