My DIY Biosand Filter

I've been thinking for a long time about how we will get drinking water out at the lake. Its a question thats got obvious applications beyond our little sabbatical. I've done a little work with Engineer's Without Borders (although not as much as I would like), and civil engineering work in developing countries primarily covers delivering clean drinking water and improving sanitation systems. Unsanitized drinking water is one of the top causes of death in children around the world. So this has been a learning experience in more than one way for me. After researching lots of different methods, I decided to build myself a biosand filter.

The foundation that has done the work to develop and promote the biosand filter has extensive plans on construction and studies on efficacy of the filter [PDF]. Their design is a bit over the top for our use though, using concrete and sheet metal to construct the thing. So my challenge was to build my own biosand filter for as cheap as possible, to make so that I could easily move it around, and design it in a way that I can repurpose the parts when we're done using it (cradle to cradle design, baby!). The design closely matches all of the recommended dimensions from the biosand design manual. Here's what I came up with for $40:

3 x 5 gal. HDPE Bucket (doesn't matter which--all HDPE (#2 plastic) is food safe)
3 x Bucket Lid
1 x 3/8" Brass Close Nipple
2 x 3/8"x3/8" Brass Hose Barbs
2 x 5/8" Neoprene Washers
4 x 5/8" Galvanized Cut Washers
5' x 3/8" ID Vinyl Tubing
6' Silicone Weatherstripping
16 x Zip Ties

Construction is rather simple, and just consists of three main steps:
1. Install a simple outlet on the bottom bucket and attaching tubing,
2. Attach the lids to the bottom of two of the buckets,
3. Drill holes through the bottom of the buckets/attached lids.

After all of this, it is just a matter of obtaining sand and gravel, and washing the silt out of those materials. I haven't done this step yet because I don't want to haul 50 lbs of sand from Kansas City to Kahola. I'll do all that once we get out there. You can find a video of this process here.

(Here's just a quick aside about the use of buckets. I would have loved to find something in a single piece that wouldn't have required joining, but I couldn't find it. 12" pipe was too expensive, a 55 gal plastic barrel was too large in diameter, and no trash cans I found were tall enough. If anyone has any ideas of a plastic alternative, I would love to hear them.)

And here's how you build the thing! (Note: some of the pictures show some out of order steps, like the holes being drilled before I say to, due to my experimenting and messing up some steps. Maybe some day I will take more helpful pictures...)

Step 1: Install the outlet.

Here are the materials, except that you will need 4 fender washers instead of just the 2 shown. The things you see there are the fender washers, neoprene washers, 3/8" pipe nipple, 3/8" hose barbs, and about 5" of 3/8" vinyl tubing. 

Drill or cut a hole in the bucket. Make sure you leave enough room for the washers to fit on the inside of bucket. 

Here's what the outlet looks like assembled. (Note again that you will need 4 metal fender washers instead of 2 to get a good seal)

Here's a view of the outlet on the inside of the bucket. 

And again on the outside. 

Step #2: Connect the Buckets. 

In order to seal the bucket to the lid I used silicone weatherstripping. It was much cheaper than big a rubber o-rings. 

Install the weatherstripping to the bottom of the bucket and attach the lid back to the bottom of the bucket. (Never mind those drilled holes...kind of did it out of order myself...)

Set a lid from one bucket on the bottom of the bucket and drill holes for zip ties. I used zip ties because they were cheap and available. You could use bolts & nuts, or sheet metal screws to connect the lids with the buckets as well (if they're galvanized). 

  

Here are some of them installed. I used 8 total to get a good seal. 

Step #3: Drill a bunch of holes. 

They don't have to be particularly uniform or a specific size. I used a 1/8th inch drill bit and just went at it. 

Here's what it looks like finished. 

Now you just have to assemble the thing and attach the outlet hose. The tube should loop over the bucket handle, and the end of the tube should sit 4" above the bottom of the top bucket. This allows the water to sit above the top of the sand to keep the biofilm that forms from being disturbed. 

The only thing left to do is to add 3" of coarse gravel at the bottom of the bottom bucket, then 3" of fine gravel. Then you fill up the rest of the bottom bucket and the middle bucket all the way full of sand. This will give you the recommended 23" of sand in the column. 

P.S. Looking around for some info while I was writing this post, I found another website that had a similar design for the biosand filter using buckets. It is supposed to only cost $20, but there isn't a lot of details on design.