Did you know that for about $30, you can make a high-quality heavy-duty water filter that can remove up to 99% of contaminants?
It's called a "Biosand" water filter. And all you really need are a few 5-gallon buckets, some plastic pipe and fitments, and 3 grades of filtering media: fine sand, regular sand, and gravel.
What's a Biosand Water Filter?
Chances are, you've probably never heard of a Biosand Filter. But you might have an idea about a Carbon Filter (the kind in your Brita filter) or a Sand Filter (one of the kinds used in swimming pools). These types of water filters work like a fine sieve that traps dirt and bacteria inside the filter media. The finer the media granules, the smaller the particles -- and microbes -- it can trap.
Sand filters are widely used in the early stages of water filtration, because sand is not only cheap, but does a really good job. And actually, ordinary sand filters aren't used only in swimming pools -- In fact, sand filters likely do most of the filtration in your municipal water system! They're generally considered effective at removing in the range of 70-80% of contaminants -- mainly larger particles but a good deal of larger microbes as well.
A BIOSAND filter is an interesting variety of sand filter that's gaining a lot of scientific momentum, especially for work in developing countries who have less access to clean water. The idea is that rather than simply pushing all the water through the sand as if it's nothing more than a sieve...
We can leave a small quantity of water in the container to keep the sand wet. And you probably know what happens when you continually leave fine sand or dirt wet -- it gradually accumulates a stinky layer of sludge on top. That top sludge layer is actually a community of friendly bacteria -- which then plays a remarkable role in the filter of eating microbial contaminants as they come through! The result is that you've turned an ordinary cheap sand filter into a high-quality water purifier that can remove up to 99% of contaminants, if built well.
The trick to turn an ordinary sand filter into a biosand filter is pretty simple in a cleverly elegant way -- You simply run a thin tube from the outflow on bottom back up to the top before it loops back down to your clean-water-collecting bottle. Via siphoning principles, the top level of water will remain at exactly the highest point of your siphon tube.
The best medium for creating the top sludge layer is fine sand -- fine, but not "too fine". Because you don't want to clog it up, you shouldn't put powdery stuff like activated carbon or diatomaceous earth in that stage. In fact, if you want to use those (they're both great filter media), it's best to put them in a separate stage AFTER the biosand filter, as he does in the video.
You can learn how to make a biosand filter in this 3 part construction video. NOTE: It's long, because it's an involved process that needs to be done to pretty exacting standards -- we're talking about your health after all. He'll show you exactly how to construct not only the bio-sand filter, but upgrade it to incorporate a carbon filter to remove some of the few common contaminants that bio-sand filters miss. (Viruses, for example, are the smallest pathogens, but activated carbon can trap them with what's essentially a static charge that contaminants "adhere" to.)
More Resources for Biosand Filters
The best resource I've found for constructing a biosand filter that achieves it's maximum efficiency is the website CAWST.org. They provide research and plans for developing countries to inexpensively manufacture their own heavy-duty community drinking water filters to the highest standards out of concrete. You can download PDF construction plans from their site.