CE311: Waste Management, Autumn 2014-2015 Tutorial Problems: Set # 3

1. An effluent sample from a dairy firm has been analyzed for bacterial content using the multiple tube fermentation test. Determine the coliform density expressed in MPN/100 mL for the sample using the Poisson equation. Determine whether there is a need of using Thomas equation and calculate result by this equation also. Compare the result obtained from these equations with the tabulated values in the Standard Methods.

Size of portion, mL Number of Positives Number of Negatives 10 4 1 1.0 5 0 0.1 3 2 0.01 2 3

[Poisson: 0.8/mL=80/100mL: Thomas: 59/100mL Standard Methods: 5-3-2: 140/100 mL=>1400/100mL. 4-5-3 Unlisted and cause of error=>Use only tabulated value. ] 2. A 1200 m long storm sewer collects storm water from a catchment area of 50 hectares where 35% area is covered by the roof (I=0.9); 20% area is covered by pavements (I=0.8); and 45% area is covered by open land (I=0.18), Determine the average I and design the diameter of the storm sewer assuming (i) Time of entry is 3 min and (ii) velocity of full flow 1.5 m/s (iii) n=0.013, s=0.013; I=run-off coefficient. [Average run-off coefficient =0.56, Q=7.2 m3/s Dia = 2.47 => Provide 2.5m dia pipe at 1:1200 slope, V=1.6m/s] 3. (a) What do you mean by hydraulically equivalent sections? (b) What are the parameters on which the cross-section of a sewer would depend? (c) Using Rational Method, calculate the peak runoff to be carried by a storm sewer line AB in the adjacent figure and check whether the pipe diameter is adequate. The drainage area is 0.2 ha and run-off coefficient is 0.8. The other relevant details are as follows: Length of the pipe line is 100 m and the level difference between A and B is 0.5 m. Time of entry at point A is 2 minute Pipe diameter is 100 mm and n = 0.015 Rainfall duration and rainfall intensity is as below.

Rainfall Duration (minute)

Rainfall Intensity (mm/hr)

2 122

2.5 108

3.0 97

3.5 89

4.0 82

4.5 77

[V=0.4m/s, tc=6.2 min, extrapolated I at 6.2 min = 60mm/hr => Q = 0.027m3/s => V=3.4m/s=>Use next available diameter at 150mm.]

B

A

4. Calculate the requisite size of a combined circular sewer serving an area of 200 ha with a population of 100000 and supplied with water at 200 lpcd. Take time of entry and time of flow of rainwater to be 5 and 15 minutes, respectively. The run-off coefficient is 0.45. Permissible maximum velocity in the sewer is 3 m/sec. Calculate pipe diameters for separate sewers also. [tc=20min, I = 84mm/hr, Qstorm=21m3/s, Qsewage=0.2 m3/s; at 2m/s A = 10.6m2=>Dia for combined sewer =3.67m=>Provide 4m dia or equivalent sewer. Separate: 4m dia + 350mm dia for sewage] 5. Given the drainage area in figure, calculate the discharge at the outfall using the rational method. Assume flow velocity at 0.9 m/s and I mm/hr=3276.5/(t+19), t is in min.

[tf=7.4min, tc=14.4min, I=98.1mm/hr, Q=0.93m3/s] 6. Calculate the size of outfall sewer below MH1 needed to serve the drainage area in figure given below. Each area has an inlet time of 10 min. The coefficient of runoff for the two housing areas is 0.45, and that for the park is 0.15. The distance between manholes is 180 m, and all pipes are set on a slope of 0.0020. I mm/hr=3276.5/(t+19), t is in min. (1 acre = 0.4047 ha).

R=0.5, A=1 ha, ti=7min R=0.4, A=2ha, ti=5min L=200m R=0.7, A=3ha, ti=4min L=200m

Assume 1m/s velocity, tf=6min, tc=16min, I =93.6 mm/hr, Q=0.473 m3/s, => 0.78m dia at outlet

=>Provide 800mm dia =>V=1.17m/s at s=0.002. OK