The present invention relates to a membrane filtration system for filtering a liquid containing impurities using microfiltration or ultrafiltration membranes, comprising a pressure vessel containing two or more membrane filter elements, connected on the filtrate side by means of a connector, and which is closed off at both ends with an end plate, wherein in one or both end plates connections are arranged for discharging filtrate or feeding back- flush and/or cleaning and/or disinfecting liquid, and wherein at both ends of the pressure vessel, either in the end plates or in the side wall of the pressure vessel connections are arranged for feeding the liquid to be filtered or discharging the back-flush and/or cleaning and/or disinfecting liquid and impurities.

Ultrafiltration and microfiltration are methods in general use for separating non-dissolved materials. In these filtration methods use is made of membrane filters. The transport of the liquid through the membrane takes place under the influence of a pressure difference. Examples of these membranes are spiral-wound membranes, tubular or capillary membranes, flat plates, etc.

The term microfiltration is used for membranes with a pore size of about 0.1 to about 10 μ and the term ultrafiltration is used for a pore size of about 1 nm to about 100 nm.

Use is made of two sorts of filtration principles, a so- called "cross-flow system" and a "dead-end" system. In the

"cross-flow system" during filtration the liquid to be filtered is conveyed at a relatively high velocity along

the membrane, a part of the liquid passing the membrane as filtrate. In this system the separated impurities are, for the greater part, discharged, as a result of which the formation of a layer of impurities on the membrane is controlled. This can be assisted by periodical back- flushing. However, the system requires a high velocity along the membrane of the liquid to be filtered. As a result additional accompanying provisions are necessary, such as larger tubes and pumps with a higher capacity, which makes this system expensive and complex.

In the "dead-end system" all the liquid to be purified passes the membrane. Instead of continuously discharging the impurities they are collected on the membrane, i.e. in the membrane filter element. The impurities which have accumulated on the membrane are discharged via periodical back-flushing. Substances with which the membrane can be chemically cleaned and/or disinfected can be added to the back-flush liquid. This system is particularly suited for purifying liquids which are only slightly contaminated. The advantage of this system is that it is more easily implemented and that the costs are lower than with the cross-flow system. For that reason the dead-end system has been given more and more preference of late. The present invention, too, relates to a suchlike system.

A dead-end system is usually realized in the form of a pressure vessel that contains several membrane filter elements, spaced in series one behind the other, the liquid to be purified being supplied from one or both ends through the connections located at the outer ends of the pressure vessel.

The problem arising in dead-end systems realized in that way is that in the pressure vessel a hindered flow occurs, whereby the distribution of the impurities over the membrane filter elements is not uniform. Further, as a

consequence in the back-flush step, the impurities cannot be removed from the membrane filter elements to a suf¬ ficient extent.

According to the present invention this problem is solved in that one or more additional connections are arranged in the side wall of the pressure vessel between the connec¬ tions at the outer ends of the pressure vessel.

The invention therefore provides a membrane filter system as described above, characterized in that in the side wall of the pressure vessel between the connections at the outer ends of the pressure vessel one or more additional connections are arranged for feeding the liquid to be filtered or discharging the back-flush and/or cleaning and/or disinfecting liquid and impurities.

In this way the flow through the pressure vessel is sig¬ nificantly improved, whereby the approach of the flow to the membrane filter elements is improved. An improved distribution of the impurities over the membrane filter elements is obtained. Back-flush is therefore more ef¬ ficient.

This offers a great number of advantages, such as:

- a better availability of the membrane filter system and a lower use of water and chemicals and lower energy expenditure as a consequence of reduced con- tamination of the membrane filter element, whereby membrane back-flush is less often required.

- lower energy expenditure as a result of a reduced pressure drop because of a better hydraulic design and a less contaminated membrane filter element. - lower membrane investment costs as the effectively available filtrate capacity can be maintained at a higher level as a consequence of the reduced con-

tamination and pressure drop.

- lower membrane replacement costs because the life span is extended by a more effective back-flush.

- better product/filtrate quality because of reduced (micro)biological growth in the membrane filter sys¬ tem due to a more effective back-flush, chemical cleaning and disinfecting.

Above advantages result in a reduction of the exploitation costs of the membrane filter system.

Preferably the additional connection(s) for feeding the liquid to be filtered or discharging the back-flush liquid are arranged at a level with a connector between the membrane filter elements.

A suchlike membrane filter system generally comprises a cylindrical pressure vessel. As a rule the pressure vessel has a length of about 2 to about 10 metres and a diameter of about 4 to about 12 inches. The membrane filter elements usually have a length of about 1 metre.

