BOD of water is expressed in ppm of Oxygen. BOD of 100 kg  water sample which requires  490 mg of acidified potassium dichromate to oxidize all the microorganisms will be

GEW of K2Cr2O7/H+=49g; GEW of O2=8g; 49g of  K2Cr2O7/H+=8g of O2;  49mg of  K2Cr2O7/H+=8mg of O2; 105g of water-80 mg of O2 106g of water-800mg of O2  BOD of water =0.8g=0.8ppm