saltwater chlorination
For years, I maintained swimming pools for my (now adult) children. I usually used a saltwater chlorination system. That’s different from a pool that’s disinfected by keeping the salt level as high as seawater. Instead, you put in a low level of salt (usually just above the threshold where the water starts to taste salty, about 3,000 ppm) and use an electrical device to liberate chloride () ions from the salt () and keep the pool chlorinated. Essentially, you generate the same ionic species that you get when you add liquid chlorinator (sodium hypochlorite) to a conventional pool, so saltwater chlorinated pools have most of the same advantages and disadvantages of conventional chlorinated pools, except that you don’t have to buy, store, and handle “chlorine” (sodium hypochlorite, which is the same as regular Clorox laundry bleach but usually at twice the concentration). You don’t even really deplete the salt; you only have to add salt when you dilute the salt concentration by adding more water to your pool.
It seemed like a great way to keep a catchment water supply conditioned, as all you need is some salt (salt isn’t hard to come by in the middle of the ocean) and energy (which we have in excess). However, if you take a more critical but still simplified look at what’s going on:
The chloride ions () are what do the disinfecting. The is just hydrogen gas which bubbles out into the air. But what of the ? That’s sodium hydroxide, otherwise known as lye, and it’s a strong base. Some of it gets used up by reacting to the , but some of it hangs around and raises the pH of the water. That’s not great, as the disinfecting properties of chloride are enhanced at lower pH. So you end up having to add pool acid (hydrochloric or “muriatic” acid, ) to the pool to bring the pH down. Now, is caustic and poisonous but, overall, I’d much rather deal with it than Clorox. It’s also much cheaper than sodium hypochlorite in industrial quantities. So I liked the idea of saltwater chlorination.
(Apologies to any chemistry nerds who were just offended by all the important details I left out.)
catchment water
Catchment water is simply water that’s collected off your roof when it rains. Free water! Falling from the sky! As someone who grew up in the Southern California desert, that’s nothing less than a miracle.
But…
When is the last time you washed your roof? Eh? And did you get it clean enough to eat (drink) off of? I’m guessing not. Bird poo lands on your roof. Mice and rats poop on your roof. Dirt lands on your roof. Volcanic ash lands on our roof, but probably not yours. Insects and their poo land on your roof. This island also has a particularly nasty nematode that causes rat lungworm disease. That lands on your roof! Worms make their way to your brain and crawl around in it. Bleah. It’s often fatal.
So catchment water has to be filtered and sanitized before you drink it. The easiest way to do that is by chlorination, and the (long term) cheapest way to chlorinate is by using saltwater chlorination.
but…saltwater??
So if you use saltwater chlorination, you now have sanitized water, but it is sanitized salt water. With a lot of chlorine in it. Not great for drinking. It also will still have a lot of yuck in it (dirt, decaying plant/insect parts…).
filtration
So next you need to filter it. If you use the right kind of filter (reverse osmosis), that will also remove the salt, and if you set things up right the backflushed salt from the filter goes back into the saltwater and maintains the salt content. Cool.
Though I brought with me most of the parts to make this system work at scale, it’s a lot of plumbing (which is not my favorite activity) and we still have municipal drinking water here when it works. So for now, I’m trying this countertop reverse osmosis filter because no plumbing is required and we’ll only need it for drinking water if the municipal system suffers a longer-term failure. This will also give us a chance to test-flight the concept before doing a lot of pipework or, worse, hiring a plumber.
By the way, as with most such systems, it’s not just reverse osmosis (RO). There are particulate filters to get rid of other stuff before stressing the RO (which will get rid of virtually all contaminants but is slow and clogs easily). There is a UV light system that sterilizes the water with short-wavelength ultraviolet light (known to kill those nematodes). There’s a carbon adsorption system. There are also mineral salts that the filter adds back in because RO water is so pure it has no taste which is pretty unpleasant and some people say is unhealthy. (Most “filters” for, say, water pitchers or refrigerators mostly do more adding of minerals to improve taste than actual filtering).
It’s one more bit of infrastructure, and one more way of being resilient in a disaster.
—2p
addendum 20250829@17:07
I just tried my first glass of water from the filter, and it was quite good.
addendum 20250830@17:02
Previously, I used the municipal tap water for testing. Today, I used the actual catchment water which, thanks for a thick layer of yellow bamboo leaves that were clogging the gutter, is slightly yellowish in hue. The water out of the filter was crystal clear and tasted great.