Ultrafiltration (UF)

Efficient removal of fine suspended solids

Ultrafiltration is a pressure-driven membrane technology for removing very fine suspended solids, colloids and most microbiological particles from water. It forms an effective filtration step within industrial water treatment and is used, among other things, as pretreatment for reverse osmosis, for surface water treatment and within membrane bioreactors (MBR).

Because dissolved salts and ions largely pass through the UF membrane, ultrafiltration has hardly any effect on the salt load of the water.

What is ultrafiltration and how does it work?

In ultrafiltration, water is passed through a (semi-permeable) membrane at low pressure. The membrane pores retain fine suspended solids, colloids, bacteria and larger organic compounds, while water and dissolved salts largely pass through the membrane.

Separation by particle size: Separation is based mainly on the size of the particles present.

Pore size: UF membranes have very small pores, typically in the order of a few tens of nanometres. This allows very fine suspended solids and most microbiological particles to be removed efficiently.

No desalination: Ultrafiltration does not remove dissolved salts in the way a reverse osmosis system does. As a result, the ion balance and conductivity of the water remain largely unchanged.

Choice of material: polymeric and ceramic membranes

Polymeric membranes are used for most industrial ultrafiltration applications. They are widely applicable and are the standard solution for applications such as surface water treatment and RO pretreatment.

Ceramic membranes are used for specific applications that require high chemical, thermal or mechanical resistance. At PCA Water, for example, they are used to separate oil from degreasing baths in the surface treatment industry.

The choice of membrane type depends on the composition, temperature and degree of fouling of the water stream to be treated.

When should you choose ultrafiltration?

Ultrafiltration is used when very fine suspended solids need to be removed from a water stream. The technology can be applied either as a stand-alone filtration step or as pretreatment for a subsequent treatment technique.

Protecting downstream systems

UF can be used as pretreatment for a reverse osmosis system, for example. Removing suspended solids and colloids beforehand reduces the load on the RO membranes and can reduce fouling.

Ultrafiltration has hardly any effect on the chemical composition of the water: dissolved salts and other small dissolved components are removed only to a very limited extent, if at all.

Removing fine suspended solids

When conventional filtration techniques are not enough to remove very fine particles, ultrafiltration can provide an additional barrier. The achievable water quality always depends on the composition of the feed water, the membranes chosen and the operating conditions.

Pretreatment for water reuse

Ultrafiltration can be part of a water reuse system, for example as pretreatment for a further membrane step such as reverse osmosis. Whether internal water reuse is possible is always determined by the required water quality and the complete treatment train.

Filtration cycle and cleaning

A UF system consists of more than just the membrane itself. To keep membrane fouling under control, filtration is combined with various cleaning steps.

Depending on the type of system, the following steps may be applied:

Filtration: during normal operation, the water is filtered through the membrane.
Backwash (BW): periodic backwashing to remove deposited material from the membrane.
Chemically Enhanced Backwash (CEB): a backwash supported by chemicals to remove more persistent fouling.
Cleaning in Place (CIP): a more intensive chemical clean, carried out periodically when the normal cleaning cycles are not sufficient.

These cleaning steps limit membrane fouling and can significantly extend membrane life.

Dead-end versus crossflow

The choice between a dead-end and a crossflow configuration depends on factors such as the fouling load, the properties of the feed water and the desired mode of operation.

Dead-end UF

In dead-end ultrafiltration, the entire feed stream is passed through the membrane. The retained contaminants stay on the membrane surface and are removed periodically by backwashing and, where necessary, chemically enhanced cleaning.

Dead-end UF is particularly suitable for water streams with a relatively low fouling load and is used, among other things, for surface water treatment and as pretreatment for RO systems.

Crossflow UF

In crossflow ultrafiltration, the feed water flows along the membrane surface. Only the filtrate or permeate passes through the membrane, while the remaining water stream is recirculated along the membrane.

The continuous flow along the membrane helps to limit deposits on the membrane surface. Crossflow is therefore used for water streams with a higher fouling load or for specific process streams.

However, the required recirculation flow means that crossflow generally uses more energy than a dead-end configuration.

In which applications do we use UF?

Our ultrafiltration solutions are widely used in industries where water quality is critical:

Surface water

Ultrafiltration can be used to treat river, canal or other surface water. Suspended solids, colloids and most microbiological particles are removed before the water is treated further or used as process water.

RO pretreatment

UF is an efficient pretreatment for reverse osmosis. By removing fine suspended solids and colloids, it reduces the fouling load on the RO membranes.

Part of an MBR

In a membrane bioreactor (MBR), ultrafiltration is used to separate the treated water from the biomass. The filtrate can be discharged or treated further, while the bacteria and activated sludge are retained in the biological system.

Surface treatment

In the surface treatment industry, ceramic membranes can be used for specific applications, such as separating oil from degreasing baths. This can extend the life of the process bath.

Pilot trials and sizing

The performance of an ultrafiltration system depends heavily on the composition and degree of fouling of the water to be treated.

For specific or hard-to-predict water streams, PCA Water can carry out pilot trials using a fully automated pilot unit. It is equipped with continuous process monitoring and data logging, so key process parameters can be tracked and analysed accurately.

The data collected can be used to determine factors such as the achievable flux, fouling rate, cleaning frequency and optimal operating settings. This makes the pilot trial a reliable basis for sizing and designing the final system.

PCA Water handles both the execution and the follow-up and evaluation of these pilot trials.

FAQ: frequently asked questions about UF

How do I know whether ultrafiltration is suitable for my application?

This depends on the composition of the water, the contaminant load and the required quality after treatment. For more complex water streams, a pilot trial can help determine feasibility and the optimal configuration.

Does UF also remove dissolved salts from the water?

No. Ultrafiltration mainly retains suspended solids, colloids and larger particles. Dissolved salts and small ions largely pass through the membrane. To remove dissolved salts, reverse osmosis, for example, is used.

What is the difference between dead-end and crossflow UF?

In dead-end UF, the entire feed stream passes through the membrane and the accumulated fouling is removed periodically. In crossflow, a large proportion of the water flows along the membrane and only the filtrate passes through it. Crossflow is therefore better suited to certain streams with a higher contaminant load, but generally requires more energy.

How are UF membranes cleaned?

Depending on the system, various cleaning methods are combined, such as backwash (BW), chemically enhanced backwash (CEB) and periodic Cleaning-in-Place (CIP).

Experience the difference with PCA Water.

Would you like to know which UF configuration is best suited to your wastewater? Get in touch with us.

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