99.9%

[matthew]

The reduction of a bacteriological or viral load is often expressed as a log reduction e.g. 2 log (99% or 1 in 100 surviving), 3 log (99.9% or 1 in 1,000 surviving), or 4 Log (99.99%  or 1 in 10,000 surviving). Therefore this will based on the testing protocols and then converting it into units that the public can understand.
In my industry regulators in other countries will give each step in the treatment process a log credit for the reduction in the contamination. So the dose of UV light applied will be defined by the target species and the number of log reductions required.

That's useful. Thanks.
1 in 1,000 is still a hell of a lot of pathogens.

Especially given, for example, E. Coli has a doubling time of 20 minutes in favourable conditions so you could get a million-fold growth in 3 hours...

Hence why having started with a count in the raw water we aim for a log reduction in the filtration, a further log reduction in a second stage filtration, 3.5 log reduction in the UV disinfection and a further 4 log reduction in the chlorine contact tank. By the time you add that up the chances of there being a surviving pathogen to repopulate the water is the influent concentration / 10^9.5. Add to that daily sampling of the water and culturing it in the lab to confirm it is clean and I trust the water from my tap more than I trust the cleanliness of the glass I am filling. 

[Tim Hall]

The reduction of a bacteriological or viral load is often expressed as a log reduction e.g. 2 log (99% or 1 in 100 surviving), 3 log (99.9% or 1 in 1,000 surviving), or 4 Log (99.99%  or 1 in 10,000 surviving). Therefore this will based on the testing protocols and then converting it into units that the public can understand.
In my industry regulators in other countries will give each step in the treatment process a log credit for the reduction in the contamination. So the dose of UV light applied will be defined by the target species and the number of log reductions required.

That's useful. Thanks.
1 in 1,000 is still a hell of a lot of pathogens.

Especially given, for example, E. Coli has a doubling time of 20 minutes in favourable conditions so you could get a million-fold growth in 3 hours...

Hence why having started with a count in the raw water we aim for a log reduction in the filtration, a further log reduction in a second stage filtration, 3.5 log reduction in the UV disinfection and a further 4 log reduction in the chlorine contact tank. By the time you add that up the chances of there being a surviving pathogen to repopulate the water is the influent concentration / 10^9.5. Add to that daily sampling of the water and culturing it in the lab to confirm it is clean and I trust the water from my tap more than I trust the cleanliness of the glass I am filling. 

Admit it, you only do all that just to wind the homeopaths up.

[mrcharly-YHT]

The reduction of a bacteriological or viral load is often expressed as a log reduction e.g. 2 log (99% or 1 in 100 surviving), 3 log (99.9% or 1 in 1,000 surviving), or 4 Log (99.99%  or 1 in 10,000 surviving).

I didn’t understand your arithmetic until I realised it is because we went completely organic a couple of years ago and now only use natural logarithms.

The bad jokes thread is that way --->
Your coat is over there.

[ravenbait]

Hence why having started with a count in the raw water we aim for a log reduction in the filtration, a further log reduction in a second stage filtration, 3.5 log reduction in the UV disinfection and a further 4 log reduction in the chlorine contact tank. By the time you add that up the chances of there being a surviving pathogen to repopulate the water is the influent concentration / 10^9.5. Add to that daily sampling of the water and culturing it in the lab to confirm it is clean and I trust the water from my tap more than I trust the cleanliness of the glass I am filling. 

I take it you work in a water works? Are you using chloramine or chlorine, out of professional interest?

Sam

[Mr Larrington]

The reduction of a bacteriological or viral load is often expressed as a log reduction e.g. 2 log (99% or 1 in 100 surviving), 3 log (99.9% or 1 in 1,000 surviving), or 4 Log (99.99%  or 1 in 10,000 surviving). Therefore this will based on the testing protocols and then converting it into units that the public can understand.
In my industry regulators in other countries will give each step in the treatment process a log credit for the reduction in the contamination. So the dose of UV light applied will be defined by the target species and the number of log reductions required.

That's useful. Thanks.
1 in 1,000 is still a hell of a lot of pathogens.

Especially given, for example, E. Coli has a doubling time of 20 minutes in favourable conditions so you could get a million-fold growth in 3 hours...

Hence why having started with a count in the raw water we aim for a log reduction in the filtration, a further log reduction in a second stage filtration, 3.5 log reduction in the UV disinfection and a further 4 log reduction in the chlorine contact tank. By the time you add that up the chances of there being a surviving pathogen to repopulate the water is the influent concentration / 10^9.5. Add to that daily sampling of the water and culturing it in the lab to confirm it is clean and I trust the water from my tap more than I trust the cleanliness of the glass I am filling. 

Admit it, you only do all that just to wind the homeopaths up.

Teh Mr Hall made me dun a roffle

[matthew]

Hence why having started with a count in the raw water we aim for a log reduction in the filtration, a further log reduction in a second stage filtration, 3.5 log reduction in the UV disinfection and a further 4 log reduction in the chlorine contact tank. By the time you add that up the chances of there being a surviving pathogen to repopulate the water is the influent concentration / 10^9.5. Add to that daily sampling of the water and culturing it in the lab to confirm it is clean and I trust the water from my tap more than I trust the cleanliness of the glass I am filling. 

I take it you work in a water works? Are you using chloramine or chlorine, out of professional interest?

Sam

I am a process engineering consultant in WTW. My current client operates a free chlorine network. Using either Chlorine Gas or Sodium Hypochlorite depending on the site consumption and surroundings. The two forms (Chlorine and chloramine) should not be mixed and a company that operates both had better have good delineation of the supply zones for each.

[HectoJ]

From what I understand, it is a question of no product being able to kill 100% of all bacteria within a given period of time. So long as you keep the concentration low enough, the bacteria will not have enough of a hold to grow to a dangerous level within another amount of time.

[hellymedic]

The reduction of a bacteriological or viral load is often expressed as a log reduction e.g. 2 log (99% or 1 in 100 surviving), 3 log (99.9% or 1 in 1,000 surviving), or 4 Log (99.99%  or 1 in 10,000 surviving). Therefore this will based on the testing protocols and then converting it into units that the public can understand.
In my industry regulators in other countries will give each step in the treatment process a log credit for the reduction in the contamination. So the dose of UV light applied will be defined by the target species and the number of log reductions required.

That's useful. Thanks.
1 in 1,000 is still a hell of a lot of pathogens.

Especially given, for example, E. Coli has a doubling time of 20 minutes in favourable conditions so you could get a million-fold growth in 3 hours...

I'm not convinced.  Shouldn't that be 2^9 = 512-fold growth?

Whoops! Well spotted!