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Hach LBOD Probe

Hach Company has just released a great new product that will make BOD measurement a whole lot easier. LDO technology has always been a great way to measure BOD but it really wasn't designed for BOD measurement. Now it is, here is a picture of the new probe.

This probe eliminates difficulties with membranes, electrolytes, and electrodes. In addition this probe has excellent accuracy and precision (when calibrated daily). The Hach LBOD is truly next generation technology, and will help to further simplify the BOD test.

Use of the probe also enables the use of Pipette Free GGA. This is exactly as it sounds. All you have to do is break the ampule and drop it in the BOD bottle with the seed and dilution water. You don't have to prepare the GGA yourself and you don't have to pour the GGA into a beaker to pipette it. As an added advantage you never have to worry about whether or not you added your GGA to the bottle. The ampule can remain in the bottle until the end of the test if you are using the Hach LBOD. This can be used with a polarographic probe but it is necessary to pour the contents into the bottle instead of just dropping it in, otherwise the membrane may rip.

The blank is a very important indicator of whether or not the BOD test is working as intended and it should not be considered an irrelevant problem. Despite what many think it is entirely possible to ALWAYS have good blanks. In fact, doing the following will practically GAURANTEE good blanks.

• Use recyclable PET BOD bottles (gains about 0.04 mg/L lower blanks on average).
• Use air calibration instead of winkler titration (gains about 0.09 mg/L lower blanks on average).
• Use a water polisher instead of deionized or distilled water (gains about 0.03 mg/L blanks on average).
• Add BOD buffering reagents as late as possible and rinse thoroughly a carboy having contained them.
• "Seat" the stoppers carefully. Do not just drop them in the BOD bottles without twisting to seat them.
• Carefully maintain your DO probe and replace approximately once per year if using a polarographic probe.
• The new LBOD probe requires hardly any maintenence and is a good alternative. Using this probe is approved in many regions. Some regions require a 4 bottle check of the users proficiency that takes about 20 minutes.

Consider this...CBOD blanks (which should never be done unless a regulator insists on it!) that average 0.26 mg/L will likely average 0.10 mg/L when following all of our recommendations on blanks. Blanks should read less than 0.10 mg/L routinely. Occasionally getting over 0.10 mg/L is okay but that should be the exception not the rule.

Likely Causes of High Blank

• Source of Water
• Bad Dilution Water Buffer
• Incorrect DO Measurement
• Dirty BOD Bottle
• Dirty Carboy
• Dirty Pipettes or Cylinder
• Dirty Probe - if measuring with a DO Probe instead of Winkler
• Super Saturation of the Blank

UnderSeeding

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.

Toxic 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.

Incorrectly Made

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)

Recommendations

• 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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