# How does residual chlorine in tap water work?

As I understand it, residual chlorine levels in tap water are below 1mg/l in the UK. Most chlorine based cleaning products suggest at least 1% solution to be effective (quite a long way off 0.5mg/l by my maths).

Given that chlorine has a very slight disadvantage to health (albeit a tiny one), if 0.5mg/l is not enough to kill harmful bacteria, why is it there at all? If it is enough to kill bacteria, how does it do it when cleaning products require what seems to be several thousand times greater concentrations to do the same job?

Actually, I think that to qualify this statment, you should use about a 3.33% solution of household (5%) bleach for disinfecting. So that's actually, 0.15% sodium hypochlorite solution.

Also, time and pH are factors. A 0.15% bleach solution will kill many bacteria within 5 minutes. But after a few days, a more dilute solution will also have killed many bacteria.

Also, given that household bleach has a pH of around 11, a 1% solution will have a pH of about 9. The optimal pH is about 6, where all of the -OCl is converted to HOCl which is roughly 80x more effective as a germicide.

So, I realize that 0.5mg/L is still about 3000 times less than 1.5g/L (= 0.15%), and that's the real question. The real answer is that the place where the bleach is added (in a municipal supply) is at the filter, where bacteria collect. Once the old filter has been bleached the fresh water supply is allowed to resume which flushes the bleach along (at tolerable levels). Also, they often use other oxidizing agent which are synergistic with HOCl, so that they can reduce their cost and improve the function.

• Great, thanks. Just a couple of things 1. Are you saying that residual chlorine is really just a by-product of the main cleaning process and it's not deliberately put in at all? 2. Is the time/dilution effect roughly equivalent. i.e would tap water have the same disinfection effect as a bleach solution after 3000x5=15,000 minutes (10-11 days)? Aug 12, 2016 at 6:49
• @Isaacson it's not so simple. With antibiotics there's a threshold or minimum inhibitory concentraton. But there are many different kinds of bacteria, each with their own MIC. But at the filter, they are not using 0.15% bleach. They can use much less and wait a few hours. I don't suppose that 0.5mg/L Would be germicidal, but that's really just an average. sometimes it's higher or lower. I avoid using tap water for growing bacteria, because it does make a little difference though. Aug 12, 2016 at 12:17
• I've marked the answer because I think it's the best I'm going to get, but still a little confused. You say "I don't suppose 0.5mg/l would be germicidal", this pretty much still leaves open the question of what's it there for then? If it does not kill germs, why don't the water companies just remove all the chlorine before piping the water out, is it that the "little difference" you mention is enough to justify it from a public health perspective? Aug 13, 2016 at 7:07
• Well, bleach isn't that bad for you. It's kind of like oxidized salt. Compared with the amount of salt we need and actually ingest, there's no way it is making any difference, health-wise. Every thing can kill you, if you have too much... too much water and you destroy your kidneys or simply drown. Too much air, too much light, too much heat, too much anything will kill. But we need all these things (in the right amount). Chlorine is an oxidizer. It breaks down into common salt. Aug 13, 2016 at 22:27
• @Isaacson it's at 0.5mg/L after the water has resumed. In the filter area I'm sure it's probably way higher, like around 0.001% bleach which reacts with germs and turns into salt. They will use just enough to kill the germs probably within 24 hours. Then they resume water flow through the filter and the residual chlorine gets diluted. Now I'm not very familiar with municipal supplies. They are all built and operated a little differently. I'm just saying this is one probable scenario. Aug 13, 2016 at 23:01