I have designed a water filtration system but I haven't build it. It is made from aquarium air pump, pipe, stone sponge, a bucket tap water. My room is about 16 cubic meter. I would like to buy a air pump 480 Litre/hour. It is about a day to clean air in my room.
How it works:
The air pump gets air and pump it into the water where bubbles are formed. You need to get a stone sponge to make bubbles smaller. The whole system consumes 5W and costs about 20 dollars.
There is an other DIY air purifier: http://helpline.awardspace.co.uk/index.php?topic=5.0