A comment was made to me recently that water does not weigh 8.34 pounds per gallon (3.78 kilograms per gallon) and that the unit of 8.34 was the result of a completely unrelated (to the weight of water) derivation. Actually, I think I understood the confusion so I thought I would go to an excellent source and simply quote (almost verbatim) the following information from Hammer and Hammer. This should provide you with all you ever wanted to know, and more, about the use of 8.34 in water and wastewater treatment.
Loadings on treatment units are often expressed in terms of pounds of BOD per day or pounds of solids per day, as well as quantity of flow per day. The relationship between the parameters of concentration and flow is based on the following conversion factors: 1.0 mg/L, which is the same as 1.0 part per million parts by weight, equals 8.34 lb/mil gal, since 1 gallon of water weighs 8.34 lb; and used less frequently, the value 62.4 lb/mil cu ft, since 1 cu ft of water weighs 62.4 lb. These relationships are defined by the following equations:
C = BOD, SS, or other constituent, milligrams per liter (mg/L)
Q = volume of wastewater, million gallons (mil gal) or million cubic feet
Calculations in Example 9-1 show that 120 gal of the sanitary wastewater described in Table 9-2 (shown below) contain 0.20 lb of BOD and 0.24 lb of suspended solids.
Sanitary wastewater from a residential community is 120 gpcd containing 200 mg/L BOD and 240 mg/L suspended solids. Compute the pounds of BOD per capita and pounds of suspended solids per capita.
Solution, using Equation 9-1
Source: Hammer, Mark J. and Mark J. Hammer, Jr. “Water and Wastewater Technology.” 4th ed. Upper Saddle River, New Jersey. 2001. (p. 316)