My GGA is too low. How can I fix it?
BOD GGA too low
The three most common reasons that GGA results may be to low are:
-The GGA is underseeded
-The seed contains toxic compounds
-The GGA is incorrectly prepared
Underseeding: GGA checks are used to determine the amount of seed needed for BOD. Though typically most users try to achieve a seed depletion of 0.6-1.0 mg/L, this is not always correct. Seed amount should be adjusted until 198 mg/L of GGA BOD is achieved.
Toxicity: Seed that contains compounds which are toxic can inhibit microbiological growth, resulting in less oxygen demand. Seed toxicity can be determined by plotting oxygen demand/uptake versus volume of seed used. This relationship should be linear; if it is not, this indicates that a toxic compound could be present. You should try a different source of seed to confirm the toxicity.
Incorrectly prepared GGA: Standard methods calls for a GGA solution that has 150 mg/L of glucose and 150 mg/L of glutamic acid. The BOD Standard Solution, 300 mg/L, pk/16 - 10-mL Voluette® Ampules (catalog number 1486510) contain 300 mg/L of glucose and glutamic acid. If you are using this standard, you should use half of the amount specified in Standard Methods to achieve the 150 mg/L GGA required.