Treatment of Aqueous Emissions

This equation indicates that every molecule of urea requires 9/2 molecules of oxygen for complete oxidation. The oxygen required for the reactions depletes the receiving water of oxygen, causing the death of aquatic life. 

The amount of oxygen used in the degradation process is called the biochemical oxygen demand (BOD). A standard test has been devised to measure BOD in which the oxygen utilized by microorganisms in contact with the wastewater over a 5-day period at 20°C is measured. 

Another test is the total oxygen demand (TOD) test, which oxidizes the waste in the presence of a catalyst at 900°C in a stream of air. Under these harsh conditions, all the carbon is oxidized to CO2. The oxygen demand is calculated from the difference in oxygen content of the air before and after oxidation. The resulting value of TOD 

The process is designed from a knowledge of physical concentrations, whereas aqueous effluent treatment systems are designed from a knowledge of BOD and COD. Thus we need to somehow establish the relationship between BOD, COD, and the concentration of waste streams leaving the process. Without measurements, relationships can only be established approximately. The relationship between BOD and COD is not easy to establish, since different materials will oxidize at different rates. To compound the problem, many wastes contain complex mixtures of oxidizable materials, perhaps together with chemicals that inhibit the oxidation reactions. 

If the composition of the waste stream is known, then the theoretical oxygen demand can be calculated from the appropriate stoichiometric equations. As a first level of approximation, we can assume that this theoretical oxygen demand would be equal to the COD. Then, experience with domestic sewage indicates that the average ratio of COD to BOD will be on the order 1.5 to 2. The following example will help to clarify these relationships. 

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