NL1016771C2 - Process for purifying water by filtration with a micro or ultra filtration membrane. - Google Patents

Description

- 1 to 5

METHOD FOR PURIFYING WATER BY FILTRATION WITH A MICRO OR ULTRA FILTRATION MEMBRANE

The present invention relates to a method for purifying water by means of dead-end filtration with a micro- or ultrafiltration membrane, wherein before, during the beginning or during each filtration period a suspension of a filtration aid to the water to be purified. is supplied so that a layer of said filtration aid is deposited on the membrane.

Micro- and ultrafiltration are known techniques for purifying water, which are used for the preparation of, for example, drinking water, domestic and industrial water.

In microfiltration and ultrafiltration, water (feed) is passed along a polymer or ceramic membrane. Under the influence of pressure as a driving force, the water is pressed through the membrane (permeate) during the filtration phase, while floating parts, colloidal parts and large organic molecules remain on the membrane. The amount of water that passes through a square meter of membrane per hour is called flux. The remaining particles cause fouling which results in an increase in the pressure drop across the membrane. The membrane is therefore cleaned periodically by briefly flowing in the opposite direction through the membrane (often called back-flushing or called "back-flush"). In this process, clean water is pressed from the permeate side to the feed side, whereby the dirt layer is removed and drained (retentate). The membrane can also be cleaned by guiding water and / or water and air along the dirt layer onto the membrane at a high speed (often called "forward flush" or "air flush"). Because such cleaning often leaves behind a small amount of contamination, cleaning with chemicals is often carried out (often called "chemical cleaning" or "enhanced backwash"). The filtration process is thus divided into several cycles, with a filtration cycle followed by a rinsing cycle. Chemical cleaning is carried out after a number of filtration and rinsing cycles.

The membranes can be operated in "cross-flow" or "dead-end" during the filtration phase. In the case of cross-flow, only a part of the feed water is discharged as permeate during the filtration phase, while the remaining part leaves the membrane element again, while in the case of dead-end all the feed water is pressed through the membranes. The membranes can be in the form of tubular, capillary or flat membranes.

The major advantage of micro- and ultrafiltration is that with the aid of these techniques a very good product quality is achieved in one process step. The removal of suspended and colloidal material (including bacteria and viruses) is absolute compared to conventional purifications. In addition, this product quality is independent of the changes in the feed. The wide usability makes micro- and ultrafiltration attractive for the preparation of both drinking water and domestic and industrial water, or as pre-purification in the preparation of demineralised water. Due to developments in membrane technology and improved operational management (including cleaning regimes), treatment of a wide range of water qualities is technically and economically feasible.

In a number of cases, the above techniques cannot be applied due to a high contamination rate and / or unstable operation. In such cases, the pressure across the membrane does not recover sufficiently after backwashing and chemical cleaning. The contamination is then so extreme that periodically the membranes must be intensively cleaned or, in some cases, replaced. In such cases, a dose of a filtration aid can sometimes help to stabilize the process. Several techniques have been described for this: "In-line coagulation"; in this technique, a coagulant is continuously metered into the feed stream. Small particles 10 present in the water are flocculated into larger ones by the coagulant. As a result, these particles can clog the pores of the membrane to a lesser extent. [P. van der Maas et al., 1999.] "Coating / body feed"; in this technique, a particle suspension is continuously metered into the feed stream. These particles (in most cases activated carbon) disrupt the cake build-up on the membrane, making it easier to separate the cake from the membrane when cleaning. 20 [Y. Matsui, 2000] "Pre-coating"; in this technique, a suspension with a high concentration of particles is metered onto the membrane before the start or during the start of the filtration cycle. The particles form a protective layer or pre-coat on the membrane. During the filtration cycle, suspended material in the feed water is captured by the pre-coat rather than the membrane. During backwashing, the pre-coat layer comes off the membrane and the captured layer with the pre-coat particles is discharged. [G. Galjaard et al., 2000]

Although micro- and ultrafiltration are very suitable for the removal of suspended and colloidal material, dissolved components such as for example ions (salt), humic acids, pesticides and organic micro-pollutants (a number of specific organic carbon compounds) are not or only partially blocked by micro- and ultrafiltration membranes.

