Desert Island

If you’re ever stuck on a desert island and have the chance to bring three things, don’t fret about potable water. Save your three wishes for a designer bikini, Netflix, or something more worth your while. According to Water Conditioning & Purification Magazine, activated carbon from coconut hull is a more efficient filter than activated carbon from bituminous coal. Below is a picture featuring this week’s topic of coconuts, and holding the coconut is a wonderful Australian traveler I follow on Instagram, Jade Phoenix.

Image from theadventuresofjade Instagram.

“Not only do coconut shells eliminate lead, but they address trihalomethanes, pesticides, herbicides, and volatile organic compounds” –Speaking Green

Some of the factors that make coconut hull an excellent filter other than the obvious long-term sustainability when compared to coal include smaller pore size, higher pore volume, less ash, higher carbon capacity, higher hardness, and lower cost. Coconut hull consists mainly of micropores less than 2nm in diameter, while coal is made up of mainly meso- and macropores which are one to two magnitudes greater in size. This means that coconut hull can capture very tiny particles, resulting in better tasting water. Coconut can filter a higher volume of water due to the number of pores in the material AND it doesn’t require a rigorous “pre-wash” to get rid of ash. With activated carbon from coal, pre-washing is required, otherwise your water will be stained black and have a gross chemical taste. Yuck! Not to mention, activated carbon from coal must be replaced more frequently than coconut because it has a low carbon capacity. Coal also just won’t clean as well because it has a lower hardness. Essentially, ground-up coconut shells can filter water better than coal, AND they are a renewable resource.

Another picture of Kauai from Jade.

Of course, you would need to dig into your imaginary island to find groundwater before the filtration step because coconuts can’t filter saltwater. In my hydrogeology course last semester we learned about the saltwater-freshwater interface that divides the groundwater and seawater like a virtual slanted line at the coast or some distance off-shore. The saltwater interface gets more complicated than just a vertical line, but that’s the general idea.

Groundwater can swell and push seawater outwards, surrounding the island in a bubble of freshwater and possibly even damaging coral reefs. That odd fact was an ongoing joke between the class because we had previously learned that coral reefs were dying due to rising sea temperatures *which they are*. But if an island ever had this issue of too much groundwater seepage, the solution is to do what we’ve been doing to Ogallala Aquifer all along: pump the water out at a faster rate than the groundwater is recharging. Then the freshwater “bubble” in the imaginary island will shrink and saltwater will move inland. But back to the main point: You need groundwater first.

Depiction from Forester Media.
If you get lucky, you might find groundwater at the surface. Screen capture from YouTube.

The article by Water Conditioning & Purification Magazine that I linked to earlier praises activated carbon from coconut as the next big prospect for water filtration in America, and that was written back in 2008. Ten years later, there’s nothing to show for it. What happened between then and now? The end of the piece, written by Ken Schaeffer and Robert Potwora, mentions that China temporarily stopped producing bituminous coal for the States and other consumers around the time of the Beijing Olympics due to stricter enforcements that were meant to limit the smog above the city. It was likely the cause of a temporary fluctuation in market dynamics. Either way, it’s a lot easier to obtain coal in the States than coconut hull. In an island setting, coconuts are harvested three times a year with no harm to coconut trees. Once the coconuts are split open and the juice and meat are removed, there are tons of coconut shells lying around that would otherwise go to waste. So it makes sense for this to be a source of activated carbon in the tropics.

Carbon is “not recommended for coliform removal or for cysts, though ironically, some of the very tight solid carbon block filters now on the market remove bacteria (though manufacturers seldom make this claim) and cysts…quite handily” –Pure Water Products

A little while ago, I mentioned that the size of an average coconut pore is super small, about 2nm in diameter. Here’s the issue: There are still some larger pores dispersed in the coconut hull. Even if you compacted the coconut hull and flushed the groundwater through it a few times, the bacteria which can be 200nm or larger would flow around the pores and end up in your drinking water. Same goes for viruses which are 20 to 300nm in size. Although urban runoff or raw sewage probably wouldn’t be of concern for drinking water on a deserted island, there are still bacteria, viruses, and parasites that can cause problems like polio, cholera, and even dysentery. Coconut won’t disinfect your water, whereas a chemical like chlorine would effectively wipe out all of those bacteria and viruses. So coconut shells aren’t perfect, okay? But they do their best. Do you live somewhere tropical? I’d love to know if anyone actually uses these coconut filters! It’s my first time hearing about them.

Thanks for reading, Jess 🙂

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