COVID-19 has brought with it some discussion of chlorine disinfection practices for drinking water, swimming pools and surfaces. So…I’m going to take the opportunity for a slightly academic twitter thread! 1/20 
You probably know that chlorine, in various forms, inactivates (or “kills”) many types of viruses and bacteria. That’s the main reason why we add it to drinking water and swimming pools, and use it to disinfect surfaces in homes, public places and hospitals. 2/20
Chlorine is a “disinfectant”. Disinfectants don’t inactivate microorganisms instantaneously, but generally require some period of time to achieve a specific number of inactivations. These inactivation kinetics were studied by British microbiologist Harriette Chick. 3/20
From Harriette Chick’s work, we have “Chick’s law”, telling us that the rate of microbial inactivation is proportional to the number of organisms remaining at any time, so the rate gradually decreases as more microorganisms are inactivated. 4/20
It follows from Chick’s law that inactivation is an exponential process, not a linear process. This is one reason why we tend to talk about “log-reductions” of microorganisms. 1 log-reduction is 90% inactivation, 2 log-reductions is 99%, 3 log-reductions is 99.9%, etc. 5/20
When a female scientist makes a brilliant contribution, it is common for a male to come along, add a few tweaks and add his name to the findings. Thus H.E. Watson built upon Chick’s findings (with her data and her permission) to produce the “Chick-Watson model”. 6/20
The Chick-Watson model gives rise to the Chick-Watson law, which boils down to the idea that a specified number of log-reductions (eg, 99.9% inactivation) can be achieved by maintaining a constant product (CT) of disinfectant concentration (C) and exposure time (T). 7/20
By comparing the CT required for any disinfectant (HOCl, OCl- and NH2Cl shown here), we can compare the effectiveness of the disinfectants for a particular microorganism (E. coli shown here). 8/20
Thus water treatment engineers refer to tables like this one to assess what combination of disinfectant concentration and exposure time they will need to achieve to ensure a specified degree of inactivation for a specific microorganism by a specific disinfectant. 9/20
Temperature is also important. As temperatures increase, generally lower CTs are required. You can see here that pH is also important, we’ll get to that on the next slide. 10/20
When we dissolve chlorine gas (Cl2) in water, it produces hypochlorous acid (HOCl).If we increase the pH above 6, the HOCl begins to convert to hypochlorite ion (ClO-). Look back at tweet 8/20 and you can see that HOCl is a much more powerful disinfectant than ClO-. 11/20
We call this HOCl/ClO- couple “free chlorine” and to make sure there’s plenty of the strong disinfectant (HOCl), drinking water and swimming pool disinfection is usually done at pH 7.5. At higher pH you lose HOCl and at lower pH, the water becomes corrosive. 12/20
Hypochlorous acid (HOCl) is very reactive and can’t generally be stored. So it is either made fresh from chlorine gas (Cl2) or by dosing sodium hypochlorite (NaOCl) which is kept stable by maintaining it at high pH. 13/20
When you buy sodium hypochlorite disinfectant at the supermarket, you’ll notice it also contains a high concentration of sodium hydroxide. The label shown here says 10 g/L. That’s about 0.25 M, equivalent to pH 13. Never acidify it since you can produce toxic chlorine gas! 14/20
Another option sometimes used for swimming pools is calcium hypochlorite, which can be attractive because it is available as a dry powder (or soluble ‘tablets’), rather than a highly toxic and corrosive gas or a liquid solution. 15/20
There are many other disinfectants that we could (and some we do) use to disinfect water. The Australian Drinking Water Guidelines list the properties of an ideal disinfectant. Nothing meets all of these properties perfectly, but chlorine ticks many of these boxes. 16/20
But the question on everyone’s lips is: Does it disinfect the virus (SARS-Cov-2) responsible for COVID-19? The answer (based on studies with closely related viruses) is “yes” and commercial sodium hypochlorite (NaOCl) solutions are ideal for disinfecting surfaces. 17/20
This understanding that a 0.1% (that is, 1 g/L) sodium hypochlorite solution, with a contact time of 1 minute is an effective disinfectant for coronaviruses is still current. This review was published in 2020 and draws conclusions specifically about SARS-CoV-2. 18/20
Now, remember the Chick Watson Law (tweet No. 7). This commercial formulation contains 52.5 g/L NaOCl. So I can confidently dilute it 1 in 50 (20 mL in 1 litre) if I can ensure an exposure time of 1 minute. Or for 6 seconds exposure time, use a 1 in 5 dilution. 19/20
If you got this far and you still don’t like chlorine-based disinfectants, you can also use 60-70% alcohol (ethanol or isopropanol) solutions for surface disinfections. But that’s another story… 20/20
