The wastewater has a DO concentration of 4 mg/L when it is discharged. The river, just upstream of the outfall, has a DO concentration of 8 mg/L. The temperatures of the sewage and river are 15 oC; at this temperature the saturated DO concentration is 10.08 mg/L. What is the initial oxygen deficit of the mixture, just downstream of the discharge point

the initial deficit of oxygen  of the mixture just downstream of the discharge point = 2.44 mg/L

Explanation:

From the diagram ; we determine the dissolved oxygen just downstream by using the equation

where;

= volumetric flow rate of waste water = 1 m³/s

= volumetric flow rate of river just upstream of discharge point = 10 m³/s

= ultimate BOD of waste water = 4 mg/L

= ultimate BOD of mixture of the stream water and waste water = 8 mg/L

∴  DO =

DO = 7.64 mg/L

Now; to calculate the initial deficit of oxygen; we use the formuale

where;

= the saturated value of the dissolved oxygen

since no presence of chloride in the stream; chloride concentration = 0 mg/L

However, we obtain the saturated dissolved oxygen from Table - 5.10  " Solubility of oxygen in water at 1 atm corresponding to the temperature at 15° C and 0 mg/L concentration of chloride = 10.08 mg/L

SO,

Thus; the initial deficit of oxygen  of the mixture just downstream of the discharge point = 2.44 mg/L

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