Thank you Imre for your answer. I am very sorry because I have not given all the data. I will edit my post now. The COD is 5000-12000 mg/L. Yes I believe the sampling approach you suggested is very essential. But the company only gave me the data which  I have mentioned all in my post.

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Thank you Imre for your answer. I am very sorry because I have not given all the data. I will edit my post now. The COD is 5000-12000 mg/L. Yes I believe the sampling approach you suggested is very essential. But the company only gave me the data which  I have mentioned all in my post.