Recycle Desiccant Dehumidifiers (Thirsty Hippos)




Introduction: Recycle Desiccant Dehumidifiers (Thirsty Hippos)

This is project for people living in tropical climates, who use 'thirsty hippo style' dehumidifiers.
Under hot, humid conditions, stored away stuff (clothes, shoes, musical instruments,..) can get moldy after a while and/or develop that moldy smell.

For bigger rooms, running the air con or electric dehumidifier works wonders, but this does not work really well for stuff stored away in enclosed spaces like cupboards or storage cabinets. This is especially problematic if you are traveling and no one is home to run the air con. The best solution to keep the air in small enclosed spaces dry tends to be using those desiccant based dehumidifiers. Here in Singapore, people refer to them as "thirsty hippos" although a range of cheaper brands are available as well.
These dehumidifiers all contain calcium chloride salt (CaCl2). This salt is extremely hygroscopic: it will suck the moist out of the air and dissolve in the process. When all the solid salts have dissolved, the humidifier won't absorb any more moist and should be disposed of.

The used dehumidifiers all end up in the domestic waste. Here, domestic waste is incinerated, but since CaCl2 won't burn, it will end up being dumped with the ashes.

This is a waste, as the chemicals don't get consumed in the process; all they do is absorbing water. It is actually really easy to recycle the CaCl2: all it takes to restore it to its solid state is simply boiling off the absorbed water again. This can be done by boiling the solution on a simple domestic cook top.

I will demonstrate how to do this, and how to construct a safe new container to put the calcium salt into to use as a humidifier.

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Step 1: Safety!

Before you consider trying this, consider the following. If you are not comfortable with any of below warnings, please don't attempt this.

Calcium Chloride is mildly hazardous. Although it is used as a food additive, it also acts as an irritant on the skin because of its aggressive water absorbing properties. You also don't want to get it flying into your eyes or ingest it...

Always wear safety gloves and safety goggles.
Keep your working area well ventilated.
Store your chemicals so that they are safe and out of children's reach.

Boiling of the solution will involve heating up the substance to temperatures exceeding those of a typical deep fryer: dangerous!

Some desiccant dehumidifier brands, like "kiwi fresh" contain a fragrance.  Try to avoid heating up those and stick to those brands that contain the pure, odourless stuff. 

I will not take ANY responsibility for accidents and/or injuries that occur while trying or as a result of this instructable!

Find out more about calcium chloride here:

Step 2: Liquids to Solids.

At this point you should have collected some used dehumidifiers. This is what mine look like, I've already cut the membrane off the top.

I use a ceramic hot pot to boil the liquid in.
They are highly available here, cheap, they resist the heat well and the salt does not corrode them.
(They do have one backdraw: the salt leaks into them, but more on that later.)

Cut open the top of a couple of dehumidifiers and pour the liquid into the hotpot. Don't overfill the pot. About half full should be fine. If it is your first batch, consider trying with a smaller quantity first, to see how it goes.

One other nice thing about the hotpot is that it comes with a nice ceramic lid. The lid usually has a hole in the top to let vapour escape. Additionally to this, I used 2 bits of skewer stick and a bit of metal wire to create spacing between the pot and the lid. Looks goofy but does the trick.  It allows steam to escape, with the lid still catching all salts spats trying to escape. I added a wire handle too, to easily lift the lid, as it gets really hot.

If you try to boil this stuff without lid, your cook top will get covered in spats of the salt. While it shouldnt damage the cooker, it's a pain to clean up: even a tiny amount left on the surface will keep the surface feeling moist. You have to wipe it multiple times with a wet cloth to get rid of all of it. Rinse the cloth well after every wipe.

With the lid in place, turn on the cooker and get the liquid to boil. "Medium" setting should be sufficient. I find that it roughly takes me about 30 minutes for the water to evaporate.

After half the liquid has evaporated, a crusty salty layer should form on the surface. Just keep the solution boiling.
The crusty layer will get thicker and thicker and the sound of the boiling liquid will change to sound more 'crispy' or 'crackly'. Keep an eye on it.
Gradually the liquid will turn into 'slush'. When this happens you need to be fast. Turn the fire lower and start stirring and break up the thick mixture by stabbing and 'cutting' it with a spoon. Use a second spoon to remove stuff that sticks to the first spoon.

