WO2007017017A1

WO2007017017A1 - Method for backwashing capillary membranes of a membrane system

Info

Publication number: WO2007017017A1
Application number: PCT/EP2006/006433
Authority: WO
Inventors: Christoph Kullmann; Dirk Volmering; Klaus Vossenkaul
Original assignee: Koch Membrane Systems Gmbh
Priority date: 2005-07-27
Filing date: 2006-07-01
Publication date: 2007-02-15
Also published as: DE102005035044A1

Abstract

The invention relates to a method for backwashing capillary membranes (2) of a membrane system that are connected with at least one open end that serves the permeate discharge with a permeate space (3) of the membrane system, and, on the outside, are surrounded by a fluid (4) that is to be filtrated. Firstly, the permeate space (3) is filled and the fluid contained in the capillary membranes (2) is displaced with a cleaning fluid (7) containing cleaning chemicals. By establishing a pressure in the permeate space that exceeds the pressure on the outside of the capillary membranes, in a backwashing operation, a portion of the cleaning fluid is displaced through the membrane walls of the capillary membranes into the fluid (4) that is to be filtrated. According to the invention, a small continuous backwash volume stream through the membranes is established, and it is adjusted in such a way that the concentration of the cleaning chemicals in the longitudinal direction of the capillary membranes (2) remains essentially constant during the backwashing operation.

Description

Method for backwashing capillary membranes of a membr^) system

DESCRIPTION

The invention relates to a method for backwashing capillary membranes of a membrane system that are connected with at least one open end that serves the permeate discharge with a permeate space of the membrane system, and, on the outside, are surrounded by a fluid that is to be filtrated.

Membrane systems for membrane filtration are used, among other things, for drinking water treatment and purification of municipal sewage and industrial wastewater. The capillary membranes have a diameter of less than 5 mm, a diameter range between 0.5 mm and 3 mm being preferred for many applications. A fluid that is to be purified flows around the outside of the capillary membranes or the capillary membranes are immersed in a tank containing the fluid to be purified. The driving force for the membrane filtration is a pressure differential that can be realized by excess pressure on the side of the untreated water and/or by a pressure reduction on the side of the permeate. The purified fluid, the permeate, discharges on the inside of the capillary membranes and reaches a permeate space, that is designed with an outlet for the permeate, at the open end of the capillary membranes.

The capillary membranes soil in the course of the operating period. In order to guarantee a steady, stable permeate stream of the membrane system, a backwashing of the capillary membranes takes place in specified intervals. A cleaning fluid is hereby supplied under pressure to the permeate space of the membrane system, streams into the capillary membranes, and permeates through the capillary membranes to the outside. By means of the backwashing, that is the reversal of the permeate flow, covering layers that adhere to the capillary membranes are dissolved and soilings are removed from the pores of the membranes. Methods for backwashing of membranes are described in JP-A 11-033 372, JP-A 11-128 701, and JP-A 04-161 232.

The cleaning action of the backwashing is not only dependent upon the duration of backwashing, the type and concentration of the cleaning chemicals, as well as the temperature, but also depends on the process conduct during backwashing. Particularly in practice, an operating mode is customary, wherein the permeate space is first filled with cleaning fluid and, subsequently, a specified fluid amount is displaced to the outside through the membrane walls of the capillary membranes by means of several short backwash pulses. An exposure time of several minutes without pressurization of the cleaning fluid is allowed for between the backwash pulses. The cleaning cycle comprising several backwash pulses is mostly repeated in intervals between one week and several weeks.

The backwashing method operating by means of backwash pulses cannot prevent that the penetrability of the membranes, that is referred to as permeability, decreases in the course of time, significant differences with respect to the permeability in the longitudinal direction of the capillary membranes being noticeable. The permeability of the capillary membranes decreases with increasing distance from the open end of the capillary membranes. The invention starts with the realization that the changing permeability of the capillary membranes results from the operating mode during backwashing. A bundle of capillary membranes has a larger membrane surface. During exposure time, a portion of the cleaning fluid leaves the membranes by diffusion or the concentration of the cleaning chemicals is diluted by fluid entering from outside. If during a backwashing pulse less cleaning fluid is supplied to the capillary membranes than is needed to completely displace the fluid contained in the capillary membranes, the concentration of the cleaning agent in the capillary membranes decreases, and areas of the capillary membranes that have a large distance from the open end of the capillary membranes receive inferior cleaning.

The effect is especially pronounced with membrane fiber bundles, the capillary membranes of which are only open at one end and are sealed at their other end. By means of the described backwashing, it cannot be prevented on a continuing basis that the permeability of the membranes noticeably worsens in the longitudinal direction of the capillary membranes.

