RU2432985C1 - Method of recovery of ultra filtration membrane elements made of hollow fibres - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to water treatment and may be used treatment of industrial effluents. Membrane element from hollow fibre operated in conditions of one-side open ultra filtration is recovered by coating hollow fibre surface on proportioning directly in filtered water of transition metal or copper ferrocyanide with particle size of 0.5-5 mcm in amount of 1-10 mg/l. Then, reverse-flow rinsing of membrane element by filtrate is carried out to remove formed deposits. Nickel or cobalt ferrocyanide is used as transition metal ferrocyanide. Deposits feature loose structure and may be readily removed from membrane surface in back flushing.

EFFECT: reduced amount of secondary effluents and frequency of chemical flushing, longer life, higher efficiency lower costs.

2 cl, 1 dwg

Description

The invention relates to the field of water treatment and water purification and can be used in the treatment of industrial wastewater containing micro- and colloidal particles of harmful, toxic, high molecular weight organic substances, radioactive waste, as well as microorganisms, bacteria and viruses.

Water purification by micro- and ultrafiltration is widely used all over the world, especially in the last decade, in connection with the advent of high-performance membrane elements, which allows to preserve the natural composition of water with a small amount of its losses. In this direction, the use of ultrafiltration membrane elements made of hollow fibers and the organization of the ultrafiltration process in a dead-end mode, which significantly saves energy and production area, is especially promising. To regenerate the membrane elements and remove the deposits formed on them, they are periodically washed briefly with a reverse flow of the filtrate, after which ultrafiltration is resumed.

To intensify regeneration, compressed air is introduced into the pressure channel of the membrane element (Shinenkova N.A. et al. Application of microfiltration for water treatment, Critical Technologies Series. Membranes, 2005, No. 4 (28), pp. 21-25).

However, only regular backflow washing, even intensified by compressed air, does not completely restore the performance of the membrane element. This requires periodic washing of the membrane elements using chemicals and detergents.

There is a method of cleaning a hollow fiber membrane element described in JP No. 2009-006209 (class B01D 65/06, published January 15, 2009) by flushing it backwash with water containing an oxidizing agent and then with water containing a reducing agent. This does not eliminate the need for chemical flushing, which, in turn, leads to a large number of secondary effluents, which also need to be cleaned.

In JP No. 05-277343 (class B01D 61/16, publ. 10/26/1993) it was proposed to add microparticles of colloidal silicon oxide to the water sent for ultrafiltration to form a loose precipitate on the surface of the hollow fiber membrane, which is easily removed during regeneration in cross flow. In this regeneration mode, a greater number of secondary effluents are formed than in the deadlock mode. In this case, the need for frequent chemical washes is also not eliminated.

From JP No. 05-168873 (class B01D 65/06, published July 2, 1993), it is known that the water to be purified is mixed with inorganic or organic chemicals such as sodium hypochlorite, sodium hydroxide, chlorine dioxide to maintain the performance of the hollow fiber membrane element , tetrasodium salt of ethylenediaminetetraacetic acid, an aqueous solution of citric acid, or a mixture thereof. These chemicals break down the substances contained in the sediment on the membrane. Of course, as a result of this, sedimentation is reduced and the backwashing of the membrane element is facilitated, but this, firstly, does not eliminate secondary effluents, and, secondly, significantly reduces the service life of the membrane itself due to the harmful effects of the aggressive environment.

A known method for the regeneration of a hollow fiber membrane element is described in JP No. 09-187769 (class C02F 1/44, publ. 07/22/1997), according to which a protective coating is applied to the surface of the membrane to reduce clogging of the membrane before the start of the ultrafiltration process in a dead end mode preferably crystalline iron oxide having good adhesion in the membrane but not clogging its pores. The optimal particle size of iron oxide is 1-2 microns. This method is the closest to the proposed. The disadvantages of this method are:

- the sediment on such a membrane coating is dense. Therefore, it is removed only during backwash with compressed air, which also increases energy consumption and requires special equipment, such as compressors;

- the inability to remove all deposits on the surface of the membrane, resulting in increased pressure drop, reduced membrane performance, which is expressed on the graph (drawing) in the form of a sawtooth drift;

- requires more frequent chemical leaching, and therefore additional reagents. As a result, the amount of secondary effluents that also need to be treated increases. Chemical washes adversely affect the life of the membrane itself;

- the need to stop the process, which reduces its overall performance.

In practice, to reduce the drift of membrane performance and pressure drop, coagulants are dosed into the filtered water, which enlarge the particles of contaminants, improve the filterability of the sediment, and facilitate its discharge. Such coagulants are, for example, aluminum and iron hydroxides. In the prototype method (JP No. 09-187769), amorphous iron hydroxide is mentioned along with crystalline iron oxide, but the results obtained using it are worse than when using crystalline iron oxide.

In the aforementioned method described in JP No. 05-277343, colloidal silica is used as a coagulant, but regeneration is carried out in a cross flow rather than a reverse flow, and nothing is said about reducing the sawtooth drift.

