Past Year Questions: Water Treatment - 2

# Past Year Questions: Water Treatment - 2 | Environmental Engineering - Civil Engineering (CE) PDF Download

Q. 13 A water treatment plant of capacity, 1 m3/s has filter boxes of dimensions 6m x 10m. Loading rate to the filters is 120 m3/day/m2. When two of the filters are out of service for back washing, the loading rate (in m/ day / m2) is ______ .    [2015 : 2 Marks, Set-II]
Ans: 144 m3/day/m2

Total water filtered in a day
= 60 x 60 x 24 = 86400 m3/day
Total surface area of filter required

Area of one filter = 6 x 10 = 60 m2

As two filters are out of service, number of filters
working = 1 2 - 2 = 10
Total surface area of filter
= 60 x 10 = 600 m2

Q. 14 Consider a primary sedimentation tank (PST) in a water treatment plant with Surface Overflow Rate (SOR) of 40 m3/m2/d. The diameter of the spherical particle which will have 90 percent theoretical removal efficiency in this tank is ______ μm. Assume that settling velocity of the particles in water is described by Stoke’s Law. Given: Density of water = 1000 kg/m3; Density of particle = 2650 kg/m3; g= 9.81 m/s2; Kinematic viscosity of water (v) = 1.10 x 10-6 m2/s. [2015 : 2 Marks, Set-I]
Ans: 22.576 μm

Q. 15 An effluent at a flow rate of 2670 m3/d from a sewage treatment plant is to be disinfected. The laboratory data of disinfection studies with a chlorine dosage of 15 mg/l yield the model Nt =Noe-0.145t where Nt = number of micro- organisms surviving at time t (in min.) and N0 = number of micro-organisms present initially (at t = 0). The volume of disinfection unit (in m3) required to achieve a 98% kill of micro-organisms is ____ [2014 : 2 Marks, Set-II]
Ans: 50.0244 m3

Q = 2670 m3/d
Nt = No. e-0.145t

Let x be the no. of microorganisms (M.O.) present initially.
98% kill of M.O. imlies that at time ‘t’ 2% of M.O. are still surviving
∴ M .O . surviving at time 't'

Q. 16 A surface water treatment plant operates round the clock with a flow rate of 35 m3/min. The water temperature is 15 °C and jar testing indicated an alum dosage of 25 mg/l with flocculation at a Gt value of 4 x 104 producing optimal results. The alum quantity required for 30 days (in kg) of operation of the plant i s ______ . [2014 : 2 Marks, Set-II]
Ans: 37,800 kg

Given data
Flow rate,  Q = 35 m3/min
Gt = 4 x 104
Alum dosage = 25 mg/lit.

Alum quantity (kg) required for 30 days
= 35 x 103 x 60 x 24 x 30 x 25 x 10-6
= 37,800 kg

Q. 17 A suspension of sand like particles in water with particles of diameter 0.10 mm and below is flowing into a settling tank at 0.10 m3/s. Assume g = 9.81 m/s2, specific gravity of particles = 2.65, and kinematic viscosity of water =1.0105 x 10-2 cm2/s. The minimum surface area (in m2) required for this settling tank to remove particles of size 0.06 mm and above with 100% efficiency is ___ .    [2014 : 2 Marks, Set-II]
Ans: 31.214 m2

For particles of size 0.06 mm,

For 100% efficient removal of 0.06 mm size particles

Q. 18 The potable water is prepared from turbid surface water by adopting the following treatment sequence.    [2014 : 2 Marks, Set-I]
(a) Turbid surface water -> Coagulation -» Flocculation -> Sedimentation -> Filtration Disinfection -> Storage & Supply
(b) Turbid surface water -> Disinfection -> Flocculation -» Sedimentation -> Filtration Coagulation Storage & Supply
(c) Turbid surface water -> Filtration -» Sedimentation - » Disinfection -» Flocculation -» Coagulation -» Storage & Supply
(d) Turbid surface water Sedimentation -» Flocculation -> Coagulation -> Disinfection Filtration -> Storage & Supply
Ans: (a)

Q. 19 :  16 MLD of water is flowing through a 2.5 km long pipe of diameter 45 cm. The chlorine at the rate of 32 kg/d is applied at the entry of this pipe so that disinfected water is obtained at the exit. There is a proposal to increase the flow through this pipe to 22 MLD from 16 MLD. Assume the dilution coefficient, n = 1. The minimum amount of chlorine (in kg per day) to be applied to achieve the same degree of disinfection for the enhanced flow is   [2014 : 2 Marks, Set-1]
(a) 60.50
(b) 44.00
(c) 38.00
(d) 23.27

Ans: (a)

In the disinfection process we have the relationship,

tCn = K
where
t = time required to kill all organism
c = concentration of disinfectant
n = dilution coefficient
k = constant

