A printed circuit board manufacturing plant is discharging the chemical 1,1,1-tr

Question

A printed circuit board manufacturing plant is discharging the chemical 1,1,1-trichloroethane (TCA) into a river that subsequently flows into a lake, the source of drinking water for a small town. The levels of TCA in this lake fail to meet drinking water objectives of 10 mu g/L. As an environmental specialist with the state department of environmental protection, you have been asked to help determine discharge requirements for the manufacturing plant. By assuming that the lake is a well-mixed system, that concentrations are at steady state, and that the only processes acting on this chemical in the lake are volatilization and biodegradation. How high can the influent concentration of TCA (Cin) be at the point where the river enters the lake? Some pertinent data are: TCA volatilization rate = 3 mu g/(m2.s); TCA biodegradation rate = 9.2 Time 10-3 mu g/(m3.s); River inflow to lake = 12.6 L/s; River outflow from lake = 12.6 L/s; Lake volume = 5.7 Times 106 L; Average lake depth = 3 m.

Explanation / Answer

lake volume = 5.7X106 liters=5.7*106*001m3 =5700 m3'

depth= 3m

area of lake = 5700/3 =1900 m2/s

Let the concentration of TCA in =x ug/l

Rate of mass in =x*12.6 ug/s

Rate of mass out =12.6*10ug/l ( out let concentration)

Rate of mass out due to Volatilization = 3ug/m2s* 1900=5700 ug/s

Rate of mass out dur to biodegradation = 9.2X10-3ug/m3.s*5700m3=52.44ug/s

At steady state Rate of mass in =Rate of mass out ( due to out let + volatilization and biodegradation)

x*12.6 = 12.6*10+ 5700+ 52.44=126+5744+52.44=5922.44

x= 5922.44/12.6=470.03 ug/l ( inlet concentration)

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