GB2143655A

GB2143655A - Water treatment process

Info

Publication number: GB2143655A
Application number: GB08417213A
Authority: GB
Inventors: John Howard Warden
Original assignee: Water Research Centre
Current assignee: Water Research Centre
Priority date: 1983-07-07
Filing date: 1984-07-05
Publication date: 1985-02-13
Also published as: GB2143655B; GB8417213D0

Abstract

In the disinfection of a water supply by dosing with chlorine, the amount of chlorine dosed is controlled in dependence on the chlorine residual in the water and on the pH. A measurement of the pH is used to correct the measurement made of the chlorine content, since the device used for this purpose is affected by the pH. The measurements are made a predetermined time after dosing.

Description

SPECIFICATION Water treatment process This invention relates generally to a water treatment process and, more particularly, to a method and apparatus for monitoring and controlling the disinfection of water by chlorination.

It is well known to disinfect water using gaseous chlorine or hyprochlorite. In both cases, it is important to use the appropriate quantity of chlorine, that is to say enough for disinfection purposes but not any significant excess since this is both wasteful and can taint the water. Where the quality of water from a source is reasonably constant, the amount of chlorine dosed to the water can be controlled in dependence on the water flow rate. This is not satisfactory, however, when the water quality varies. In such cases, it is necessary to control the chlorine dose in response to the residual chlorine in the water at some predetermined interval of time after dosing. Typical chlorine monitors for this purpose comprise a copper platinum electrode system mounted in a cell through which is passed the water whose chlorine content is to be measured.It is important to keep constant the water flow rate through the cell, and a flow control valve is normally used for this purpose. It is also important to keep the pH of the water at a constant value and, for this purpose; the water is dosed with buffers.

Investigation of this known procedure has shown it to have a number of disadvantages. In particular, the dosing with buffer, whilst critically important to the obtainment of accurate results for chlorine level, gives rise both to problems and to expense.

In accordance with the present invention, it has been found that the prior necessity for using buffers can be completely avoided, with attendant advantages. In particular, it has been found that the pH of the water can be allowed to fluctuate and that satisfactory residual chlorine measurements can still be obtained by detecting the pH and correlating the direct chlorine measurement output in dependence on the pH, i.e. "correcting" the chlorine measurement to allow for the effect, on the measurement, of any particular pH.

According to one aspect of the present invention, there is provided a method of chlorinating water which comprises dosing water with chlorine in an amount dependent on the residual chlorine content of the water measured after a predetermined interval of time, characterised in that the pH of the water is also measured, and the chlorine dose is coiitrolled in dependence on both the measured residual chlorine and the pH.

The invention further includes apparatus for chlorinating water which comprises means for dosing the water with chlorine, means for measuring the amount of chlorine remaining in the dosed water after an interval of time, and means for controlling said dosing means in dependence on said measuring means, characterised in that the apparatus further includes means for measuring the pH of the dosed water and wherein said control means controls said dosing means in dependence also on said measuring means.

In the method and apparatus of the invention, the output from the chlorine monitor and the output from a pH measuring probe (both preferably located in a single measuring cell) are preferably fed into a micro-processor based device which uses the two signals to compute the true chlorine content of the water and so appropriately control the chlorine dosing. This completely avoids the prior art use of buffers and all the various disadvantages inherent therein.

The microprocessor and associated circuitry and controls can, of course, be calibrated by using aqueous solutions of chlorine of known strength and pH.

In the method of the invention we refer to dosing the water with chlorine, by which we include the use of gaseous (or liquid) chlorine and of hyprochlorite. The chlorine residual measurement is made after chlorine dosing.

Experiments have shown that, to be meaningful, the measurement should be made only after a delay of from about 5 to about 30 minutes. In accordance with a preferred feature of the present invention, therefore, the water sample for analysis may be withdrawn from the water flow some distance downstream from the chlorine dosing. Alternatively, after dosing the water, the sample may be stored (for example in a holding tank or coil) for an appropriate period before the measurement is made.

The method and apparatus of the invention can be used to advantage in both large and small scale disinfection plants. In accordance with further aspects of the invention, however, special advantages are achievable in certain small scale operations. Some water sources, which may be springs or surface waters, for example, are remote from national power supplies and the like. When water is drawn from these sources, it must nevertheless be disinfected. The method and apparatus of the present invention are particularly suited for use in the treatment of remote, small scale water sources.

In one preferred such arrangement, chlorine dosing is controlled using a valve whose position is changed only at discrete time intervals, power being needed only to change the position and not to maintain it. An arrangement involving a stepping motor is satisfactory for this purpose. The microprocessor (or data-logger) can be on-site, or it may be at a control station some distance from the site, with telemetry of signals to and from the site.

The chlorine monitor and pH meter can be actuated only at intervals, in order to reduce the overall power requirements of the system. The system can be powered by one or more batteries, and/or by a locally generated power supply, e.g.

wind power, solar power or hydraulic power.

