GGA is too Low

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There are a two main reasons that the GGA might be to low. One is that the GGA might be underseeded, the other is that the GGA might be made incorrectly.



A common reason for low GGA results is that the [[GGA}]] might be underseeded. The real purpose of the GGA is to set the seed concentration correctly. Many people have mistakenly thought that if the oxygen depletion of the seed is 0.6 to 1.0 mg/L, then that is all there is to worry about, but this is incorrect. The most important function of the GGA is to tell us when we have enough seed in our seeded samples. Regardless of the oxygen depletion of the seed, the seed amount should be adjusted until 198 mg/L of GGA is obtained on average.

Toxic Seed

Sometimes the seed has toxic compounds in it that inhibits the growth of the microbes. When the growth is inhibited there will naturally be less oxygen demand that there would be if the microbes were not inhibited by toxics. The best way to detect the toxicity of the seed is to plot the oxygen uptake versus the mL of seed that is used. If the uptake is not linear, then there is toxicity in the seed. The only choice at this point is to get rid of the toxic seed and get a better seed.

Incorrectly Made

Another cause of low GGA is that it was incorrectly prepared. The GGA must be prepared precisely. When Standard Methods says that the GGA must be 150 mg/L of glucose and 150 mg/L of glutamic acid, that is what it must be. Even small changes in this contribution can lead to grossly different BOD values.

One way to avoid this issue altogether is to purchase a commercially prepared GGA. Many different companies prepare GGA. Choose one with the following characteristics:

  • The GGA should be ampouled not bottled
  • The GGA should come from a known supplier with a good reputation
  • The GGA should contain ONLY glucose and glutamic acid (no preservatives).
  • Not premeasured. You should be testing your pipetting technique as well as the seed.

The following GGA is from a respected source. It is at twice the concentration recommended by Standard Methods which results in superior stability, but requires half the amount for the same BOD for example 3 mL of Hach GGA is the same as 6 mL of the formulary in Standard Methods. For example if you use 6 mL normally you should use 3 mL instead.

Hach Glucose and Glutamic Acid (GGA)


  • Use ampouled commercially prepared GGA
  • If you make your own GGA
    • Dry the glucose and the glutamic acid prior to dissolving it
    • Pay special attention to weighing the glucose and the glutamic acid
    • Carefully pipette the GGA solution into your test bottles
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