Water Treatment MCQ Quiz - Objective Question with Answer for Water Treatment - Download Free PDF

Concept:

The settling velocity of sand particles is given by Stoke's Law,

\({{\rm{V}}_{\rm{s}}} = \frac{{g \times \left( {G - 1} \right) \times {d^2}}}{{18\;ν }}\)...... (1)

G -Specific gravity of sand particles

ν - Kinematic viscosity

d - Diameter of sand particles

Calculation:

Given:

Let V₁ and V₂ be settling velocities of larger and smaller grain respectively

V₁ = 0.5 mm/sec

Diameter of larger grain d₁ = 0.075 mm

Diameter of smaller grain d₂ = 0.0075 mm

g is constant, specific gravity identical for both grains, both the particles are falling through water hence ν remains same. Hence equation 1 becomes

⇒ \(\frac{{{V_1}}}{{{V_2}}}\; = \;{\left( {\frac{{{d_1}}}{{{d_2}}}} \right)^2}\)

⇒ \(\frac{{{0.5}}}{{{V_2}}}\; = \;{\left( {\frac{{{0.075}}}{{{0.0075}}}} \right)^2}\)

⇒ V₂ = 0.005 mm/sec

Explanation:

(i) Alum is the name given to the aluminium sulphate with its chemical formula is Al₂(SO₄)₃.18H₂O.

(ii) The alum when added to raw water, reacts with the bicarbonate alkalinities, which are generally present in raw supplies, so as to form a gelatinous precipitate (floc) of aluminium hydroxide.

Al₂(SO₄)₃.18H₂O + 3Ca(HCO₃)₂ → 3CaSO₄ + 2Al(OH)₃ + 6CO₂

(iii) From the above equation, it becomes evident that the addition of alum to water imparts permanent hardness to it, in the form of calcium sulphate.

(iv) The CO₂ gas which is evolved, causes corrosiveness.

(v) On addition to water, alum react with HCO₃^-, CO₃^2-, and OH^- and form aquametalllic cation and reduces the alkalinity of solutions, thus decreases the pH of water.

Explanation:

Grit Chamber Dimensions ​10 m × 1.5 m × 0.75 m

i.e.Surface area = 10 m × 1.5 m

Flow Rate (Q) = 750 m³/h

We know,

a)  \(\rm Surface \ loading \ rate = \dfrac{Q}{{Surface\;Area}}\)

\(= \dfrac{{750}}{{10 \times 1.5}}\)

= 50 m3/hr/m² or 50,000 l/hr/m²

b)  \(\rm Detention \ time= \dfrac{{Volume}}{Q}\)

\(= \dfrac{{10 \times 1.5 \times 0.75}}{{750}} \times 60 = 0.9\; \rm min\)

Additional Information

Grit chambers:

(i) These are long narrow rectangular channels with an increased cross-sectional dimension that decelerate flow and allow grit to settle.

(ii) These units are provided in order to carry out the removal of the inorganic suspended solid and to pass forward the suspended solid in PST.

(iii) These chambers are designed in the form of a long channel, which may either be rectangular or parabolic in cross-section.

(iv) The grit chamber is a settling tank with a provision to control flow velocity.

(v) A velocity control device known as proportional weir flow and Parshall flume is provided at the outlet of the grit chamber to maintain constant flow velocity irrespective of the flow rate.

(vi) If the rectangular section is adopted, the proportional weir is provided in front of it and if the parabolic section is adopted, a parshall flume is provided in front of it.

Concept:

The unit operation in water treatment includes:

• Screening
• Aeration
• Flocculation
• Filtration
• Disinfection
• Softening
• Deferrization
• Deffluoridation.

There are many treatment processes available but typical raw water treatment technologies usually include the following steps:

Intake of Raw Water: 

• Naturally and untreated raw water present in the environment comes from many sources that can be natural or man-made such as rivers, lakes, ponds, oceans, and streams, etc., when a plant draws water from the surface, they pull it in through a screen to eliminate the larger objects.

Screening: 

• Screening is the first unit operation of a water treatment plant. Screening is the process to remove the big floating particles having a size that lies between approx 10 mm to 150 mm depending upon the types of screen used in the screening process. In this process generally, paper, wooden pieces, Plastic bottles, Glass bottles, etc. are removed. Screening is done with the help of a device known as a screen.

