2.2. Generalized Mass Balance

Try these first

1. 1.

   What does mass balance account for in a system?

   Hint

   Think about input, output, and change in the system.

2. 2.

   What is meant by unsteady state?

   Hint

   Consider how concentrations evolve in a flowing water system.

3. 3.

   What does the mass balance equation help to determine?

   • The rate of pollution generation
   • Input vs. output in a system
   • Total production of a chemical

   Hint

   This equation balances in terms of input and output.

4. 4.

   True or False: In a steady state, the concentration of pollutants changes over time.

   • True
   • False

   Hint

   Think about what steady means in terms of flow and concentration.

5. 5.

   Consider a lake with an inflow of 500 L/day and an outflow of 300 L/day. If a pollutant is generated at 100 mg/day and lost at 30 mg/day due to evaporation, calculate the change in concentration over a day assuming the lake's volume is constant.

   Hint

   Use the mass balance equation and factor the pollutant generated and lost.

6. 6.

   If a segment of river initially has a concentration of 200 mg/L and receives an additional inflow of 100 mg/L from tributaries, calculate the new concentration assuming the flow is steady.

   Hint

   Consider both the main river flow and tributary contributions for average concentration.