Membrana Filtration


Bubble point test [BP] The bubble point procedure is a commonly used test method to characterize a membrane and monitor product consistency and quality. The bubble point itself is a determination of the minimum pressure (bubble point) at which a wetting liquid is pressed out of the pore system of a membrane while forming a steady bubble chain. The bubble point test procedure is also a common method of determining the maximum pore size.

Backwash [BW]
Backwash is a very effective membrane cleaning mechanism where a strong reverse filtrate flow is applied for up to one minute. This is applicable for all membrane types tolerating negative TMP but is not used for spiral wound membranes such as reverse osmosis.

Cross-flow Filtration
In cross-flow filtration, a fluid (feed) stream runs tangential to a membrane, establishing a pressure differential across the membrane. The concentration polarisation of rejected fluid ingredients close to the membrane surface is limited. Remaining particles continue to flow across the membrane, "cleaning it," while the water to be filtered flows through the membrane wall.

Chemical Enhanced Backwash [CEB]
This is a term established for a cleaning method of micro and ultrafiltration modules. This is where a chemical cleaning agent is added to the backwash flow. That cleaning solution remains in the membrane module for a short period of soaking time. The cleaning agent will be discharged together with components of the fouling layer by a final backwash.

Cleaning in Place [CIP]
CIP is common for all kinds of process equipment. Contrary to CEB the cleaning solution flow is conducted across the membrane surface. CIP needs more equipment and a longer interruption of filtration service than CEB but allows the use of elevated temperatures and special cleaning agents.

Cut-Off [MWCO]
The MWCO describes the retention performance of a membrane. It is used as “nominal” value that means 90% matter of the indicated size is rejected or “absolute” if the rejection is higher than 99 %. For rejection of macromolecules the expression Molecular Weight Cut Off MWCO is used.

Dalton [Da]
This is the unit for the molecular weight (1 Da = 1 g/mol) which is widely used to characterize ultrafiltration membranes by determination of retention rates for macromolecules of different molecular mass.

Dead-end filtration
In the dead-end filtration technique all the fluid passes through the membrane, and all particles larger than the pore size of the membrane are retained on its surface. Trapped particles will start to build up a "filter cake" on the surface of the membrane, which has an impact on the efficiency of the filtration process.

Downstream side
The downstream side refers to the filtrate side of the membrane.

The feed is the unfiltered fluid that is fed to the membrane module.

Filtration is a process of removing particles of different diameters from a fluid or a gas by passing it through a permeable material, such as a membrane.

The filtrate is the fluid or gas that has already passed through the membrane and has been filtered.

Flux is a term that is commonly used for the flow rate of water/liquid through a membrane related to surface area, expressed in l/(m² h) . It characterises the flow performance of the membrane independent of the size of a system.

Deposition of substances on the surface of the membrane or inside of membrane pores is called fouling. Fouling can cause a decrease in both the amount of filtrate produced and the quality of the filtrate. The performance of fouled membranes can usually be restored with cleaning procedures.

Forward Flush [FF]
Forward Flush creates a flow along the inside of the membrane that removes particles. Here, the filtrate outlet port is closed and water will be discharged through the concentrate port for a short period.


Membranes characterized as hydrophobic are not wetted by water. They have little or no tendency to absorb water so that a droplet remains on the surface. Water does not penetrate the membrane easily.

Hydrophilic membranes have an affinity for water. Their surface chemistry allows these materials to be wetted spontaneously. Water flows easily through the membrane.

Inner diameter [ID]
A value characterizing the inner geometry of a capillary or tubular membrane. The wall thickness and the outer diameter of a membrane are the other essential parameters for characterization of a capillary or tubular membrane.

Inside-out is a term describing an operating mode of filtration membranes of tubular or capillary shape. The Feed flows in an axial direction inside the tube and penetrates through the membrane wall to the outside. The retention takes place at the inner surface of the tubular membrane.

The lumen is the inside or the inner volume of a capillary or tubular membrane.

Maximum Pore Size [PGmax]
PGmax is a calculated value obtained from the results of the bubble point test. The maximum pore size is theoretically the largest pore within the pore structure of a membrane.

Microfiltration is a way of removing contaminants in the size range of 0.1 to 10.0 µm from fluids or gases by passing the substance through a microporous medium such as a membrane. There are two techniques used in microfiltration: dead-end filtration, where microfiltration is widely used, and cross-flow filtration, using a tangential flow for the fluid being filtered.

Natural Organic Matter [NOM]
Natural Organic Matter is a collective term describing all broken down organic matter that comes from plants and animals in the environment. NOM content in water is not harmful or toxic itself but it may influence microbiological growth or create harmful compounds if oxidized by chlorine.

Outside-In describes an operating mode of filtration membranes of tubular or capillary shape. The Feed flows outside and between the membranes in the shell side of the module and penetrates through the membrane wall to the inside. The retention takes place at the outer surface of the membrane.


Permeability is based on TMF and is primarily expressed in l/(m² h bar). It can be calculated by division of Flux through TMP. The determination of permeability is useful to characterize the performance of membrane filtration systems independent of changes in the driving pressure.

Pore Size
The pore size is usually stated in micrometers (µm) and refers to the diameter of particles a membrane is likely to retain to a defined degree of efficiency. Specifically it is the size of the opening in the membrane wall.

Scaling is a special kind of fouling caused by precipitation of salt crystals by concentration polarization if the solubility limits of contained salts are exceeded.

Silt density index [SDI]
The Silt Density Index is an empirical test used to characterize the fouling potential of a feedwater stream. The test is based on measuring the rate of plugging a 0.45 micron filter using a constant feed pressure for a specified period of time.

A very practical characterization technique for a microfiltration membrane. The intention is to evaluate the achievable filtrate output during the life-time of a filter: i.e. the total volume of a specific fluid that passes through the membrane before it must be replaced. The terms dirt holding capacity or high loading capacity are also used.

Trans-membrane flow [TMF]
A measurement of the transmembrane flow is carried out to document the membrane flow characteristics. The TMF is defined as the initial volume of liquid passing through the membrane wall within a given unit of time, related to surface area and pressure, and expressed in ml/(min cm² bar).

Trans-membrane-pressure [TMP]
TMP is the driving force for membrane filtration processes. It is calculated as the difference between the feed and filtrate pressures.

Ultrafiltration is a process similar to microfiltration. The main difference is the "tighter" retention
behavior: the ultrafiltration membrane retains much smaller particles from the passage through the membrane compared to microfiltration membranes. Typically the particle size is measured by molecular weight, and ultrafiltration membranes have retention ranges from 1,000 to 500,000 Dalton with pore sizes in the range of 5 to 100 nm.

Upstream side
The upstream side is the feed side of the membrane.