1. What is filtration?
Filtration is a solid–liquid separation process using a porous medium (cloth, membrane, mesh).
Solids remain as cake, liquid passes as filtrate.
Used in chemical & pharma for product recovery, clarity improvement, impurity removal.
Driven by vacuum, pressure, gravity, or centrifugal force.
2. What is the principle of filtration?
Based on size exclusion: solids larger than pore size are retained.
Driven by a pressure difference across the filter medium.
Involves cake resistance + media resistance controlling flow rate.
Efficiency depends on pore size, DP, viscosity, and particle size.
3. Types of industrial filters.
Nutsche / ANFD (Agitated Nutsche Filter Dryer)
Filter Press (Plate & Frame / Chamber / Membrane)
Sparkler filter
Leaf filter
Candle filter
Bag / Cartridge filters
Membrane filters (UF, MF, NF, RO)
Rotary vacuum drum filter
Application chosen based on solids %, clarity, cake thickness, and process requirements.
4. What is a Nutsche filter?
A batch pressure or vacuum filter with a vertical cylindrical vessel.
Operates as filtration + washing + drying unit.
Used for API crystals, sensitive products, and solvent processes.
Ensures closed system operation and high purity.
5. What is an ANFD (Agitated Nutsche Filter Dryer)?
Advanced version of Nutsche with agitation + filtration + washing + drying in one machine.
Provides uniform cake washing, drying under vacuum or nitrogen, and heel removal.
Ensures GMP, closed handling, and solvent containment.
Eliminates multiple equipment transfers.
6. Difference between pressure filtration and vacuum filtration.
Pressure Filtration:
Uses positive pressure to push liquid through cake.
Faster, suitable for viscous slurries and fine particles.
Vacuum Filtration:
Uses negative pressure to pull filtrate.
Suitable for large volumes, low-viscosity slurries.
Used in sparkler, leaf, and rotary drum filters.
7. What is cake filtration?
Filtration where solids build a cake layer on the cloth.
Cake acts as an additional filter medium, improving clarity.
Filtration rate decreases as cake thickness increases.
Common in filter press, Nutsche, ANFD.
8. What is depth filtration?
Filtration inside the thickness of the medium, not just on the surface.
Particles are trapped within the porous internal structure.
Used for fine polish filtration, e.g., cartridges, sand filters.
Good for removing colloids, fines, and turbidity.
9. What is surface filtration?
Filtration occurs on the surface of the cloth or membrane.
Forms a clear boundary cake.
Used in Nutsche, filter press, sparkler, leaf filters.
Faster washing and discharge of cake.
10. What is filter cloth?
A porous medium used to retain solids while passing filtrate.
Materials include PP, polyester, nylon, PTFE, cotton.
Cloth selection depends on:
Particle size
Chemical compatibility
Filtration rate
Temperature and solvent resistance
Crucial for clarity, cake formation, and cycle time.
11. How to select a filter cloth?
Based on particle size distribution of solids.
Consider chemical compatibility with solvent.
Select proper pore size / mesh size.
Consider cake release characteristics (sticky vs free-flowing).
Evaluate temperature resistance and cloth strength.
Optimize for filtrate clarity + filtration rate.
12. Why pore size is important in filters?
Determines clarity of filtrate.
Controls solids retention and cake formation.
Too large → solids pass → cloudy filtrate.
Too small → slow filtration → long cycle time.
Must match crystal size distribution and process requirement.
13. What is filter cake?
Solid layer deposited on filter medium during filtration.
Acts as secondary filtration layer improving clarity.
Properties depend on particle size, compressibility, and solids %.
Impacts cycle time, washing efficiency, and moisture.
14. What is cake resistance?
Resistance offered by cake layer to filtrate flow.
Increases with cake thickness and compressibility.
A key parameter in Darcy’s equation.
High resistance → slow filtration and higher DP.
15. What is Darcy’s law?
Fundamental filtration equation relating flow rate to pressure drop and resistance.
Formula:
Q = (ΔP × A) / (μ × R)
Where:
Q = Filtration flow rate
ΔP = Pressure difference
A = Filtration area
μ = Filtrate viscosity
R = Total resistance (media + cake)
Used to size filters and predict filtration time.
16. What is filtration rate?
Volume of filtrate collected per unit time.
