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Asked by ruhel123 to Cathy, Jaz, Mark, Roma, Rory on 13 Jun 2014.
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Jaz Rabadia answered on 13 Jun 2014:
Great question Ruhel!
I’d like to know the answer to that – I think we should let Mark our water and sewerage expert answer that one 🙂
Jaz
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Mark Greaves answered on 15 Jun 2014:
Hi Ruhel,
That’s a very interesting question!
There are three things here, 1. how to treat sewage, 2. how to treat water for drinking and 3. the implication of treating sewage for drinking water.1. Sewage is treated mostly by physical and biological processes. The first stage is removing the unwanted bits with big screens, then the remaining sewage is settled to allow the brown stuff to be removed and processed. Following this is the clever bit, removing the harmful bugs. One process is called the Activated Sludge Process invented 100 years ago at Davyhulme WwTW in Manchester. Finally the sewage is settled again to get the last of the brown stuff out. There are more possible treatments but one that is appearing more and more is UV disinfection before water is discharged to the environment.
2. Drinking water is produced using physical and chemical processes. Raw water enters a treatment plant where we dose chemicals (coagulants) to help get all the bits out in large filters, usually made up of sand. Then we usually dose more chemicals, to adjust the pH for instance, before a second set of sand filters. This second stage helps removes substances such as manganese. The final part is more chemical addition, this time for disinfection. We tend use chlorine or sodium hypochlorite for this. Water is then sent into the network for use by the public.
3. I’ve been told that there is no reason why we couldn’t produce drinking water from the back end of a sewage works but, I think the public may consider that a bit dodgy. The reality is that treated sewage is discharge to rivers, lakes and seas. Our drinking water is taken from rivers, lakes and seas. Thanks to the dilution of the water courses it is less apparent that some sewage is recycled and used for drinking water.
Do you remember David Walliams swimming the River Thames? During that swim, due to heavy rain, around 500,000 cubic metres of sewage was discharged into the river. Thames Water can treat water from the River Thames at their desalination plant, google it for more information.
Sorry for the long reply, it was a good question. If you want to know more feel free to ask.
Mark
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