Finally, the last step in the BOD procedure involves inoculating the sample with various dilutions along with standards and blanks for quality control. The following quality control must always accompany each BOD run:
- 2 carboy water blanks
- 4 standards
- 2 seeded blanks
- optional control sample
Usually, many laboratories will include one set of QC for every ten samples. Additional QC is necessary for more than 10 samples.
1. Water blanks - carboy water is withdrawn to the rim of the bottles.
2. Standards - 4, 6, 6, and 8 mls of standard solution are added to separate bottles. An additional 2 mls of seed solution is added to each bottle and the bottles are then filled to the rim with carboy water.
3. Seeded blanks - 2mls of seed solution are added to each of 2 BOD bottles. The bottles are filled to the rim with carboy water.
4. Samples- 4-5 bottles are usually necessary for each sample. Some samples may have to be diluted in order for the DO range to be detected by the meter. Observation of the sample will usually give an indication to its dilution. Clean samples usually require small dilutions whereas wastewater samples will need high dilutions due to their high BOD values. Once reasonable dilutions have been determined, the specific volume of sample is added to each bottle along with 2 mls polyseed. The bottles are filled to the rim with carboy water.
Once all the bottles have been filled, the initial DO's of each solution is determined on the meter and recorded. Once recorded, the bottles are capped with ground glass stoppers to avoid excess bubbles and capped. The bottles are placed in an incubator at approxiamtely 20 degrees celsius where they will remian for five days.
From here, you can continue on the the next step, the Analysis section or go back to the Procedure or Introduction page.