Chapter 5 - Surface Water
1. A liter water sample collected for analysis accidentally has an insect weighing 0.1 g trapped in the bottle. The initial DO is 10 mg/L. Assume that 10% of the insect's fresh weight is biodegradable and has the formula CH2O. Assume microbes are present to decompose. If the laboratory analyzes the sample after decomposition, what DO will be measured?

2. 5 day BOD test: A 15.0-ml sample of sewage is mixed with water filling 300-ml bottle. The mixed sample has DOi =
8.0 mg/L. When measured at 5 days, DO5 = 5.0 mg/L. What is BOD5?

3. A standard five-day BOD test is run using a mix consisting of four parts distilled water and one part wastewater. The initial DO of the mix is 9.0 mg/L and the DO after five days is 1.0 mg/L. What is BOD5?

4. The following data have been obtained in a BOD test that is made to determine how well a wastewater treatment plant is operating. What percentage of the BOD is being removed by this treatment plant? If this is a secondary treatment plant that is supposed to remove 90 % of the BOD, is the plant operating properly?
DOi (mg/L) DOf (mg/L) Volume of wastewater (mL) Volume of dilution water (mL)
Untreated sewage 7.0 1.5 4 296
Treated sewage 8.0 3.2 12 288

5. The dilution factor, P, for unseeded waste and water = 0.030 and the DOi = 9.0 mg/L. At 5 days, DO5 = 3.0 mg/L. The reaction constant is kd = 0.22/day What is BOD5? What is the ultimate CBOD? What is the remaining oxygen demand after 5 days?

6. If the BOD5 for a wastewater is 200 mg/L and the ultimate BOD is 300 mg/L, what is the reaction rate constant k?

7. Assume a sample of domestic wastewater with 30 mg/L of nitrogen as TKN. Assume very few new cells of bacteria are formed during nitrification of waste and NBOD can be found by stochiometric analysis. Find nitrogenous oxygen demand.

8. A wastewater treatment plant discharges 1.1 m3/s of treated effluent with a BOD = 50.0 mg/L into stream . The stream has a Q = 8.70 m3/s and BOD = 6.0 mg/L. The deoxygenation constant, kd = 0.20/day. Estimate ultimate carbonaceous BOD of river just downstream from outfall. Assume CSTR.

9. A stream has a DO of 9 mg/L, an ultimate biological oxygen demand of 12 mg/L and an average flow of 0.2 m3/s. Industrial waste with 0.0 DO and an ultimate biological oxygen demand of 20,000 mg/L and a flow rate of 0.007 m3/s is discharged into the stream. What is the ultimate biological oxygen demand immediately below the point of discharge? What is the dissolved oxygen in the stream immediately below the point of discharge?

10. A wastewater discharge has DO of 2.0 mg/L and discharge of 1.10 m3/s. The river, into which it is discharged, has DO of
8.3 mg/L and flow rate of 8.70 m3/s with a temperature of 20 C. Assume CSTR. Find initial dissolved oxygen deficit, Do, of mixture just downstream from the point source.

11. A stream has a reoxygenation constant of 0.35/day and a velocity of 0..42 m/s. At the point at which an organic pollutant is discharged, the stream is completely saturated with oxygen at 10 mg/L (Do =0). Below the waste outfall, the ultimate biological oxygen demand is 25 mg/L and the deoxygenation constant is 0.18/day. What is the dissolved oxygen 75 km downstream? (assume that the volume of the wastewater is very small compared to the main stream flow)

12. A city of 210,000 people deposits 4 m3/s of sewage having a BOD of 28 mg/L and a DO of 1.8 mg/L into a stream that has a discharge of 30 m3/s and a flow velocity of 5.0 m/s. Just upstream of the release point, the river has a BOD of 3.6 mg/L and a DO of 7.6 mg/L The saturation value of DO is 8.5 mg/L The deoxygenation coefficient is kd= 0.61/day and the reaeration coefficient kr = 0.76/day. Assume CSTR conditions.

a. What are the DOi and the ultimate BOD just downstream of the effluent outfall?
b. What is the time and distance to reach the minimum DO (use Eq. 5.32 in your text to find the critical time)?

13. A phosphorus-limited lake has a surface area of 8 x 107 m2. It is fed by a 15.0 m3/s stream that has a phosphorus concentration of 10.0 mg/m3. Effluent from a point source adds 1 g/s of phosphorus. The phosphorus-settling rate is estimated at 10 m/yr. What is the average lake phosphorus concentration?

14. Consider a lake with 1 x 108 m2 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 m3/s and its phosphorus concentration is to be determined by you. The lake is also fed by a stream with discharge of 10 m3/s and a phosphorus concentration of 0.005 mg/L. If the phosphorus settling rate is 15 m/yr, what is the average phosphorus concentration in the lake? What should be the maximum phosphorus concentration of effluent from the treatment plant in order to keep the average lake concentration below 0.020 mg/L? Assume the lake has an outlet.