Blast, I think you will probably need a holding tank between the downspout and the filter to buffer the water flow.

From what I understand about slow-sand filters, they need a constant flow of water to form and maintain the biological layer ("Schmutzdecke"). Or at least they need to stay full of water once the layer is established. And the layer takes weeks to fully develop.

See the bottom of this page under "Important":
http://www.refugeecamp.org/learnmore/water/slow_sand_filter.htm

Tom,

Good idea on adding a buffer tank before the sand filter.

I've seen the link you posted before. I like that system but it requires some pretty good height differences to keep it all gravity-fed.

For the biofilm, I've read it takes up to four weeks to start purifying the water. My plan is to have two sand fliters set up so that I can alternate cleaning them without losing the ability to purify water.

-Blast

To remove disolved solids and another step in purifing your water you might want to concider solar distillation.

Here are some links.

http://www.thefarm.org/charities/i4at/surv/sstill.htm
http://www.rotarnet.com.au/users/9/9520/SolarWater/swp.html
http://www.davesplanet.net/solarstill/