Problems Mass Balance
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Transcript of Problems Mass Balance
Department of Civil and Environmental Engineering CVNG 3007 Environmental Engineering I Worksheet 1
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1. (Q3, Incourse test, 2011) A city discharges 7.2 MLD of wastewater with a BOD5 of 295 mg/L into a stream. Answer the following questions, each being independent of the other.
a. If the BOD5 of the stream is zero, what is the BOD5 of the diluted wastewater after discharge if the streamflow is 0.15 m3/s?
b. What stream discharge is required to reduce the BOD5 of the diluted wastewater to 40 mg/L?
c. To what value must the BOD5 of the wastewater be reduced by treatment so that the BOD5 of the diluted wastewater is 40 mg/L if the streamflow is 0.06 m3/s?
2. (Example 1.6, Masters, pg 15). A bar with volume 500 m3 has 50 smokers in it, each smoking 2
cigarettes per hour. An individual cigarette emits, among other things, about 1.4 mg of formaldehyde (HCHO). Formaldehyde converts to carbon dioxide with a reaction rate coefficient k of 0.40/h. Fresh air enters the bar at the rate of 1000 m3/h, and stale air leaves at the same rate. Assuming complete mixing, estimate the steady‐state concentration of formaldehyde in the air. At 25° C and 1 atm of pressure, how does the result compare with the threshold for eye irritation of 0.05 ppm?
3. Show that the formula for the fraction removal of a pollutant undergoing first‐order decay in
two identical CFSTRs, connected in series, is: , where c0 and c2 are the influent
and effluent concentrations, k is the decay coefficient and tCFSTR, the nominal hydraulic retention time in each reactor. Hence show that for n identical CSTRs in series, the resulting fraction of removal is given by:
4. (Q2, Incourse test, 2011) The concentration of a particular reactant is reduced from an initial
concentration, c0, of 200 mg/L to a final concentration, ce, of 15 mg/L as the liquid flow containing the reactant passes through two identical CFSTRs connected in series. If the reaction in the reactor follows first‐order kinetics and the liquid flow rate is 0.025 million litres per day (MLD), what is the volume of each reactor? Assume that the reaction rate constant has a value of 0.4 d‐1. Recall that the fraction removal achieved in one CFSTR, with a mean hydraulic retention time, t, is given as:
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5. (Example 1.7 Masters, pg 16) Every year, herons, seagulls, eagles, and other birds mass along a
4.75 km stretch of stream connection a lake to the ocean to catch the fingerling salmon as they migrate downstream to the sea. The birds are efficient fishermen and will consume 10,000 fingerlings per kilometre of stream each hour regardless of the number of the salmon in the stream. In other words, there are enough salmon; the birds are only limited by how fast they can catch and eat the fish. The stream’s average cross‐sectional area is 20 m2, and the salmon
Department of Civil and Environmental Engineering CVNG 3007 Environmental Engineering I Worksheet 1
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move downstream with the stream’s flow rate of 700 m3/min. If there are 7 fingerlings per m3 in the water entering the stream, what is the concentration of salmon that reach the ocean when the birds are feeding?
6. A wastewater treatment plant must disinfect its effluent before discharging the wastewater to a
near‐by stream. The wastewater contains 4.5 x 105 faecal coliform colony forming units (CFU) per litre. The maximum permissible faecal coliform concentration that may be discharged is 2,000 faecal coliform CFU/L. It is proposed that a pipe carrying the wastewater be used for the disinfection process. Determine the length of pipe required if the linear velocity of the wastewater in the pipe is 0.75 m/s. Assume that the pipe behaves as a steady‐stage plug‐flow system and that the reaction rate constant for destruction of the faecal coliform is 0.23 min‐1.
7. (Example 1.9 Masters, pg 20) Consider the 10 x 106 m3 lake analyzed in Example 1.5, which,
under conditions given, was found to have a steady‐state pollution concentration of 3.5 mg/L. The pollution is non‐conservative with reaction‐rate constant kd of 0.20/d. Suppose the condition of the lake is deemed unacceptable. To solve the problem, it is decided to completely divert the sewage outfall from the lake, eliminating it as a source of pollution. The incoming stream still has flow Qs of 5.0 m
3/s and concentration of 10 mg/L. With the sewage outfall removed, the outgoing flow Q is also 5.0 m3/s. Assuming complete‐mix conditions, find the concentration of pollutant in the lake one week after the diversion, and find the new final steady‐state concentration.