Poorly treated municipal wastewater is discharged to a stream. The riv - PDF document

5 5. Poorly treated municipal wastewater is discharged to a stream. The river flow rate upstream of the discharge point is Q u/s = 8.7 m 3 /s. The discharge occurs at a flow of Q d = 0.9 m 3 /s and has a BOD concentration of 50.0 mg/L. Assum ing that the upstream BOD concentration is negligible. ( a) What is the BOD concentration just downstream of the discharge point? Solution : / 333 (0)() 0.9508.70.9 4.7 usddtotaldown down down QQCQC mmgmm C sLss mg C L       7 7. A pollutant which degrades according to the equation dC/dt = - kC is said to react according to _______ First Order ______ kinetics. 8. In a material balance problem, the situation where the rate of accumulation is zero is referred to as ________ Steady State _______. Lafayette College Department of Civil and Environmental Engineering CE 321: Introduction to Environmental Engineering Fall 20 10 SOLUTIONS - Homework # 3 - SOLUTIONS Due: Monday, 9/20/10 SOLUTIONS 1. Calculate the hydraulic residence times (the retention t ime) for Lake Superior and for Lake Erie using data in Table 4 - 3 (Mihelcic and Zimmerman) . Solution : U sing information provided in Table 4.3. 93 3 9 93 3 9 12,00010 ()180 6710 46810 ()2.6 18210 Vm forSuperioryr m Q yr m forErieyr m yr       2 2. The Rappahannock River near Warrenton, VA, has a flow rate of 3.00 m 3 · s - 1 . Tin Pot Run (a pristine stream ) discharges into the Rappahannock at a flow rate of 0.05 m 3 · s - 1 . To study mixing of the stream and river, a conservative tracer is added to Tin Pot Run. If the instruments that measure the tracer can detect a concentration of 1.0 0 mg·L - 1 , what minimum c oncentration must be achieved in Tin Pot Run so that 1.0 mg·L - 1 of the tracer can be measure after the river and stream mix? Assume that the 1.0 0 mg·L - 1 of tracer is to be measure after complete mixing of the stream and Rappahannock has been achieved and that no tracer is in Tin Pot Run or the Rappahannock above the point where the two streams mix. What mass rater (in kilogram per day) of tracer must be added to Tin Pot Run? (Answer 263.52 or 264 kg · day - 1 ) Given: Q RR = 3.00 m 3 /s, Q TPR = 0.05 m 3 /s, de tection limit = 1.0 mg/L Solution : a. Mass balance diagram (NOTE: Q out = Q RR + Q TPR = 3.05 m 3 /s) b. Mass balance equation C RR Q + C TRP Q TPR = C out Q out Since C RR in = 0 this equation reduces to: C TPR Q TPR = C out Q out (if no tracer is added to Rappahannock , the C RR would be 0) ଷ ହ ହ = 61 mg/L c. Mass Rate of Tin Pot is = C TPR Q TPR = ( 61 mg/L) 0.0 5 m 3 /s ଺ ସ ௘௖ ௗ = 264 kg/d Q OUT = 3.05 m 3 /s C OUT = 1.0 mg/L Q RR = 3.0 m 3 /s C RR Q TPR = 0.05 m 3 /s C TPR = ? Rappahannock River Tin Pot Run