FRERE pressurized and submerged ultrafiltration systems for industrial water treatment

Pressurized vs. Submerged Ultrafiltration: How to Choose the Right UF System

Тамыз 12 2026, -ы/і арқылы WANGZEYU, 11 мин оқу уақыты

Pressurized and submerged ultrafiltration systems can both provide reliable particulate and microbial control, but they differ in hydraulic design, footprint, energy use, cleaning strategy, and application fit. This guide explains the key selection factors for RO pretreatment, municipal water, industrial process water, and wastewater reuse projects.

Ultrafiltration (UF) is widely used to remove suspended solids, colloids, microorganisms, and other fine contaminants from water. It can serve as a standalone clarification process or as pretreatment for reverse osmosis (RO), nanofiltration, ion exchange, and other downstream technologies.

However, choosing an ultrafiltration membrane is only part of the design decision. Engineers must also determine how the membrane system will operate.

Two of the most common configurations are pressurized ultrafiltration and submerged ultrafiltration. Both use a membrane barrier, but their hydraulic arrangement, operating conditions, cleaning methods, and infrastructure requirements are different.

There is no universally better configuration. The correct choice depends on the feedwater, treatment objective, plant layout, operating philosophy, and total lifecycle cost.

What Is Pressurized Ultrafiltration?

In a pressurized UF system, feedwater is pumped into closed membrane modules. The applied pressure drives water through the hollow-fiber membrane, while suspended solids, colloids, and microorganisms are retained.

Depending on the membrane and module design, filtration may operate in an inside-out or outside-in direction. The modules are normally installed in racks and connected to feed, filtrate, backwash, air-scour, drain, and chemical-cleaning lines.

Pressurized UF systems are often considered for:

Pretreatment before RO or NF
Municipal drinking-water treatment
Industrial process-water production
Surface-water and groundwater clarification
Seawater desalination pretreatment
Projects requiring a compact, modular treatment train

Because the modules are enclosed, flow, pressure, transmembrane pressure, and filtrate quality can be monitored within a controlled piping system. Individual racks or modules may also be isolated for inspection or maintenance, depending on the system design.

What Is Submerged Ultrafiltration?

In a submerged UF system, membrane modules are installed directly inside an open tank or basin. Filtrate is normally withdrawn by suction or hydraulic head, while aeration beneath the membrane modules helps limit solids accumulation and controls fouling.

Submerged UF is often considered for:

Tertiary wastewater treatment
Reclaimed-water production
Advanced municipal wastewater treatment
Large-scale municipal water projects
Retrofit projects with usable existing basins
Applications that favor low-pressure filtration

The tank becomes part of the filtration process. This can reduce the need for enclosed pressure vessels, but the design must account for tank hydraulics, membrane access, aeration distribution, solids removal, water level, and lifting space for maintenance.

Pressurized and Submerged UF at a Glance

Selection factor	Pressurized UF	Submerged UF
Hydraulic arrangement	Feedwater is pumped through enclosed modules	Membranes are immersed in a tank and filtrate is withdrawn by suction or hydraulic head
Typical installation	Modular rack with closed piping	Membrane cassette or module installed in an open basin
Common application fit	RO pretreatment, drinking water, industrial process water	Wastewater polishing, reuse, municipal treatment, basin-based retrofits
Driving force	Positive feed pressure	Suction pressure or available hydraulic head
Fouling control	Backwash, relaxation, air scour, CEB and CIP as required	Aeration, relaxation, backwash, CEB and CIP as required
Footprint consideration	Compact and highly integrated equipment layout	Can make use of existing tanks, but basin and maintenance space are required
Maintenance access	Modules can often be isolated within a rack	Modules may need to be lifted from the basin for inspection
Expansion approach	Additional modules or racks	Additional membrane cassettes or basins

These are general tendencies rather than fixed rules. Actual performance depends on membrane design, operating flux, feedwater quality, cleaning protocol, recovery target, redundancy, and control strategy.

Seven Factors That Should Drive the Selection

1. Feedwater Quality and Variability

The first question is not which membrane configuration looks simpler. It is what the system must treat.

