People have asked me about risks of transmission of COVID-19 via water. I’ve been reading up and this morning, I watched a US-based webcast giving a research update on this topic. Here are a few key points that I can share. 1/20 
First, I must point out that I am not a virologist, or even a microbiologist. My field is water quality engineering with a focus on chemical contaminants. So I’m describing only what I’ve learnt from the expertise of others (some shown here). 2/20
Coronavirus Infectious Disease 2019 (COVID-19) is caused by infection with the virus known as SARS-CoV-2. There’s still much we don’t know about SARS-CoV-2, but as the name suggests, it’s a very close relative of SARS-CoV, responsible for the 2002/03 SARS outbreak. 3/20
Understanding the close relationship between SARS-CoV and SARS-CoV-2 is helpful since we now know a lot about SARS-CoV and can make some reasonable extrapolative assumptions about some characteristics and behaviour of SARS-CoV-2 in water and the environment. 4/20
SARS-CoV and SARS-CoV-2 are examples of “enveloped viruses”. Unlike, non-enveloped viruses, their genetic material is packaged inside a lipid “envelope”. The integrity of this enveloped structure is believed to be essential for the virus to maintain infectious viability. 5/20
You may have read the excellent thread by @PalliThordarson, explaining how soap disrupts this lipid envelope and thus inactivates the virus. If you haven’t, you really should. 6/20
Enveloped viruses can survive for a while in water, but they’re much less persistent than the non-enveloped viruses known for causing waterborne diseases (eg. hepatovirus A, rotavirus, norovirus, adenovirus, coxsackievirus and poliovirus). 7/20
SARS-CoV can survive intact for weeks in cold (4 deg C) sterile water. But it is notably less stable in warmer natural waters such as lake or river water. Survival in sewage is expected to be even shorter –perhaps a couple of days. After all, sewage contains plenty of soap! 8/20
Ideally, sewage will be treated at a sewage treatment plant and that will lead to inactivation through a range of processes including biological predation and exposure to sunlight. If available, disinfection with chlorine or UV radiation are expected to be very effective. 9/20
We don’t yet know the precise doses of chlorine required to inactivate SARS-CoV-2, but enveloped viruses tend to be among the most susceptible microorganisms, -much more than non-enveloped viruses and even more than common bacteria such as E. coli. 10/20
So unless people are exposed to relatively fresh and very poorly treated sewage, risks from SARS-CoV-2 exposure via sewage are likely to be low. And, those would be circumstances likely to lead to greater risks from other waterborne viruses (and bacteria). 11/20
If raw drinking water sources, such as rivers, did become contaminated with SARS-CoV-2, towns and cities that have well-managed drinking water supplies would still be safe. Treatment plants that use filtration and disinfection should effectively inactivate the virus. 12/20
The use of chlorine to disinfect drinking water is well-established. The chlorine dose required to achieve a specified degree of virus inactivation (eg. 99%) is conventionally described by the product of the disinfectant concentration (C) and exposure time (T). 13/20
Viruses that are relatively resistant to chlorine require higher CT than viruses that are less resistant. To achieve 99% inactivation of more resistant viruses a drinking water treatment plant might maintain chlorine concentration ≥ 0.5 mg/L for at least 30 min at pH < 8. 14/20
So by targeting effective removal of resistant viruses, such as norovirus or rotavirus, a CT will be established that will be even more effective for more susceptible viruses, as SARS-CoV-2 is expected to be. 15/20
I haven’t seen any disinfection CT studies for SARS-CoV-2 yet. But in time, we should expect to have CT values for a range of common water disinfectants including free chlorine and chlorine dioxide. Let me know if you come across a good study. 16/20
Another concern I’ve read, is that a COVID-19 outbreak might be so severe that there’ll be shortages of people to operate our water treatment plants. That might be a good argument for slowing down the pandemic, to ensure it doesn’t hit all plant operators simultaneously. 17/20
But many modern drinking water treatment plants can be operated with relatively few staff available, and in some cases, can be largely operated remotely, -via logging in through a computer from any location. That may prove to be helpful during a pandemic outbreak. 18/20
So my conclusions and recommendations: Person-to-person transmissions, and transmission via some contaminated surfaces, are the main exposure risks for COVID-19. Waterborne exposure is possible, but unlikely to become a significant route. 19/20
If you live in a town or city with reliable drinking water, it won’t help to switch to bottled water. Keep an eye on the official public health advisories and keep washing your hands with warm soapy water. 20/20
