Non Linear Depletions
This is a clear case of toxicity. If it is required to perform BOD on these samples, then you must use the lowest possible dilution in order have a semi-accurate BOD. If toxicity is a recurring problem, it might be necessary to run COD instead on these types of samples. Try to explain this to your regulator, and ask to be given an exemption for this particular sample, or at least be allowed to run COD instead.