Yes, biodegradable compounds are degraded by microorganisms in standard BOD analysis. If you diluted the water sample by 10 times, you have to multiply the final result with the same dilution factor. But be careful about the minimum (eg. 0 mg/l) and maximum BOD value (eg. 1000 mg/l) that can be measured through your BOD reactor (If you are measuring BOD through a BOD unit). You can only measure the samples within that range. If the range is exceeded, you have to avoid that reading and take only the readings within the range.

Hence you are testing a sample with an unknown BOD value, you have to make a dilution series and do the BOD experiment. If your BOD is higher than 1000 mg/l and your equipment can only measure BOD 0 mg/l - 1000 mg/l, you cant get a BOD value. So the dilution is very important in this test.

Thanks a lot for the explanation!

However my samples are in the range... the point is: When I made tukey’s test to compare the results for BOD5 of each filter, I identify that filter 1 had similarity with Blank samples. So I wanna know explain why BOD5 analysis for filter 1 have similarities whit BOD dilution water.

For instance, can I say the treatment for filter 1 is good cause it has BOD5 similarities with dilution water?

Probably yes. It's also better if you could duplicate or triplicate the experiment and verify your findings if you are doing a research.