Introduction: DIY 12 Volt Chlorine Producing Unit for Water Purification

Today’s project is a device used to create a practically inexhaustible supply of the same disinfecting solution as the calcium hypochlorite makes. IF you want to go off grid, you have to have a supply of safe drinking water.  This instuctable shows how I built an inexpensive way of purifying water.

I have found that American missionaries that work in the third world are an excellent supply of information in what some call “appropriate technology” This is people centered, small scale, labor intensive, energy efficient, environmentally sound devices and processed. It’s a lot like “Macgyverisms” from my favorite 80’s TV show. He had advanced knowledge and primitive supplies and was able to cobble together 1900’s level tech with modern scientific principles.

One famous example of this is the CD3WD which is a collection of appropriate tech, Travis Hughley and barrelponics (which I AM going to build one day) and Safe Water International Ministries the developers of the CPU that is the focus of today’s article.

SWIM has developed the CPU to provide a chlorinating solution for water disinfection in third world countries. I would highly recommend you check out their website and consider donating to their mission as they are doing wonderful work.

If you want a CPU, but don’t have a DIY gene or interest in building one yourself, SWIM sells complete CPU tool box kit which includes an instruction card, a chlorine test kit, 2 mixing bottles, a salt measuring cup, and a couple of solution droppers. All you need to provide is 12 volts of electricity, salt, and water. A donation of $150 to their ministry would support this kit.

I wanted to turn this into a project so I gave a $50 donation and received the anode and cathode from them. They will email you the plans for free if you contact them on the website, and they have a technology link online with the instruction manual and a basic lesson plan for teaching this to others. I will warn you, the cathode and anodes are the main expense in the CPU, and you may have a hard time finding a supplier. One is a titanium mesh; the other is a mixed metal oxide (ruthenium).

However, I must tell you that their primary mission is to provide these units for missionary work in third world countries, so if demand causes a supply backlog, I would imagine they would fill that need first.

Basically what happens is when you bridge the electrodes with a salt water solution and apply an electrical charge to them you start a chemical reaction called electrolysis. In this particular process the water bubbles and produces a caustic chlorine solution roughly half as strong as laundry bleach. After the 9/11 attacks many municipal water treatment plants converted to this process so that they could remove their one ton chlorine gas tanks from their sites to mitigate their attractiveness as a terrorist target.

Per the SWIM for Him website the directions for use are:

  1. Mix salt & water solution ( approx. ¼ cup salt to 16 oz water)
  2. Connect wires to 12 volt source (negative, or black, to negative; positive, or red, to positive)
  3. Pour the saltwater solution through the CPU into another bottle (observe the bubbling process)
  4. Carefully repeat the pour-through process 5 times.
    1. This is different from a chlorate cell as that the water free flows through the unit which does not allow chlorates or perchlorates to form. This process operates at a different voltage and a much lower temperature.
  5. Unhook the battery wires.
  6. Rinse the CPU by pouring clean water through it.
  7. Add 10 drops of the solution for each gallon of drinking water.
  8. Wait one hour before drinking.

Additionally, if you are using solar power to charge your battery, you will want to recharge it for 3-5 hours after this process to ensure it is fully charged.

You will also need a test kit to ensure you use enough chlorine solution to properly sanitize your water supply.

This was a very simple project and I built the device in under an hour, I have to wait about 24 hours for the sealant to set, and then another 30 minutes in finishing touches. Please watch the video below to see all the steps.


Knuffelboom (author)2012-09-23

The chemistry behind this is wrong, this proces will not purify your water.

In reality you're not making chlorine as in the pure element, but HCl at the positive electrode and NaOH at the negative electrode. Those two compounds are in high concentrations indeed toxic to microorganisms. Buuuuuuuut as soon as the liquid reaches the electrodes again it makes the HCl and NaOH mix, by that neutralizing each other. Since HCl is a strong acid and NaOH a strong base, they will make H2O + NaCl again, which is exactly the same as the salt and water you started with.

Conclusion is that you're indeed electrolysing the ions in the water, but reversing the reaction again. At the end you're ending up with exactly the same mixture as you started with, I'm sorry.

You could overcome this problem by having the electrodes in seperated compartiments by placing a semipermeable membrane between the electrodes for example. One should note though, that high concentrations of HCl and NaOH have a very low and high pH respectivly, which could cause damage to your throat and digestive system. I highly recommend to use commercially bought products for important health issues like purification of drinking water.


I apologize for resurrecting an old thread.  However, the actual reaction here is as follows:

2Na++2Cl- + 2H20 + 2e- --> H2(g) + 2NaOH + Cl2(g)

HOWEVER, if the reaction products are not kept separate, the Cl2 will react with the NaOH to produce NaClO, sodium hypochlorite, AKA bleach.  If the chlorine and the NaOH are kept separate, and the H2 and Cl2 are allowed to recombine above the reaction chamber, then you can indeed produce HCl - as a gas.  This is called the Chloralkali process, and has been used for well over one hundred years.

The full reaction for bleach production is as follows:

2Na++2Cl- + 2H20 + 2e- --> H2(g) + 2NaClO + H2O

tngun (author)Knuffelboom2012-09-23

This has been in use for the past 10 years on several continents, purifying water for entire villages, it is the same tech, (but I will admit cruder) as chlorine generators in water treatment plants and pools.

I test the product with chlorine test strips and it is not neutralized as you say. Also you are only using drops per gallon just as you would bleach. This is established protocol, and has been used by backpackers and campers for decades. I would agree that it could cause damage if you drank it straight, but anyone who would is an idiot.

surpcrepair (author)2012-08-24

It says use salt and water.. can you use saltwater from the ocean for this rather then having to have salt and water..

In a survival situation i dont see massive amounts of salt being that easy to get but in some areas salt water would be very easy to get..

tngun (author)surpcrepair2012-08-24

You can

PS118 (author)2012-08-03

I'm a little confused about the chemistry here.
Where does the sodium go?

(Oh yeah. I gave you 5 stars, BTW!)

JACTChemist (author)PS1182012-08-09

I believe this method is called "Electrolysis". In reality he is not synthesizing Chlorine as Cl2, instead he is making a chlorinated compound called "hydrochloric acid". The chemistry behind it is as followed:
H2O + NaCl > H2O + [Na+] + [Cl-] > 12V > HCl + NaOH

Another method is by adding several drops of Bleach or Clorox to the gallon of water that you wish to decontaminate. Or you could boil the water for several minutes and use the battery and a UV Bulb to do a UV Water Purifier.


e1ioan (author)2012-08-02

How many times can the CPU be used before the electrodes go bad?

tngun (author)e1ioan2012-08-02

I have had mine a since right before the Alabama tornadoes last year, have used it several time in practice and haven't noticed anything. I did email the designer and this is what he said:

"We've been using some CPUs almost 10 years and made hundreds of gallons of chlorine with them and not had any problems with electrode corrosion. In some water conditions the electrodes might develop a film or layer of build up on them, but that can be easily cleaned off by a soak and rinse of vinegar or Coca-Cola. We recommend doing that every couple of months of use. Hope this answers your questions. "

About This Instructable




Bio: David is a professional firearm instructor and Emergency manager, his website is devoted to teaching individuals how to be better prepared for life and life ... More »
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