As will be understood, in the case of remotely sited systems, an arrangement can be included to raise an alarm if the chlorine residual falls too low (indicating inade quate disinfection) of if the pH varied outside a normal range (indicating severe fouling of the water).

After analysis of a water sample, it may be necessary or desirable to de-chlorinate it prior to disposal. For this purpose, we prefer to pass it through a granulated activated carbon (GAC) column. A system of the invention may also include other water treatment or water-testing devices. For example, modules for colour removal may be incorporated.

In order that the invention may be more fully under stood, one example thereof is shown in the accompanying schematic drawing.

Referring to the drawing, there is shown schematically a flow of water 1, moving the direction of arrow 2. Upstream is a dosant supply line 3 including a control valve 4 and actuator 5 therefor. Line 3 is fed with disinfectant from reservoir 6 which may include a level sensor 7 to provide a signal when replenishment is required. Downstream of dosant line 3 is sample withdrawal line 10 in which are disposed chlorine measurement cell 11 and pH measurement probe 12. Line 10 terminates at granulated activated carbon column 13, from which is a liquid outlet 14 to waste. Before (or upstream of) cell 11, a contact unit or mixing coil 15 may be provided in line 10.

The system also includes a microprocessor 20 and a battery 21 and, optionally, power generation means 22 for charging the battery 21. Electrical connections from the battery are not shown, but some other electrical connections (in discontinuous lines) are shown, including connection 30 (microprocessor to valve actuator 5), connections 31 and 32 both from the microprocessor to the chlorine cell 11 and pH probe 12, respectively, connection 33 from micro-processor 20 to telemetry output unit 40 (to transmit data to a distant control station). There is also shown electrical connection 34 from sensor 7 to telemetry output unit 40.

In operation, water is drawn from a source (not shown) in the direction of arrow 2 as a flow 1.

Sampling line 10 is downstream of dosing line 3. It may be spaced a sufficient distance downstream for a delay of from 5 to 30 minutes to occur, after dosing, before sampling and analysis. Alternatively, the contact unit or mixing coil 15 can be used to delay passage of the sample to the cell 11 and probe 12. The sample is withdrawn from flow 1 (under the action of a pump, for example, or by gravity) into line 10, wherein its chlorine content and pH are measured by cell 11 and probe 12, respectively. The sample then passes through the column 13 to waste. The electrical outputs of cell 11 and probe 12 are passed to the microprocessor 20 along respective electrical connections 31,32.

Microprocessor 20 is programmed to determine the true chlorine content of the water, by taking account of the pH and of the apparent chlorine content as determined by cell 11. Then, via connection 30, the actuator 5 is caused to alter valve 4 (or not) as may be required. Thus, if there is too little chlorine in the sample water, valve 4 will be opened slightly more to admit more dosant to flowing stream 1. The microprocessor may also (as desired) send its output to the telemetry output 40 for transmission to a distant control station.

In a preferred arrangement, actuator 5 is so designed as to be able to operate various different chlorine dosing devices. For example, it may comprise a horizontal shaft movable axially, the movement being utilisable to alter a valve, raise or lower a flexible tube, alter a stop or in various other ways.

In order to conserve power, the system may include one or more timers or the like (not shown) so that the monitoring process occurs only at preset intervals.

Claims

CLAIMS:

1. A continuous method of chlorinating a water supply, which comprises dosing the water with chlorine in an amount dependent on the residual chlorine content of dosed water measured a predetermined interval of time after dosing, characterised in that the pH of the dosed water is also measured, and the continued chlorine dosing is controlled in dependence on both the measured residual chlorine and the pH.

2. A method according to claim 1, wherein said interval of time is from about 5 to 30 minutes.

3. A method according to claim 1 or 2, wherein said residual chlorine and said pH are measured substantially simultaneously.

4. A method according to claim 3, wherein the residual chlorine and pH are measured in a cell, and electrical signals indicative thereof are fed to a microprocessor programmed to provide an output for appropriately controlling the chlorine dosing.

5. Apparatus for chlorinating water, which comprises means for dosing the water with chlorine, means for measuring the amount of chlorine remaining in the dosed water after an interval of time, and means for controlling said dosing means in dependence on said measuring means, characterised in that the apparatus further includes means for measuring the pH of the dosed water and wherein said control means controls said dosing means in dependence also on said pH measuring means.

6. Apparatus according to claim 5, wherein said chlorine measuring means comprises a two-metal electrode cell characterised in that the said pH measuring means is also included in said cell.

7. Apparatus according to claim 5 or 6, wherein said chlorine measuring means and said pH measuring means both provide electrical signal outputs indicative of their respective measurements, characterised in that a microprocessor is provided to receive and correlate the signals to provide an out Dut for controlling said dosing means.

8. Apparatus according to claim 5,6 or 7, characterised in that said measuring means are downstream of said dosing means so that, in use, said interval of time is from about 5 to about 30 minutes.

9. Apparatus according to any of claims 5 to 8, also including one or more timing devices characterised in that the measuring means are only operative at predetermined time intervals.