katadyn Micropur Forte
Just to go one bigger, i have 500grams of this in powder form (MF 50000P) recently purchased from a local French chemist (around 3 week order time) which cost 84euros and treats 50000 litres! Even cheaper than the liquid per litre if you can use it within its shelf life, can't leak/evaporate like the liquid may. This is mainly a good idea for me in my campervan, to both pretreat water added to the camper tank, and keep the tank in a good condition without using a strong bleach type treatment. This is for all the water to be used, not just for drinking, and should last ages in a tub the size of a big coffee mug.
The downside is dosing. The measurement spoon is 10grams sufficient for 1000l obviously, unless I put a big water tank on the roof to fill the campers tank from when it needs a top up
I'll only need a fraction of this depending how much water I put in at a time.
My solution is to put 10grams in a litre nalgene filled with clean water. If i put 200l in the watertank, I need to add 200ml of micropur from the nalgene. Since I only got the stuff today, I don't know if this will work, will 1 litre carry 10grams of the stuff fully dissolved, will it eat into the nalgene? We'll see. I doubt there will be a problem since chemical strengths for purification are a lot lower than for cleaning bleaches which I could also be using.
As long as I know how much water I put into the tank though!?!