Inside-Out Ultrafiltration (UF) Membrane — What It Is, How It Works & Where It Fits

Inside-out ultrafiltration membrane (also called an inner-pressurized UF) is a hollow-fiber membrane in which raw water enters the lumen of each fiber and permeate flows from the inside to the outside through the membrane wall under a low trans-membrane pressure (TMP). Particles, colloids, and bacteria are retained on the lumen side, while clarified water passes the pores and is collected on the shell side.

1) Inside-out ultrafiltration membrane — working principle & flow direction

Flow path for inside-out UF: feed → fiber lumen → filtration across the wall → permeate collected on the outside → concentrate leaves via the lumen. End-potting (e.g., epoxy) seals both ends so feed cannot bypass into the permeate channel. Compared with outside-in, inside-out typically operates at lower pressure and allows quick backwash/air scour to recover flux.

2) Construction & operating essentials

• Hollow fibers in parallel with sealed potting separate feed and permeate safely.
• Low TMP at ambient temperature is sufficient; stable TMP is key to lifetime.
• Backwash & chemical cleaning: periodic backwash with permeate, optional air scour, and CIP (acid/alkaline) restore performance after fouling.

3) Benefits & limitations

• Benefits: low energy, high automation, up to ~95% water recovery (application dependent), compact footprint, and simple O&M.
• Limitations: oil/viscous feeds or high fouling loads may clog fibers; reliable pretreatment and a realistic cleaning strategy are necessary.

4) Typical applications

• RO pretreatment: seawater/brackish water, surface water, and well water polishing (turbidity/Silt Density Index control).
• Drinking water polishing: mineral/spring water and municipal polishing before disinfection.
• Process water: pure/ultrapure water trains (incl. post-EDI polishing) for bacteria/colloid removal.
• Water reuse & ZLD steps: municipal/industrial reuse, cooling/condensate recovery.
• Pharma/electronics/food & beverage: bioburden control, clarification, haze removal.
• Public & residential: point-of-entry/point-of-use systems, aquaculture recirculation water.

5) Inside-out vs outside-in UF (quick comparison)

Aspect	Inside-Out UF	Outside-In UF
Feed path	Feed in lumen; permeate flows inside → outside	Feed on shell side; permeate flows outside → inside
Operating pressure	Generally lower	Typically higher
Backwash/air scour	Fast and effective	Depends on module design
Where it fits	RO pretreatment, polishing, reuse	Similar domains—choose by feed quality & footprint

6) Selection checklist (what to confirm before sizing)

• Feed quality: turbidity/SS, colloids/organics, oil, iron/manganese.
• Targets & scale: permeate flow, recovery, outlet specs (turbidity/SDI/bacteria).
• Polymer & module: common materials include PVDF and PES—check chemical and thermal limits.
• Pretreatment & dosing: coagulation/clarification, fine filtration, oxidants, acids/alkalis/reductants (ensure compatibility with the membrane).
• Cleaning strategy: backwash interval, CIP recipes/temperature, safety handling.
• Lifecycle cost: energy, chemicals, consumables, labor and planned downtime.

7) Design & O&M quick notes

• Set backwash correctly based on ΔP/flux trend; add air scour where allowed by the module.
• CIP trigger lines: run cleaning when normalized flux drops or ΔP rises to your threshold (e.g., ~10–15% change).
• Normalization: correct for temperature/viscosity before judging performance.
• Safety & compliance: chemical storage/dosing, neutralization & discharge, hygiene and lock-out/tag-out during service.

8) FAQs

Q1. Is an inside-out ultrafiltration membrane better than outside-in?
Each has its place. Inside-out often wins for low-pressure operation and quick backwash, but the best choice depends on feed quality, space, and existing utilities.

Q2. What recovery can I expect?
With suitable pretreatment and tuning, water recovery can reach around 95% in many applications; confirm with pilot data and vendor guidelines.

Q3. How does UF compare with NF/RO?
UF is a screening process that removes suspended solids, colloids, and bacteria at low pressure. NF/RO target dissolved salts and small organics and require higher pressure/energy.

Q4. Can UF be used directly for drinking water?
UF sharply lowers turbidity and microbes, but potable use typically requires post-disinfection (UV/chlorine) and compliance with local regulations.

Q5. How do I minimize fouling?
Stable pretreatment, optimized coagulant dosing, disciplined backwash/CIP, and excluding oil/large particles. Track TMP/flux/SDI to act before fouling becomes irreversible.

9) Next steps (RFQ & resources)

Een offerte aanvragen · RO sizing calculator · UF/RO systems · UF consumables & accessories · Stainless-steel tanks & skids

Engineering notice

Always confirm membrane chemical limits, compatibility and cleaning windows with the specific module supplier. Pilot testing is strongly recommended when feed quality varies seasonally or contains oil/organics.

Summary

This guide explained what an inside-out ultrafiltration membrane is, how it works, and when to choose it over outside-in. Combine realistic pretreatment, conservative flux and clear cleaning triggers to achieve reliable SDI control and long membrane life—then scale with pilot data for confident CAPEX and OPEX planning.

References & further reading

• WHO – Guidelines for Drinking-water Quality · ISO 20468-1 – Guidelines for water treatment systems