DE20005681U1 - Electrolytic cell for disinfecting water - Google Patents

Description

Electrolysis cell for water disinfection

The invention relates to an electrolysis cell with a high specific electrode surface for disinfecting drinking water, industrial water and waste water. It is particularly suitable for the anodic oxidation of inorganic and organic water constituents with the aim of breaking down persistent germs and other undesirable organics and producing oxidized species in the water that have an oxidizing and disinfecting effect. The latter are able to prevent recontamination on the way to the consumer and/or serve as a disinfectant themselves, for example for mixed and untreated water or for devices and containers, e.g. in the food industry or in the medical field. This cell is also well suited for combination with a system for disinfection using UV radiation. This means that the advantage of very cost-effective disinfection using UV radiation can be combined with the advantage of achieving a depot effect through electrochemical treatment.

In most cases, electrochemical water treatment can make use of the ingredients contained in the water, which provide a certain electrical conductivity and are also suitable for the formation of oxidizing species, such as chlorides and sulfates. However, electrolysis cells are required that have a high specific electrode surface with the smallest possible distance between the electrodes.

However, it is also possible to add small amounts of neutral salts such as chlorides or sulfates to the water to be treated in order to improve the electrical conductivity and to increase the possible content of oxidizing species.

The electrolysis cells used to date for such applications mainly consist of plate-shaped electrodes that are combined to form electrode packages. Monopolar circuits with current leads to the individual electrode plates are predominant. Such electrolysis cells have the disadvantage that in order to achieve a cell voltage that is low enough for economic use, the electrodes must be sufficiently dimensioned for the current transport within the electrode plates. The use of poorly conductive or very high-quality electrode materials, e.g. made of precious metals, is therefore not feasible at all or only at unacceptably high costs, since the electrode thicknesses and thus the electrical resistances cannot be reduced arbitrarily.

In order to achieve a sufficiently high mass transport of the ingredients to be electrochemically changed in dilute aqueous solutions such as natural water, the spaces between the electrodes must be very intensively flowed through. In the case of reversible redox reactions, as is largely the case with the formation of oxidizing species, their reduction at the cathode is also accelerated in undivided electrolysis cells with the intensification of the mass transport. For example, the hypochlorite formed anodically from the chloride can be reduced cathodically back to chloride.

There are also problems with the carbonate hardness contained in natural water. Since an alkaline boundary layer forms on the cathode surfaces, the bicarbonates are converted to the less soluble carbonates and lime deposits form on the cathode surfaces. This can be prevented by periodically reversing the polarity of the electrolysis current. However, this requires a symmetrical structure of the electrolysis cell in the sense that the anode and cathode surfaces must be made of the same materials. Active layers made of

Metal oxides, which change irreversibly under cathodic stress, can therefore- ······.·· . . .; ....

cannot be used. When using platinum-coated valve metals, e.g. platinum-coated titanium electrodes, such a desired symmetrical structure is possible, but under cathodic stress, if the platinum layer is not completely free of pores, it will soon be damaged and eventually detached due to the formation of titanium hydride beneath the platinum layer.

For the reasons mentioned above, the usual plate electrodes made of valve metals with active layers made of precious metals or precious metal oxides cannot generally be used in monopolar electrode circuits and when periodic polarity reversal is required, as they are not sufficiently (long-term) resistant.

The invention specified in the protection claim was based on the problem of providing an electrolysis cell suitable for water treatment, in which the stated disadvantages of previous electrolysis cells for water treatment do not occur and with which better economic efficiency in the disinfection and oxidation of water can be achieved.

This problem was solved by a bipolar electrolysis cell according to claim 1, whereby the electrolysis cell can be used in an undivided version as well as in a divided version using separator membranes. Both versions have in common that a tightly packed electrode package is clamped in a plastic housing, which is formed from the base frame 1 and the side clamping plates 2 and has an inlet 3 and an outlet 4 for the water to be treated, at a distance of 10 to 50 mm from the inlets and outlets so that it completely fills the rectangular cross-section of the electrode package through which the flow passes. The cell is elongated in the direction of flow, the ratio of the length between the inlets and outlets to the width of the cross-section through which the flow passes should be at least 3 to 1. The electrode package consists of two electrode edge plates 6 provided with power supply lines and at least one bipolar electrode plate 5.

In the undivided version, a spacer 8 is clamped between every two electrode plates. In the split version, a separator membrane 9 is clamped between every two electrode plates and a spacer 8 is clamped between the electrode plate and the membrane. The current is fed to the electrode edge plates through the contacts 7.

The number of bipolar electrodes and thus the number of individual cells connected electrically in series can be optimally adapted to the available direct voltage. Preferably, 3 to 12 bipolar-connected individual cells are used. If only a relatively low direct voltage is available, e.g. when operating such electrochemical disinfection systems using solar power, several bipolar electrode segments connected electrically and hydrodynamically in parallel can be arranged in an electrode package.

Thin-film electrode plates made of precious metal foils with a thickness of 10 to 100 pm are preferably used. This makes it possible to accommodate very high specific electrode surfaces of more than 1 m ² per liter in an electrode package using spacers with a thickness of less than 1 mm.

But high specific electrode areas are also possible when using electrode plates made of coated materials. The use of electrode plates coated with doped diamond and up to 2 mm thick made of ceramic material, e.g. silicon, or valve metals, e.g. niobium, has proven particularly advantageous.

