Over 200 Rosemount Analytical 1054B automatic pH compensation chlorine analysers are planned for installation in the West of Scotland Water Supply network
Chlorine or Hypochlorite are widely used in Municipal and Industrial applications for disinfection of drinking water supplies, taste and odour control, bleaching etc.
Applications include algae control in closed loop water systems as well as potable water treatment.
The UK water supply system maintains water quality to the high standards demanded by the consumers by careful addition of chlorine in the distribution network.
The objective for best water quality for the customer, is to add chlorine into the distribution system to a level where it protects the water in the supply network, but by the time of delivery to the tap, the concentration is minimal.
In the West of Scotland Water Supply system, the large distances between water sources, treatment plant and service reservoirs make this doubly difficult, because of significant and variable losses of chlorine from solution.
This can be affected by the water demand pattern, storage time, flow rates, temperature and the weather.
The West of Scotland Water Authority have developed a scheme to address this problem by monitoring chlorine levels at various points throughout their distribution network.
Information is sent via telemetry to various control centres then allows decisions to be made about the level of chlorine dosing, and whether this should be adjusted at the available injection points.
The chlorine monitoring task was made more complex by the variable pH level occurring in the water supply in this area, dependant on the water source, the reservoir level, and the rainfall.
A simple chlorine measurement cell would give an unreliable indication without taking account of these pH variations.
When chlorine is added to potable water supplies, typically with no significant levels of ammonia, amines or bromide/iodide compounds, it reacts with the water to produce both Hypochlorous acid (HOCl) and the Hypochlorite ion (OCl?).
The simple electro-chemical cell used to measure chlorine levels has a different sensitivity to the two types of reaction product.
Since the ratio of the HOCl and OCl? depends on water pH level, the calibration of the cell will vary with pH.
If the pH of the sample is held constant to within 0.2 pH units, then the chlorine measurement can be done in known conditions to a known calibration.
This is the basis of the conventional approach used to overcome this measurement problem. At the monitoring station a water sample is extracted from the delivery line, and a pH sensor in the sample measures the water pH.
A control system causes additions of a buffer solution, usually sodium acetate, via a peristaltic injection pump, to bring the pH level of the treated sample down to typically 4.6 pH units.
Now the Chlorine measurement cell in this sample will have a known sensitivity, and the output measured will give a calibrated reading.
However, the sample of the water treated with sodium acetate cannot be returned to the potable water delivery system, so must be drained away to storage for subsequent collection and disposal - it cannot just be allowed to drain into the soil or watercourses.
This system was considered unsatisfactory for the wider use planned for such monitoring stations, because of the need for special handling and disposal of this sample stream, as well as needing supplies of buffer solution and maintenance on the peristaltic pump.
Both the initial cost and the regular attendance requirement meant that the overall cost of the increased quality monitoring was too high.
However, developments in microprocessor based sensor technology have enabled reliable chlorine monitoring systems, where the water pH level can be used to correct the cell calibration.
This makes possible the use of direct measurement techniques, which do not require buffer solutions, or specially treated sample streams.
So the West of Scotland Water Authority, in co-operation with Engineering Consultants Entec UK Ltd and P.B.
Kennedy and Donkin, investigated the use of such systems and their application to this network.
Again a sample stream is taken from the main delivery line, and is monitored by both pH and Chlorine sensors.
Chlorine measurement is made with an electrochemical cell, where the chlorine diffuses through a membrane, into the cell electrolyte, and a reaction takes place at the cathode.
The cell itself contains a temperature sensor, which provides a temperature compensation input into the electronics.
The pH measurement sensor is connected to the same electronic control unit as the Chlorine sensor.
Within this electronics, the pH measurement is monitored to ensure that the level is within the range over which the calibration is valid, and the microprocessor then selects the calibration look-up table to be applied to the Chlorine cell output.
By processing the chlorine output in this way, the correction required for the pH value of the water is automatically applied.
The sample stream is not affected chemically at all, and can be returned to the potable water flow, if desired, or diverted to a normal drain. The buzzwords describing this approach would be "Multi-Variable Technology", where two variables are monitored and used together to achieve the required calculated measurement.
The unit selected for installation by the West of Scotland Water Authority, the Rosemount Analytical 1054B, provides pH compensation over the range of pH from 5 to 9.5.
It has a liquid crystal display for the operator to interrogate the information he wants, and electronic outputs of each variable, to provide analogue data for recording or telemetry.
The unit also has two alarm outputs, field configurable as High/Low on either the pH level or the Chlorine level, or selectable as a Fault signal.
Over 200 of these automatic pH compensation chlorine analysers are planned for installation in the West of Scotland Water Supply network.
This is part of a major quality monitoring investment to improve the water supply system in this area, linked with telemetry and Chlorine dosing stations.
The project could not have been considered using buffer solution techniques of chlorine measurement, where it would have involved frequent, often weekly visits to remove waste sample liquids and replenish the required chemicals, plus check the operation of the pumps. The major advantages seen for the microprocessor based system compared to a buffer solution system are: * No buffer chemicals on site requiring top-up * No moving or wearing parts, as with peristaltic pump systems * No collection of chemical waste required * Water in the monitoring stream can be returned to the flowline if needed, or alternatively drained into a soakaway The Electrochemical cell used, the Rosemount 499A CL, does require membrane replacement and electrolyte renewal, but this is typically undertaken on a 3-4 monthly basis.
