GB2202550A

GB2202550A - Disinfection of water supplies

Info

Publication number: GB2202550A
Application number: GB08706283A
Authority: GB
Inventors: David John Ecob
Original assignee: Water Research Centre
Current assignee: Water Research Centre
Priority date: 1987-03-17
Filing date: 1987-03-17
Publication date: 1988-09-28
Also published as: GB8706283D0

Abstract

An apparatus to disinfect water comprises a reverse electrodialysis (RED) cell 1 to generate electric power and an electrolysis cell 6 electrically powered by the RED cell to generate chlorine or hypochlorite disinfectant from brine discharged to the electrolysis cell 1 from the reverse electrodialysis cell. The concentrated brine solution is preferably made up to 250 g/l of sodium chloride, and the flow rate of the input is set to produce a dilute stream output having a concentration of 25 g/l of sodium chloride for passage to the electrolysis cell. The apparatus is self-contained and may be used to disinfect water at remote sites, the only inputs needed being salt and water. <IMAGE>

Description

DISINFECTION OF WATER SUPPLIES The present invention relates to disinfection of water supplies. More particularly, but not exclusively, it relates to disinfection of small supplies at remote sites by use of a self-contained unit which is safe to use.

Within Britain alone, there are many thousands of water supplies, many of which are either private or, if in the public sector, operate on a comparatively small scale. Many of these sites are remote and difficult for access by vehicles. Nevertheless, the water must be disinfected and presently, this is carried out by using sodium hypochlorite as the disinfectant chemical and using battery power to supply electrical requirements.

Handling this comparatively dangerous chemical can prove hazardous and difficult, while using batteries for electrical power is expensive and again difficult in some cases.

It is known to electrolyse sea water to produce hypochlorite for water disinfection. There is now being developed the reverse electrodialysis (RED) system of generating electrical power utilising the free energy difference between salt solutions of different concentrations.

It is an object of the present invention to overcome the above disadvantages inherent in the use of small, possibly remote, water supplies by combining facets of the above two processes to produce a selfcontained disinfection unit and process.

According to a first aspect of the present invention, there is provided an apparatus to disinfect water comprising a reverse electrodialysis (RED) cell to generate electric power and an electrolysis cell electrically powered by said RED cell to generate chlorine or hypochlorite disinfectant from brine discharged to said electrolysis cell from said reverse electrodialysis cell.

According to a second aspect of the present invention, there is provided a method of disinfecting water comprising passing at least some of the water to a RED cell as a dilute input thereto, preparing a concentrated salt solution, passing said concentrated salt solution to the RED cell, passing the dilute output of the RED cell to an electrolysis cell, applying at least part of the electrical output of the RED cell to the electrolysis cell to generate chlorine or hypochlorite disinfectant therein, and passing the chlorinated output of the electrolysis cell to any remainder of the water.

Preferably, the salt is sodium chloride, and in this case, the concentrated solution may be made up to 250 g/l sodium chloride.

Advantageously, the flow rate is set to produce a dilute stream output having a concentration of 25 g/l of sodium chloride for passage to the electrolysis cell.

The weakened concentrated stream may be recirculated to the preparation step for restoring its concentration by addition of further salt.

According to a third aspect of the present invention, there is provided a method of disinfecting water comprising the steps of passing at least some of the water to an RED cell, preparing and passing to the RED cell a more concentrated salt solution, operating the RED cell under such conditions as to produce chlorine gas (or sodium hypochlorite) at an appropriate electrode, collecting said chlorine (or sodium hypochlorite) and introducing it to the water to be disinfected.

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: FIGURE 1 shows schematically a self-contained disinfection plant embodying the invention; and FIGURE 2 illustrates diagrammatically the principle of reverse electrodialysis.

Referring firstly to Figure 2, there is illustrated the principle of operation of a reverse electrodialysis cell. It comprises a pair of electrodes 1 separated by a number of membranes 2 which have a low electrical resistance, coupled with a high permselectivity. Alternate membranes are permselective for anions and cations. A concentrated salt solution is fed between pairs of opposite type membranes while a dilute or even salt-free solution is passed between the membrane pairs between the flow paths of the concentrated salt solution. As can be seen diagrammatically from Figure 2, a proportion of the anions migrate to the left (as seen in Figure 2) while a proportion of the cations migrate to the right, in each case from the more concentrated solution to the dilute solution. Thus, a current is generated between the electrodes 1.