If you don’t break it up at this point, it will form a solid hard layer stuck to the container, and the only way to get it off would be by dissolving it in water again. It feels similar to how caramel can get really hard and stuck to the pan if it cools down in place.

Keep it moving until it forms separate chunks. As water continues to evaporate, the chunks will solidify and dry. At this point there is no more risk they will stick together and you can stop stirring. Turn off the cooker completely and let it cool to room temperature.
When approaching room temperature, the salt will start to absorb moist, so you might want to put them into a temporary airtight container, or get on with making the dehumidifier.

If you used the ceramic pot, you might need to store that in an airtight container too.
Take a look at the picture of the hot pot while on the fire. The white stuff on the bottom and sides are cacl2 slowly leaking through the ceramic.
It turns out the cacl2 slowly leaks into and through the ceramic material. This is ok to boil of the water, it only leaks really slowly. But when it is exposed to humid air again later, the salt inside the ceramic will absorb the moist, expand and the pot will crack.
I discovered this when I tried to use the hot pot itself as dehumidifier container. I did put it inside a bigger, plastic container.
Very happy I did that: the pot broke after about a week: a big crack formed, starting in the middle running all the way up.
Now I store the hotpot after use inside an airtight container and only take it out right before usage.

Let's build the dehumidifier.

Step 3: Build the Dehumidifier Case

The off-the-shelve dehumidifiers are really safe: they are covered with a pretty strong membrane that lets vapour in but won’t let the liquid out. It's ok to tip over the container or drop it from moderate height.
I'm not sure what the membrane is made off. I would be great to use the same material too. (About 1-2 months ago, I sealed a newly recycled dehumidifier inside a clear plastic vegetable bag (HDPE?), for later use. Yesterday I opened the bag, and to my surprise the dehumidifier was completely dissolved. That could be a clue... :)
Anyway, once the membrane has been cut open the original package is not really suitable anymore for safe use.
Let's make a safe new dehumidifier.

I'm using a clear plastic pot with a screw top and a handle. They are used to package breakfast oats, and by now, I have loads of these. Try to find something similar.

If you have access to a 3D printer:
Cut open the top as shown in the picture. Don't damage the handles hinges.
Print the mesh screen for the top and the inner container for the salt chunks on your 3D printer of choice. I used openscad to design them. The design is fully parameterized and can be adjusted to fit any size container.
Instructions for printing and source files can be found on the thingiverse (

If you don't have access to a 3D printer:
Drill holes in the top of the container. Don't damage the handles hinges.
For the inner reservoir, you are on your own. I guess it is a little trickier without a 3D printer, but shouldn’t be a problem.
Something like cut up empty soda bottles could do the trick nicely.
(Actually, you can also decide to skip the inner reservoir altogether. It will work as well with the solid salt submerged after a while, be it not as good.)

Put the reservoir inside you container and spoon in the CaCl2. Don't overfill it: make sure the liquid won't overflow as water gets absorbed.
Screw on the top, presto.
I like to hang the humidifier, so it can't tip over. If you hang it, secure it properly: it will get heavier as water gets absorbed..


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4 Discussions


6 months ago

Did this in a glass casserole in the microwave and it worked great. Remember to stir every minute or so!


4 years ago on Step 3

in relation to the material that decomposed quickly...

PLA or polylactic acid, is biodegradable, 3d printable, and will decompose in under a month in a heat compostable bin.

so maybe it was PLA.

but otherwise this is a great idea, have you tried pyrex instead of ceramic?


6 years ago on Introduction

I use this sort of humidifier in my gun cases. As you said they just go to the dump. I'm going to give this a try using a solar oven to evaporate the water. Living in the desert there is plenty of sunshine. Already have solar water heaters an solar panels, might just as well use it this way.


6 years ago on Introduction

Instead of heating, is it easier to microwave the calcium chloride solution?