Softners - Waste Water Engineering Questions and Answers

This set of Waste Water Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Softners”.

1. ________ is the removal of calcium, magnesium and certain other metal cations in hard water.
a) Disinfection
b) Sedimentation
c) Softening
d) Cleaning
View Answer

Answer: c
Explanation: Softening is the removal of calcium, magnesium and certain other metal cations in hard water. The product water that is soft water extends the life of plumbing.

2. Soft water can foul plumbing.
a) True
b) False
View Answer

Answer: b
Explanation: Hard water leads to fouling of plumbs by building up lime scales and cause galvanic corrosion. In industries, effluent flow from the regeneration process can lead to scale precipitation.

3. ________ form insoluble salts.
a) Aluminium ions
b) Sodium ions
c) Calcium ions
d) Manganese ions
View Answer

Answer: c
Explanation: The rinse water contains calcium or magnesium ions which form insoluble salts and leaves a coating of insoluble stearates on tub and shower surfaces, commonly called soap scum, in hard water areas.

4. Which of these methods does not remove hardness?
a) Reverse osmosis
b) Ion exchange resins
c) Chelating agents
d) Sedimentation
View Answer

Answer: d
Explanation: The methods such as reverse osmosis, ion exchange resins and chelating agents are used to remove hardness. Sedimentation can be used to remove suspended solids.

5. Ion exchange resins are ______
a) Inorganic polymers
b) Organic polymers
c) Sandy materials
d) Liquids
View Answer

Answer: b
Explanation: Ion exchange resins are usually organic polymers. It contains anionic functional groups to which divalent cations bind strongly than monovalent cations.

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6. Ion exchange resins contain cation functional groups.
a) True
b) False
View Answer

Answer: b
Explanation: Inorganic materials called zeolites also exhibit ion-exchange properties. They are the organic polymers and contain anionic functional groups to which divalent cations bind more strongly.

7. _____ is added to hard water to make it softer.
a) Lime
b) Chlorine
c) Hydraulic acid
d) Sulphuric acid
View Answer

Answer: a
Explanation: Lime softening is the process in which lime is added to hard water to make it softer. It has several advantages over the ion-exchange method but requires full-time, trained personnel to run the equipment.

8. ________ is used in detergents to soften water.
a) Hydraulic acid
b) Sulphuric acid
c) Citric acid
d) Chlorine
View Answer

Answer: c
Explanation: Citric acid is used to soften water in soaps and laundry detergents. A commonly used synthetic chelator is ethylenediaminetetraacetic acid (EDTA).

9. Which is the commonly used chelator?
a) Rhodamine B
b) EDTA
c) Chlorine
d) Bromine
View Answer

Answer: b
Explanation: A commonly used synthetic chelator is ethylenediaminetetraacetic acid (EDTA). Chelators are used in chemical analysis, as water softeners and are ingredients in many commercial products such as shampoos and food preservatives.

10. Calculate the amount of softener resin required for the following information.
Magnesium: 80 ppm as CaCO₃
Capacity: 47 Kg/m³
Correction factor: 0.9
Flow rate: 135 m³/hr
Operation hours: 20 hrs
a) 4.5 m³
b) 3.5 m³
c) 5.1 m³
d) 2.5 m³
View Answer

Answer: c
Explanation: Magnesium load= Magnesium as CaCO₃ x Flow rate x Operating hours/1000. Resin Volume= Magnesium load/ (Exchange capacity x Correction factor). Resin Volume =[(80 x 135 x 20)/1000]/(47 x 0.9)= 5.1 m³.

11. Calculate the theoretical softener resin bed depth for the following data:
Magnesium Load: 130 ppm as CaCO₃
Capacity: 47 Kg/m³
Correction factor: 0.9
Flow rate: 135 m³/hr
Operation hours: 20 hrs
Linear velocity: 16.6 m³/h/ m³
Space velocity: 20 m³/h/ m³
a) 1.5 m
b) 1.2 m³
c) 2.6 m
d) 2.5 m
View Answer

Answer: b
Explanation: Magnesium load= Magnesium as CaCO₃ x Flow rate x Operating hours/1000. Resin Volume= Magnesium load/ (Exchange capacity x Correction factor). Resin Volume = [(130 x 135 x 20)/1000]/(47 x 0.9)= 1.8 m³. Bed depth = Linear Velocity/Space velocity. Bed depth = 20 /16= 1.2 m.

12. Calculate the area of the softener vessel required for the following information.
Magnesium Load: 180 ppm as CaCO₃
Capacity: 47 Kg/m³
Correction factor: 0.9
Flow rate: 135 m³/hr
Operation hours: 20 hrs
Linear velocity: 16.6 m³/h/ m³
Space velocity: 20 m³/h/ m³
a) 8.5 m²
b) 11.6 m²
c) 9.7 m²
d) 8.5 m³
View Answer

Answer: b
Explanation: Magnesium load= Magnesium as CaCO₃ x Flow rate x Operating hours/1000. Resin Volume= Magnesium load/ (Exchange capacity x Correction factor). Resin Volume = [(180 x 135 x 20)/1000]/(47 x 0.9)= 11.6 m³. Bed depth = Linear Velocity/Space velocity. Bed depth = 20 /16= 1.2 m. Area = 11.6/1.2= 9.7 m².

