– N = 8
1. Water is flowing with a velocity of 0.5 m/s in a pipeline having uniform cross sectional area
and a length of (90+N) km, calculate the water retention time in the pipe line. (4 Points)
2. (a) If (1.5+N*0.1) mg/L chlorine concentration is maintained for 1 hour, then what is the
CT value? (b) If one has to reduce the time of exposure to (25+N) min, what should be the
chlorine concentration to achieve the same CT value? (c) If you have 105
organisms per mL
and if you achieve 3 log removal, how many organisms will remain in water? (9 Points)
3. There are two different suspended particles with settling velocities of 0.1 and 0.2 mm/s.
The particles are present at 60:40 ratio with an overflow rate of (15+N*0.2) m/d. What is
the overall removal efficiency of the system? (6 Points)
4. Determine the volume of a flocculation tank and the required power input using the
following parameters: Water retention time is 15 minutes, water flow rate is (110+N*2)
litre/sec, water temperature is 200 C. Assume the tank has equal length, width and depth. (5
Points)
5. If the settling velocity of a particle is 2 mm/sec and the overflow rate of a horizontal flow
sedimentation tank is (0.3+N*0.1) cm/sec, what percent of the particles are retained in
water? (3 Points)
6. If water contains (15+N*0.4) gm of carbon, 5 gm of nitrogen and 2 gm of phosphorus,
which one is the limiting nutrient and why? How much of other two nutrients (not limiting)
will be the leftover after utilizing the limiting nutrient? (6 Points)
7. Consider a lake that is fed by mix of two streams: one stream has a flow rate of N m3
/s and
BOD concentration of 10 mg/L. The other is a sewage outfall that discharges 0.6 m3
/s and a
BOD concentration of 40 mg/L. Assuming the pollution is completely mixed in the stream,
find the resulting BOD concentration. (3 Points)
2
8. A wastewater has a dissolved oxygen concentration of 2.0 mg/L and a discharge rate of 1.1
m3
/s. The river into which it is being discharged has DO equal to 8.3 mg/L, a flow rate of
(8+N*0.1) m3
/s and a temperature of 200C. Assuming complete and instantaneous mixing,
estimate the initial dissolved oxygen deficit of the mixture of wastewater and river water
just downstream from the discharge point. The saturated valued of DO at 200C is 9.1 mg/L.
(4 Points)
9. A complete mixed activated sludge process aeration tank treats 5 MLD sewage having the
influent and effluent soluble BOD concentrations of 220 and 20 mg/L respectively. The
volume of the aeration tank is (500+N*10) m3
and the mean cell residence time is 10 days.
MLVSS in aeration tank is 3,400 mg/L, MLVSS/MLSS = 0.8 and sludge wastage rate is 1
m3
/hour. Calculate the concentration of return sludge suspended solids. (6 Points)
10. The 5 day BOD of a sample of water has been measured as (70+N) mg/L. The reaction
rate constant k is 0.15/day. What is the ultimate BOD of this sample? (5 Points)
11. A city has a population of (140,000+N*10), where the average solid waste generation rate
is 1.25 kg/capita/day. The average uncompacted density of the solid waste is 145 kg/m3
and the truck compactor density rating is 480 kg/m3
. What volume of compacted solid
waste could be collected by the trucks from the city in 7 days? (4 Points)
12. Consider the case where a noise level of (92+N*1) dBA exists for 4 minutes and is
followed by a reduced noise level of 65 dBA and 40 dBA for 24 minutes and 12 minutes
respectively. What is the equivalent continuous equal energy level for the 40-minute
period? Assume a 2-minute sampling interval. (5 Points)
