7/25/2019 CIVE4016-5-2007-2

1/5

UNIVERSITY OF MAURITIUS

FACULTY OF ENGINEERING

PAPER NO

BECE/06S/8

EXAMINATION

Second Semester 2006/07

BEng (Hons) Civil Engineering

Level 4

DATE

Thursday

17 May 2007

SERIES

May 2007

SPECIAL PAPER

Wastewater Engineering

[CIVE 4016(5)]

TIME

9:30 - 11:30 Hours

This paper contains FIVE (5) Questions.

Candidates are required to answer ANY FOUR (4)Questions

7/25/2019 CIVE4016-5-2007-2

2/5

Question 1

(a) Discuss the role of primary treatment in the treatment of domesticwastewater.

[8 marks]

(b) Two or more rectangular sedimentation tanks are to be provided for theprimary treatment of sewage at a rate of 3000m3/d. The detention period is tobe 2 hours and the overflow rate is not to exceed 30m3/m2/d. The maximumpermissible horizontal flow through velocity is to be 0.30m/min. Whatshould be the dimensions of the tanks?

[12 marks]

(c ) Explain the effect on the operation of primary sedimentation tank as a result

of the failure of the grit removal system.[5 marks]

Question 2

An extended aeration activated sludge process is used to treat wastewater from asmall housing estate. The following information is available:

Number of housing units : 300

Number of persons/household : 5

Domestic water consumption : 200 L/c/d

Loading factors : 90g BOD5/c/d

: 80g SS/c/d

Aeration Unit

Hydraulic Retention Time (HRT) : 18h

Mixed Liquor Suspended Solid (MLSS) : 3000mg/L

Treated Effluent

BOD5 : 20mg/L

SS of treated effluent : 30mg/L

Sludge wasted : 0.5kgMLSS/KgBOD removed.

(Continued next page)

Page 1 of 4

7/25/2019 CIVE4016-5-2007-2

3/5

Question 2(Continued)

Calculate

(i) the BOD5and the SS of the raw sewage. [5 marks]

(ii) The BOD5and the SS%, removal efficiency of the plant. [4 marks]

(iii) Volume of aeration tank. [4 marks]

(iv) F/M ratio [4 marks]

(v) Sludge age in days [4 marks]

(vi) Air requirements assuming 4% transfer efficiency and that 24L of air contains(32 x 0.21)g of O2. [4 marks]

Question 3

(a) Wastewater is treated in a conventional continuous flow stirred tank reactorwith sludge recycle and wasting from the mixed liquor at steady state. Plotthe effluent substrate and MLSS concentration as functions of the sludge age,

c, if the influent BOD5is 250mg/L. Use:

Y = 0.4 (mg/mg of BOD5) Kd= 0.05 d-1 Xe =20mg/L

m= 4d-1 Ks = 120 mg/L Se= Ks(1+Kdc)

c(m Kd) 1

HRT = 8h

Comment on your results and discuss the aeration requirements.

[15 marks]

(b) Describe the effect of bulking on the activated sludge process describedabove.

[10 marks]

Page 2 of 4

7/25/2019 CIVE4016-5-2007-2

4/5

Question 4

(a) Briefly describe the physical and biological characteristics of the various typesof stabilization ponds which might be used for sewage treatment in tropical

countries.[12 marks]

(b) An agro-industrial wastewater has been found to have the following averagecharacteristics:

Flow rate : 1000 mg/L

Suspended Solids : 650 mg/L

BOD5 : 1800 mg/L

pH : 5.5

Assuming that nutrient concentrating are not limiting, estimate theperformance characteristics and calculate likely pond dimensions for a seriesof stabilisation layers treating this wastewater in a tropical country. The serialeffluent BOD5should not exceed 75 mg/L.

Facultative pond BOD5 removal, r , may be estimated from surface loading,

5in kg BOD5/ha/d using the following equation:

r = 0.725 s + 10.75

(s > 400 kg/ha/d)

[13 marks]

Page 3 of 4

7/25/2019 CIVE4016-5-2007-2

5/5

Question 5

Given the following setting data for a biological sludge from an activated sludgepilot plant, estimate the maximum concentration of the aerator mixed liquor

biological suspended solids that can be maintained if the sedimentation tankapplication rate has been fixed at 25 m3/m2.d and the sludge recycle rate Qris equalto 40%. Assume that the solids wasting rate Qris negligible in the pilot plant study.

MLSS X

(mg/L)

Initial Settling

Velocity, m/h

2000 4.273000 3.51

4000 2.77

5000 2.13

6000 1.28

7000 0.91

8000 0.67

9000 0.49

10000 0.37

15000 0.15

20000 0.07

30000 0.027

[25 marks]

END OF QUESTION PAPER

sg/

Page 4 of 4