Water Treatment – DM Water MCQs With Answer

Introduction: Demineralized (DM) water is a critical component in pharmaceutical manufacturing, used where ionic purity is essential for product quality and process consistency. This blog provides focused multiple-choice questions on DM water treatment tailored for M.Pharm students studying MIP 203T – Pharmaceutical Production Technology. The questions cover principles of ion-exchange, reverse osmosis, electrodeionization, resin chemistry, regeneration, monitoring parameters (conductivity, resistivity, TOC), microbial control, and regulatory expectations. These items are designed to test conceptual understanding and practical considerations in producing, validating, and maintaining DM water systems—helpful for exam preparation and applied learning in industrial water management.

Q1. Which process is primarily responsible for removal of dissolved ionic species to produce demineralized water?

• Membrane filtration excluding ions
• Ion-exchange using cation and anion resins
• Activated carbon adsorption
• Ultraviolet oxidation

Correct Answer: Ion-exchange using cation and anion resins

Q2. In a mixed-bed ion exchange unit for DM water, the resin bed contains:

• Only strong acid cation resin
• Only strong base anion resin
• A physical mix of cation and anion resins to polish water
• Granular activated carbon and sand

Correct Answer: A physical mix of cation and anion resins to polish water

Q3. Which parameter is most commonly used to infer ionic purity of DM water in real time?

• pH measurement
• Conductivity (or resistivity) measurement
• Total Organic Carbon (TOC)
• Dissolved oxygen

Correct Answer: Conductivity (or resistivity) measurement

Q4. Which regeneration chemical pair is standard for strong acid cation and strong base anion resins respectively?

• NaOH for cation and HCl for anion
• HCl for cation and NaOH for anion
• H2SO4 for anion and NaCl for cation
• NaHCO3 for cation and H2O2 for anion

Correct Answer: HCl for cation and NaOH for anion

Q5. Which unit operation is most effective for removing silica that can pass through softening and basic ion exchange steps?

• Sand filtration
• Reverse osmosis (RO)
• Chlorination
• Zeolite softening

Correct Answer: Reverse osmosis (RO)

Q6. Electrodeionization (EDI) differs from conventional ion exchange because it:

• Relies solely on chemical regeneration with acids and bases
• Uses electrically driven ion migration to continuously regenerate resins without chemical regenerants
• Removes only organics and not ions
• Is a form of activated carbon adsorption

Correct Answer: Uses electrically driven ion migration to continuously regenerate resins without chemical regenerants

Q7. What is the typical resistivity of ideal deionized water at 25°C?

• 0.1 ohm·cm
• 18.2 megohm·cm
• 1000 ohm·cm
• 1 microsiemens/cm

Correct Answer: 18.2 megohm·cm

Q8. Which contaminant is primarily measured by TOC in DM water systems?

• Ionic hardness (Ca2+, Mg2+)
• Total organic carbon representing organic impurities
• Colloidal silica
• Residual chlorine

Correct Answer: Total organic carbon representing organic impurities

Q9. Breakthrough in an ion-exchange column is defined as:

• Complete removal of fouling organisms from the resin
• The point when target ions begin to appear in the effluent at specified concentration
• The moment when resin is completely dry
• The initial loading of fresh resin into the column

Correct Answer: The point when target ions begin to appear in the effluent at specified concentration

Q10. Which of the following is a major cause of resin fouling in DM water systems?

• High-quality prefiltration and softening
• Colloidal silica, iron, organics, and microbial biofilms
• Properly controlled regeneration cycles
• Use of dechlorination steps

Correct Answer: Colloidal silica, iron, organics, and microbial biofilms

Q11. For pharmaceutical DM water intended as feed to purity-critical systems, which microbial specification is most relevant?

• Endotoxin levels (EU/mL) and viable counts
• Only total dissolved solids (TDS)
• pH variability across seasons
• Sulfate concentration exclusively

Correct Answer: Endotoxin levels (EU/mL) and viable counts

Q12. Which pretreatment is essential when feed water contains residual chlorine to protect anion exchange resins and RO membranes?

• Chlorination at higher dose
• Activated carbon to remove free chlorine
• Direct acid dosing
• Heating to 100°C

Correct Answer: Activated carbon to remove free chlorine

Q13. In DM water systems, the conversion between conductivity and resistivity is:

• Directly proportional if temperature is constant; resistivity is the reciprocal of conductivity
• Irrelevant because they measure different contaminants
• Dependent on TOC only
• Only valid for organic-free water

Correct Answer: Directly proportional if temperature is constant; resistivity is the reciprocal of conductivity

Q14. A common strategy to control microbial growth in DM water storage tanks and distribution loops is:

• Maintaining stagnant water for long periods
• Maintaining circulating flow, sanitization (thermal/chemical), and using sanitary materials
• Eliminating all filtration steps
• Adding nutrients to favor benign flora

Correct Answer: Maintaining circulating flow, sanitization (thermal/chemical), and using sanitary materials

Q15. Which regulatory pharmacopeia specification is directly relevant when selecting water for pharmaceutical preparations requiring DM water as feed?

• USP and EP monographs for Purified Water and Water for Injection
• EPA drinking water standards only
• Food additive GRAS listings exclusively
• Cosmetic ingredient lists

Correct Answer: USP and EP monographs for Purified Water and Water for Injection

Q16. Which analytical method is most appropriate for detecting trace ionic contaminants like sodium in DM water at low ppm levels?

• Colorimetric pH paper
• Ion chromatography or atomic absorption spectroscopy
• Standard turbidity meter
• Visual inspection

Correct Answer: Ion chromatography or atomic absorption spectroscopy

Q17. The principal advantage of integrating RO before ion-exchange for DM water production is:

• RO increases bioburden dramatically
• RO reduces ionic and particulate load, reducing resin fouling and regenerant consumption
• RO negates the need for any downstream polishing
• RO increases silica content in feed to resins

Correct Answer: RO reduces ionic and particulate load, reducing resin fouling and regenerant consumption

Q18. When monitoring DM water quality, an unexplained sudden rise in conductivity most likely indicates:

• Improved resin performance
• Resin exhaustion or a leak in pre-treatment allowing ions through
• Lower temperature of water
• Decreased feed water hardness

Correct Answer: Resin exhaustion or a leak in pre-treatment allowing ions through

Q19. Which statement about silica behavior in ion-exchange systems is correct?

• Silica is never removed by ion-exchange and is irrelevant to DM water
• Silica can pass through cation exchangers and be removed inefficiently by anion resins unless pretreated or removed by RO
• Silica is only removed by UV irradiation
• Silica does not foul resins and requires no control

Correct Answer: Silica can pass through cation exchangers and be removed inefficiently by anion resins unless pretreated or removed by RO

Q20. Which practice is essential during resin regeneration to ensure optimal DM water quality after the cycle?

• Skipping rinse steps to save water
• Thorough backwash, proper contact time with regenerant, and extensive rinsing to remove residual regenerant
• Leaving resin in contact with regenerant indefinitely
• Regenerating only once per year irrespective of capacity

Correct Answer: Thorough backwash, proper contact time with regenerant, and extensive rinsing to remove residual regenerant

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