Operating Modes

We recommend a basic equipment set up with a standard sequence of automatic operating steps in most of the applications to utilize and maintain the performance capacity of 3M™ Liqui-Flux™ Membrane Modules for ultrafiltration.
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3M™ Liqui-Flux™ Membrane Modules for water ultrafiltration have at least three connecting ports for feed, concentrate and filtrate. Some types offer a second filtrate connection in the lower section that can be used alternatively or in parallel for additional purposes such as draining or sampling.

In order to remove built up particles from the membrane surface, the module needs to be backflushed regularly. Because some residues slowly accumulate during continuous operation, the module should be run through chemically enhanced backwash cycles. This is where cleaning chemicals are added to the back flush water so that the module requires fewer backflush cycles.

A variety of backflush operating procedures are explained schematically with only one module. Large-scale installations with a number of modules in parallel are operated the same way.

Basic Operating Modes

All in all, 3M™ Liqui-Flux™ Membrane Modules for ultrafiltration can be used in a variety of applications by adapting the operating mode accordingly. 3M has the experience, the laboratory equipment and test units to provide additional solutions to your specific applications. Please contact us if you have further questions.

Dead-End Filtration

In normal operation, 3M™ Liqui-Flux™ Membrane Modules for ultrafiltration operate with the feed entering the bottom feed port to the inside (lumen) of the hollow fiber membranes. By using a slight pressure, the water is driven through the membrane wall and the filtrate exits the module at the filtrate port near the top of the module. Because the concentrate port is closed this mode is commonly named as dead-end operation.

After a certain time period of filtration the built up particle layer will be removed from the membrane surface by a physical cleaning step, which is the regular backwash.

Backwash Top

Being a source of particle free water, the filtrate is applied as a medium for a regular backwash. A very effective measure to maintain stable operation is to change the backwash outlet direction regularly, alternating between top and bottom concentrate port with each new backwash.

Backwash Bottom

After a longer sequence of several filtration and backwash procedures, some ingredients of the feed water may remain on the membrane surface and form a fouling layer. Such a layer may be removed effectively by adding cleaning chemicals to the backwash water.

Chemical Enhanced Backwash Bottom

A so called Chemical Enhanced Backwash (CEB) shall be executed automatically. It requires limited duration and inexpensive and simple chemicals (e.g. caustic lye, hydrochloric acid, sodium hypochlorite).

Chemical Enhanced Backwash Top

The flushed particles may be drained from both ends of the capillaries through the top and bottom concentrate ports. This can be done consecutively or alternating between the ports.

Inline Coagulation

In many cases it is possible to enhance the filtrate quality and to minimise the CEB demand by dosing a small quantity of coagulant into the feed line of the ultrafiltration unit. While applying this in-line coagulation, dissolved organics may also be separated by using a 3M™ Liqui-Flux™ Membrane Modules for ultrafiltration. In some cases up to 60 % of Natural Organic Matter (NOM) could be removed which would otherwise penetrate the membrane.

Optional Operating Modes

All in all, 3M™ Liqui-Flux™ Membrane Modules for ultrafiltration can be used in a variety of applications by adapting the operating mode accordingly. 3M has the experience, the laboratory equipment and test units to provide additional solutions to your specific applications. Please contact us if you have further questions.

Integrity Testing

An integrity test of the hollow fiber membranes inside the 3M™ Liqui-Flux™ Membrane Modules for ultrafiltration may be conducted easily and accurately by feeding pressurized air to the shell side of the module. Larger quantities of bubbles at the top concentrate outlet would indicate a membrane defect. This testing procedure requires special attention on the air pressure. A detailed description of this process can be found in our operating manual.

Forward Flush

It is also possible to flush the lumen of the hollow fiber membranes using feed water with a Forward Flush. In this case the concentrate port at the top of the module is opened and the filtrate ports are closed to allow flushing by high fluid velocity along the membrane lumen.

Besides the basic equipment and operation modes there are some additional treatment steps, which may be applied depending on the specific requirements of the various applications.

Alternating Feed Flow Direction

For heavy loaded feed water quality there are some optional operating modes.

To load the membrane surface more evenly one may alternate the feed flow between the top and the bottom port as already described for the backwash outlet.

Continuous Concentrate Discharge

In addition to that it may be useful to switch from the dead-end operation to a semi dead-end mode where a small high loaded fraction of the concentrate is allowed to exit at the ends of the membrane thus providing some tangential flow which avoids the rapid build up of an inhibiting cover layer.

Concentrate Recirculation

Another possibility to deal with heavily loaded feed flows may be to recirculate the concentrate from outlet to the feed water inlet. This mode provides much higher shear forces to the membrane surface.