How can I make sure my UV-purificator kills bacteria?
Not having the equipment needed to actually count bacteria, you can use things you know bacteria does to find out how much they were affected.

My approach would be thus:


Select a model bacteria. There are a few candidates, but probably the most readily available is lactobacilli, which can be found in yogurt and (loosely translating terms of art) is a Gram-stainable fermentor-sometimes-breather.
Infect the water with a known quantity of bacteria and a food source (lactobacilli eat lactose - ie from milk) and equally divide it into a control and experimental group...
You'll need to find an alternative way to estimate the bacterial content in the volume of water used compared to a control colony. Since lactobacilli produce lactic acid, you could use the pH value of the water (either with an indicator or a meter) to detect bacterial metabolism, compare that to the control's pH value.
Record the UV exposure time from the experimental group, and isolate the control group from any potential UV light source, such as the sun.


Even if you find that, for example, your particular strain of lactobacilli is not sensitive to your UV light source, that's a perfectly scientific result! How did you obtain it? The source likely knows what kind of bacteria it is. Perhaps the UV light merely interferes with bacterial metabolism, and doesn't kill it. Unanswered questions are wonderful, you may have provided a place for further research to take place. You could then use the two groups to infect a food source (ie milk) and compare post-exposure growth, depending on how much time is available.