13. Calculate the diameter of the softener vessel required for the below given data.
Magnesium Load: 500 ppm as CaCO₃
Capacity: 57 Kg/m³
Correction factor: 0.9
Flow rate: 135 m³/hr
Operation hours: 20 hrs
Linear velocity: 16.6 m³/h/ m³
Space velocity: 20 m³/h/ m³
a) 5.2 m
b) 5.0 m
c) 7.6 m
d) 9.8 m
View Answer

Answer: a
Explanation: Magnesium load= Magnesium as CaCO₃ x Flow rate x Operating hours/1000. Resin Volume= Magnesium load/ (Exchange capacity x Correction factor). Resin Volume =[(500 x 135 x 20)/1000]/(57 x 0.9)= 26 m³ Bed depth = Linear Velocity/Space velocity. Bed depth = 20 /16= 1.2 m. Area = 26 /1.2= 22 m². The diameter for the vessel is 5.2 m.

14. Calculate the fast rinse flow of the regenerant liquid for a softener with the below given data.
Magnesium Load: 10 ppm as CaCO₃
Capacity: 47 Kg/m³
Correction factor: 0.9
Flow rate: 135 m³/hr
Operation hours: 20 hrs
a) 4.5 m³ /hr
b) 7.2 m³/hr
c) 1.7 m³ /hr
d) 7.5 m³/hr
View Answer

Answer: b
Explanation: Magnesium load= Magnesium as CaCO₃ x Flow rate x Operating hours/1000. Resin Volume= Magnesium load/ (Exchange capacity x Correction factor). Resin Volume =[(10 x 135 x 20)/1000]/(47 x 0.9)= 0.6 m³ Velocity is considered as 12-16 m³/hr/m³. In this case we consider 12 m³/hr/m³. Thus the fast rinse flow = Resin volume x velocity = 0.6 x 12 = 7.2 m³/hr.

15. Calculate the regenerant flow required for the softener system with the given below data.
Calcium Load: 85 ppm as CaCO₃
Capacity: 37 Kg/m³
Correction factor: 0.9
Flow rate: 135 m³/hr
Operation hours: 20 hrs
a) 25 m³/hr
b) 28 m³/hr
c) 12 m³/hr
d) 21 m³/hr
View Answer

Answer: d
Explanation: Calcium load= Calcium as CaCO₃ x Flow rate x Operating hours/1000. Resin Volume= Calcium load/ (Exchange capacity x Correction factor). Resin Volume =[(85 x 135 x 20)/1000]/(37 x 0.9)= 7.0 m³ Velocity is considered as 3 m³/hr/m³. Thus the regenerant flow = Resin volume x velocity = 7 x 3 = 21 m³/hr.

16. Calculate the rinse volume of the regenerant liquid for the softener system with the given below data.
Calcium Load: 60 ppm as CaCO₃
Capacity: 37 Kg/m³
Correction factor: 0.9
Flow rate: 35 m³/hr
Operation hours: 10 hrs
a) 1.5 m³/hr
b) 3.5 m³/hr
c) 4.9 m³/hr
d) 2.5 m³/hr
View Answer

Answer: b
Explanation: Calcium load= Calcium as CaCO₃ x Flow rate x Operating hours/1000. Resin Volume= Calcium load/ (Exchange capacity x Correction factor). Resin Volume = [(60 x 35 x 10)/1000]/(37 x 0.9)= 0.7 m³ Velocity is considered as 5 m³/hr/m³. Thus the rinse volume = Resin volume x velocity = 0.7 x 5 = 3.5 m³/hr.

17. Calculate the fast rinse period of the regenerant liquid for a softener system with the given below data.
Calcium Load: 40 ppm as CaCO₃
Capacity: 37 Kg/m³
Correction factor: 0.9
Flow rate: 235 m³/hr
Operation hours: 10 hrs
a) 25 mins
b) 30 mins
c) 20 mins
d) 15 mins
View Answer

Answer: a
Explanation: Calcium load= Calcium as CaCO₃ x Flow rate x Operating hours/1000. Resin Volume= Calcium load/ (Exchange capacity x Correction factor). Resin Volume =[(40 x 235 x 10)/1000]/(37 x 0.9)= 3.0 m³ Velocity for fast rinse is considered as 12-16 m³/hr/m³. In this case we consider 12 m³/hr/m³. Thus the fast rinse flow = Resin volume x velocity = 3 x 12 = 36 m³/hr. Velocity for rinse is considered as 5 m³/hr/m³. Thus the rinse volume = Resin volume x velocity = 3 x 5 = 15 m³/hr. Fast rinse period = Total rinse volume/ Fast rinse flow = 15/36 = 0.42 hrs= 25 mins.

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