Commercial swimming pools should have a system installed to automatically dose the disinfection, pH control and coagulation chemicals into the pool water circulation system. Manually dosing chemicals (sometimes referred to as ‘hand-dosing’) is a hazardous activity that can be easily avoided by the installation, use and maintenance of such systems.
Hand-dosing chemicals should only be undertaken in exceptional circumstances, after a thorough and robust risk assessment has been conducted, and only by people trained and competent to do so.
Automatic dosing systems also provide a much more reliable level of control over the pool chlorine and pH levels.
Written procedures should be established for day tank filling, mixing or diluting chemicals and cleaning injectors. There should also be built-in safeguards to cover those periods when the plant is not attended.
If the plant is to be shut down for longer than 60 hours, valves infilling lines between the day and bulk tanks should not be closed, as decomposition products might otherwise build up. After such a shutdown, the whole of the dosing system should be flushed through gently with low-pressure water.
Chemical dosers should be interlocked with the circulation pumps and the circulation of water through the system, so that dosing stops if there is pump failure.
Day tanks are vessels for holding the chemical solution, from where they are pumped into the circulation system, usually via an injector. They should be constructed from UV-stabilised polyethylene and ideally be fitted with:
- High and low level indicators and alarms
- Overflow pipe
- Water inlet form header tank
- Drain valve
Preparing dosing chemicals
- Chemicals should always be added to water and never the other way around when preparing solutions.
- Non-liquid chemicals should be kept dry until dissolved in water.
- Calcium hypochlorite should be kept away from all other chemicals in its preparation for dosing.
- Calcium hypochlorite should be dissolved in water at a ratio of 1:33
- Sodium hypochlorite can be dosed at its delivery concentration (10-15%)
- If hydrochloric acid is not being dosed direct from a container, dilution should be introduced by filling the day tank with a known quantity of water, adding a known quantity of concentrate, and mixing thoroughly.
- Any sludge formed from the incomplete dissolving of chemicals should be cleared periodically.
Usual practice is to dose disinfectant prior to filtration. This is achieved by injecting a solution into the circulation pipework by means of a chemical dosing pump. A common type of pump used for this purpose is a diaphragm pump, which uses a rubber disc (the diaphragm) to create suction in the chemical feed line when it moves backwards, and then creates a pressure in the feed line on the discharge side when it moves forwards. See image below.
Dosing disinfectant before the filter(s) prevents inadvertent mixing of disinfectants and acids (which should be added post-filter). However, with ozone and ultraviolet systems (which removes residual disinfectant), dosing is always after the ozone or UV treatment.
The pH correctant (in the UK, usually acid-based), is usually dosed after filtration, in a similar manner to that described above for disinfectant dosing.
A typical automatic dosing system will comprise of an electronic control unit connected to probes that are being supplied with a representative sample of pool water via a sample line taken from the circulation pipework.
The sample line is usually taken from the circulation pipework in the plant room, prior to the pre-pump strainer(s). In older pools, it may be taken form extract points in the pool wall.
These probes are constantly monitoring the levels of free chlorine and pH, and are electrically interlocked to pumps that turn on or off according to the readings from the probes and the parameters that have been pre-programmed into the control unit. See below for a labelled image of a typical control unit.
It is recommended that a service contract is arranged for the ongoing service and maintenance of automatic dosing systems. As technology improves, these systems are becoming ever-more complicated and sophisticated.
Chemical dosing should be continuous, 24 hours a day. The automatic dosing system should be backed up by regular monitoring and verification.
Commercial swimming pools should have a system installed to automatically dose the disinfection, pH control and coagulation chemicals into the pool water circulation system. Manually dosing chemicals (sometimes referred to as ‘hand-dosing’) is a hazardous activity that can be easily avoided by the installation, use and maintenance of such systems. Automatic dosing systems also provide a much more reliable level of control over the swimming pool chlorine and pH levels.
A typical automatic dosing system will comprise of an electronic control unit connected to probes that are being supplied with a representative sample of pool water via a feed line taken from the circulation pipework. These probes are constantly monitoring the levels of free chlorine and pH, and are electrically interlocked to pumps that turn on or off according to the readings from the probes and the parameters that have been pre-programmed into the control unit.