Important feedwater parameters include:

Turbidity and total suspended solids
Particle-size distribution
Colloidal and organic loading
Oil and grease
Algae and biological activity
Temperature and seasonal variation
pH and chemical composition
Potential membrane foulants or cleaning limitations

A relatively stable, pre-screened feed may suit a compact pressurized system. A wastewater stream with higher solids loading may favor a submerged configuration with continuous air scouring and a process tank designed for solids management.

This is not an automatic rule. Pretreatment, membrane chemistry, flux, recovery, and cleaning strategy can change the result. Feedwater analysis should therefore come before equipment selection.

2. Treatment Objective and Downstream Protection

If UF is used before RO, the objective is usually to provide consistently low-turbidity feedwater and reduce particulate or colloidal loading on the RO membranes.

In this case, the UF system must be evaluated as part of the complete pretreatment train. The design should consider:

Required filtrate quality
RO feed requirements
Upstream coagulation or media filtration
Final cartridge or security filtration
UF integrity monitoring
The consequences of a feedwater upset

Pressurized UF is frequently selected for RO pretreatment because it integrates naturally with closed, pumped treatment trains. Submerged UF can also be used before RO, especially where a basin-based process or wastewater-reuse scheme already exists.

If UF is used for tertiary wastewater polishing or municipal reuse, the ability to manage variable solids and use existing tanks may become more important than integration with a high-pressure downstream process.

3. Available Footprint and Existing Infrastructure

A pressurized UF rack can provide a compact equipment footprint, but it still requires space for pumps, valves, manifolds, chemical systems, electrical controls, and maintenance access.

A submerged system may use an existing basin, which can be attractive in a retrofit. However, the apparent space saving can be misleading if the project also requires new tanks, blower equipment, lifting structures, walkways, or additional solids-handling capacity.

The correct comparison should include the entire installed system rather than only the membrane modules.

4. Energy Consumption

Pressurized UF requires feed pressure, so feed-pump energy is an important operating cost. Submerged UF operates at a lower filtration pressure, but aeration for membrane scouring can represent a significant part of its energy demand.

Energy performance depends on:

Operating flux
Transmembrane pressure
Feedwater temperature
Backwash frequency
Air-scour rate
Pump and blower efficiency
Recovery rate
Cleaning frequency
Plant turndown and operating schedule

For this reason, it is risky to claim that one UF configuration always consumes less energy. Energy should be modeled under the project's actual design and seasonal conditions.

5. Fouling Control and Cleaning Strategy

All membrane systems foul. The goal is not to eliminate fouling completely, but to control it predictably and restore performance without damaging the membrane.

Pressurized systems may use filtration cycles followed by backwash, air-assisted backwash, relaxation, chemically enhanced backwash (CEB), and periodic clean-in-place (CIP).

Submerged systems commonly combine membrane relaxation, permeate backwash, continuous or intermittent air scouring, CEB, and periodic recovery cleaning.

The cleaning strategy must match the dominant foulants. For example, mineral scale, organic matter, biological growth, oil, and metal precipitates may require different chemical and hydraulic responses.

Before selecting a membrane, confirm:

Which cleaning chemicals are permitted
Maximum chemical concentration and exposure time
Cleaning temperature limits
Required neutralization and disposal arrangements
Whether cleaning can occur without stopping the entire plant

6. Recovery, Backwash Water, and Waste Handling

High water recovery is valuable, but it should not be pursued at the expense of rapid fouling or unstable operation.

Both configurations generate backwash water, chemical-cleaning waste, and a concentrated solids stream. The amount and characteristics of this waste depend on the operating cycle and feedwater.

The project should evaluate:

Net recovery rather than gross filtration output
Backwash-water availability
Backwash and drain-tank capacity
Wastewater return or disposal route
Chemical waste handling
Impact on upstream and downstream process balance

A configuration that produces slightly more filtrate but creates a difficult waste stream may not deliver the lowest total cost.

7. Operation, Maintenance, and Expansion

The best UF system is one the plant team can operate reliably.

Pressurized racks usually offer clear module-level piping and isolation, but they may contain more valves, instruments, and automated sequences. Submerged systems can simplify the filtration pressure arrangement, but membrane lifting, tank access, aeration maintenance, and confined operating spaces must be considered.