In the preparation of high quality drinking water or industrial water (for example demineralized water for cooling purposes), additional purification steps are used to remove these dissolved components. With such a purification, micro and ultrafiltrations can be considered as pre-treatment or "polishing" techniques that act as a "disinfection screen" for the later steps. The use of multiple purification steps makes water preparation expensive.

As noted above, in the purification of water, certainly if disinfection is necessary, the use of micro or ultrafiltration will often be considered. In many cases, however, some of the dissolved components will also have to be removed, because a number of these dissolved components can cause problems. For example, ammonium and assimilable organic carbons (AOC) can cause the growth of bacteria in the distribution system, in plants of the customers or in the subsequent purification. Another dissolved substance that can cause problems is, for example, manganese which after oxidation can give a black deposit that is difficult to remove.

Recently the use of separate ion exchange columns for the removal of dissolved substances [C. Charnock, 2000 and S.G.J. Heijman et al. 1999] and color [S. Ver-bych et al. 2000, W.H. Höll et al. 2000]. An additional step in the purification of water is therefore used here.

The object of the invention is therefore to provide a method with which both suspended and colloxal material and dissolved components can be removed in one purification step.

This object is achieved according to the invention by dosing ion exchanger particles to the membrane during water purification by means of dead-end filtration with a micro- or ultra-filtration membrane before or during the filtration.

Thus, the invention provides a method for purifying water by means of dead-end filtration with the aid of a micro- or ultra-filtration membrane, wherein the filtration is periodically interrupted to clean the membrane and wherein before, during the start, or during each A filtration aid is supplied to the water to be purified during the filtration period, so that a layer of said filtration aid is deposited on the membrane, characterized in that the filtration aid comprises particles of an ion-exchange resin with a particle size of 0.5 to 50 µm.

With the method of the invention, in one purification step the water is completely purified from floating and colloidal material and specific dissolved components such as for example hardness, manganese, ammonium, assimilable organic carbon and color.

The method of the invention can be carried out with the aid of the known micro and ultrafiltration membrane materials and embodiments.

The membrane can for example be a spiral wound membrane, a flat membrane or a capillary membrane. For large-scale production (> 250 m3 / h), it is preferable to use compact membrane modules which are provided with a large number of capillary membranes (hollow fibers, with a fiber diameter of, for example, 0.5 mm). Examples of materials of such capillary membranes are hydrophilic polyether sulfone (PES), polysulfone (PS), polypropylene (PP), celluloacetate (CA), polyacrylonitrile (PAN) polyvinylidene fluoride (PVDF) or polyvinylpyrrolidone (PVD).

In carrying out the method of the invention, the membranes are operated in "dead-end".

101 6771 '- 6 -

The ion exchange particles can suitably be added to the water to be purified in the form of a suspension of the particles in water.

Both ion-exchange resins and anion-exchange resins can be used as ion-exchange resins in the process of the invention. It is also possible to use a mixture of these resins. The choice of the resin to be used is determined by the analysis of the water to be purified and the purpose of use of the purified water.

The ion exchange resin particles have a particle size of 0.5-150 μπι, preferably of 0.5-20 μιη. The particle size of the ion-exchange resin is considerably smaller than the particle size of 0.2-1.2 mm of the conventional ion-exchange resins in separate columns for removing substances. The suitable particle size of the resin to be used according to the invention depends on the type of synthetic resin and the desired retention of the specific solutes.

The ion exchange resins to be used are preferably microporous. Examples of suitable ion exchange resins are Duolite AP 143/1093 (from Rohm & Haas) and Amberlite IRP69 / IRP64 (from Rohm & Haas).

The layer of ion exchange resin particles can be deposited by continuously dosing the particles during each filtration period. The suspension of resin particles can also be dosed before or during the start of each filtration period. If the suspension is supplied before the start of the filtration period, a layer of deposited particles is present on the membrane before the filtration begins. The layer of resin particles is preferably applied to the membrane by dosing a suspension of ion exchange resin particles with a relatively high concentration for a short time before or at the beginning of the filtration period (<5 minutes for a filtration period of 15 to 60 minutes) pulse rate). Dosing of the suspension of particles can also be continued after this pulse dosing during the filtration cycle in a low concentration. The concentration of the resin particles in the suspension 1 01 6771 ** - 7 - is determined by the set dosing time and the desired layer thickness. A suitable layer thickness is 3 to 5 times the diameter of the dosed synthetic resin particles.