L = length of pipe ; v1, = velocity of flow

where W1 =  weight of disinfectant per day;
Q1 = discharge per day,

Q. 20 A settling tank is designed for a surface overflow rate of 30 m3/day.m2. Assuming specific gravity of sediment particles = 2.65. Density of water, γw = 1000 kg/m3, dynamic viscosity of water mwater = 0.001 Ns/m2 and stokes law is valid. The approximate minimum size of particles which can be completely removed is  [2013 : 2 Marks]
(a) 0.01 mm
(b) 0.02 mm
(c) 0.03 mm
(d) 0.04 mm
Ans: (b)
To calculate minimum size of particles equating settling velocity to overflow rate, we get,

Q. 21 A water treatment plant, having discharge 1 m3/sec, has 14 filters to treat the water. Each filter is having 50 m2 area, but due to backwashing activity 2 filters are non operational. Calculate hydraulic loading rate in m3/day.m2. [2013 : 1 Mark]
Ans: 144m3/day.m2

Q= 1 m3/sec
No. of working filters = 12
S.A. of each filter = 50 m2

Q. 22 A town is required to treat 4.2 m3/min of raw water for daily domestic supply. Flocculating particles are to be produced by chemical coagulation. A column analysis indicated that an overflow rate of 0.2 mm/s will produce satisfactory particle removal in a settling basin at a depth of 3.5 m. The required surface area (in m2) for settling is   [2012 : 2 Marks]
(a) 210
(b) 350
(c) 1728
(d) 21000
Ans: (b)

Overflow rate, V0 = 0.2 x 10-3 m/s Water required to treat raw water,

Q. 23 Chlorine gas (8 mg/L as Cl2) was added to a drinking water sample. If the free chlorine residual and pH was measured to be 2 mg/L (as Cl2) and 7.5, respectively, what is the concentration of residual OCl- ions in the water? Assume that the chlorine gas added to the water is completely converted to HOCI and OCh Atomic Weight of Cl: 35.5

(a) 1.408 x 1CL5 moles/L
(b) 2.817 x 10-5 moles/L
(c) 5.634 x 10 6 moles/L
(d) 1.127 x 10-4 moles/L     [2011 : 2 Marks]

Ans: (a)

Free chlorine residual = 2 mg/l
[Free chlorine residual]

Q. 24 Consider the following unit processes commonly used in water treatment; rapid mixing (RM), flocculation (F), primary sedimentation (PS), secondary sedimentation (SS), chlorination (C) and rapid sand filtration (RSF), The order of these unit processes (first to last) in a conventional water treatment plant is  [2011 : 1 Mark]
(a) P S -R SF-F-R M -S S-C
(b) PS -F-R M -R S F-S S -C
(c) PS F SS RSF - RM - C
(d) P S -R M -F-SS -R SF-C
Ans:
(d)

Rapid mixing is the process done to disperse the coagulant into the entire mass of water after that flocculation occurs. Flocculated particles are then removed in secondary sedimentation tank followed by rapid sand filter. Water is than chlorinated and sent for supply.

The document Past Year Questions: Water Treatment - 2 | Environmental Engineering - Civil Engineering (CE) is a part of the Civil Engineering (CE) Course Environmental Engineering.
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## FAQs on Past Year Questions: Water Treatment - 2 - Environmental Engineering - Civil Engineering (CE)

 1. What are the different methods used for water treatment?
Ans. There are several methods used for water treatment, including coagulation, flocculation, sedimentation, filtration, disinfection, and desalination. Coagulation involves adding chemicals to water to remove suspended particles, while flocculation helps to form larger particles that can be easily removed. Sedimentation is the process of allowing particles to settle at the bottom of a container, and filtration involves passing water through various media to remove impurities. Disinfection is done to kill harmful microorganisms, and desalination is used to remove salt and other minerals from seawater.
 2. What is the purpose of disinfection in water treatment?
Ans. The purpose of disinfection in water treatment is to kill or inactivate harmful microorganisms, such as bacteria, viruses, and parasites, that may be present in the water. Disinfection helps to prevent the spread of waterborne diseases and ensure that the water is safe for consumption. Common disinfection methods include chlorination, ozonation, and ultraviolet (UV) radiation.
 3. How does coagulation help in water treatment?
Ans. Coagulation is an important step in water treatment as it helps to remove suspended particles, such as dirt, clay, and organic matter, from the water. During coagulation, chemicals called coagulants are added to the water, which neutralize the electrical charges on the particles and cause them to come together and form larger particles called flocs. These flocs can then be easily removed through sedimentation or filtration processes.
 4. What is the role of sedimentation in water treatment?
Ans. Sedimentation is a crucial step in water treatment that allows particles to settle down at the bottom of a container. After coagulation and flocculation, the water is left undisturbed in a sedimentation tank, which allows the flocs to settle to the bottom due to gravity. The settled particles, along with any other impurities, are then removed, leaving behind clearer water. Sedimentation helps to remove larger particles and improve the overall clarity of the water.
 5. What is the purpose of desalination in water treatment?
Ans. The purpose of desalination in water treatment is to remove salt and other minerals from seawater or brackish water, making it suitable for various purposes, including drinking, irrigation, and industrial use. Desalination is particularly important in areas where freshwater resources are scarce or contaminated. Common desalination methods include reverse osmosis, distillation, and electrodialysis.

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