The invention also provides a pressure vessel suitable for membrane filtration, in which at the outer ends of the pressure vessel connections for discharging the filtrate or feeding the back-flush and/or cleaning and/or disinfec¬ ting liquid and connections for feeding the liquid to be filtered or discharging the back-flush and/or cleaning and/or disinfecting liquid and impurities are arranged, characterized in that one or more additional connections for feeding the liquid to be filtered or discharging the back-flush and/or cleaning and/or disinfecting liquid are arranged.

Various kinds of membrane filter elements can be used in the present invention, such as, for example, spiral-wound membranes, capillary or tubular membranes. These membranes

contain a central tube which has openings for discharging the filtrate. The liquid to be cleaned is fed axially to the filter. The filtrate moves through the membrane, and subsequently to the discharge tube.

The membrane filter elements are arranged in the pressure vessel, such that the fed liquid flows primarily through one membrane filter element to the adjacent membrane filter element.

It is common practice to arrange the connections for feeding the liquid to be filtered which are to be placed on the outer ends of the pressure vessel, in the end plate of the pressure vessel. Because of the filtrate connection already present here or for a better availability, they can also be placed in the side wall on the outer end of the pressure vessel. However, this does not change the flow through the pressure vessel .

The filtrate can be discharged on one side as well as on both sides of the pressure vessel.

The membrane filter system according to the invention can be used for filtering various kinds of liquids. Examples of this are surface water, process water, wash water, cooling water, salt solutions, etc.

In general part of the filtrate itself is used as back- flush liquid, provided where appropriate with substances suitable for chemically cleaning and/or disinfecting the membrane filter system.

The membrane filter system according to the invention can be used specifically for purifying slightly contaminated liquids. The particles filtered out of this are, among other things, bacteria, solid substances, floating par¬ ticles, colloids, etc., depending on the applied pore size

of the membrane.

When cleaning large quantities of liquid several membrane filter systems can be coupled. These couplings can be arranged in various configurations (i.a. in series, paral¬ lel or combined) .

The invention will now be explained in more detail with reference to the following figures. These figures serve merely to elucidate the invention and should not be con¬ sidered as a restriction thereof. For clarity's sake the figures are not shown to scale.

In the figures:

figure 1 represents a membrane filter system according to the state of the art :

figure 2 represents a membrane filter system according to the present invention,

figure 3 shows the flow through the membrane filter system according to the state of the art,

figure 4 shows the flow through the membrane filter system with the filter method according to the invention.

Figure 1 shows an embodiment of a membrane filter system according to the state of the art . A number of membrane filter elements (2) are incorporated in a pressure vessel (1) . On the filtrate side the membrane filter elements are connected to each other via a connector (6) . The pressure vessel is closed off on both sides by means of an end plate (3) . In the side wall the usual connections (4) are arranged for feeding the liquid to be purified or for discharging the back-flush liquid and impurities. In the end plates (3) connections (5) are arranged for dischar-

ging the filtrate or feeding the back-flush and/or the cleaning and/or the disinfecting liquid.

Figure 2 shows a membrane filter system according to the invention. This system corresponds to that of figure 1 with the exception that in the side wall of the pressure vessel (1) connections (7) are arranged for feeding the liquid to be filtered or discharging the back-flush li¬ quid.

Figure 3 shows the flows occurring in a membrane filter method that makes use of a membrane filter system accor¬ ding to figure 1.

During filtration the liquid to be purified is supplied via the connections (4) . The liquid to be purified flows axially into the membrane filter element and subsequently flows axially through the membrane filter element to adjacent elements. The filtrate flows through the membrane and subsequently to the filtrate discharge tube. The filtrate leaves the pressure vessel via the connections (5) . The impurities remain behind in the membrane filter elements (2) .

The pressure vessel is periodically back-flushed to remove the accumulated impurities. The flow direction is then reversed. The back-flush liquid is fed via the connections (5) in the end plate (3) . The back-flush liquid flows through the membrane and subsequently axially through the membrane filter elements and moves the impurities which were left behind through the membrane filter elements (2) to the outer ends of the pressure vessel. The back-flush liquid containing the impurities leaves the pressure vessel via the connections (4) .

Figure 4 shows the flows which occur with a membrane filter method according to the present invention.

The filtration takes place in the same way as in figure 3, except that the liquid to be filtered is also fed via the connections (7) .

The back-flush, too, takes place in the same way as in figure 3, with the exception that the back-flush liquid containing the impurities likewise leaves the pressure vessel via the connections (7) .

RD/FL