UV purification and algae?

Newbie question here: I understand that algae need sunlight to live, and so it's advisable to cover one's rainwater holding tank with something that will block sunlight so that algae won't grow in it.  But I also understand that one can purify one's water with UV light for drinking purposes.  Can anyone explain to me how these two facts coexist?  Does it have to do with time?  I.e., algae take longer to grow, and UV purification should only take one or two days?  

My understanding is that it is a matter of UV intensity. UV sterilisation using sunlight is not possible outside equatorial regions, and even there you need to put the water into small bottles where the UV can penetrate through the entire container.

EDIT: I should add that I have never tried this myself.

In addition to intensity, UV is blocked/absorbed by even a thin layer of (most types of)plastic.  So if you use transparent/translucent plastic tanks, almost none of the UV in sunlight will reach the water.  Even with direct sunlight, the UV is absorbed by the first couple inches of water and whatever is in it.

UV sterilization uses MUCH more intense UV light, and it's inside a tube that means that the bacteria, algea, etc. is never more than an inch or so from the light.

Every reference to uv light that i have seen is with a lightbulb like peter refers to. In addition, the water needs to be filtered prior to going thru the light. Any debris (algae, particles) will cast a shadow so that some of the water doesnt get hit with the light.

Its interesting if sunlight could be used. Maybe i havent seen it referenced because i havent looked.