Chapter 4, Problem 4.44P

Effluents from metal-finishing plants have the potential of discharging undesirable quantities of metals, such as cadmium, nickel, lead, manganese, and chromium, in forms that are detrimental to water and air quality. A local metal-finishing plant has identified a wastewater stream that contains 5.15 wt% chromium (Cr) and devised the following approach to lowering risk and recovering the valuable metal. The wastewater stream is fed to a treatment unit that removes 95% of the chromium in the feed and recycles it to

the plant. The residual liquid stream leaving the treatment unit is sent to a w aste lagoon. The treatment unit has a maximum capacity of4500 kg wastewater/h. If wastewater leaves the finishing plant at a rate higher than the capacity of the treatment unit, the excess (anything above 4500 kg/h) bypasses the unit and combines with the residual liquid leaving the unit, and the combined stream goes to the waste lagoon.

		nijlkg Cr/h) [no water]95C< of Cr in feed Io unit
Wastewater
m|(kg/h)0.0515 kg Ci/kg	ms
		mj nifttkg/h)

rajtkg/h)To waste lagoon

1. Without assuming a basis of calculation, draw and label a flowchart of the process.

Wastewater leaves the finishing plant at a rate fill = 6000 kg/h. Calculate the flow' rate of liquid to the waste lagoon. /n₆(kg/h), and the mass fraction of Cr in this liquid, j&(kg Cr/kg).

Calculate the flow rate of liquid to the waste lagoon and the mass fraction of Cr in this liquid for wi| varying from 1000kg/h to 10,000 kg/h in 1000 kg/h increments. Generate a plot of.t6 versus mt. (Suggestion: Use a spreadsheet for these calculations.)

The company has hired you as a consultant to help them determine whether or not to add capacity to the treatment unit to increase the recovery of chromium. What w ould you need to know to make this determination?

What concerns might need to be addressed regarding the waste lagoon?