UNIVERSITY OF NAIROBI
FIFTH YEAR EXAMINATIONS FOR BACHELOR OF SCIENCE
IN ENVIRONMENTAL AND BIOSYSTEMS ENGINEERING
FEB 517: DESIGN OF WASTE MANAGEMENT SYSTEMS
DATE 27 October 2009
TIME 8.30 to 10.30 (2
hours) …………………………………………………………………………………
……………
INSTRUCTIONS
i. Attempt any 4 questions.
ii. You are allowed up to 5 minutes to read the question paper before the examination starts.
iii. Use of notes or text books is not permitted in the examination room.
iv.
Provide brief but concise answers.
v. Credit will be given for explaining your answers, reasoning and calculations.
vi.
Candidates will require a scientific calculator.
vii.
You may make appropriate assumptions where the data given is inadequate. State and
justify any such assumptions made.
viii. Marks for each correct answer are given. The marks are in percentage. All questions
carry equal marks.
ix. Candidates are advised to spend 30 minutes on every question.
x. Candidates are required to sign the examination attendance register.
xi. All answers are to be given in the answer books provided. Record your registration
number on the cover of the answer book. Check to ensure your registration number is
correct and legible. If any doubts exist you may record the number twice on the cover
page. Record your registration number on every extra answer book or extension sheet you
may use.
xii. Candidates will not be allowed to leave the examination room in the first and last 15
minutes.
xiii. Direct any questions you may have regarding the conduct of the examination to the
invigilators present.
xiv. Good luck!
QUESTION ONE
Noise is measured on a logarithmic scale called a Decibel. The reference noise power level
and noise pressure level for a Decibel are 10 -12 Watts and 10 -6 Pascals, respectively.
a. Briefly discuss the impact on people of the following products of environmental noise:
 Loudness


Hearing damage
Speech interference
(3 marks)
b. With the aid of mathematical expressions explain the meaning of the following terms:
 Noise pressure level
 Noise power level
 Noise intensity
(3 marks)
c. If two noise sources of equal pressure of 2 000 micro-Pascals each are combined, what
will be the resulting noise pressure level in dB?
(4 marks)
d. The average value of recorded noise pressure levels at a given place cannot be calculated
by computing a simple arithmetic mean of the Decibels . Provide a methodology for
computing the average noise pressure level dB.
(3 marks)
e. Explain the meaning of the term equivalent continuous noise level, Leq.
(3 marks)
f. Four noise pressure level readings were recorded for a room as 40, 50, 62 and 72 dB.
Calculate the average noise pressure level for the room.
(4 marks)
g. Using the same data in question 1(f) above compute the equivalent continuous equal
noise energy level (L eq )
(5 marks)
QUESTION TWO
After 3 days the BOD of a sample of sewage is 150 mg/l at a temperature of 15 degrees
Centigrade. K (20) = 0.1.
a. Calculate K (15).
(6 marks)
b. Estimate the sewage 3-day BOD at a temperature of 20 degrees Centigrade.
(6 marks)
c. Estimate the sewage 5-day BOD at a temperature of 25 degrees Centigrade.
(6 marks)
d. Explain the importance of the following operations in the determination of BOD in the
laboratory:



Incubation at controlled temperature
Dilution with aerated water
PH adjustment
(7 marks)
QUESTION THREE
Suppose after graduation you are contracted to do a feasibility study for a sewage treatment plant
of a town. The design data given are as follows:
 The population of the town is 30 000 people.
 The sewage treatment plant will handle both domestic and industrial wastewaters.
 Domestic sewage is produced at the rate of 240 litres per capita per day.
 Domestic sewage has a BOD load of 72 mg/l per capita.
 The domestic sewage volume will increase by 10 % during the lifetime of the sewage
treatment plant because of population growth.
 The industrial wastewater to be treated comes from dairy and sugar mill wastes.
 Dairy wastewaters have a BOD load of 1100 mg/l and a volume of 3 million litres per
day.
 Sugar mill wastewaters have a BOD load of 1500 mg/l and a volume of 2.4
million litres per day.
 The sewage treatment plant effluents will be discharged into a nearby stream.
 The stream has a dry weather flow of 4500 litres per second.
 The stream saturation dissolved oxygen content is 9 mg/l.
 Assume the dissolved oxygen content of waste water is nil
 Assume the stream water upstream of the mixing point is pure.
 National Environmental Management Authority (NEMA) requires the minimum
dissolved oxygen content in a stream to be 4 mg/l.
 The stream self purification constant (f) has a value of 3 (Note, f = Kr/Kd).The stream
self purification ability is to be estimated using the stream coefficients of deoxygenation (Kd) and re-oxygenation (Kr).
 The stream coefficient of de-oxygenation at the mixing temperature is estimated at 0.1.
 The stream coefficient of re-oxygenation at the mixing temperature is estimated at 0.3.
 K (T) at the mixing temperature is 0.1.
a. Calculate the total BOD load per litre of wastewater that will be treated by the
sewage treatment plant.
(3 marks)
b. Calculate the total sewage discharged into the sewage treatment plant( in litres per
second) after allowing for population growth.(3 marks)
c. Calculate the dissolved oxygen content (DO) at the point where the stream mixes with the
sewage(4 marks)
d. By making suitable assumptions, estimate the dissolved oxygen deficit at the point of
mixing.(4 marks)
e. By making suitable assumptions, estimate the maximum value of sewage BOD load that
can be permitted to discharge into the stream.(4 marks)
f. Estimate the 5 day BOD load value of this maximum BOD load.(4 marks)
g. Calculate the degree of waste water treatment(as a percentage change in BOD) required
before it is discharged into the stream.(3 marks)
QUESTION FOUR
Many estates in the city of Nairobi are unsewered and use septic tanks to treat domestic
wastewater.
a. Using diagrammatic sketches explain the functions as well as operations of the following
units of a septic tank:
 The anaerobic chamber
 The aerobic chamber
 The soak away pit
(5 marks)
b. What are the major limitations of a septic tank?
(5 marks)
c. By making suitable assumptions estimate the required depth of the soak away pit of a
septic tank. The design data for the septic tank are as follows:


The household size is 8 persons.
Wastewater is generated at a rate of 150 litres per
person per day.
 The soak pit should not have any standing water in
it at the end of the day.
 The cross sectional area of the soak pit should not
exceed 4 m2
 The soil infiltration capacity is 10 cm per hour
(5 marks)
d. Septic tanks can be designed to provide biogas for the household.
 Name 5 main advantages of biogas
 Compare and contract constant volume and
constant pressure biogas reactors
(10 marks)
QUESTION FIVE
Various equipment and facilities are used by the industry to control the quality of the
workplace environment.
a. Explain the term “50 per cent cut size” as used in the design of cyclone dust separators
(4 marks)
b.
Using a sketch explain the operation of a gravity dust separator
(5 marks)
c. Under what circumstances should waste incineration be used as a method for air
pollution control
(4 marks)
d. A series of lagoons is to be installed to treat the waste water from a food
processing factory. Name possible sources of lagoon odours and suggest ways of abating
them.
(4 marks)
e. A factory producing 2000 m3of waste water per day is to be constructed to discharge
effluent on land with a soil infiltration capacity of 10 cm per hour. How much land will
be required?
(4 marks)
f. What are the main disadvantages of disposing effluent over Land?
(4 marks)