| There are a two main reasons that the GGA might be too 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 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.
Freeze-Dried Commercial Seed
This is a special problem. The freeze-dried commercial seed should not be used. Almost inevitably an analyst will fail to achieve the required 198 mg/L on his or her GGA. The best advice if you are using freeze-dried commercial seed is to stop using it. Use influent, primary effluent, or dirt instead. If there is some overriding reason to use freeze-dried commercial seed then simply use the seed then add half a gram of dirt or influent to the mix. That will put some viable microbes into the seeding mixture. One can also try to use an increased amount of the seed...but often it will require so much seed to get a good response that it would be better to just use an alternative source of seed.
Sometimes the seed has toxic compounds in it that inhibit the growth of the microbes. When the growth is inhibited, there will naturally be less oxygen demand than 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 try another seed.
Another cause of low GGA is that it was incorrectly prepared. The GGA must be prepared precisely. If Standard Methods says that the GGA must be 150 mg/L of glucose and 150 mg/L of glutamic acid, then 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).
- Some vendors provide pre-measured GGA, using it will eliminate pipetting errors.
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