How to Build a Biosand Water Filter

The biosand filter is a practical method for affordable water treatment around the world.

The biosand filter (also known as a slow-sand filter) is a low-volume version of the filtration stage used in many municipal water treatment plants. It physically removes small waterborne particles through mechanical action (trapping particles between sand grains) and biological action (a bacterial layer captures and consumes organic materials).



For small villages and individual households in the Third World, the biosand filter is a practical alternative to expensive treatment plants and distribution systems. A properly maintained biosand filter removes most waterborne pathogens (disease-causing organisms) and many contaminants at very low cost.

Things You'll Need

• Concrete or sturdy metal pipe measuring 8 inches in diameter, and 5 feet in length
• End cap for pipe
• Drill
• 1-inch drill bit (designed for pipe material)
• 3 PVC 90-degree elbows, 1-inch
• 4-foot section of 1-inch PVC pipe
• PVC pipe cutter (or hacksaw)
• PVC primer
• PVC cement
• Gravel
• Coarse sand
• Fine sand
• Perforated disk (or plate) to match pipe diameter
• Water

Instructions

1. Attach end cap to pipe.

2. Drill a 1-inch hole in the pipe, slightly above the end cap.

3. Cut two 3-inch lengths from the PVC pipe. Trim the remainder of the pipe to 3.75 feet.

4. Use PVC primer and then PVC cement to attach one 3-inch length of PVC pipe to the hole you drilled in the filter pipe.

   Cement a 90-degree PVC elbow to the end of the 3-inch length of PVC pipe, with the open end of the elbow facing up (away) from the capped end of the filter pipe.

   Cement the long piece of PVC pipe into the open end of the 90-degree elbow.

   Cement another 90-degree PVC elbow to the top of the long PVC pipe, with the open end of the elbow facing away from the filter pipe.

   Cement the second 3-inch length of PVC pipe to the open end of this elbow.

   Finish by cementing the last elbow to the open end of the second 3-inch length of PVC, with the elbow's open end facing down toward the capped end of the filter pipe, like a faucet.

   Allow the cement to dry.

   The result will be a PVC gooseneck pipe running upward from the capped end of the filter pipe to a point slightly below its fill level. This allows the filter pipe's internal water pressure to drive the filtered water out of the PVC pipe for collection.

5. Stand the filter pipe up on its capped end.

6. Fill the bottom 4 to 5 inches of the filter pipe with gravel.

7. Pour coarse sand onto the gravel until you've filled the next 4 to 5 inches of the filter pipe.

8. Fill the next 3 feet of the filter pipe with fine sand.

9. Place a perforated disk (e.g., a sink strainer, or a stiff metal disk with a dozen or more 1/8-inch holes) on top of the fine sand. This will act as a diffuser to keep water from churning up the top layer of sand and disturbing the biologically active "schmutzdecke."

   The schmutzdecke forms as microscopic parasites attach themselves to the top layer of sand in order to feed upon trapped organic materials. This biological layer provides a finer sieve than the sand, but it takes days or weeks to grow to its full potential.

10. Pour in your source water carefully. As the water reaches the sand and gravel layers, pressure from the water above will force it up through the PVC pipe.

    As the schmutzdecke forms, the filter will become more effective.

Tips & Warnings

• Use stand-off posts under the diffuser. Alternatively, screw 1/8- to 1/4-inch metal screws into the interior of the filter pipe, so that they hold the diffuser 2 to 4 inches above the top layer of sand. This will help the schmutzdecke to form.

• Use PVC primer and cement in a well-ventilated location.

• The biosand filter removes 90 percent or more of waterborne pathogens, but some water will require additional disinfection. If possible, have the water from the filter tested to see whether biosand filtration is adequate. Chlorine, iodine drops, pasteurization, and ultraviolet light exposure will all be more effective after biosand filtration.

References

• Biosandfilter: The Biosand Filter
• Fort Lewis College: Biosand Filter