Indicates filtration efficiency and equipment capacity.
General relation:
Filtration Rate ∝ ΔP / (Viscosity × Resistance)
Faster rate desired but must be balanced with clarity and cake formation.
17. Factors affecting filtration rate.
Pressure / Vacuum level applied
Viscosity of slurry
Particle size and nature (compressible vs non-compressible)
Cake thickness
Filter cloth pore size
Temperature (affects viscosity)
Solids concentration
18. Why vacuum is used in filtration?
Provides continuous, steady flow for large volumes.
Useful for low-viscosity slurries.
Lower energy compared to pressure filtration.
Ensures safe handling of flammable solvents in closed systems.
19. Why pressure is used in filtration?
Needed for viscous or fine particle slurries.
Produces faster filtration by increasing driving force.
Achieves lower cake moisture and better washing.
Used in ANFD, pressure Nutsche, filter press.
20. What is pre-coat in filtration?
Layer of filter aid (e.g., Celite) applied before filtration.
Creates a porous, uniform layer to capture fines.
Prevents filter cloth blinding.
Enhances clarity, flow rate, and cake release.
21. Why filter aid (e.g., Celite) is used?
Helps trap fine particles that would pass through cloth.
Forms a porous pre-coat layer for higher clarity.
Reduces cloth choking and increases filtration rate.
Improves cake discharge and prevents sticking.
Useful for colloidal or gelatinous solids.
22. What is heel volume?
Residual slurry or solids left at the bottom after filtration.
Occurs in Nutsche filters and ANFD.
Affects yield and must be minimized.
Removed through agitation, reslurry, or nitrogen blow-down.
23. What is slurry?
Mixture of solids suspended in liquid.
Filtration behavior depends on:
Solids %
Viscosity
Particle size
Temperature
Proper slurry conditioning improves cycle time and clarity.
24. What is wet cake?
Solid cake containing residual filtrate after filtration.
Moisture content depends on:
Pressure or vacuum level
Cake thickness
Particle size
Filtration time
Further dried by nitrogen blow, vacuum drying, or thermal drying.
25. How to reduce cake moisture?
Increase pressure/vacuum applied.
Lower slurry viscosity by heating.
Optimize cake thickness.
Use membrane squeezing (filter press).
Use nitrogen blowing in Nutsche/ANFD.
Improve particle size distribution upstream.
26. How to improve filtrate clarity?
Use finer cloth pore size.
Apply pre-coat.
Reduce feed rate to avoid channeling.
Maintain constant DP.
Improve upstream crystallization to grow larger crystals.
Use depth filter cartridges for polishing.
27. What is filter blinding?
Permanent blockage of filter medium pores by fines, oils, or sticky solids.
Leads to high DP, low flow, and longer cycle time.
Prevented by proper pre-coat, cloth washing, and feed conditioning.
Requires chemical cleaning or cloth replacement when severe.
28. What causes filter cloth choking?
Excessive fines or small particles.
High slurry viscosity.
Sticky or compressible solids.
Inadequate cloth washing or pre-coating.
Leads to slow filtration and poor clarity.
29. What is candle filter?
Pressure filter with vertical porous elements (candles).
Filtration occurs on the candle surface → cake builds outward.
Suitable for high clarity, fine solids, and solvent filtration.
Offers automatic cake discharge and backwashing.
30. What is a sparkler filter?
Vertical plate-and-frame pressure filter.
Contains multiple horizontal filter plates with cloth.
Used for high-volume polishing filtration.
Provides clear filtrate but requires manual cake removal.
31. What is a leaf filter?
A pressure or vacuum filter with multiple vertical filter leaves.
Used for large-volume slurry filtration with good clarity.
Offers easy cake discharge and minimal cloth area.
Common in chemical, pharma, and edible oil industries.
32. What is a membrane filter?
A thin polymer membrane that separates particles by size exclusion.
Used in sterile filtration, water purification, and solvent filtration.
Available in 0.2 µm, 0.45 µm etc.
Ensures high clarity and microorganism removal.
33. What is a HEPA filter?
High-Efficiency Particulate Air filter.
Removes 99.97% particles ≥ 0.3 µm.
Used in cleanrooms, HVAC, sterile areas, isolators.
Provides contamination-free environment in pharma.
