My GGA is too high. How can I fix it?

Document ID

Document ID TE886

Published Date

Published Date 27/05/2017
Question
My GGA is too high. How can I fix it?
Summary
BOD GGA too high
Answer
There are two main reasons that the GGA might be high. The GGA might be overseeded, or the GGA might be made incorrectly.

The most common reason is due to incorrectly prepared GGA, which must be prepared precisely. Double-check the concentration of glucose and glutamic acid of your standard to ensure that it matches your calculations. Standard Methods calls for 150 mg/L of glucose and 150 mg/L glutamic acid. The common Hach BOD Standard Solution, 300 mg/L, pk/16 - 10-mL Voluette® Ampules(catalog number 1486510) contains twice the concentration recommended by Standard Methods, to help ensure stability. If using it, use half the amount for the same BOD (3 mL of Hach GGA is the same as 6 mL of the formulary in Standard Methods).

Although not a common reason for high GGA results, it is possible that the GGA is overseeded. The GGA is expected to be 198 +/- 30.5 mg/L, but this is not the theoregical oxygen demand value. The theoretical value is actually about 307 mg/L. It is entirely possible that a more robust population of seed organisms could consume more than the expected 198 mg/L. One solution to this is to change your seed source to a less robust seed or try using less seed. Be sure that the oxygen depletion of the seed is around 0.6-1.0 mg/L in the seeded bottles.