Question 1. A river has a heavily-polluted discharge of wastewater from industrial area entering it at a flow rate of 0.5 m³/s as shown in Figure 1. The 5-day BOD and DO of this discharge at 18.7°C are 168 mg/L and 1.9 mg/L respectively. The river flow upstream of the discharge may be taken as 5.6 m³/sec and have a 5-day BOD of 2.4 mg/L and DO of 7.4 mg/L at 15.6°C. The river may be taken as having a width of 11m and a depth of flow of 1.5m in a rectangular channel. The following information is derived from the laboratory results and river field survey: o Deoxygenation constant (K₁) = 0.33day-¹ at 20°C (e-base) o Reaeration constant (K₂) = 0.57 day-¹ at 20°C (e-base) o K₁ and K₂ at temperature T can be changed using the following equations: (T-20) K₁,7 = K₂0 (1.135) (T K2,7 = K₂0 (1.024) (-20) 0.5 m³/s Industrial area 5.6 m³/s Mixing zone/n1. Current discharge condition: a) Calculate DOM, BODM, initial DO deficit (Do), ultimate BOD, and BOD5 loading rate (kg/day) at the mixing point. b) Calculate the distance downstream (xc) where the minimum dissolved oxygen concentration will occur and show it in the graph of DO concentration against distance downstream. c) Provide the table for DO concentration against distance downstream (DO Sag curve) including DO deficit, Dc, DOmin, Tc, and Xc for the first 220 km downstream of the mixing point. d) Construct a clearly-labelled DO Sag curve (plotted with a point and value of DOmin, Dc, and Tc) of DO concentration against distance (km) downstream from the mixing point for the first 220 km downstream of the mixing point, making clear the values of all the parameters you have used. (refer to the graph presented in Tutorial solution Q1.14) Assume there are no other discharges downstream. Dc, DOmin, Tc, and Xcare same variables used in Lecture Note and Tutorials 2. Management Scenario 1: e) Calculate the allowable ultimate BOD (mg/L), BODs concentration, and allowable BOD loading (kg/d) at the mixing zone if the minimum DO concentration must be kept at 4.5mg/L at a point where the minimum dissolved oxygen concentration will occur. f) Calculate the amount of BOD5, in kg/d, of wastewater (qwBODw) and the allowable BODs concentration of wastewater (BOD) that can be discharged to/nthe mixing zone if the minimum DO concentration must be kept at 4.5mg/L at a point where the minimum dissolved oxygen concentration will occur. g) For Management Scenario 1, provide the table for DO concentration against distance downstream (DO Sag curve) including DO deficit, Dc, DOmin, Tc, and Xc for the first 250 km downstream of the mixing point. h) For Management Scenario 1, plot a clearly-labelled DO Sag curve (plotted with a point and value of DOmin, Dc, and Tc) of DO concentration against distance (km) downstream from the mixing point for the first 220 km downstream of the mixing point if the minimum DO concentration must be kept at 4.5mg/L at a point where the minimum dissolved oxygen concentration will occur. i) Plot both the DO Sag curve for Management 1 from g), and the DO sag curve produced for current discharge condition from d) together for the comparison. (refer to the graph presented in Tutorial solution Q1.17) (Recommend the DO Sag curves presented in a separate page in landscape orientation for a good visualisation) 3. Management Scenario 2: j) Calculate the allowable ultimate BOD (mg/L), BOD5 concentration, and allowable BOD loading (kg/d) if the DO concentration must be kept at 6.5mg/L at a point 110km downstream. k) Calculate the amount of BOD5, in kg/d, of wastewater and the allowable BOD5 concentration of wastewater that can be discharged based on the current wastewater flow rate if the DO concentration must be kept at 6.5mg/L at a point 110km downstream below the mixing point M. 1) For Management Scenario 2, provide the table for DO concentration against distance downstream (DO Sag curve) including DO deficit, Dc, DOmin, Tc, and Xc for the first 220 km downstream of the mixing point. m) Plot the DO Sag curves for Management 2 from i), for Management 1 from g), and the DO sag curve produced for current discharge condition from d) together for the comparison. (Recommend the DO Sag curves presented in a separate page in landscape orientation for a good visualisation)/n4. Management Scenario 3: As shown in Figure 2, based on current discharge condition, in order to increase DO concentration in a tributary stream before the mixing zone, the engineer constructed a free- falling three-step weir at the outfall before the mixing zone. The weir height is 1.4m. The tributary stream is less than moderately polluted (refer to Tutorial Q1.18 & 1.19). 0.5m³/s Industrial area 5.6m³/s weir M Figure 2/nn) Calculate DOM, BODM, initial DO deficit, ultimate BOD, and BOD5 loading rate (kg/day) at the mixing point for Management Scenario 3 o) For Management Scenario 3, generate the table for DO concentration against distance downstream (DO Sag curve) including DO deficit, Dc, DOmin, Tc, and Xc for the first 250 km downstream of the mixing point. p) For Management Scenario 3, construct a clearly-labelled DO Sag curve (plotted with a point and value of DOmin, Dc, and Tc) of DO concentration against distance (km) downstream from the mixing point for the first 220 km downstream of the mixing point, making clear the values of all the parameters you have used. q) Plot the DO Sag curves for Management 3 from 0), the DO Sag curves for Management 2 from i), Management 1 from g), and the DO sag curve produced for current discharge condition from d) together for the comparison. (Recommend the DO Sag curves presented in a separate page in landscape orientation for a good visualisation) Evaluation and comparison of Management Scenarios r) Generate the comparison table for initial DO deficit, BODw, the amount of BOD5, in kg/d, of wastewater (qwBOD), BODM, ultimate BOD, BOD loading rate (kg/d), Dc, and DOmin for current discharge and Management Scenarios 1 - 3 s) Provide the combined table for DO concentration against distance downstream (DO Sag curve) including DO deficit, Dc, DOmin, Tc, and X. for the first 220 km downstream of the mixing point for current discharge and Management Scenarios 1-3. t) Provide the analysis and discussion in terms of effectiveness of controlling the wastewater discharge (based on BODw and the amount of BOD5, in kg/d, of wastewater) by comparing Management Scenarios with current discharge condition. The following Streeter-Phelps relationship may be assumed: Do 0₂ K.L K₂ - K₁ - ( e-K₁² - e-K ₁² ) + Do₂ е 9.00 -ky

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