Consider a lake with 100×10 

6

  m 

2

  of surface area for which the only source of phosphorus is the effluent from a wastewater treatment plant. The effluent flow rate is 0.4 m 

3

 /s, and its phosphorus concentration is 10.0mg/L(10.0 g/m 

3

 ). The lake is also fed by a stream having 20 m 

3

 /s of flow with no phosphorus. If the phosphorus settling rate is estimated to be 10 m/yr, estimate the average phosphorus concentration in the lake. What level of phosphorus removal at the treatment plant would be required to keep the average lake concentration below 0.010mg/L ?