GGA is too High
Although not a common reason for high GGA results, it is possible that the GGA is overseeded. Why is this a problem? The GGA is expected to be 198 +/- 30.5 mg/L but that is not the theoretical 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. So what is the solution to this Situation? Change your seed source to a less robust seed, or perhaps use less seed. (Keep the oxygen depletion of the seed around 0.6-1.0 mg/L in the bottles that are seeded.)
This is the most common cause of high GGA. Human beings make mistakes, this is certainly a well known fact. 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.
- 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).
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 (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.
- Use ampouled commercially prepared GGA
- If you make your own GGA:
- Pay special attention to weighing the glucose and the glutamic acid
- Carefully pipette the GGA solution into your test bottles