1. A well mixed lake of 107 m" of volume is loaded with 300 kg of N-NH4+.

- How much oxygen is consumed to oxidize completely this load?

- How much time is needed if the water temperature is 15°C?

- Suppose that the lake water is initially oxygen saturated, which oxygen concentration will be reached at the end of the oxidation process?

2. A shallow lake with a surface of 10h nr. an average depth of 2 m, an affluent inflow of 3 m/s and a initial value of the phosphorus concentration of 0.1 mg/1, is loaded with 100 kg/y of phosphorus.

- What will be the concentration at the steady state condition?

- Which is the order of magnitude of the phosphorus settled in the lake?

3. BOD, is 25 mg/1 at 25°C. Find the BOD, at 20°C.

4. The reaeration ratio of a river is 0.8 (1/day) at 15°C. Find the rate at 20°C.

5. A municipal waste water treatment discharges secondary effluent to a surface stream. The waste water has a flow of 100 1/s, a BOD, concentration of 30 mg/1 at 20°C, an O, concentration of 2 mg/1 and a temperature of 25°C. The stream has a summer minimum flow of 1 mVs, BOD, of 3 mg/1 a temperature of 22°C and an oxygen saturation concentration. Complete mixing is almost instantaneous. The velocity of river water is 0.2 m/s, and the depth of 0.8 m.

- Find the critical oxygen concentration and the distance from the treatment plant where the situation is most critical.

- Suppose a winter condition and evaluate the effect in this condition.

6. Consider a completely mixed shallow lake with an inflow of 401/s, an average depth of 3 m and an area of 150,000 nr. The average wind speed on the area is approximately 5 m/s. The inflow water is characterized by an oxygen concentration of 8 mg/1 and no BOD. The lake is impacted by a waste water discharge that produces 120 kg/day of BOD. The bottom of the lake is sandy and the Secchi depth is 2.25 m. The fraction of daylight is 0.5 while ¡ for dominant species of phytoplankton is 2 1 /day. Due to the nutrient limitation ¿u. can be estimated to 1 1/day. by using the model portrayed in Section 3C.3. The chlorophyll-a concentration is found to be 20/u.g/l on average for the period considered. The O./chl-a ratio a, is estimated to be 0.2. A value of A, = 0.21/day can be used. Assuming a temperature of T - 20°C determine the BOD, and the oxygen concentration in the lake.

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