GB2299077A

GB2299077A - Water disinfection

Info

Publication number: GB2299077A
Application number: GB9506761A
Authority: GB
Inventors: Ian Walker
Original assignee: WRC PLC
Current assignee: WRC PLC
Priority date: 1995-03-20
Filing date: 1995-03-20
Publication date: 1996-09-25
Also published as: GB9506761D0

Abstract

A method of obtaining a measure of the chlorine demand of water comprises analysing the water for two or more water quality parameters and determining from these analyses a measure of the chlorine demand. This method can be used to disinfect water by dosing the water with an amount of chlorine in excess of the measure of the chlorine demand.

Description

WATER DISINFECTION This invention relates to water disinfection and, more particularly, to water disinfection using chlorine.

Chlorine is widely used as a disinfectant during the production of drinking water. It has a number of advantages: for example, it is effective, it is relatively easy to apply, and it provides a persistent residual to the customer's tap. It also has some disadvantages: for example it produces undesirable by-products, and its correct dosing can be difficult to control in waters of varying quality.

When chlorine is added to a natural water, it begins to react with substances contained within the water.

This process can be rapid in the case of, for example, ammonia or less rapid in the case of, for example, certain organic compounds. These reactions are responsible for what is termed the "chlorine demand" of the water. To achieve effective disinfection, it is necessary to first overcome this demand to ensure that there is enough "free" chlorine left in solution to inactivate the micro-organisms. If the chlorine demand varies, due to changes in the water quality, it can be difficult to always ensure the presence of an appropriate quantity of free chlorine. The presence of free chlorine can, of course, be assured by adding a considerable excess of chlorine (i.e. an amount much greater than the chlorine demand). Then dechlorinating agents (such as sulphur dioxide) can be added to "trim" the concentration of chlorine back to an acceptable level.However, this procedure has the drawback of requiring more chemicals and equipment, and it is also likely to encourage the formation of undesirable by-products.

An alternative approach is to match chlorine dosing with variations in chlorine demand. However, this is not always possible with current monitoring equipment and treatment plant configurations. On-line continuous monitoring of chlorine demand has not been developed to any degree of sophistication. A limited range of chlorine demand monitors is commercially available, but these depend on a feedback mechanism, i.e. they add chlorine, allow contact and then measure how much chlorine has been used.

Thus, they are not predictive. They have not found widespread use in the water industry.

We have now found a way of obtaining a measure of chlorine demand of water, in advance of chlorination if desired, whereby the amount of chlorine needed to effect disinfection can be determined and the appropriate chlorine dose thus provided, even for water flow whose chlorine demand varies.

In accordance with the present invention, the water is analysed for two or more water quality parameters which significantly affect the chlorine demand, and from these analyses a measure of chlorine demand is determined.

By monitoring these water quality parameters, the chlorine demand can be followed and the amount of chlorine varied in dependence on the measured demand.

There are a number of water quality parameters which can be used for the purposes of the invention. We prefer to use the ammonia concentration and the organic carbon concentration, but other possible parameters include colour, turbidity, oxidisable iron and oxidisable manganese, for example.

Ammonia reacts stoichiometrically with chlorine and thus directly affects chlorine demand. The amount of chlorine required to neutralise a measured concentration of ammonia can be readily predicted if factors such as pH and temperature are taken into consideration. Ammonia can be measured on-line using existing electrochemical technology, as will be clear to those skilled in the art.

Organic carbon will also react with chlorine in proportion to the organic carbon concentration. On-line measurement of organic carbon directly is complex and can be unreliable, and whilst this technique can be used, we prefer instead to measure a surrogate. One example of a surrogate is the amount of ultra violet light that the water absorbs.

This can suitably be measured at a specific wavelength: eg.

254 nm (UV254). Monitors are currently available for this purpose, as will be clear to those skilled in the art.

It will also be necessary when using the parameters of organic carbon and ammonia, and for most other parameters too, to measure or control the pH and the temperature at which the determinations are made.

The invention includes a method of obtaining a measure of chlorine demand which comprises determining two or more water quality parameters and obtaining a measure of the chlorine demand therefrom.

The invention also includes a method of disinfecting water which comprises obtaining a measure of chlorine demand by the method of the invention, and then dosing the water with an amount of chlorine in excess of the chlorine demand.

Both methods can be carried out on static or flowing water.

The invention further includes a device for obtaining a measure of chlorine demand which includes at least two monitors each for a different water quality parameter, and means for providing a measure of chlorine demand from the measured parameters.

One embodiment of device of the invention combines monitors for ammonia, UV254, pH and temperature. The signals from these are fed into a controller that operates on software which converts the signals into another signal that is proportional to the predicted chlorine demand of the water. This is used to control the chlorine dose. A chlorine monitor can be used to measure the chlorine concentration at a fixed interval following the dosing point, and this will be used to trim the dose.

The system can operate on so-called "fuzzy logic" and can use neural networks which allow it to learn and self calibrate to the specific type of water being examined.

The accompanying schematic drawing illustrates a device of the invention and its operation. The device comprises four monitors: ammonia (1), UV254 (2), pH (3) and temperature (4). The monitors sense conditions in the water (10) and provide signals to a control (5). Control (5) is connected to a chlorine doser (6) to control the dosing of chlorine (via line 7) to the water 10.

Claims

CLAIMS:

1. A method of obtaining a measure of the chlorine demand of water, comprising analysing the water for two or more water quality parameters, and determining from these analyses a measure of the chlorine demand.

2. A method according to claim 1, wherein one of the water quality parameters is ammonia concentration.

3. A method according to claim 1 or 2, wherein one of the water quality parameters is organic carbon concentration.

4. A method according to claim 3, wherein the organic carbon concentration is determined by measuring the ultra violet absorption of the water.

5. A method according to claim 4, wherein the ultra violet absorption is measured at a wavelength of 254nm.

6. A method according to any preceding claim, wherein at least one of the water quality parameters is colour, turbidity, oxidisable iron concentration and/ or oxidisable manganese concentration.

7. A method according to any preceding claim, further comprising measuring the pH and/or temperature at which the analysis is carried out.

8. A method according to claim 7, further comprising controlling the pH and/or temperature at which the analysis is carried out.

9. A method of disinfecting water, comprising obtaining a measure of chlorine demand by a method according to any one of claims 1 to 8, and dosing the water with an amount of chlorine in excess of said measured chlorine demand.

10. Apparatus for obtaining a measure of chlorine demand, comprising at least two monitors each capable of measuring a different water quality parameter, and means for providing a measure of chlorine demand from the combined parameters.

11. Apparatus according to claim 10, wherein one of the monitors is for measuring ammonia concentration, and another of the monitors is for measuring organic carbon concentration.

12. Apparatus according to claim 10 or 11, wherein the means for providing a measure of chlorine demand comprises a controller, which controls the amount of a chlorine dose fed to the water.

13. Apparatus according to any one of claims 10,11 or 12, further comprising a chlorine monitor for measuring the chlorine concentration of the water.

14. A method of obtaining a measure of the chlorine demand of water substantially as herein described with reference to and as shown in the accompanying drawing.

15. A method of disinfecting water substantially as herein described with reference to and as shown in the accompanying drawing.

16. Apparatus for obtaining a measure of chlorine demand of water substantially as herein described with reference to and as shown in the accompanying drawing.