Questions to ask include:

Can one train be maintained while the others remain online?
Is lifting equipment available for membrane removal?
Are spare-module and spare-part requirements manageable?
Can operators easily monitor normalized permeability and transmembrane pressure?
Is future capacity expansion expected?
Does the site have adequate technical support for pumps, blowers, instruments, and controls?

Which Configuration Fits Common Applications?

RO Pretreatment

Pressurized UF is often a practical choice for a new, compact RO pretreatment system because it fits a closed and modular process layout. Submerged UF may be suitable when the project includes an equalization basin, clarifier, biological process, or wastewater-reuse tank upstream of the RO system.

Municipal Drinking Water

Both configurations can be used. The decision depends on raw-water variability, plant capacity, available hydraulic head, existing civil works, redundancy requirements, and the operator's preferred maintenance strategy.

Industrial Process Water

Pressurized UF is frequently selected where the system must connect directly to existing industrial piping and protect downstream RO, ion exchange, or process equipment. Chemical compatibility and the effect of production interruptions should receive particular attention.

Wastewater Reuse

Submerged UF can be attractive for tertiary treatment and basin-based reuse projects, especially when existing tanks are available. Pressurized UF may be preferred when wastewater has already received effective solids removal and the project requires a compact packaged treatment step.

Plant Retrofit

Do not assume that reusing a tank automatically makes submerged UF less expensive, or that a packaged pressurized skid automatically minimizes project work. A retrofit survey should examine civil condition, hydraulic profile, electrical capacity, pipe routing, crane access, controls, and waste connections.

Common UF Selection Mistakes

Choosing by Membrane Pore Size Alone

Nominal pore size does not describe the complete system. Membrane structure, integrity, fouling behavior, flux, hydrodynamics, pretreatment, and cleaning all affect practical performance.

Comparing Only Membrane-Module Price

The lowest module price may not produce the lowest lifecycle cost. Compare pumps, blowers, tanks, racks, piping, chemical consumption, labor, membrane replacement, downtime, waste handling, and downstream protection.

Using Maximum Flux as the Design Flux

A high published flux under favorable conditions should not automatically become the design value. Sustainable flux depends on actual feedwater, temperature, recovery, cleaning frequency, and required availability.

Ignoring Feedwater Upsets

Average water quality is not enough. Seasonal algae, storm events, oil contamination, process spills, and upstream equipment failures can determine the real design requirement.

Treating Cleaning as an Afterthought

Cleaning tanks, chemical compatibility, dosing equipment, waste neutralization, and downtime should be designed before commissioning—not after permeability begins to decline.

FRERE UF Options

FRERE offers both pressurized and submerged ultrafiltration membrane product pathways for different treatment configurations.

Frere Prisma Pressure Ultrafiltration Membrane

The Frere Prisma series uses PVDF hollow-fiber membranes in an external-pressure module configuration. It is positioned for municipal and industrial water-treatment applications requiring modular integration, stable filtrate quality, and compact system planning.

Frere Lumina Immersion Ultrafiltration Membrane

The Frere Lumina series uses PVDF composite membrane fibers in a high-density submerged module design. It is positioned for applications such as reclaimed-water reuse, advanced wastewater treatment, and municipal water projects where an immersion configuration fits the process layout.

Final membrane selection and system design must be confirmed against the actual feedwater analysis, required capacity, target filtrate quality, operating temperature, chemical-cleaning conditions, available footprint, and downstream process.

Conclusion

Pressurized and submerged ultrafiltration systems use the same fundamental separation principle, but they solve project constraints in different ways.

Pressurized UF often provides compact modular integration and a natural fit with RO pretreatment or industrial process-water systems. Submerged UF often provides advantages in basin-based treatment, wastewater reuse, and projects designed around low-pressure filtration and aeration-based fouling control.

The right decision should not be based on one specification or one equipment price. It should be based on feedwater behavior, treatment objectives, installed infrastructure, cleaning requirements, system availability, and total lifecycle cost.

Not sure which UF configuration fits your project?

Send FRERE the following information for an initial technical evaluation:

Your application, feedwater analysis, and required product-water quality
Design flow, operating temperature, operating schedule, and available footprint
Existing process layout and the downstream system that the UF stage must protect

FRERE can evaluate whether a pressurized or submerged UF configuration is more appropriate and recommend a membrane pathway for further technical confirmation.