Zwaplat

Being bored, I've been thinking a bit. Microbial growth is probably one of the biggest problems in watercooling systems, and people try all kinds of things to prevent it from happening.
Now, with my education I know quite a few tricks to kill that nasty stuff. I'll throw up some ideas here and give my thoughts on what might work.

Demineralized water: it's almost impossible to obtain pure H2O. Also, the AD (aqua destillata) you buy is made one way or another. One will kill microorganisms, the other will reduce their number at best. It is good enough to prevent corrosion (no ions in the water), but some microorganisms are really though and can survive with just the few bits and pieces of 'food' that are left in the water and in your tubing.
Adding antifreeze: as I said before, there are some really though buggers out there. Doesn't help against everything.

Sterilizing chemicals:
Chlorine is one of the easiest to use and most effective chemical against microbial growth. Splash on some 1/10 diluted chlorine, and there won't be much left alive. Would work wonders in a WC system (cheap, not terribly dangerous to humans if you watch out), were it not for the problem of corrosion. Big nono, unless you manage to make your whole system out of plastic.

Alcohol, and namely ethanol. 70% ethanol will kill most microorganisms (the only things that survives it, iirc, are bacterial spores), and the great part about it is that it doesn't provide a corroding environment. Not sure about the heat capacity of 70% ethanol...prolly not near as much as water.

What to do then ? Well, here are a few solutions, one better than the other.
-Autoclave your system. Yes, that's right, autoclave it. Your tubes, your blocks, your reservoir, your radiator and your cooling liquid. Just make sure it's all heat resistant. The only part not sterile is the pump, and you could just rinse it for a bit with sterile water. Biggest problem: who's got an autoclave at home ?

-Bake everything at 180°C for two hours...no wait, that won't work, would it ? Melting tubes, cooling liquid boiling away, we don't want that.

By now, you must be thinking "Great, what an idiot, nothing works". Well, you're right...sort of. But I am, too. If you want to keep your WC system growth free and don't mind spending some time, read on.

First of all, you need sterile AD. Just buy AD, and boil it for about 15 minutes. That'll kill most of what's left in it. Just keep it at boiling point and don't let too much steam escape.
Next, sterilize your system with a chemical: chlorine (1/10 dilution), 70% ethanol, or even Dettol.
Now you've still got that nasty chemical in there, and you don't want that. Also, you don't want to "infect" the system again. No problem at all, just rinse it thouroughly with your freshly boiled water. (use small amounts and repeat several times, the more the better)
So, now you've got a sterile system (on the inside, that is). Just top it off with freshly boiled water and close it. If that doesn't do the trick, I don't know WHAT will!

Last words of warning though: avoid sneezing, coughing, talking and even breathing heavily when you're working around your sterile stuff. You'd be surprised at how fast you can contaminate your work again.

Also, none of the above stuff described here is meant to be used for medicinal purposes. Do not attempt to clean a wound or tools to be used around a wound in any of these ways. You'll probably make matters worse.

Raziel

Did anyone tried to add methanol to prevent bacterial growth?

I added "chloorhexidine". It works fine for me.

Xploited Titan

I wanted to add methanol, but in the end, just added WaterWetter. Although my system didn't turn long enough to check for microbial growth, I can assure you WaterWetter does sh*t against corrosion

Zwaplat

Good idea. But I'm not sure if it stays active though.
If I didn't need to study atm, I'd be writing a longer article on getting and keeping a WC system sterile. Will be a nice project for February.