Why do I have to adjust pH–won the dilution water make the pH OK?
This gets back to the “BOD Pyramid”. You need to have a happy, healthy population of bugs present in order for the BOD test to work, and pH extremes CAN be harmful if not fatal to bugs. Sure, if your influent pH is 10, and you only use 3 mLs of it, the addition of 297 mLs of dilution water could bring the pH into the method-specified range (6.5 to 7.5 for samples <6.0 or > >8.5). But…are there any viable bugs? You could have a significant BOD loading in the sample, but if there aren’t any (living) bugs present, the BOD will appear to be less than the detection limit because no oxygen will be used. Certainly, there are some natural systems (some rivers/streams) with pH above 8.5 that DO conbtain a viable supply of bugs. The reason for the Styandard Methods requirement is to BE SURE that all corners of the pyramid are covered—by adjusting the origional sample pH to 6.5-7.5, making your dilutions, and then seeding thye dilutions..
