21) Aeration of water is done to remove
a) odour
b) colour
c) hardness
d) turbidity
22) It is better to provide separate system of sewerage if
(a) rainfall is throughout the year and intensity is less
(b) streets are narrow
(c) topography of city is flat
(d) city is in rocky area
23)Which one of the following valves is seldom used in water distribution systems because of high head loss characteristics ?
a) Butterfly
b) Globe
c) Plug
d) Sluice
24) The ratio of minimum hourly flow to the average flow of sewage, is
a) 1/3
b) ½
c) 2/3
d) 3
25) Testing of sewer pipes may involve
a) water test
b) mirror test
c) ball test
d) All these tests
26) The self cleansing velocity normally adopted for sewer design is
a) 0.5 m/s
b) 0.8 m/s
c) 1 m/s
d) 10 m/s
27) Which one of the following is a wrong statement about chemical precipitation method?
(a) It is to be used when sewage flow has high seasonal variation.
(b) It is recommended when the suspended solids from industrial waste, are to be precipitated.
(c) It produces small volume of sludge.
(d) It is suitable when sludge conditioning for dewatering is required.
28) The pH of fresh sewage is usually
a) less than 7
b) more than 7
c) equal to 7
d) equal to zero
29) A single stock system without deaerators can be used upto ______ storeys.
(a) 3
(b) 5
(c) 7
(d) 9
30) Most of the bacteria in sewage are
a) anaerobic
b) parasitic
c) saprophytic
d) pathogenic