34. What is sterile filtration?
Filtration to remove microorganisms using 0.2 µm membrane filters.
Used for heat-sensitive solutions (e.g., enzymes, APIs).
Ensures sterility without heat or chemicals.
Final step before aseptic filling.
35. What is micron rating?
Indicates pore size of filter media in micrometers (µm).
Determines the smallest particle that can be retained.
Lower micron rating → higher clarity.
Must match process safety and purity requirements.
36. What is differential pressure in filters?
Difference in pressure across the filter medium.
Indicates how hard filtrate is pushing through cake + cloth.
Higher DP → cake buildup or cloth choking.
Used for process monitoring and cycle termination.
37. Why DP increases during filtration?
Cake thickness increases.
Cloth pores start clogging.
Slurry viscosity increases due to cooling.
Solids concentration rises with time.
Signal to stop filtration or start washing/drying.
38. What is backwashing?
Reversing flow of filtrate or clean liquid through filter medium.
Removes trapped solids and unclogs pores.
Used in candle filters, cartridge filters, leaf filters.
Restores flow rate and reduces DP.
39. What is CIP in filters?
Clean-In-Place system for online cleaning without opening the filter.
Removes residual solids and product.
Ensures GMP compliance and batch changeover cleanliness.
Uses water, solvents, detergents, or caustic solution.
40. How to check filter integrity?
By performing bubble point test, diffusion test, or pressure hold test.
Ensures membrane or cloth is undamaged and not leaking.
Mandatory for sterile and critical fluid filtration.
Prevents contamination and batch failures.
41. Bubble point test – what is it?
A test to check integrity of membrane filters.
Measures the minimum pressure needed to force air bubbles through a wetted membrane.
Low bubble point indicates damaged membrane or enlarged pores.
Mandatory for sterile filtration validation.
42. What is filter validation?
Ensures the filter performs consistently and reliably for its intended use.
Includes: integrity testing, microbial retention tests, chemical compatibility, and flow rate tests.
Required for pharma GMP and regulatory compliance.
Documents proof that the filtration process ensures product quality.
43. What is a bag filter?
A simple gravity or pressure filter using fabric bags as the medium.
Used for coarse filtration and removal of large particles.
Easy to operate and replace.
Common in utilities, water systems, and chemical processes.
44. What is a cartridge filter?
Cylindrical filter element used for fine and polishing filtration.
Available in 1–50 µm (depth filters) or 0.2–0.45 µm (membrane filters).
Quick replacement, compact, and gives high clarity.
Common in solvent, water, and sterile filtration.
45. What is a basket filter?
Strainer-type filter with a perforated or mesh basket inside.
Removes large debris, fibers, and solids from liquids.
Protects pumps and downstream equipment.
Used in utilities, chemical industries, and pre-filtration.
46. What is sizing in filtration?
Determining required filter area and capacity based on slurry load and filtration rate.
Depends on:
Solids concentration
Filtration rate
Cake thickness
Required cycle time
Critical for selecting the right filter equipment.
47. How does viscosity affect filtration?
High viscosity → slow filtration, higher DP, wet cake.
Low viscosity → faster filtration, better clarity.
Temperature control reduces viscosity and improves flow.
Must be considered for solvent selection and process design.
48. How does temperature affect filtration?
Higher temperature lowers viscosity → faster filtration.
Too high temperature may cause solvent evaporation or product degradation.
Low temperature increases viscosity → poor flow and slow filtration.
Used to optimize slurry handling for efficiency.
49. Why filtrate becomes cloudy?
Filter cloth pores too large.
Cloth damaged or torn.
Very fine particles or colloids.
High feed velocity causing channeling.
Slurry viscosity causing incomplete retention.
Fixed by finer cloth, pre-coat, or slower feed.
50. Difference between polishing filter and primary filter.
Primary filter:
Handles high solids load.
Removes bulk solids and forms cake.
Examples: Nutsche, filter press, sparkler.
Polishing filter:
Removes traces of fines to achieve high clarity.
Examples: cartridge filter, membrane filter.
Used downstream of primary filters.
51. What is crossflow filtration?
Filtration where feed flows parallel to the membrane surface, not perpendicular.
Minimizes cake buildup and reduces fouling.
Used in UF, MF, NF, RO systems.
Provides stable flux and clarity over long operation.
