METHOD FOR THE PURIFICATION OF WATER BY MEANS OF MEMBRANE FILTRATION

Abstract: The present invention relates to a method for the purification of water by means of membrane filtration, in which a suspension of particles of a filtration aid which forms a deposit layer on the membrane is added to the water, the membrane is periodically cleaned in order to at least partially remove the deposited layer of filtration aid with pollutants caught therein, in which the filtration aid is only added during a part of the period between the periods in which the membrane is cleaned and the filtration aid is added in a relatively high concentration. The method of the invention makes a stable process operation at high flux possible in the ultra filtration of highly polluted water.

Description: Method for the purification of water by means of membrane filtration The present invention relates to a method for the purification of water by means of membrane filtration, in particular ultra- or microfiltration. Ultra- and microfiltration are known techniques for the purification of water, which are used for the preparation of for instance drinking-water, domestic- and industry water. In ultrafiltration and microfiltration water (feed) is passed along past 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 stage, while suspended matter, colloidal matter and large organic molecules are left behind on the membrane. The quantity of water passing a square meter of membrane per hour is called flux. The remaining particles cause fouling which becomes evident from the increase in pressure drop over the membrane. Periodically the membrane is therefore cleaned by flushing through the membrane in opposite direction for a short period of time (often called backflush). Here clean water of the permeate side is pressed to the feed side, as a result of which the dirt layer is removed and discharged (retentate). A membrane can also be cleaned by passing water and/or water and air past the dirt layer on the membrane at high speed (often called forward flush or air flush). Because such cleanings often leave a small quantity of pollution behind, cleaning is often carried out with chemicals (often called chemical cleaning or enhanced backwash). During the filtration stage the membranes can be operated in cross-flow or dead-end. In cross-flow only a part of the feed water is discharged as permeate during the filtration stage, while the remaining part leaves the membrane element again, whereas in dead-end all the feed water is pressed through the membranes. The membranes can be designed as tubular, capillary or flat membranes. The major advantage of ultrafiltration is that by using said technique a very good production quality can be achieved in one process step. The removal of suspended and colloidal matter (including bacteria and viruses) can be called absolute in comparison to the conventional purifications. Additionally said production quality does not depend on the changes in the feed. The broad applicability renders ultrafiltration attractive for the preparation of both drinking-water and domestic- and industry water, or as pre- purification in the preparation of demi-water. Because of the developments in the membrane technology and improved process operation (including cleaning regimes) the treatment of a large range of water qualities is technically and economically feasible. In a number of cases in practice, however, it has appeared that serious membrane fouling, and as a result reduction of the productivity occurs. As a solution to this, flocculation is often used prior to filtration, with varying success. The flocculants are continuously dosed during the entire filtration stage. The improvement in membrane performance is usually attributed to the coagulation of colloids in the feed water, as a result of which the blockage of the membrane is reduced. Also the dosing of certain adsorption agents such as heated iron oxide particles in ultrafiltration as a result of which the flux is increased as a side effect, is described [Chang Y-J, Choo K-H, Benjamin M.M., Reiber S. combined adsorption - UF process increases TOC removal. Journal AWWA vol 90, 1998]. The measures described above are not always sufficient to prevent too high a degree of fouling of the membranes. It is an object of the present invention to reduce blockage of the membranes in the treatment of highly polluted water, so that at a certain pressure a higher flux can be attained or a smaller membrane surface can be used. According to the invention, this object is achieved with a method for the purification of water by means of membrane filtration, in which a suspen- sion of particles of a filtration aid, which forms a deposit layer on the membrane, is added to the water, the membrane is periodically cleaned in order to at least partially remove the deposited layer of filtration aid with the pollutants caught therein, in which the filtration aid is only added during a part of the period between two periods in which the membrane is cleaned and the filtration aid is added in a relatively high concentration (for instance 10 mg/l feed or higher). The method of the invention gives a strongly improved flux in comparison to the methods of the state of the art. The method of the invention can be used for both dead-end and for cross- flow filtration. The used membrane elements can include tubular, capillary or flat membranes. The particles of the filtration aid (here also called pre-coat) can be all particles that give a layer on the membrane, which is porous to such an extent that a sufficient flux is obtained, while suspended and colloidal particles are effectively retained. Examples of suspensions of particles that can be added to the water to be treated for forming a layer of filtration aid, are suspensions of iron hydroxides, of aluminium oxides and hydroxides, of sintered iron oxide particles, of pulverized activated carbon and of clay particles or other mineral particles. According to a preferred embodiment of the invention a suspension of iron hydroxide particles is used that is prepared by adjusting a solution of iron(lll)chloride to a pH of approximately 10. In the invention, suspensions of particles having a relatively high concentration are used. In case of iron hydroxide a concentration of for instance 10 mg/l feed or higher, such as some tens to hundreds mg/l is used. In common dosages of flocculants relatively low concentrations are used (for instance some milligrams FeCI 3 /l feed). The filtration aid is only dosed during a part of the filtration stage. It could be considered not to load the membranes with feed water during the period in which the filtration aid is dosed. This can be realised by applying the filtration aid with the help of relatively clean water (for instance permeate), instead of feed water. Apart from flux improvement, using the new method has considerable advantages in comparison to a flocculant dosage. The coagulation kinetics do not play a part any more and because the filtration aid in principle can be re-used, the sludge flow can be less. Furthermore it is possible to select or design the filtration aid (pre-coat) on the adsorption of specific pol- lutants (including starch derivatives, organic micro compounds or AOC). Dosing a filtration aid moreover offers the possibility to monitor the membrane integrity more easily. The method of the invention is now elucidated by the following example. Surface water (channel water) which based on experience can be qualified as water that is hard to treat (Twentekanaal, inlet point Elsbeekweg, WMO), was treated by means of ultrafiltration. In the experiment use was made of capillary membranes (Stork Superfil E 015-010 membranes in a 9833 SMMP/1-3 module, total membrane surface 2.4 m 2 ). A solution of iron(lll)chloride was used as filtration aid, which solution was adjusted to a pH of 10, so that a almost stable suspension of iron hydroxide particles was formed. The dosage of said suspension was such that after dosing an iron concentration of 45 mg/l was obtained. The dosing time and period of time of the filtration stage, depended on the flux used: at a flux of 25 l/m 2 .h the dosing time was 3 minutes and the filtration stage lasted 24 minutes and at a flux of 50 l/m 2 .h the dosing time was 1.5 minutes and the filtration stage lasted 12 minutes. Furthermore a chemical cleaning using an acid was carried out periodically. In the experiment use was made of an already described measuring protocol and device for unambiguously comparing membrane performance [Galjaard G., Schoonenberg F., Jonge J. de, Meetprotocol Quick-Scan: vergelijken van MF/UF membraanprestaties {Measuring protocol Quick- Scan: comparing MF/UF membrane performance). Kiwa report SWE 98.008, December 1998]. During the experiment it appeared that with the channel water without the use of flocculants and flake aids even at a low flux (25 l/m 2 .h) no stable ultrafiltration process was possible. Despite frequent chemical cleaning the trans-membrane pressure (TMP) exceeded the maximally allowable value within some days. Only very long soaking and cleaning at extreme pH's offered some recovery. Flocculant dosage initially resulted in a limited effect but here as well no stable operation was achieved. When using the filtration aid according to the invention, a very stable operation even at a high flux (up to at least 50 l/m 2 .h) was achieved. Also the frequency of chemical cleaning could be significantly reduced (from approximately 8 hours to approximately 16 hours). The results are illustrated in the accompanying figures which show the change of the trans-membrane pressure (TMP) as a function of the time at a flux of 25 l/m 2 .h (figure 1) or 50 l/m 2 .h (figure 2). Figure 1 shows the results obtained with a membrane module in which no dosing was used (line a) and with a membrane module in which first a continuous flocculant dosage (FeCI 3 , corresponding to 3 mg/l iron) was used, and after a manual chemical cleaning was carried out after 20 hours, an iron hydroxide dosage according to the invention was used (line b). Figure 2 shows the results for a membrane module in which no dosing is used (line a) and for a membrane module in which iron hydroxide dosage according to the invention was used (line b). The figures clearly show that using a filtration aid according to the invention results in stable operation at a high flux.

