All pool disinfection systems should be designed to match the expected rate of disinfectant consumption at the worst conditions expected. For some pools this may be bright sunshine at 40˚C and standing room only.
The higher the pool turnover rate, the easier it is to circulate the disinfectant, measure and respond to demand. Some systems, such as chlorine gas and liquid chlorine, are quite flexible in the amount that can be injected per hour. Systems such as salt chlorinators and erosion feeders (BCDMH, trichlor) must be correctly sized on the basis of pool volume, flow rate and anticipated bather load and have a reserve capacity to cope with peak situations.
Design of Dosage Systems
Metering pumps are used to dispense liquid systems at pressure into the circulation system. Usually the stroke volume and frequency can be adjusted to change feed rates. These pumps require priming to ensure that air bubbles are not present in the lines, which may cause ineffective pumping.
Gas systems (chlorine and carbon dioxide) use valves and an injector into a circulation loop. The feed rate can be set using a sight glass valve on the cylinder.
Points of Dosing
There are varying arguments about the merits of where chemicals should be dosed.
Disinfectants, when dosed before the filter, have the advantage of continuously disinfecting the filter media preventing colonisation of organisms, such as Pseudomonas and Legionella. The disadvantage is that more chloramines may be created and disinfectant consumption may increase. However, if disinfectant is injected after filtration, regular superchlorination will also control filter colonisation. Where ozonation is present, disinfection should take place after the removal of ozone.
Control systems analyse disinfectant and pH levels using a sensor and electronic meter, and when outside the set parameters, sends a signal to the pump or solenoid valve to allow more chemical to be injected or released.
Controllers are divided into two types:
1. Proportional controllers that feed faster when the measured concentration is far away from the set point. Conversely, the addition rate slows when the pool condition is close to the set point.
2. Feed wait control. Chemical addition is performed at the same rate when away from the set point.
Either type of controller should have facilities to minimise controller bounce (that is, dampening of signal variations). The pumps should be adjusted accordingly to deliver chemical at an appropriate rate per hour given the pool turnover characteristics in order to minimise overdosing.
Some control systems can also measure electrical conductivity and operate a dump valve to ensure dilution of pool water and control of TDS.
Relevant markings to Australian Standards or international standards prescribed by Standards Australia should be present on the controller. The units should be mounted in a safe area and not directly subject to accidental water splashes, such as may happen when cleaning electrodes. The mains power supply to the controller should have safety circuit breakers fitted, both for the pool operator safety and to provide some protection of the controller electronics.
Disinfectant and acid should not be added simultaneously. The controller system itself, or some other means, should prevent acid and disinfectant contact.
Pumps and other chemical delivery units should be constructed from materials rated for use with the pool chemicals being delivered. Close attention should be paid to tubing used for disinfectant and acid. Chemical delivery tubing should be inspected at least weekly.
pH is measured by a glass electrode that selectively measures the relative hydrogen (acid) activity and sends a reading in millivolts to a pH meter/controller. The meter/ controller converts this into pH units.
There are two commonly used methods of automatically analysing disinfection: Direct Chlorine Residual Measurement (Amperometric) and Oxidation-Reduction Potential Measurement (ORP, Redox, Rh).
Direct Chlorine Residual Measurement (Amperometric)
This method uses a chlorine sensor to estimate the actual concentration of free chlorine by measuring the hypochlorous acid component. Because pH affects the ratio of hypochlorous acid/ion, it should be kept constant so that the free chlorine is measured accurately.
Oxidation-Reduction Potential Measurement (ORP, Redox, Rh)
This method uses a platinum electrode to measure the relative oxidative strength of the water. When the pH is kept constant, there is generally a close relationship between free chlorine and ORP readout. Because pH affects the ratio of hypochlorous acid/ion, it should be kept constant so that the free chlorine effect is measured accurately.
At higher levels of free chlorine residual (> 3 ppm) ORP becomes less sensitive. Accordingly, disinfectant residual becomes increasingly difficult to control at higher levels using ORP controllers.
Other factors may also affect ORP measurements. For example, the presence of cyanuric acid lowers the ORP value. Other substances in the water may also have an effect, but where these factors are constant, they will not adversely affect the operation of the system. The values selected for the control will reflect these factors.
Changes in concentration of combined chlorine can influence ORP readings. At a given free chlorine concentration and pH, water without combined chlorine will have a higher ORP than water with combined chlorine. It is therefore better to set minimum ORP readings when there is no combined chlorine in the water and when bather loads are low. The free chlorine residual must also be set to at least above the minimum regulatory limit when setting ORP. When combined chlorine is formed during the course of the day, extra free chlorine residual will be then maintained in the system until the combined chlorine is destroyed. The free chlorine level should not drop below the original set point when ORP control equipment is set up this way.
Analyser Cell/Probe Buffering
Both ORP and direct chlorine residual methods of measurement or control work best when the pH is kept stable below 7.6 in chlorinated pools. Where this is not possible, and precise chlorine control is required, buffered cell analysers can be used. Buffered cell analysers inject a pH buffer into the water at the point of measurement to give a stable pH reading (usually around pH 6), which ensures accurate measurement of free chlorine. Most pools should not need buffered cells.
ORP sensors are often used to control bromine levels. Because bromine is less sensitive to pH change, automatic systems should work well throughout the normal pH range for swimming pools. For direct bromine residual measurements, some chlorine sensors can also measure bromine. Check with the manufacturer for the compatibility. Operators should also note that bromine might affect the response of a silver/silver chloride sensor.
Calibration of Sensors
Because it is difficult to obtain stable chlorine solutions, primary calibration of equipment is usually not done. Comparing the readout with the result obtained from a DPD photometer or comparator, and realigning the readout to match the DPD test, is all that is required.
Calibration is not required for ORP sensors that are used to control disinfection, as the individual readings are electrode-and site-specific. Finding the correct minimum ORP setting for each pool requires monitoring of pool performance and correlation with measured water quality and disinfection parameters with ORP readings measured.
Primary calibration of a pH sensor should be done with two standard solutions. Standard solutions should cover the swimming pool pH range. These standard solutions can be obtained from scientific suppliers. Solutions commercially available as pH 7.01 and pH 9.01 are recommended, as this will produce an accurate response in the desired range. Measuring the pool water with a separate pH meter or phenol red indicator and adjusting the controller accordingly can be used to make secondary calibration of controllers.
Location of Sensors
Sensors should be located at a point that is indicative of the actual swimming conditions. Sensors can be inserted directly into the circulation loop, subject to manufacturer’s specifications on pressure and flow velocities. Alternatively, a loop can be created which side-streams a small flow to a wall mounted sensor installation.