This makes it possible to combine the advantages of the diamond coating, which result from the very high electrode potential, with the advantages of this cell construction. This makes it possible to use electrode plates made of doped silicon, coated on both sides with doped diamond, despite the relatively poor electrical conductivity.

In divided electrolysis cells, microporous membranes, e.g. made of plastics, are suitable as separator membranes, but ion exchange membranes are preferred. The latter have the advantage that the current is transported either only by cations when using cation exchange membranes or by anions when using anion exchange membranes. By selecting the separator membrane appropriately, it is possible to promote desired electrode reactions or to discriminate against undesirable ones.

Fabrics made of suitable plastics or perforated plastic films, mechanically stretched, which have a sufficiently large free cross-section in the direction of flow are suitable as spacers. However, spacers made of ceramic materials can also be used advantageously.

The thickness of the spacers, which determines the distance between two electrode plates, can be changed and adapted to a large extent according to the electrical conductivity of the water to be treated. This means that if the conductivity is poor, the smallest possible distance is chosen, but this must still ensure sufficient flow through the spaces between the electrodes. For water with a higher salt content and thus better conductivity, the thickness of the spacers can be chosen to be larger, which also reduces the loss currents in the water-filled supply and discharge areas before and after the electrode packages, where a short-circuit current flows.

In order to keep the inlets and outlets to the individual electrode spaces open and to reliably prevent a short circuit of the flexible foil electrodes in this area, the spacers and, in the split design, also the separator membranes are dimensioned so that they protrude 5 to 30 mm beyond the electrode plates on both sides in the direction of flow.

Since in the split design the anode chambers and cathode chambers are flowed through in parallel and the pressure differences between the inlet and outlet are identical for both partial flows, in the event of remaining leaks in the edge area of the electrode packages the fluid exchange between the cathode and anode chambers remains low and has hardly any adverse effect on the effectiveness of the disinfection and the formation of oxidizing species.

If this liquid exchange has an adverse effect on certain applications, particularly when treating more conductive waste water, the separator membranes and, if applicable, the thin-film electrodes must be sealed against each other by means of sealing strips inserted at the sides in the edge area. Surprisingly, however, it has been shown that this additional effort is only absolutely necessary in very few cases.

Figures 1 and 2 each show a preferred embodiment of an electrolysis cell for water disinfection. Figure 1 shows an undivided version with a total of 6 bipolar connected individual cells, a) and b) in longitudinal section, c) in cross section at the level of the electrode package. The housing of the electrolysis cell consists of a base frame 1 and clamping plates 2 attached on both sides. The base frame contains

The inlet 3 and the outlet 4 are for the water. The electrode stack consists of the bipolar thin-film electrodes 5 acting on both sides and the edge electrodes 6 acting on one side. The latter are electrically connected to the contact elements 7 for supplying current. The thin-film electrodes are separated and positioned by the spacers 8.

Fig. 2 shows a divided electrolysis cell with three bipolar connected individual cells. The separator membranes 9 are located between two thin-film electrodes, the distances between which and the electrodes are fixed by two spacers 8 each.

In both versions, the spacers, and in the split cell, the separator membranes, protrude above and below the upper and lower edges of the electrodes. The electrode packages are fitted into the housing under tension in such a way that they cannot move due to the back pressure of the water. For better positioning, the cross-section is also narrowed in the inlet and outlet areas of the housing.

Claims (8)

1. Electrolysis cell for the disinfection of water in undivided or divided by separator membranes design, consisting of: - a plastic housing formed from the base frame 1 and lateral clamping plates 2 with a flow-through rectangular cross-section, - an inlet and outlet 3, 4 for the water to be treated, - a ratio of the length between the inlets and outlets to the width of the flow cross-section of at least 3:1, - an electrode package clamped between the clamping plates at a distance of 10 to 50 mm from the inlets and outlets and filling the entire free cross-section, consisting of at least one bipolar electrode plate 5 and two electrode edge plates 6, - the spacers 8 clamped between two electrode plates in the undivided version, the separator membranes 9 clamped between two electrode plates in the split version and one spacer 8 between the electrode plates and the membranes, as well as the contacts 7 for supplying current to the electrode edge plates. 2. Electrolysis cell according to claim 1, characterized in that several bipolar electrode segments connected electrically and hydrodynamically in parallel are arranged in an electrode package. 3. Electrolysis cell according to claims 1 and 2, characterized by the use of thin-film electrode plates made of precious metal foils with a thickness of 10 to 100 µm. 4. Electrolysis cells according to claims 1 and 2, characterized by the use of electrode plates coated with doped diamond and up to 2 mm thick made of ceramic material, e.g. silicon, or of valve metals, e.g. niobium. 5. Electrolysis cell according to claims 1 to 4, characterized by ion exchange membranes as separator membranes in divided electrolysis cells. 6. Electrolysis cell according to claims 1 to 5, characterized by spacers made of plastics or ceramic materials. 7. Electrolysis cell according to claims 1 to 6, characterized in that the spacers and, in the split design, also the separator membranes, project beyond the electrode plates on both sides in the direction of flow. 8. Electrolysis cell according to claims 1 to 7, characterized by lateral seals between the electrode plates or between the electrode plates and the separator membranes.