An industrial wastewater is discharged into a river about 0.5 km downstream from
ID: 1920826 • Letter: A
Question
An industrial wastewater is discharged into a river about 0.5 km downstream from an outfall (discharge) coming from a desalination plant. Upstream of the desalination plant's outfall the river has a flow of 17.8 m3/sec and a total dissolved solids (TDS) concentration of 260 ppm. The water coming from the desalination plant's outfall flows into the river at a rate of 1800 L/sec and has a TDS concentration of 4,450 ppm. The industrial wastewater discharge has a flow of 5.0 m3/sec and a TDS concentration of 860 ppm
a) Calculate the flow in the river just downstream of the desalination plant's outfall (m3/sec).
b) Calculate the flow in the river just downstream of the industrial wastewater discharge (m3/sec).
c) Calculate the TDS in the river just downstream of the desalination plant's discharge (ppm).
d) Calculate the TDS in the river just downstream of the industrial wastewater discharge (ppm).
Explanation / Answer
a)
Flow just upstream of desalination plant = 17.8 m3/s
Flow from desalination plant = 1800 L/s = 1.8 m3/s
Flow in river just downstream of desalination plant = 17.8 + 1.8 m3/s = 19.6 m3/s
b)
Flow just upstream of industrial discharge = 19.6 m3/s
Flow from industrial discharge = 5.0 m3/s
Flow in river just downstream of industrial discharge= 19.6 + 5.0m3/s = 24.6 m3/s
c)
We can't simply add ppm upstream and from discharge to get the net ppm downstream.
ppm is a property which depends on per unit volume, hence net ppm downstream will be weighted mean of upstream ppm and discharge ppm
TDS just upstream of desalination plant= 260 ppm
TDS from desalination plant = 4450 ppm
TDS in river just downstream of desalination plant = (260 * 17.8 + 4450 * 1.8)/( 17.8 + 1.8) = 645 ppm
(d)
TDS just upstream of industrial discharge= 645 ppm
TDS from desalination plant = 860 ppm
TDS in river just downstream of desalination plant = (645 * 19.6 + 860 * 5)/( 19.6 + 5) = 689 ppm