Starting from the problem explained above, at the bottom of the invention is the object to give a method for backwashing capillary membranes that guarantees an effective cleaning of the capillary membranes over their entire length with as low a consumption of cleaning fluid as possible. In particular, the backwashing method should also be suitable for membrane modules, the capillary membranes of which are only open at one end.

Subject matter of the invention and solution of this object is a method for backwashing capillary membranes according to claim 1. According to the invention, firstly, the permeate space is filled with a cleaning fluid containing cleaning chemicals, and the fluid contained in the capillary membranes is displaced. Subsequently, by establishing a pressure in the permeate space that exceeds the pressure on the outside of the capillary membranes, in a backwashing operation, a portion of the cleaning fluid is displaced through the membrane walls of the capillary membranes into the fluid to be filtrated, a small continuous backwash volume stream through the membranes being established that is adjusted in such a way that the concentration of the cleaning chemicals in the longitudinal direction of the capillary membranes remains essentially constant during the backwashing operation. According to the invention, it is not operated by means of short backwash pulses as in the state of the art, but a constant backwash volume stream is established. In order to not increase the amount of cleaning fluid needed, in the method according to the invention, a small backflow volume stream is established which compensates for diffusion losses of cleaning chemicals and prevents a back-dilution of the cleaning fluid contained in the capillary membranes by fluid that enters from the outside to the inside through diffusion, defects in the membranes and the like. The backflow volume stream should be adapted to the permeability of the membranes and the membrane surface. According to a preferred embodiment of the invention, it is operated with a backflow volume stream that, based on the membrane surface loaded with cleaning fluid, is less than 20 l/(m²h) and preferably less than 10 l/(m²h). With capillary membranes that are used for membrane filtration in wastewater treatment, a continuous backflow volume stream of less than 5 1/ (m²h) already proves to be sufficient .

A preferred embodiment of the method according to the invention provides for the variation of the volume stream of cleaning fluid supplied to the permeate space, a large fluid volume stream for filling the permeate space and displacing the fluid contained in the capillary membranes being established first, which is afterwards reduced to a lower value for backwashing. The pressure in the permeate space can be controlled in order to control the backwash volume stream.

It is practical to continuously carry out the backwashing over a time period of about 10 minutes to 120 minutes. At least in wastewater treatment, it is sufficient to repeat the backwashing every one to two weeks.

The cleaning action can be further improved, when an air or gas stream is introduced into the fluid to be filtrated during backwashing of the capillary membranes, which generates flow turbulences on the outside of the capillary membranes. The ascending gas bubbles improve the mass transfer on the outside of the capillary membranes and facilitate the removal of the deposits on the capillary membranes that dissolve as a result of the backwashing.

If it is desired to archive the operating state rapidly, the permeate space can be backwashed with an explicitly higher backwashing rate and with pure permeate at the end of the backwashing operation.

In the following, the invention is illustrated by means of a drawing that only represents one embodiment. In the figures :

Fig. 1 shows schematically a membrane system with capillary membranes that is immersed and used for wastewater treatment,

Fig. 2 shows schematically the time dependent backflow volume stream during backwashing of the membrane system.

The membrane system shown in Fig. 1 operates while immersed and has several modules 1, each with a bundle of capillary membranes 2. The capillary membranes 2 are connected with an open end that serves the permeate discharge with a permeate space 3 of the membrane system and are surrounded on the outside by a fluid that is to be filtrated. It can be seen in Fig. 1 that, on their other end, the capillary membranes 2 end in the fluid to be filtrated in a freely moving fashion. On the free end, the capillary membranes 2 are individually sealed. However, the method described in the following is also suitable for membrane modules, the capillary membranes of which are each connected to a permeate space on both ends . In filtration operation, the pressure on the side of the untreated water, that is the pressure in the fluid 4, exceeds the pressure in the permeate space 3, and a permeate stream directed from the outside to the inside through the membrane walls of the capillary membranes 2 results. The purified fluid, the permeate, discharges on the inside of the capillary membranes and reaches the permeate space 3, from which the permeate is pumped off by means of a permeate pump 5, at the open end of the capillary membranes. In the course of time, a deposit forms on the outside of the capillary membranes 2. Furthermore, the pores of the capillary membranes soil. From time to time, the soilings are removed by a backwashing of the capillary membranes 2.

