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EXAMPLE: The BOD5 of an effluent from a municipal sewage treatment plants is 25mg/L and the effluent discharge is 4 ML/d. the receiving stream has a BOD5 of 2 mg/L and the stream flow is 40 ML/d. compute the combined 5-day BOD in the stream just below the mixing zone.

SOLUTION: Applying cd = (csQs + cwQw)/(Qs +Qw)

cd = (2 X 40 + 25 X 4)/ 40 X 4 = 180/44 = 4.1mg/L

Where cd represent the diluted BOD5 in the combined flow

EXAMPLE: A river has a dry-weather discharge of 100 cfs and a temperature of 25oC. Compute the maximum discharge of cooling water at 65oC that can be discharged from a power plant into the stream. Assume the legal limit on the temperature increase in stream is 2oC

SOLUTION: The maximum allowable stream temperature is 25 + 2 = 27oC

Applying mass balance equation cd = (csQs + cwQw)/(Qs +Qw)

27 = (25 X 100 + 65 X Qw)/100 + Qw

200 = 38Qw : Qw = 5.3 cfs

The discharge of warm water cannot be exceed 5.3 ft3/s if the stream temperature is not to increase more than 2oC

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