Monitoring across large geographical areas
Microprocessor technology uses the water pH level to correct cell calibration in chlorine monitoring which makes these units more suitable for potable water treatment in extended distribution systems.
In the West of Scotland Water Supply system, large distances between water sources, treatment plant and service reservoirs make this difficult, because of significant and variable losses of chlorine from solution. Water demand patterns, storage time, flow rates, temperature and the weather are also factors.
The West of Scotland Water Authority (WoSWA) developed a scheme to monitor chlorine levels at various points throughout its wide distribution network. Information is sent via telemetry to control centres to monitor 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 WoSWA's water supply area which is dependent on the water source, the reservoir level, and the rainfall. A simple chlorine measurement cell would be unreliable without taking account of these pH variations.
Free residual chlorine
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, the chlorine measurement can be done in known conditions to a known calibration. This is the basis of the conventional approach 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 adds 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. 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 be allowed to drain into the soil or watercourses.
Buffer system rejected
This system was considered unsatisfactory by WoSWA for the wider use planned for such monitoring stations, because of the need for special handling and disposal of this sample stream, plus the required supplies of buffer solution and maintenance on the peristaltic pump. The initial cost and the regular attendance requirement meant that the overall cost of the increased quality monitoring was too high.
However, microprocessor based sensor technology allows the water pH level to be used to correct the cell calibration and these direct measurement techniques do not require buffer solutions, or specially treated sample streams.
A sample stream from the main delivery line is monitored by both pH and chlorine sensors. Chlorine measurement is made with an electrochemical cell, 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 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. The correction required for the pH value of the water is automatically applied. The sample stream is not affected chemically and can be returned to the potable water flow, if desired, or diverted to a normal drain.
This 'Multi-Variable Technology' monitors and uses two variables together to achieve the required calculated measurement. For its network, WoSWA selected the Rosemount Analytical 1054B which provides pH compensation over the range of pH from 5 to 9.5. It has a liquid crystal display 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 WoSWA supply network. The project could not have been considered using buffer solution techniques of chlorine measurement, which would have involved frequent, often weekly visits to remove waste sample liquids and replenish the required chemicals, plus checking pump operation. The major advantages of the microprocessor based system compared to a buffer solution system are:
- No buffer chemicals on site
- No moving or wearing parts
- No collection of chemical waste.
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.