Coagulation: 

• When various chemicals are added to remove large and bulky suspended solids is called a coagulation process. This process starts with adding specific chemicals and takes out all finer particles present in the water by combining them and converting them into heavier particles that lead to easy settling in the water.

Flocculation: 

• After completion of the coagulation process, the flocculation process starts, where the coagulated particles are slowly mixed (stirred) together and create visibly, settleable particles.

Sedimentation: 

• The process in which solids particles settle down and lead to the formation of a sludge blanket after the particles of that solid are mixed and the sludge is pumped out from the bottom into de-watering or sludge handling operations. Then, the de-watering process takes place in which all the water is taken out from the sludge with a filter press. Mostly water from this process is reused (added to the clarifier).

Filtration: 

• It is a process in which water overflows into gravity sand filters these filters cover big areas because which include finely crushed silica sand with jagged edges as they are kept around 3-5 feet of sand. The sand is typically used in the filter at a depth that is packed tightly.

Disinfection: 

• Water disinfection means the removal, deactivation, or killing of pathogenic microorganisms. Microorganisms are destroyed or deactivated, resulting in the termination of growth and reproduction. When microorganisms are not removed from drinking water, drinking water usage will cause people to fall ill.
• Sterilization is a process related to disinfection. However, during the sterilization process, all present microorganisms are killed, and both harmful and harmless microorganisms are. Generally, chlorination is done for disinfection.

Explanation:

The correct answer is Filtration.

Key Points

Filtration

• Filtration is a process used to separate solids from liquids or gases by using a filter medium that allows fluid to pass through but not solid. 
• Filtration can be mechanical-biological or physical. 
• It does not help in disinfecting water. 
• If you want to disinfect the water you can boil it or you can use alum for chlorine tablets. 

Boiling

• Boiling is used to kill pathogenic bacteria, viruses, and protozoa. 

Chlorination

• Chlorine kills a large variety of microbial waterborne pathogens.
• Chlorination is the process of adding chlorine to the water to disinfect it and kill pathogens. 
• Amount of chlorine required for water disinfection around 1-16 milligrams per litre of water.

Coagulation

• Alum acts as an electrolyte that helps in settling the suspended matter in water.
• The process of adding alum to disinfect water is called coagulation. 

Concept:

• Water softening: It is the process of hardness removal from the water. It is caused by multivalent cation and affects water quality.
   
• Lime soda method: It is a water softening method in which lime and soda ash are added to the water, which causes the precipitation of multivalent cation as CaCO3.
• Precipitation of CaCO3 occurs only when the pH of water is greater than 9, so in case of less pH alkalinity is added to the water. In this process small amount of Ca2+ and Mg2+ precipitates very late, which will create incrustation in the pipe, so to avoid this recarbonation is done to dissolve back this small amount of cation.
• Due to this, the method does not give zero hardness.
   
• Ion Exchange Process: Ion-exchange resin, (zeolite) exchanges one ion from the water being treated for another ion that is in the resin (sodium is one component of softening salt, with chlorine being the other). Zeolite resin exchanges sodium for calcium and magnesium. It can produce water with zero hardness.

Concept:

Coagulation:

• Coagulation is a process used to neutralize charges and form a gelatinous mass to trap or bridge particles thus forming a mass large enough to settle or be trapped in the filter.
• Coagulation destabilizes the charges of the particles. Coagulants with charges opposite to those of the suspended solids are added to the water to neutralize the negative charges on dispersed non-settable solids such as clay and organic substances.
• Coagulant chemicals are substances that promote the coagulation of liquids into solids.

Coagulants used for wastewater treatment:

• Aluminium chloride
• Ferric chloride
• Ferric and ferrous sulphate
• Aluminium chlorohydrate

Coagulants used for water treatment:

• Alum
• Ferrous sulphate
• chlorinated ferrous sulphate
• Sodium aluminate

Ferric chloride is widely used for sewage treatment and Alum is widely used for water treatment.

