Oxygen Depletion by Organic Waste

Q.
Upstream of a sewage outfall, a river contains 7mg/liter Dissolved Oxygen. Some distance downstream of the outfall, however, dissolved oxygen has been diminished to 4mg/L due to organic waste decomposition by microbes. What is the approximate amount of organic matter (CH2O) that must have been degraded to account for the this consumption of dissolved oxygen? Hint: Oxidation of organic material by organisms during respiration given by: CH2O + O2 -> CO2 + H2O Not sure how to set up this problem or what formula to use.
A.
1 mole of CH2O = 30 grams
1 mole of O2 = 32 grams
The mass ratio of CH2O to O2 is 30/32 or 0.94 gram organic matter / 1.00 gram O2. The ratio of 0.94 / 1.00 does not depend on the units used. It could be milligrams, grams, etc. The change in DO is 7mg/L – 4mg/L = 3mg/L dissolved O2.
To find the milligrams of organic matter degraded:
(3mg/L dissolved O2)(0.94g organic matter / 1 g O2) = 2.82g organic matter / liter reacted with dissolved oxygen.
[Please note that the milligrams of O2 used and of organic matter degraded are about the same: 3mg and 2.8mg.]
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