During this process, the concentrated solution becomes less concentrated, at the expense of the dilute solution which becomes more concentrated.

Referring now to Figure 1, a portion of a raw filtered water supply is taken off the main at 3 and fed as the dilute supply to an RED cell. A separate water supply is mixed in vessel 4 with sodium chloride (common salt) to produce a saturated solution at an approximate concentration of 250 g/l. This is fed to the RED cell as the concentrated supply.

The apparatus is shown diagrammatically as having a single pair of electrodes 1, but obviously, to generate any appreciable power, a plurality of electrodes will be necessary together with a corresponding plurality of flow path between them.

The concentrated output of the RED cell, now less concentrated, is recirculated by means of pump 5 to vessel 4 where the previous concentration is restored by addition of further sodium chloride. At least part of the dilute output of the RED cell, which now contains sodium chloride in an amount of possibly 25 g/l is passed to an electrolysis cell 6 where chlorine is liberated at the anode. The chlorine is either dissolved in the water or combines to form sodium hypochlorite in the water, and the output of the electrolysis cell is returned to the water supply main at 7. The concentration is sufficient to disinfect the water supply.

The electrical power required to operate the electrolysis cell is provided by the output of the reverse electrodialysis cell, and in fact, any excess output of the RED cell may be used to power the pump 5 and also possibly any telemetry, instrumentation, etc.

As can be seen, the apparatus and method require only a supply of raw filtered water and a quantity of common salt or some other salt. Sodium chloride is non-hazardous, safe to handle and cheap.

It is estimated that the requirement for sodium chloride will be 2.0 kg/1OO cubic metres/day. The salt solution is used to produce electricity by reverse electrodialysis, and this electricity is used to produce a disinfectant chemical (either chlorine or sodium hypochlorite) from the brine output of the RED cell. Thus the system is completely self-contained and safe to operate.

In the above description, it has been assumed that the RED cell is operated to maximise the electrical output for operation of an electrolysis cell generating chlorine. However, the operation conditions of the RED cell may be changed so that electrical generation efficiency is reduced by virtue of the cell acting also as an electrolysis cell itself. In this case, the chlorine generated in the reverse electrodialysis cell may be collected and used to disinfect the water, with the electrolysis cell being omitted.

Claims

CLAIMS:

1. A method of disinfecting water comprising passing at least some of the water to a RED cell as a dilute input thereto, preparing a concentrated salt solution, passing said concentrated salt solution to the RED cell, passing the dilute output of the RED cell to an electrolysis cell, applying at least part of the electrical output of the RED cell to the electrolysis cell to generate chlorine or hypochlorite disinfectant therein, and passing the chlorinated output of the electrolysis cell to any remainder of the water.

2. A method as claimed in claim 1, wherein the salt is sodium chloride and the concentrated solution may be made up to 250 g/l sodium chloride.

3. A method as claimed in either claim 1 or claim 2, wherein the flow rate of said input is set to produce a dilute stream output having a concentration of 25 g/l of sodium chloride for passage to the electrolysis cell.

4. A method as claimed in any one of the preceding claims, wherein the weakened concentrated stream may be recirculated to the preparation step for restoring its concentration by addition of further salt.

5. A method of disinfecting water comprising the steps of passing at least some of the water to an RED cell, preparing and passing to the RED cell a more concentrated salt solution, operating the RED cell under such conditions as to produce chlorine gas (or sodium hypochlorite) at an appropriate electrode, collecting said chlorine (or sodium hypochlorite) and introducing it to the water to be disinfected.

6. A method of disinfecting water substantially as herein described with reference to the accompanying drawings.

7. An apparatus to disinfect water comprising a reverse electrodialysis (RED) cell to generate electric power and an electrolysis cell electrically powered by said RED cell to generate chlorine or hypochlorite disinfectant from brine discharged to said electrolysis cell from said reverse electrodialysis cell.

8. An apparatus to disinfect water substantially as herein described with reference to the accompanying drawings.