NCL Technical Tips

GGA Calculations

Part 1: Seed Controls

Remember,the material must be seeded, so you must also set up “seed controls”.

If you are using BOD Seed: Prepare the seed according to directions. Standard Methods requires a depletion of at least 2.0 mg/l in seed controls. Some states require a depletion of at least 40% but less than 70%. In either case, you should set up AT LEAST three different seed dilutions. For BOD Seed, we recommend starting with 5 ml, 10 ml, and 15 ml. (The balance of the 300 ml is dilution water).

DO NOT PIPET THE SEED INTO A DRY BOTTLE!

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

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. If you are required to have a 40-70% depletion, you must throw out the 5 ml and the 10 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.

Part 2: The GGA Test

Most chemists use 6 ml of BOD Standard and 2 ml of seed, the balance being dilution water. Most people set up two or three identical bottles.

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

The depletion due to seed is the average seed depletion per ml (from Part 1) 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/6ml = 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 you air-calibration value on your bench sheet every day.
6. A weak seed usually causes HIGH GGA results

Questions:
Give us a call at 1-800-648-7836

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