Explanation:

For a slow sand filter

The rate of filtration is 100 - 200 lit/hour/sqm

Some important points about screens in the screening process

• The coarse screen consists of parallel iron rods are placed vertically or at a slight slope at about 25 to 50 mm apart
• The fine screen is usually made of woven wire mesh with an opening not more than 6 mm square
• The spacing between two bars in medium size screen ranges from 15 to 40 mm
• The clear spacing between the bars may be in the range of 15 mm to 75 mm in case of a mechanically cleaned bar screen
• However, for the manually cleaned bar screen, the clear spacing used is in the range 25 mm to 50 mm

Explanation:

Given Data:

Village population = 200

The average rate of water demand = 100 litres per capita per day

average filtration rate = 100 liter/hour/m² = 100 x 24 liter/day/m²

Ratio of maximum demand to average demand = 1.5

Design dicharge = 1.5 x Village population x average rate of water demand

= 1.5 x 200 x 100 = 30000 liter/day/m2

Area of rapid sand filter = \(\frac{Design \ discharge}{Average \ filtration \ rate}\)

Area of rapid sand filter = \(\frac{30000}{100\ \times 24}\)

Area of rapid sand filter = 12.5 m2

Additional InformationFor Rapid Sand Filter

• Numbers of filters (N) = 1.22 \(\sqrt (\)Q), where Q is in MLD
• Total cross-sectional area of perforations = 0.2% of filtered area
• Cross-sectional area of on lateral is 2 or 4 times the total cross-sectional area of perforation.

Explanation:

• The efficiency of the sedimentation tank indicates the overall percentage removal of suspended matter at a given overflow rate or surface loading rate.
• The efficiency of the sedimentation tank increases if the overflow rate reduces (more time available to particles for settle).

overflow rate, \({V_s} = \frac{Q}{{BL}}\)

From the equation, it is clear that if the surface area (B × L) of the tank increases, the overflow rate reduces, and efficiency increases for a given discharge.

 Important Points

Efficiency depends on the following parameter during sedimentation:

• The velocity of flow: Efficiency increases if the velocity of flow reduces
• Viscosity: Efficiency increases if viscosity reduces (Viscosity changes but we can not do)
• Size of particle: Efficiency increases if the size of particle increases.

The correct answer is Carbonates of calcium.

 Key Points

• Hardness in water is that characteristic which prevents the formation of sufficient leather or foam, when such hard waters are mixed with soap.
• It is usually caused by the presence of calcium and magnesium salts present in water, which form scum by reaction with soap.
• Hard waters are undesirable because they may lead to greater soap consumption, scaling of boilers, causing corrosion and incrustation of pipes, making foods tasteless, etc.
• If bicarbonates and carbonates of calcium and magnesium are present in water, the water is rendered hard temporarily, as this hardness can be removed to some extent by simple boiling or to full extent by adding lime to the water. Such a hardness is known as temporary hardness or carbonate hardness.
• When such waters are boiled carbon dioxide gas escapes out and the insoluble calcium carbonate gets precipitated. Magnesium carbonate being fairly soluble in water won’t get removed by boiling. The temporary hard waters, therefore do cause deposition of calcium scales in boilers.

Concepts:

As per IS 2296: 1982, the surface water quality standards are classified into 5 categories which are specified below:

1. Class A – Drinking water without conventional treatment but after disinfection.

1. Class B – Water for outdoor bathing.
2. Class C – Drinking water with conventional treatment followed by disinfection.
3. Class D – Water for fish culture and wild life propagation.
4. Class E – Water for irrigation, industrial cooling and controlled waste disposal.

Answer:

The coarse screen is normally kept inclined at about 45°– 60° to the horizontal, so as to increase the opening area to reduce the flow velocity.

Explanation:

• Screening is done to carry out the heavy suspended solids from the water like plants, animals, stone, trees, etc.
• Screening is generally adopted for the treatment of surface water.
• Screening is done with the help of the device termed as screens.
  • Coarse screen
  • Fine screen

Coarse screen:

• Coarse screen is in the form of the bar of size 10 - 25 mm, having spacing of 20 - 100 mm center to center in between them.
• These Screens are always at an inclination of generally 45° - 60°, as it helps in affecting cleaning of the screens by racking mechanism. more ever it also helps in increasing the efficiency of screening.

from the figure, it is clear that by providing screen at inclination area increases and velocity reduces to maintain constant discharge.

∵ sinθ = A / A' ⇒ A' = A / sinθ 

and θ provided is less than 90° 

∴ A' > A

∵ we know that discharge = A' × V'

∴ if area increases then velocity reduces to maintain constant discharge.

Fine Screen:

• The fine screen is in the form of wire mesh of size 10 mm
• Under normal treatment operation, the fine screen is generally avoided as they get frequently clogged requiring which require frequent cleaning which is turn operational cost.