Question

A manufacture in Ontario has been fined because it has been releasing 5 L/s effluent having zinc concentration of 0.1 mg/L into a river. Upstream of the factory, the stream water rate is 100 L/s with zinc concentration of approximately zero. The factory has been forced to reduce the zinc concentration of the effluent to below 20 μg/L. The engineer that is an employee of the factory recommends that they divert parts of the stream into the site and dilute the effluent to bring its zinc concentration to the required level. a) Calculate the present concentration of zinc several miles downstream of the plant where the zinc is diluted in stream completely. b) Compute the amount of water needed to be diverted to the site to achieve the required zinc concentration of effluent. c) Determine the concentration of zinc downstream of the plant where the zinc is diluted in stream completely if the engineer’s plan is put into operation.

Answer 1

a) The effluent is 5L/s x 0.1 mg/L = 0.5 mg/s.

The stream flow is 100 L/s,

so the down stream concentration is 0.5mg/s / 100 L/s =

0.005 mg/L or 5 ug/L

b) 0.1 mg/L x 5 L/s = 0.020 ml/L x V L/s =

V = 25L/s,

but that is a combined flow,

need to add 20 L/s

c) The water comes out of the stream and back into it, so since there is no reduction of mass going into the stream, nor is there an increase in the volume of water in the stream, the down stream concentration will not change.