Guide

How a natural swimming pool stays clean without chlorine

A natural pool stays clear for one reason: something other than algae eats the food first. Every design choice comes back to that idea.

Algae is a food problem

Algae needs light, warmth and nutrients. You can't take away the light or the warmth in a swimming pool, so you take away the food. The food is mostly nitrogen and phosphorus, and it comes from leaves, pollen, dust, bird droppings, sunscreen and anything else that falls in and rots.

A chlorine pool kills whatever grows on that food. A natural pool starves it instead, by handing the food to things you want: plants and bacteria.

The four workers

1. Plants. Water plants pull nutrients straight out of the water to grow. I aim for half the water surface in plants and never less than 40 percent.
2. Bacteria. Beneficial bacteria live as a thin film on every surface underwater. They break down waste and lock up nutrients.
3. Gravel and shale. This is where the bacteria live. I use expanded shale because each piece is porous, so a small load holds a huge amount of surface for bacteria.
4. Moving water. Water has to be pulled through the shale and past the roots over and over. Still water grows algae and mosquitoes. Moving water carries oxygen to the bacteria and food to the plants.

The water loop

In our pool, a ring of expanded shale 5 to 7 feet wide surrounds the swim zone. At the bottom of that ring, sleeved perforated pipe collects water that has soaked down through the shale and the roots. The pump pulls from that pipe and from a skimmer, sends the water through a UV light, and returns it to the swim zone over a waterfall. Every trip, the water gets scrubbed by bacteria, fed to plant roots and loaded with oxygen on the way down the falls.

Where UV fits

A UV sterilizer kills the single-cell algae that turns water green and cloudy, and it knocks out bacteria from things like bird droppings. It does nothing to string algae. I consider UV essential on every system, but it is insurance, not the filter. The plants, bacteria and shale are the filter.

When algae wins anyway

It will happen, especially in the first one to three years while plants fill in. Net string algae out by hand. If you need a shock, use hydrogen peroxide, not chlorine or algaecide. It breaks down and leaves water and oxygen behind. Chlorine would kill the bacteria you spent a season building.

The first summer is the messy one. It takes a full season for the bacteria to colonize the shale. By the second summer it's really working.

See it on your own yard

The designer sizes the plant zone, filter and pump for any pool you sketch, so the balance comes out right before you dig.