or rrc
Faculty of Engineering
and Technology
Course code: CIVE
514
Course Title: Design
ofwater and wastewater structures
and distributionsystem
Date:
25th November 2022
Time
2 HOURS
Venue
BLOCK 124 room 001-002
OVERALL MARKS:
[801
The following is provided:
Examination answer book
Instructions to candidates:
I. You are given 10 Minutes reading time and you should not urite anything duringthis
time.
2. Answer all the 3 questions
3. Cell phones or any other communicationsdevice are strictly prohibited.
4. You are allowed to bring a non-programmablescientific calculator.
5. All answers should be written in the Answer Book(s) provided.
6. Show enough steps in calculations tojustify your answer.
7. All rough work must be written in the answer book(s)provided.
not part of the work
8. A line should be drawn through any rough work to indicate that it is
to be graded.
answers.
9. A black or blue pen must be used for written
There are 10 pages including this one
Question 1
treatment facility. Below is the data collected for the
new water
a) A city is planning for a
purpose:
Design population: 120 000
170 L/cap.day inclusive of other demands.
Average daily demand:
daily demand
Maximum day demand: 1.5 average
2
g = 1.139 x 10 -3N.s/m
dimensions and the power requirement of the
Design a rapid mixer unit by calculating the
O (10)
mixer at water temperature of 1O C:
o Number of unito
o Detention time: 60 seconds
o Velocity gradient: 800/s
Tank configuration: Square in plan with depth = 1.35 times its width
b) A cross flow, horizontalshaft,paddle-wheeledflocculation basin is to be designedfor
a flow of 3060 m3/d. (15)
A mean velocity gradient0 37/s at ICC), and a detention time f 40 min.
The GTvalue should be from 50 000 to 100.000.
The tapered flocculation is to be provided, and three compartments Sqyare in profile of
equal depth in series are to be used.
Width of Flocculator must be 15 m so as to adjoin the settling basin
The G values determined from laboratory tests for the three compartments are 70/s
G2 = 30/s and G3 = 10/s
g = 1.139 x 10 -3N.s/m2
i) Determine the GT value
The basin dimensions
iii) The power imparted into each section (5)
c) Design slow sand filter units given the following data:
Present population 9000
Population growth rate —7% per annum
Design period 20-years
Average demand 80 1/cap.d
Maximum day demand 2.0 x average day demand
Filters to be operated for.24 hous
3 2
Filtration rate 6.0 m /m .d and with one filter
out of operation for cleaning, the
filtration rate in the operating units should
not exceed 8 m3m2.d.
Determine the number of units and the
sizes of each unit (10)
Length to width ratio is 1.5
Question 2 1251
Completely mixed high rate activated sludge plant is to treat 14000 rn3/d of industrial
(Figure 1.0 below). 1lie primary effluent going to the activated sludge reactor has
a BODSof 1200 mg/L that must be reduced to 140 mg/L prior to discharge to a municipal
sewer. Pilot plant analysis gave the following results:
• Mean cell residence time = 5 d
MLSS concentration in reactor = 6000 mg/L VSS, Y=0.5 kg/kg, kd = 0.03/d
Determine:
i) The hydraulic retention time
ii) Volume of the activated sludge reactor
iii)The volumetric loading rate in kg BOD5/m3.d to the reactor
iv) F/M ratio in the reactor [3]
v) The mass of solids wasted each day, at an underflow solids concentration Xu = 12 000
mg/L [4]
vi) The volume of solids wasted each day at an underflow solids concentration Xu = 12
000 mg/L [3]
vii) The sludge recirculation ratio
viii) The volume of solids that must be wasted each day, if the solids are wasted
directly from activated sludge reactor instead of from the underflow
[2]
System Boundary
Aeration tank Secondary
clarifier i
Primary effluent. I Effluent
Q, Xo, So
v,x,s i
Sludge!
return 1 Sludge I
Xu, S
i underflow I Qu,
i
Sludge I
waste
Figure 1: Activated sludge process
3
Question 3
Determine the size of a two-stage trickling filter using the NRC equations. Assume both
filters have the same efficiency of BOD5removal and the same recirculation ratio of 75%and
rl = r2= 2.0 respectively.Only the size of the first stage filter should be
determined.
Wastewater tem erature 250c
m3/d 5 000
BOD5 m
Influent 200
Desi effluent BOD5 m [L 25
D th of each filter 2.5 m
Recirculation ratios for both filters 2
