Wt is the bod(biological oxigen demand) for fresh water?
Most pristine rivers will have a BOD below 1 mg/L. Moderately polluted rivers may have a BOD value in the range of 2 to 8 mg/L. Municipal sewage that is efficiently treated by a three-stage process would possess a value of about 20 mg/L or less. Untreated sewage varies, but averages around 600 mg/L