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Focus On... Seawater Chlorination
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Dear <<First Name>>
The chemistry of the chlorination of seawater is more complex than many people realise and although the measurement of chlorine residuals is possible in seawater (and therefore automatic control of chlorine dosing), better results will be obtained if this is fully understood. Did you know that...
...when you dose chlorine into seawater it is bromine that does the disinfection?
...DPD 1 measures free chlorine or total bromine and not free bromine?
Is it Chlorine or is it Bromine?
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Seawater contains about 70ppm dissolved bromides, most of which are sodium bromide. When you put chlorine in water it displaces (because it is more reactive) the bromine from the bromide and becomes a chloride. So for up to about 70ppm of total chlorine dosed what you actually have in the water is free bromine and combined bromine (NOT free and combined chlorine) so it is the total bromine that actually does the disinfection1.
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So why does everyone call it chlorination when technically it is bromination? Mainly because most people don't know this interesting bit of chemistry. So what? Normally it makes no difference at all as bromine is an effective disinfectant, however there can be a lot of confusion when it comes to monitoring residuals and controlling dosing. Choosing the correct sensor to control the dosing is crucial as is choosing the correct DPD test.
Pi offer a specialist range of seawater chlorination controllers, but to choose the right controller we need to understand the chemistry going on. A technical note on the same subject is available here.
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Free Chlorine and Total Bromine
Due to the confusion on what is being measured, it is easy for an engineer to specify the wrong equipment and calibrate it incorrectly. For example, it is common for a free chlorine sensor to be specified for seawater chlorination control. Most electrochemical free chlorine sensors will react to free bromine (not all so be careful!) but this isn't necessarily what you need for bromination control.
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Most authors agree that whilst the disinfection capability between free chlorine and combined chlorine differs, when it comes to free bromine and combined bromine, both forms of the chemical are equally good at disinfection so a better measurement would be total bromine, which requires a total bromine sensor.
DPD and Seawater Chlorination
To add to this already confusing environment we need to look at calibrating online sensors or using handheld photometers to track the residual. DPD is used extensively to measure chlorine residuals and it also reacts to bromine so can be used for both, however, DPD 1 measures FREE chlorine or TOTAL bromine. The situation can therefore arise where you have an online instrument such as a CRONOS® or CRIUS® specified as free chlorine, actually measuring free bromine but calibrated as a total bromine (against DPD 1)! Typically the best results are obtained by specifying a total bromine (total chlorine) sensor and calibrating it using DPD 1. That, however, isn't the end of the story!
When specifying an analyser it is crucial that we suppliers know that it is for use with seawater because the physical and chemical make-up of seawater is very different to potable or process water and this can affect what we would supply to our customers.
The effect of Salinity on Membrane Sensors
It is crucial for us to know if you are going to use a Pi sensor in seawater so we can provide you with a saltier electrolyte. Osmosis means that water moves from a low solute concentration to a higher solute concentration across a semi-permeable membrane. The electrolyte in our sensors is saltier than potable or process water so osmosis forces water into the end of the sensor, which the sensor is designed to cope with, however, with seawater the process is reversed and the water in the electrolyte can be forced out of the sensor into the sample. To solve the problem we supply electrolyte especially designed for seawater, with a higher salinity.
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Estuarine Waters
Many seawater chlorination applications are estuarine in nature (partly seawater and partly fresh water) and it is the degree of dilution which determines which sensor and which electrolyte you should use.
Seawater has approximately 70ppm bromides and so up to 70ppm chlorine the replacement will be 100%. If the seawater is 50% fresh water then up to 35ppm chlorine will give 100% displacement.
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For example, if we looked at a 2ppm residual then the water could be only 3% seawater and 97% fresh water and you would still be measuring bromine, so a total bromine sensor calibrated with DPD 1 would be appropriate. For any water that is contaminated with seawater, the seawater electrolyte is likely to be the most appropriate.
The Solution!
If all of this is too much to take in and remember, then don't worry! Just remember to talk to Pi for any online chlorination application and we will do the rest... guaranteed! Please don't hesitate to give us a call.
References
1 White's Handbook of Chlorination and Alternative Disinfectants, 5th Edition, Wiley - page 874, pages 122-129.
Focus On
'Focus On' are a series of emails providing technical information regarding instrumentation, process measurement in potable, waste, process and pool waters. If you want to, you can unsubscribe.
Previous Focus Ons are available below:
Other technical notes are available from Pi and can be downloaded from here. For more information on Pi's products, visit our website.
Best Regards,
Amy Ryan
Marketing
Process Instruments
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