After a certain filtration time or after a certain pressure difference has been achieved across the membrane, the membrane is backwashed. The layer with particles with therein and thereon the captured substances (both suspended and colloidal material and dissolved substances) comes loose from the membrane and is flushed out of the membrane device. The concentrate stream with loosely rinsed particles is collected in a separation tank in which the synthetic resin is separated from the rest of the liquid. The synthetic resin particles are returned to the suspension tank from which the particles can be re-dosed. After the particles have been used a number of times, they are regenerated with saline, after which they can be used again.

i U i o d έ ί 1¾ - 8 -

Literature

Chang Y-J, Choo K-H, Benjamin. ,, M.M, Reiber S. (Combined adsorption - UF process increases TOC removal, Journal AWWA 5 vol 90, 1998.

Galjaard G., Paassen van J., Buijs P., Schoonenberg G., Enhanced Pre-Coat Engineering. The Solution for Fouling ?, Proceedings of the Conference on Membranes in Drinking and Industrial Water Production, Volume 1, biz. 605-610, ISDN 10 0-86689-060-2, October 2000, Desalination Publications L'Aquila, Italy.

Galjaard G., Schoonenberg F., de Jonge J., Quick-Scan measurement protocol: comparing MF / UF membrane performance. Kiwa report SWE 98.008, December 1998.

15 Maas P. van der, Leiting E., Dost S., Eppinga E., van Hoof S., Customizes water supplies: producing hig quality process water from surface water using membrane technology, Proceedings AWWA membrane Technology Conference, Long Beach (Calif .) 1999.

20 Y. Matsui, Effect of operational modes on the removal of a synthetic organic chemical by powder-activated carbon during ultrafiltration, Proceedings of the Conference on Membranes in Drinking and Industrial Water Production, Volume 1, biz. 215-224, ISDN 0-86689-060-2, October 2000, Desalinations Publications l'Aguila, Italy.

S. Verbych, M. Bryk, A. Alpatova, Extraction of heavy metal ions from waqter solutions, Proceedings of the Conference Innovations in Conventional and Advanced Water Treatment Processes, September 2000.

30 W.H. Höll, C. Bartosch, X. Zhao, S. He, Elimination of trace heavy metals from drinking water by means of weakly basic anion exchangers, Proceedings of the Conference Innovations in Conventional and Advanced Water Treatment Processes, September 2000.

35 S.G.J. Heijman, A.M. van Paassen, W.G.J. van der Meer, R. Hopman, Adsorptive removal of natural organic matter during 1016771 «- 9 - drinking water treatment, Water Science and Technology, Volume 40, no. 9, biz. 183-190, 1999, Elsevier GB.

C. Charnock, O. Kjonno, Assimilable organic carbon and biodegradable dissolved organic carbon in Norwegian raw and 5 drinking waters, Water Res. Volume 34, No. 10, p. 2649-2642, 2000 Elsevier GB.

^ 016771¾

Claims (3)

A method for purifying water by means of dead-end filtration using a micro or ultra-filtration membrane, wherein the filtration is periodically interrupted to clean the membrane, wherein before, during the beginning, or during each A filtration aid is supplied to the water to be purified during the filtration period, so that a layer of said filtration aid is deposited on the membrane, characterized in that the filtration aid particles of an ion-exchange resin with a particle size of 0.5 to 5 0 μτη. 2. Method according to claim 1, wherein the filtration aid is added in a high concentration before or during the start of each filtration period. 3. Method as claimed in claim 1 or 2, wherein after a certain time has been filtered or after a certain pressure drop has occurred over the membrane, the layer of ion exchange resin with therein or thereon the impurities captured therein is removed from the membrane, the ion exchange resin is separated and , possibly after regeneration, is used again as a filtration aid. 30 ***** * 0 1 S7 7 taf