I am at dilemma not to use too many chlorine or similar. I treat 1 larger bottle, hydration bladder with NaDCC or sodium hypochlorite. I drink half water from it. Water is disinfected and chemically treated. If I add new water(from some stream) do I need to add another NaDCC pill to disinfect it or is in old water enough NaDCC left so it will clean newly added water without adding new? I am not sure if NaDCC, chlorine stays in water for longer periods or reacts and there is not enough left. I usually use very good water, with not cows, pastures, human cabins up. If it could be bad I would add another tablet or drop. I had some bad experiences with water that should be good, so I use precautionary measures, but I don't want to over do it.

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    Do you believe in homeopathy? How many dilutions are reasonable? Clearly putting one tablet into your water bottle once won't work very well.
    – Jon Custer
    Aug 1, 2023 at 16:19
  • @JonCuster Used better wording than me in my answer. "Dilution" is the word. So, Alex, Each dose of chemical, whatever its form, works for a certain amount of water to be treated. Therefore the total quantity of water has to be calculated. You also need to add the required amount of time, EACH time. Example: If you treated the water and the manufacturer said "let sit for 30 minutes", you'll have to wait an additional 30 minutes the moment you had any water. That's why water filters like Sawyer, Katadyn, etc. are much better IF you don't have to deal with viruses.
    – Diablo
    Aug 28, 2023 at 16:45

2 Answers 2


Generally no, it doesn't.

Chemical disinfection requires a certain concentration of the active chemical, and by diluting it you've got a weaker solution. This is like using a tablet for more water than it says - and I would assume you're using the correct dose to start with. Some have quite a narrow range.

Specifically NaDCC 3.5mg tablets are used in low-risk areas, 8.5mg for certain hazards. That's the dose for 1 litre. So on that basis it might seem like you could start strong and dilute. But you couldn't.

Disinfectants that bubble or otherwise give off gas will have finished doing so before you top up, which is even worse as there's nothing left. Any gas left dissolved in your water will get diluted, and probably be too weak. NaDCC generates chlorine, and one manufacturer says the chlorine dissipates in 30 minutes - after that there's nothing left to clean the new water.

Chemical treatments also take time to work. Some are quicker than others, but even if you add more, you'll have to wait. Or if you've initially made it up strong enough to handle the dilution, you still shouldn't be drinking it immediately, and have to go without.

If I was relying on finding and treating water, I'd carry 2 bottles.


This answer is wrong and might be dangerous!

The answer is YES

Why is this? Very simple: Why take a chance with your health? Now let me give some more facts...

I treat 1 larger bottle, hydration bladder...>

OK, but what is "large"? What is Hydration bladder" to you? there are many different sizes out there and what you need to take into consideration is that any chemical will be effective up to a certain quantity of water. Therefore, you need to make sure you stay within those limits.

Also, you need to give the chemical the proper time to do its job. And you need to add even more time the cloudier and colder the water is. Still, some people are getting sick. Why? Simply because they forgot to disinfect the mouth (inside and outside) of the water bottle, as well as the inside sides of the cap! That's why you need to add the pills (chemical) and avoid screwing the lid tight before shaking the bottle to make sure the chemical covers everything. Then you tighten the lid and wait.

SAWYER & KATADYN You could avoid all this by using a Sawyer Squeeze or Katadyn BeFree filter. Both use hollow fibers and no need to wait any amount of time. The Sawyer will last way over 100,000 gallons and the Katadyn way less. Another key difference between both is that you can screw the Sawyer directly to a bottle or add it in line with any bladder-type container. You can't do any of that with the Katadyn. The Katadyn is also using its own fragile pouch and many people are complaining about punctures. And the Katadyn is useless without its own bag...

OTHER PRODUCTS There are many other products that also use hollow fibers. One of the best known is the "LifeStraw" however, no serious hiker (or outdoors enthusiast) is using it. The joke is "How do you know someone is using a life straw?.. Answer: His t-shirt or jacket is damp and full of mud" LOL

WARNING A filter will not remove any viruses. So depending on where you're going, you may still need to use chemicals or a SteriPen.

ECO-FRIENDLY The environmental cost of producing those chemicals is very high. First, everything related to their production, then they either come in a plastic/papper wrapping or glass bottle. A water filter is way more eco-friendly.

BOTTOM LINE Yes, you should treat the water again depending on how much is the TOTAL water quantity, and why take a chance anyway? A good filter is better, faster, more eco-friendly, and costs way less in the end.

  • 1
    The big Yes at the start of your answer does not make sense. It is in conflict with the general advice in your answer and particularly the bottom line.
    – Willeke
    Aug 27, 2023 at 7:34
  • @willeke You make no sense and I'm reading to challenge your outdoor knowledge anytime in front of an expert pannel. You;re getting close to harassment now..
    – Diablo
    Aug 28, 2023 at 16:40
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    You start with saying yes you can add new (not clean) water to cleaned water, to follow with all reasons not to add that water. That is not outdoors knowledge, that is language understanding.
    – Willeke
    Aug 28, 2023 at 20:04
  • 2
    This answer is incorrect, and dangerously so.
    – Rory Alsop
    Aug 28, 2023 at 21:08
  • Even if the answer wasn't dangerously wrong, it doesn't actually answer the question. Quote from the Q: "I treat 1 larger bottle, hydration bladder with NaDCC or sodium hypochlorite". You say yes, implying that you're addressing that means of cleaning water, but then hint briefly no, and talk about alternatives (which may be valid options, but aren't the subject of the question). I don't often downvote, but not answering the question and providing dangerous advice are two reasons that I do.
    – Chris H
    Aug 31, 2023 at 15:33

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