Question

N a bod test using 5% dilution of the sample (15 ml of sample and 285 ml of dilution water ) , dissolved oxygen values for the sample and dilution water blank bottles after five days incubation at 20oc were 3.80 and 8.80 mg / l , respectively . Dissolved oxygen originally present in the undiluted sample was 0.80 mg / l . The 5 - day 20oc bod of the sample is (i) 116 mg / l (b) 108 mg / l (c) 100 mg / l (d) 92 mg / l

Answer 1

Dilution is needed to prevent the depletion of entire oxygen present in in BOD bottle in less than incubation period of 5 days. For a valid BOD test APHA manual specifies some min DO of 1 mg/L to be present in the bottle after end of 5 days and DO consumed should be at least 2mg/L during incubation. Suppose saturation DO is 8 mg/l and expected BOD is 400 mg/L then, volume of sample to be added in 300 ml BOD Bottle has the following range:

minimum ml sample = [(8 mg/L 6 mg/L) x 300 mL]/400 mg/L=1.5 ml

maximum ml sample = [(8 mg/L 1 mg/L) x 300 mL]/400 mg/L=5.25 ml

Also, excess dilution reduces precision.