What steps are necessary to transform raw water from rivers, lakes, and groundwater sources into safe drinking water? Drinking Water Treatment involves a series of physical and chemical processes designed to remove impurities, pathogens, and harmful contaminants before water reaches consumers. Understanding treatment methods such as coagulation, sedimentation, filtration, and disinfection is essential for protecting public health and ensuring a reliable supply of safe water. These concepts are highly relevant for UGC-NET/JRF, SLET, ARS, GATE, and other competitive examinations.
Use this curated MCQ bank to test your conceptual understanding, identify weak areas, and sharpen your exam readiness.
Syllabus Outline
- Major water pollutants and their associated health impacts on human health.
- Drinking water standards and the need for water treatment facilities.
- Unit operation of water treatment plant (e.g. coagulation, flocculation, sedimentation, filtration, and disinfection: chlorination, ozonation, UV irradiation).
- Membrane filtration (microfiltration, ultrafiltration, nanofiltration, reverse osmosis), adsorption, and ion exchange.
- Principles, mechanisms, design considerations, operation, and maintenance of each treatment process.
Quick Study Guide
Drinking water purification relies on physical, chemical, and engineering unit operations to remove suspended solids, pathogens, and toxic dissolved species from raw water to meet safe potability standards.
- Coagulation, Flocculation, and Colloidal Stability: Natural turbid waters contain microscopic colloidal particles that remain suspended due to negative surface charges (zeta potential) that cause mutual repulsion. Coagulation introduces trivalent metallic salts (e.g. alum) to neutralise these surface charges, allowing particles to come together. Flocculation involves gentle mechanical agitation to promote particle collisions, growing them into larger, heavy aggregates called flocs.
- Sedimentation and Settling Dynamics: Flocculated water enters a sedimentation basin where gravity removes suspended solids. Settling behaviour follows two primary regimes: Type I settling (discrete particles that sink independently at a constant velocity) and Type II settling (flocculant particles that coalesce while sinking, increasing in mass and settling faster). Basin efficiency is governed by the Surface Overflow Rate, the design fluid velocity rising through the tank.
- Rapid Sand Filtration Mechanics: Clarified water passes through a dual-media filter bed composed of anthracite coal and sand. Unlike simple surface sieving, rapid sand filtration acts as a deep physical-chemical process where sub-micron particles are trapped throughout the bed via inertial impaction, sedimentation, and surface adsorption. When the bed becomes clogged (head loss accumulation), it is cleaned using high-velocity water and air backwashing to fluidise the media and flush out trapped debris.
- Disinfection Kinetics and the CT Concept: Pathogen inactivation is governed by the Chick-Watson Law, which dictates that disinfection efficiency depends on the disinfectant concentration (C) and the contact time (T). This CT factor determines the kill rate of bacteria and viruses. Using chlorine can generate carcinogenic secondary pollutants called Disinfection Byproducts, such as trihalomethanes, which form when free chlorine reacts with natural organic matter.
- Advanced Unit Operations for Dissolved Contaminants: Conventional treatment cannot remove dissolved toxic ions or micro-pollutants. Specialised geogenic removal uses adsorption on activated alumina to strip fluoride and arsenic from groundwater. For desalination or advanced pure-water production, Reverse Osmosis (RO) applies hydraulic pressure greater than the natural osmotic pressure to force water molecules through a semipermeable membrane, rejecting dissolved salts and heavy metals.
Test Your Knowledge
This quiz contains 25 concept-based MCQs on “Drinking Water Treatment“. Each question has a single correct/most appropriate answer.
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1. Which of the following processes/unit operations of conventional wastewater treatment is the correct order starting from the inflow?
a) Coagulation & Flocculation – Sedimentation – Filtration – Disinfection
b) Sedimentation – Coagulation & Flocculation – Filtration – Disinfection
c) Preliminary treatment, Primary clarifier – Aeration – Secondary clarifier – Filtration – Disinfection
d) Preliminary treatment, Aeration – Primary clarifier – Activated sludge process – Secondary clarifier – Filtration – Disinfection
2. Choose the correct order with an increasing dose of coagulants
I – Restabilization or charge reversal
II – No destabilisation
III – Destabilisation by sweep floc
IV – Destabilisation by charge neutralisation
a) I, II, III, IV
b) IV, II, I, III
c) II, IV, I, III
d) III, I, IV, II
3. The magnitude of the buoyant force acting on a particle in water depends on which factors?
a) Weight and shape of the particle
b) The density of the water and the mass of the particle
c) The density of the water and volume of the particle
d) The density, weight, and shape of the particle
4. What is the key mechanism by which inter-particle contact occurs in a simple coagulation and flocculation process?
a) Brownian motion, stirring and precipitation
b) Thermal motion, Centrifugal and Gravitational effects
c) Gravitational, solvation and drag force
d) Thermal motion, bulk fluid motion and particle settling
5. Inactivation of microorganisms/pathogens in water distribution systems is classified as:
a) Primary disinfection
b) Secondary disinfection
c) Tertiary disinfection
d) Quaternary disinfection
6. At which zone of coagulation and flocculation is turbidity efficiently removed?
a) Restabilization or charge reversal
b) No destabilisation
c) Destabilisation by charge neutralisation
d) Destabilisation by sweep floc
7. Various species are formed in coagulation and flocculation with alum due to hydrolysis of alum salt such as Al3+, Al(OH)2+, Al(OH)2+, Al(OH)3(s), Al(OH)4–, Al(OH)5-2. Which species is predominant at pH above 8?