Before a backwashing of the capillary membranes, firstly, the permeate pump 5 is turned off and a valve 6 contained in the permeate pipe is closed. Thereafter, cleaning fluid 7 is pumped into the permeate space 3 of the membrane system from a cleaning storage tank, streams into the capillary membranes 2, and permeates through the capillary membranes to the outside. The cleaning fluid contains cleaning chemicals that in the system scheme shown in Fig. 1 are supplied to the cleaning tank 8 via a pipe 9. In the following, the conduct of the backwashing process is explained in more detail.

In a first process step, the permeate space 3 is filled with the cleaning fluid 7, and the fluid contained in the capillary membranes is displaced by the cleaning fluid 7. Thereafter, in a backwashing operation, a portion of the cleaning fluid is displaced through the membrane walls of the capillary membranes 2 into the fluid 4 that is to be filtrated. During the backwashing operation, a small continuous backwash volume stream 10 through the membranes is established, and it is adjusted in such a way that the concentration of the cleaning chemicals in the longitudinal direction of the capillary membranes 2 remains essentially constant during the backwashing operation. The backwash volume stream 10 is adjusted in such a way that losses of cleaning chemicals due to diffusion through the membrane walls are compensated for, and a dilution of the cleaning fluid contained in the capillary membranes by water, which enters by osmosis, diffusion, membrane defects and the like through the membrane walls of the capillary membranes into the capillary membranes 2, is prevented. In practice, it is practical to operate with a backflow volume stream 10 that, based on the membrane surface loaded with cleaning fluid, is less than 20 l/(m²h) and preferably less than 10 1/ (m²h) .

It can be seen in Fig. 2 that the volume stream of cleaning fluid supplied to the permeate space is varied, a large fluid volume stream 11 for filling the permeate space and displacing the fluid contained in the capillary membranes being established first, which thereafter is reduced to the lower value 10 for backwashing. The backwash volume stream 10 of, for example, 3 l/(m²h) to 5 1/ (m²h) is maintained over a time period of 10 minutes to 120 minutes. For controlling the backwash volume stream 10, controlling the pressure in the permeate space is advantages. In order to assist the cleaning action, it is practical to introduce an air or gas stream 12 into the fluid to be filtrated during backwashing of the capillary membranes, that generates flow turbulences on the outside of the capillary membranes and removes soilings that are dissolved from the membrane surface .

For termination of the backwashing, the backwashing pump 13 is turned off and the cleaning fluid that is contained in permeate space 3 and in the capillary membranes 2 is pumped off as well as returned to the cleaning storage tank 8. Subsequently, the membrane filtration is continued by switching on permeate pump 5 and opening valve 6. In difference to Fig. 1 and the corresponding description, a pump being able to operate bi-directionally can be used as backwashing pump 13. It is additionally possible to use the permeate pump 5 for backwashing, if it is able to operate bi-directionally. The cleaning chemicals might be added to the liquid stream by a portioning inlet set-up.

Claims

PATENT CLAIMS

1. Method for backwashing capillary membranes of a membrane system that are connected with at least one open end that serves the permeate discharge with a permeate space of the membrane system, and, on the outside, are surrounded by a fluid that is to be filtrated, comprising the following process steps:

1.1) with a cleaning fluid containing cleaning chemicals, the permeate space is filled and the fluid contained in the capillary membranes is displaced;

1.2) by establishing a pressure in the permeate space that exceeds the pressure on the outside of the capillary membranes, in a backwashing operation, a portion of the cleaning fluid is displaced through the membrane walls of the capillary membranes into the fluid to be filtrated, a small continuous backwash volume stream through the membranes being established that is adjusted in such a way that the concentration of the cleaning chemicals in the longitudinal direction of the capillary membranes remains essentially constant during the backwashing operation.

2. Method according to claim 1, characterized in that the backflow volume stream, based on the membrane surface loaded with cleaning fluid, is less than 10 l/(m²h).

3. Method according to claim 2, characterized in that the backflow volume stream is less than 5 1/ (m²h) .

4. Method according to any of claims 1 to 3, characterized in that the volume stream of cleaning fluid supplied to the permeate space is varied, a large fluid volume stream for filling the permeate space and displacing the fluid contained in the capillary membranes being established first, which is afterwards reduced to a lower value for backwashing.

5. Method according to any of claims 1 to 4, characterized in that the pressure in the permeate space is controlled in order to control the backwash volume stream.

6. Method according to any of claims 1 to 5, characterized in that the backwashing is carried out continuously over a time period of 10 minutes to 120 minutes .

7. Method according to any of claims 1 to 6, characterized in that during backwashing of the capillary membranes, an air or gas stream is introduced into the fluid that is to be filtrated that generates flow turbulences on the outside of the capillary membranes.