Claims: Claims 1. Method for the purification of water by means of membrane filtration, in which a suspension of particles of a filtration aid which forms a deposit layer on the membrane is added to the water, the membrane is periodically cleaned in order to at least partially remove the deposited layer of filtration aid with the pollutants caught therein, in which the filtration aid is only added during a part of the period between the periods in which the membrane is cleaned and the filtration aid is added in a relatively high concentration. 2. Method according to claim 1 , in which the membrane is an ultra- or microfiltration membrane. 3. Method according to any one of the preceding claims, in which the deposited filtration aid is removed by back washing and/or chemical cleaning. 4. Method according to any one of the preceding claims, in which the filtration aid consists of iron hydroxide particles, aluminium hydroxide or aluminium oxide particles, sintered iron oxide particles, pulverized activated carbon, clay particles or other mineral particles. 5. Method according to claim 5, in which the filtration aid consists of iron oxide particles which are formed by adjusting a solution of iron chloride to a pH of 9 to 13. 6. Method according to any one of the preceding claims, in which the particles of a filtration aid in a concentration of 10 mg/l or higher are added to the water to be treated.

Inventors: BUIJS PAULUS JOANNES, GALJAARD GILBERT, BUIJS, PAULUS, JOANNES, GALJAARD, GILBERT
Applicants: BUIJS PAULUS JOANNES, GALJAARD GILBERT, KIWA NV, BUIJS, PAULUS, JOANNES, GALJAARD, GILBERT, KIWA N.V.
Application year: 2000
Main classification: C02F1
IPC codes: B01D37, B01D61, B01D65, B01J20, C02F1