a) Al3+
b) Al(OH)2+
c) Al(OH)3(s)
d) Al(OH)4–
8. Which of the following is not a limitation of natural coagulants such as crude Moringa oleifera (drumstick) seed extract?
a) It increases colloidal particles
b) It increases organic matter and chlorine demand
c) It causes colour, taste & odour problems due to long periods of water storage
d) Unable to coagulate orthophosphate & nitrates
9. Flotation is a physical process used in water treatment to remove
a) Particles that are denser than water
b) Pertinacious material from the water
c) Colloidal particles, including clay and bacteria
d) Particles that are lighter than water
10. Which species is more potent in the disinfection process?
a) HOCl
b) OCl–
c) Cl2 (g)
d) Cl2 (aq)
11. Perikinetic flocculation is controlled by:
a) Brownian motion
b) Both Brownian motion and Intrinsic properties of the system
c) Intrinsic Properties of the system
d) Mechanical mixing
12. The majority of the suspended impurities get removed in a slow sand filter by:
a) Absorption
b) Adsorption
c) Coagulation
d) Filtration
13. In the sedimentation tank, the drag force on a particle is directly proportional to:
a) Density of the liquid
b) Velocity of the particle
c) Volume of the particle
d) Projected area of the particle
14. Flotation is a physical process used in wastewater treatment to remove
a) Particles that are denser than water
b) Pertinacious material from the water
c) Colloidal particles, including clay and bacteria
d) Particles that are lighter than water
15. Sedimentation is a physical process used in wastewater treatment to remove
a) Particles that are denser than water
b) Particles that are less dense than water
c) Pertinacious material from the water
d) Colloidal particles, including clay and bacteria
16. Chlorination of water and wastewater may result in the formation of:
a) Chlorox
b) Chloramines
c) Hypochlorites
d) Chlorinated hydrocarbons
17. Control of the disinfection process is usually based on maintaining total residual chlorine of at least ______for a contact time of at least 30 minutes at design flow.
a) 10 mg/L
b) 0.1 mg/L
c) 2.5 mg/L
d) 1.0 mg/L
18. Extract of Seeds of Moringa Oleifera (drumstick) has the potential to replace chemical coagulant since it contains about 36% natural coagulative______________.
a) Protein
b) Carbohydrate
c) Lipid
d) Glycolipids
19. Bone char filters are effective for the removal of:
a) Organic matter and decreased chlorine demand
b) Heavy metals such as arsenic and fluoride
c) Pathogenic microorganisms such as bacteria and viruses
d) Colloidal particles and colour-causing molecules
20. Chemical coagulant can be used to reduce______ between colloidal particles.
a) Electrostatic attraction
b) Intermolecular or Van der Waals force of attraction
c) Gravitational force of attraction
d) Electrostatic repulsion
21. The enmeshment of colloidal particles in a hydrolysed chemical coagulant precipitate is known as:
a) Perikinetic floc
b) Orthokinetic floc
c) Sedimentation
d) Sweep floc
22. The random motion imparted to the particle by collisions between the molecules of the fluid surrounding the particle and the particle is known as:
a) Floatation
b) Sedimentation
c) Brownian motion
d) Sedimentation coefficient
23. Laboratory studies using the _____ are adequate to select the optimum coagulant dose for water treatment.
a) Column test
b) Hydraulic flocculation test
c) Gravimetric test
d) Jar test
24. The rate of aggregation of destabilised colloidal particles in a simple coagulation and flocculation is dependent upon:
a) Size of the particles
b) Charges on the particles
c) Amount of bulk fluid and density of particles
d) The rate at which collisions occur between particles
25. The resultant force (Fnet) responsible for floating particles in a fluid can be theoretically estimated using the following formula (Fg = gravitational force, Fb = buoyant force and Fd = Drag force).
a) Fnet = Fg – Fb – Fd
b) Fnet = Fg + Fb – Fd
c) Fnet = Fg – Fb + Fd
d) Fnet = Fb – Fg – Fd
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References
- Metcalf & Eddy (2014). Wastewater Engineering: Treatment and Resource Recovery, McGraw-Hill Education, 5th edition.
- De, Anil Kumar and De, Arnab Kumar (2024). Environmental Chemistry, New Age International, 11th Edition.
- Singh, J.S., Gupta, S.R., Singh, S.P. & Singh, R. (2026). Ecology, Environmental Science and Conservation, S Chand Publishing, 2nd Edition.
- Erach Bharucha (2017). Environmental Studies, Universities Press, 4th Edition.
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