Not an expert here, but I feel pretty confident that you can indeed treat 1.5L of water with just one Micropur tablet -- by increasing treatment time accordingly. OTOH, if the water is very cold, do people really want to extend treatment time from 4 to 6 hours -- overnight treatment excepted??
Just the other day, there was a poster here who wrote that he never treats his water -- and nothing ever happens! That's the thing about water treatment -- we can't see the baddies and unless we do lab tests, we really don't know...