I read a bit periodically about slow sand filters. I understand the concept, and that the method depends on the bacteria layer on the top.

What I don't understand is how one would keep the bacteria layer moist (which it has to be, or it becomes ineffective).

Specifically, how do you think you would do this? I'm thinking some kind of controlled trickle, but how?

Sue

Recirculation?

Something to catch water under with tube pumping water (solar powered?) back onto the sand?

-john

Take a look at the cross-section image on this page:
http://en.wikipedia.org/wiki/BioSand_Filter

The outlet comes out the bottom and then up to a level that is above the sand. When water stops flowing in the top, the filter doesn't drain completely, only to the level of the outlet.

That being said, I understand that they like best to have a continuous flow. It also takes some period of time with water flowing through it for a brand new filter to develop the layer of bacteria.

This is a nice concise description you found thseng.

In places like Haiti where people need not only clean water but
employment, the building and selling of biosand filters by
nationals has been a boon to local economies.

It may be possible to inoculate a filter using an existing filter's smutzedecke, as long as the same water source
is used by both filters. This might speed up the time it takes
for the bacterial disinfection action to start. Still might be
several days.

I remember hearing there is evidence that chemical eating bacterial grow and can remove oil and other man made materials.

Thank you, thseng!

Using the fact that water seeks its own level is the simplest way to do it!

I didn't think of that...

I didn't think of looking at Wikipedia, either...

Sue

Okay, next question...

Sometimes I see these short lengths of concrete pipe at construction sites (early phase) that I've wondered if they would be good for this kind of filter. They appear to be 20-24" wide and about the same in length, with a molded lip.

I don't know what kind of pipe they're called, though. But do you think three of these (assuming you know what I mean), could be stacked to form a tight enough column for a sand filter? And, since I don't know what they're called, I don't know if any of them come with holes suitable for a pipe, or not. I wonder how hard it would be to drill a hole?

It seems you could set the first one in place, seal the bottom with concrete, add the bottom gravel, coarse sand, and top with fine sand, then add the second length, fill with more fine sand, and then put the diffuser in the third one, and top with a lid with a hole for a pipe, or even no pipe if you would be around to fill it with a bucket.

I also just passed another construction site and coveted some of the wide turquoise plastic (probably sewer?) pipe that was at least 18" wide, maybe more. (You have probably noticed by now that I am obviously in the wrong line of work.) If I happened to find one of these mysteriously lying in my front yard (not likely), would it be suitable for potable water? Or would contaminant leaching be an issue?

Thanks for the ideas, guys!

Sue

My Grandma has always said that we should learn something new every day. Thanks Sue!

Keep in mind though that the biosludge (bacteria) layer needs oxygen. It is best if the water continuously falls onto a splash plate above the bacteria layer. The picture in Wikipedia is the next-best thing but the water level can't be over 2 inches deep over the biosludge. It needs to be that shallow so that oxygen can diffuse through the water to the bacteria.

-Blast

If you follow the links from wikipedia to Manz Water Info, their Guidance and Construction Manuals go in great detail on this topic, with pretty pictures

Thanks for the tip, EMP.

Sue