Dear Anil,

Water at 20 C holds only 9.2 mg/L dissolved oxygen (DO) at saturation. For the BOD test, we want at least 1-2 mg/L of oxygen depletion after incubation in the dark for 5 days, but also must have 1 mg/L DO remaining. Thus, we can only measure a BOD of 1-7 mg/L without dilution. Domestic wastewater (sewage, or “blackwater”) typically has a BOD of 200 mg/L or higher in the USA, where we use large amounts of household water; it is typically higher elsewhere. Domestic greywater (sewage without toilet water) still has a high BOD here (>100 mg/L). This is why dilution is needed.

Best of luck with your work!

- Peter

Hi Anil,

Peter provided a very good answer. I came across a similar question before, follow this link for more inputs: https://www.researchgate.net/post/Exactly_why_do_we_dilute_waste_water_samples_to_find_BOD_and_how_to_decide_dilution_factor

I hope this helps.

Kindly go through this research article on Grey water which may help you a lot.

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