It is also known to use salts of transition metals, for example ferric chloride, as a coagulant introduced into water before filtering it on membranes (patent RU No. 2222371 (class B01D 65/08, published on January 27, 2004). This allows to reduce the degree of contamination of membranes However, backwashing uses an aggressive reagent such as chlorine, which worsens the surface of the membrane itself and significantly shortens its service life. This patent did not raise the issue of reducing sawtooth drift.

The problem to which the invention is directed, is to eliminate the disadvantages inherent in the above methods and, above all, the prototype.

This object is achieved in that in a method for regenerating a hollow fiber membrane element operating in dead end ultrafiltration mode, comprising forming a coating on the surface of the hollow fiber and flushing the membrane element with filtrate to remove deposits, according to the invention, the coating is formed by dosing directly into filtered water transition metal or copper ferrocyanide with a particle size of 0.5-5 μm in an amount of 1-10 mg / l. Nickel or cobalt ferrocyanide is preferably used as the transition metal ferrocyanide.

Nickel and copper ferrocyanides are used as adsorbents in the neutralization of liquid radioactive waste containing cesium radionuclides.

For example, in patent RU No. 2223923 (class C02F 9/02, published February 20, 2004) nickel ferrocyanide is used as a corrective additive in the treatment of highly toxic radioactive waste and wastewater containing ¹³⁷ Cs and ⁹⁰ Sr radionuclides in an apparatus equipped with a mixing zone and separation zone. This apparatus contains alternating two-layer cermet membrane and separator elements. This allows you to intensify the process and at the same time simplify it. The description does not say anything about the possibility of using a hollow fiber membrane element operating in a dead-end ultrafiltration mode, and, accordingly, to reduce a sawtooth drift.

In patents RU No. 2134458 (class G21F 9/06, publ. 08/10/1999) and RU No. 213991 (class G21F 9/06, publ. 07/27/1999) it is said that nickel or copper ferrocyanides are introduced into the purified water before ultrafiltration. However, these compounds are administered only when radionuclides are present in water, i.e. for their intended purpose. It does not mention hollow fiber membrane elements operating in dead end ultrafiltration mode. In addition, this document does not disclose how regeneration is carried out, there is no information about the sawtooth drift and the possibility of its reduction.

It was found that ferrocyanides of transition metals and copper, being crystalline rather than amorphous substances, form coatings on the surface of membrane elements made of hollow fibers, which contribute to poor adhesion of deposits, the formation of deposits with an unconsolidated loose structure and with improved filterability (easily and completely removed from the surface membranes during backwash with filtrate, even without compressed air, which manifests itself in a minimum sawtooth drift (see figure 1, curves 2 and 3). Ferrocyanides of various types were tested. transition metals, but the best results were obtained using cobalt or nickel ferrocyanide, which, in turn, can significantly reduce the number and frequency of chemical leaching, and therefore significantly reduce the contact of the membrane with aggressive reagents and thus increase its service life. the method is also the minimum number of secondary effluents, which ultimately leads to a simplification of the process, the ability to carry out a filter-regeneration cycle in a continuous mode, increase its productivity and reduce energy consumption. An additional advantage is that if radionuclides are present in the filtered water, they will reliably bind. This allows you to use the proposed method for the regeneration of membrane elements both in the treatment of non-radioactive wastewater and in the treatment of liquid radioactive waste.

If the particle size of the ferrocyanides is less than 0.5 μm and their number is less than 1 mg / l, then they easily pass through the pores of the membrane, not lingering on its surface.

If the particle size of ferrocyanides is more than 5 μm and their number is more than 10 mg / l, then the sediment becomes less loose, which leads to an increase in sawtooth drift.

The drawing shows the dependence of the differential pressure of the membrane on the duration of its operation. Curve 1 - using conventional coagulants (aluminum or iron hydroxides), curve 2 - using copper ferrocyanide, curve 3 - using nickel or cobalt ferrocyanide, curve 4 - using crystalline iron oxide, as in the prototype. As can be seen from this drawing, when using nickel or cobalt ferrocyanide, the pressure drop, and therefore the sawtooth drift, is minimal, when using copper ferrocyanide it is slightly higher, but significantly lower than when using aluminum or iron hydroxides, as well as crystalline iron oxide. When using ferrocyanides of other transition metals, the results were in the region between curves 2 and 4, which also exceeds the results of the known inventions.

Claims (2)

1. The method of regeneration of a hollow fiber membrane element operating in dead-end ultrafiltration mode, comprising forming a coating on the surface of the hollow fiber and backwashing the membrane element with filtrate to remove deposits, characterized in that the coating is formed by dosing transition metal ferrocyanide directly into the filtered water or copper with a particle size of 0.5-5 microns in an amount of 1-10 mg / L. 2. The method according to claim 1, characterized in that nickel or cobalt ferrocyanide is used as the transition metal ferrocyanide.