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Technical Tips

GGA Calculations

BOD Standard (NCL# B-12D)
Glucose-Glutamic Acid Standard

This standard is prepared in accordance with Standard Methods, 20th and 21st editions. Products of this type need to be seeded if you want correct and consistent results.

1.Acceptable depletions and dilutions: Different states and different regulators have different criteria for acceptable depletions. At the very least, you should comply with Standard Methods.

a. Depletion in all bottles must be at least 2.0 mg/l

b. Remaining DO must be at least 1.0 mg/l

2. How many bottles should I set up? Most regulators will tell you that you need to only run one seed control and one BOD standard to comply. However, we STRONGLY DISAGREE with this idea. As long as your results are within the required range, you would be OK. However, when your results are out of compliance, there is NO WAY to troubleshoot the problem. At a minimum, we recommend that you run at least three different dilutions of the seed (regardless of seed source). We also recommend that you run three identical bottles of BOD Standard. These bottles typically contain 6 ml of BOD Standard and 2 ml of seed, although the amount of seed is occasionally varied.

3. How do I prepare my seed and standard bottles? Use dilution water for seed and standard bottles (must contain nutrients). Fill each bottle approximately 2/3 full with dilution water. Then add Standard and/or seed. Next, fill bottle with dilution water. Record ml of each constituent, Initial DO and Final DO.

4. Seed Control Calculations:

Record ml of seed, initial DO, and final DO.

Example:
ml seed 5 ml seed 10 ml seed 15 ml seed
Initial DO 8.40 8.40 8.38
Final DO 6.68 5.30 4.03
Depletion 1.72 3.10 4.35


Next, divide each depletion by the ml of seed. This gives you the depletion per ml of seed. It can be very revealing to do this, especially if you use a weak seed. *Note that if you are required to have at least a 2.0 mg/l depletion, you must throw out the 5 ml results.

Dividing this out, we get:

For 5 ml sample: 1.72/5 = 0.34 mg/l depletion per ml of seed
For 10 ml sample: 3.10/10 = 0.31 mg/l depletion per ml of seed
For 15 ml sample: 4.35/15 = 0.29 mg/l depletion per ml of seed

Next, we average the usable results: (0.31 + 0.29)/2 = 0.30 <-- This is the average depletion per ml of seed.

Since Standard Methods requires the depletion due to the seed to be between 0.6 and 1.0, you should add either 2 ml or 3 ml of seed to the standard.

5. GGA Calculations:

Record ml of seed, ml of standard, initial DO, and final DO.

Example:
Bottle # 1 2 3
ml Standard 6 6 6
ml Seed 2 2 2
Initial DO 8.40 8.39 8.42
Initial DO 3.56 3.81 3.68
(-) Final DO 4.84 4.58 4.74
(-) Depletion due to seed 0.60 0.60 0.60
Net depletion due to GGA 4.24 3.98 4.14
x dilution factor (50) 212 199 207


The depletion due to seed is the average seed depletion per ml times the number of ml of seed used in the GGA sample, in this case 2 ml x 0.30 = 0.60.

The dilution factor is the total capacity of the BOD bottle (300 ml) divided by the ml of sample
(300 ml/6 ml = 50).


Notes:

1. NEVER pipet seed material into a dry BOD bottle. Always have some dilution water in first.
2. The method, duration of seed aeration and the settling time are not as important as the fact that they should be consistent.
3. Always set up at least three DIFFERENT dilutions for seed controls, and at least two identical GGA bottles.
4. Most operators use 1 or 2 ml of seed and 6 ml of GGA Standard.
5. Record your air-calibration value on your benchsheet every day.

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