52. What is ultrafiltration (UF)?
Membrane filtration with pore size 1–100 nm.
Removes proteins, colloids, polymers, and fine particles.
Retains macromolecules; lets water and small solutes pass.
Used in biotech, wastewater, pharma purification.
53. What is microfiltration (MF)?
Membrane filtration with pore size 0.1–10 µm.
Removes bacteria, suspended solids, and fine particles.
Used for pre-filtration before UF/RO systems.
Low pressure and high flow application.
54. What is nanofiltration (NF)?
Membrane filtration between RO and UF.
Removes divalent salts, organics, dyes, pesticides.
Retains hardness ions (Ca²⁺, Mg²⁺) and organics.
Used in water softening, solvent purification.
55. What is reverse osmosis (RO)?
High-pressure membrane filtration that removes ions, salts, dissolved solids.
Requires osmotic pressure > natural osmotic pressure.
Formula:
Applied Pressure > π
(π = osmotic pressure)
Produces high-purity water for pharma, utilities, boilers.
56. What is flux rate?
Volume of filtrate passing per unit membrane area per time.
Formula:
Flux (J) = V / (A × t)
V = filtrate volume
A = membrane area
t = time
Indicates membrane productivity; declines due to fouling.
57. What is membrane fouling?
Accumulation of particles, organics, salts, or microbes on membrane surface.
Causes flux decline, high pressure drop, and shorter membrane life.
Requires periodic CIP, backwash, or chemical cleaning.
58. Causes of membrane fouling.
High solids load.
Colloidal particles and organics.
Biofilm formation (biofouling).
Scale deposition (CaCO₃, MgSO₄).
Improper pre-treatment or pH control.
59. What is diafiltration?
Process of adding fresh solvent while filtering to wash out impurities.
Used to remove salts or small molecules from product.
Common in biotech, protein purification, enzyme solutions.
60. What is filter area?
Total surface area available for filtration.
Higher filter area → faster filtration and lower cycle time.
Used to size filters for batch or continuous operation.
Critical parameter for Nutsche, filter press, leaf filter design.
61. What is cake compressibility?
Measures how cake resistance changes with pressure.
Compressible cake → resistance increases at higher pressure → slower filtration.
Incompressible cake → stable resistance → smooth filtration.
Important for selecting pressure vs vacuum filtration method.
62. What is permeability?
Ability of filter cake or media to allow liquid flow through it.
High permeability → fast filtration, low moisture.
Low permeability → slow filtration, wet cake.
Depends on particle size, porosity, and compressibility.
63. What is tortuosity?
Describes complexity of pore pathways within filter media.
Higher tortuosity → longer liquid path → slower flow.
Affects clarity and pressure drop.
Critical for membrane and depth filter design.
64. What is flow-through filter vs dead-end filter?
Flow-through (Crossflow):
Feed flows parallel to membrane.
Lower fouling, stable flux.
Used in UF/MF/NF/RO.
Dead-end filter:
Feed flows perpendicular to media.
Cake builds on surface.
Used in Nutsche, filter press, sparkler.
65. What is a filter press?
Batch filtration system using plates + cloth to form cake under pressure.
Produces dry cake and clear filtrate.
Suitable for high solids slurries.
Common in chemical, pharma, ETP, and mineral industries.
66. Plate & frame filter press – working principle?
Slurry pumped between plate-frame chambers.
Solids deposit as cake; filtrate exits through channels.
Easy cake washing and cloth changing.
Good for thin cakes and clarity control.
67. Chamber filter press – difference?
Plates form built-in chambers without frames.
Larger cake volume than plate & frame.
Faster operation and better dewatering.
Suitable for high solids and thicker cakes.
68. What is cloth washing?
Cleaning filter cloth to remove embedded solids.
Prevents cloth blinding and restores filtration rate.
Done using water spray, CIP, chemical wash, or steam.
Extends cloth life and improves clarity.
69. What is filter cake washing?
Passing clean solvent or water through the cake to remove impurities and mother liquor.
Improves product purity and yield.
Washing modes: displacement washing, percolation washing, reslurry washing.
70. Why washing efficiency matters?
Ensures minimal impurity retention in cake.
Reduces solvent consumption during washing.
Improves drying efficiency and product quality.
Critical for pharmaceutical API purity and regulatory compliance.