In all disinfecting systems which incorporate automatic chemical dosing, the following precautions should be considered as appropriate:
- interlocking the dosing system electrically with the water circulating pumps, to prevent the continuation of dosing, should the pumps fail
- incorporating into the circulation system a fail-safe, flow measuring device capable of detecting a reduction or cessation of flow and interlocking this with the dosing pumps to prevent continuation of dosing
- switching off the power supply to the chemical pumps and/or the monitoring equipment when swimming pool water circulation is stopped
- siting the pool water circulation pumps below the level of the pool water, to minimise the risk of the pumps losing their prime
- siting the calcium/sodium hypochlorite and acid injection points as far apart as possible (preferably a minimum of 1 m); ideally the hypochlorite injection point should be located before the filter and the acid dosing point after the filter and heat exchanger (although, this is not possible if using UV or ozone disinfection systems)
- designing dosing lines so that they are protected from damage, and so that they cannot, inadvertently, be connected the wrong way round
- displaying notices warning of the risks of mixing calcium/sodium hypochlorite and acids, and the importance of maintaining pool water circulation during dosing
- ensuring that pressurised chemicals in the line are safely relieved before breaking the delivery line for maintenance work to be carried out
Operation, cleaning and calibration of the probes
Automatic monitoring systems vary in their levels of sophistication. Some are fairly simple: once the chemical pumps’ stroke rate and speed have been set, pumping is activated/deactivated according to the discrepancy between the programmed set-point and the readings obtained from the sample. At the other end of the spectrum, there are controllers that are programmed to predict its response to readings in different circumstances and adjust accordingly in order to prevent under or over dosing, i.e. they are ‘self-tuning’. They can also be connected to computer software to enable better communication in addition to being able to monitor and adjust from anywhere.
Systems also vary in their requirements for cleaning and/or calibration. Some systems will need neither as they are self-cleaning. Refer to the manufacturer’s instructions to find out what type of on-going maintenance your system requires. If it does need to be cleaned and/or calibrated by the user, this usually involves isolating the incoming feed, disconnecting the leads from the control unit to the probes, unscrewing the probes and either dipping them into, or wiping them with a probe cleaning solution.
If calibration is being done at the same time, this usually involves navigating into the appropriate menu screen on the control unit and following the step-by-step instructions. For disinfectant calibration, a sample of pool water is taken from the sample line (not the swimming pool itself) and a free chlorine test is carried out using a DPD 1 tablet in the usual way. The control unit reading is then overwritten with the reading obtained from the DPD 1 test using the on-screen menus.
For pH calibration, the probe is dipped into a solution with a known pH value (supplied by the manufacturers of the system) and then dipped into another solution with either a higher or lower pH value than the first solution. As with chlorine calibration, the operator is required to follow the manufacturer’s instructions specific to the type of equipment and follow the on-screen instructions carefully. It is recommended that pool plant operators request that the service engineers provide them with a tutorial on the automatic control unit during one of their service visits. It is worth remembering that most probes require replacement on an annual frequency.
Operators may need to change the set-point parameters occasionally. This may be because there has been a contamination issue that requires the disinfectant levels to be at the top of the recommended range for a period of time (as would be the case following a liquid faecal release into the swimming pool).
The in-line filter will get dirty, grimy and clogged up over time. This will then mean that the probes are no longer obtaining accurate chlorine and pH readings of the water. The filter should be isolated from the incoming feed and the assembly unscrewed so that the filter can be taken out and either cleaned or replaced with a fresh one (some in-line filters are only designed to be used once, then discarded). This task should be done as often as is necessary, but once per month is usually sufficient.
The entire dosing system should be inspected on a weekly frequency. The inspection should be recorded and any issues dealt with as a priority. Things to look for include:
- Build-up of residue around joints and injection points
- Split chemical feed lines (or areas where a split would likely occur, such as twists and kinks etc.)
- Build-up of sediment in chemical feed lines and chemical storage tanks
- Missing or damaged sheathing of chemical feed lines
It should be noted that the above list is not exhaustive and additional items may need to be added, according the nature and operation of the system.