I’ve been reading that with a humidifier you have to watch for growth of bacteria and algae in the tank and add additives to stop the growth. But how exactly do bacteria and algae grow in a water tank with no food? Algae and bacteria need organic material to grow.
Some bacteria use photosynthesis, but they still need various minerals from the environment.
So why is bacterial and algae growth in a humidifier an issue?
Your tap water has lots of minerals. And some small amount of “food” from your pipes.
It’s one of several reasons they recommend using distilled water in humidifiers. Bacteria won’t grow in distilled water.
Algae can’t grow in the tank if it’s not exposed to light. Slime and fungus can grow in the tank, and bacteria can feed on the slime or the fungus, or team up and form a joint venture.
Some algae can live in the dark, but even if you are feeding sugar-water to your humidifier, it’s unlikely. In the wide world out there, there’s probably an algae that teams up with a slime or fungus, but normally if you have algae in a dark tank, it’s because it died there.
It’s amazing how algae can find nutrients, even where you wouldn’t expect it to.
We had a closed water cooling system for a laser that was filled with distilled water. it was exposed to light, because the laser beam actually went through the cooling water, but the water system was sealed.
It still developed algae, no matter how careful they were with filling and sealing it.
Humidifiers still suck in air from the room, which contains dust, a lot of which is human skin, also dirt, pollen, and other particulates. So even if you start with distilled water and a sanitized humidifier, you’re going to get some sort of microbial growth eventually.
At work I get to evaluate procedures used to measure how impurities in water scatter light. This scattering is called turbidity. It is useful to have very low turbidity standards to evaluate instruments. A good low turbidity standard is pure water. (I am convinced that there is no such thing as absolutely pure water anywhere on this planet.) My low turbidity standards prepared from my best filtered, deionized water and stored in pristine, sealed, acid-washed vials stored in the dark start to deteriorate after just a few weeks and the measured turbidity begins to rise. I suppose some of this deterioration may be due to the precipitation of carbonate salts from trace impurities in my source water but I doubt it. I suspect it is mostly because of the growth of some kind of living organisms. This is supported by the fact that the deterioration is suppressed, but not eliminated, by keeping the vials in the dark. The increased turbidity is not visible by just looking at them but it is measurable. My question has always been, “Just what the hell are these little bastards eating?” Even if it were algae, wouldn’t it need some nutrients in addition to light energy to grow? Also, I am sure there is plenty of dissolved oxygen and carbon dioxide in my supposedly pure water.
Have you considered keeping them under strong UV light?
It’s easier to just make new ones. Though this does sound like an interesting experiment. Management can’t tell if I am doing something useful or just playing around.
Dooooooo ittttt @Ynnad c’mon, for science!
I hike a lot in the Sierra Nevada, an area where the water quality is studied quite extensively. The purest water is found not in babbling brooks, but in the top few inches of calm exposed high altitude lakes, where the water is subject to constant sterilizing UV bombardment.
This gets to the answer best I think. There is food in the water; it’s just that like the bacteria themselves, it’s microscopic (or at least very small). Having a fan draw air in from all over the house would be one of the best ways to pick up dust and organic debris to contaminate (enrich) the water with.
The same general process happens with aquariums. You can start them with essentially sterile water/gravel/plastic plants, and within a few weeks you’ll feel a slimy bacterial layer coating everything. The ingredients and source bacteria are floating around everywhere.
In addition to the other things mentioned, the humidifier filter (which is less of a filter and more a wicking membrane to increase the surface area of wet stuff and get more water into the air) is likely made of something that some kind of microorganism can eat.
Is that really true? I know that there are bacteria that can metabolize almost anything to some degree, but where there is a compelling reason to design it inhibit bacterial growth, is it impossible to find a filter material that’s not biodegradable?
I think it’s something of a cost tradeoff. They do resist bacterial growth, but not perfectly. Simpler and cheaper to just replace the filter every month or two.
But I’m not sure if that’s because they are actually metabolizing the filter, or because the filter provides a suitable surface for growth.
In a whole-house humidifier attached to your furnace, the filter pad may just be an aluminum mesh to provide surface area. The water trickles down from above since it can’t effectively wick up from below. Since they’re connected directly to plumbing it’s easy enough to supply with a trickle of water and drain out any excess. Drum humidifiers use a sponge-like material, which could be all plastic since it’s constantly moving and doesn’t need much wicking power.
Table-top or console evaporative humidifiers tend to use pads made out of paper, like compressed paper towels formed into a honeycomb grid shape. That’s great for wicking, but it’s cellulose, which is also food. I assume they’re treated to prevent microbes but that can only go so far.
Steam and ultrasonic humidifiers don’t have pads at all, but the water jugs can grow gunk over time. In a steam humidifier it would be killed by boiling before blowing into the room, though there’s nothing to sterilize the water jug itself. In the ultrasonic one everything is simply atomized and blown into the air, whether that’s microbes or minerals.
This. Air is full of bacteria food. Including other bacteria.
Then there are extremophile bacteria 
“ We’ve found that D. radiodurans not only enjoys living in the cores of nuclear reactors, but it can survive exposure to everything from toxic chemicals and corrosive acids to extreme heat above the boiling point of water, subzero temperatures in the Antarctic, and the vacuum of space.
D. durandurans is another highly robust organism. It survived the 1980s and still refuses to die.