Before Austrian researchers introduced the concept of the natural swimming pool in the 1980s, there were few ways to DIY your own pool, and after you spent an extraordinary amount having a chlorine pool professionally built, you had to resign yourself to the use of chemicals or saltwater to keep the water clean. That has all changed now, and if you have access to heavy equipment to dig a hole (and are up for one of the heftiest DIY projects), you can create your own natural swimming pool that is free of chemicals for a fraction of the cost of a conventional pool.
Instead of sanitizing chemicals, a natural swimming pool is an ecosystem that relies on the biological filter created by the roots and foliage of aquatic plants to purify the water. These plants occupy a water garden that is either sequestered in a separate area of the pool or arranged around the perimeter. The garden pond is shallower than the swimming area, and underwater pumps circulate water between the two zones through mesh filters or skimmers, so part of the installation involves laying water circulation pipes and conduit for electrical wires. If you're going to need professional help for any part of the project, it will likely be for this part.
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Planning a DIY Natural Swimming Pool
Organic pools are generally at least twice as large as conventional ones since you have to devote 50 percent of the surface area of the water to the plant-occupied regeneration zone, which is where the water gets purified. At a minimum, the entire pool area should be 500 square feet; anything less than that will become overrun with algae.
The pool can take any shape and may even resemble a pond, but the easiest to build is one with a rectangular swimming area surrounded by shallow pools that slope gradually up to ground level. You can also enclose both the swimming and regeneration zones within a rectangular area, and following a swimming pool design developed in France, you can border two sides with a conventional pool curb to make the swimming zone and leave the other two sides open to form a shoreline for the plants. You can also create separate pools for the water garden and connect them to the swimming area through openings in the pool wall, but this is more work and calls for more circulation pumps.
You'll want to put some thought into the pool liner, which is needed not just to keep water in the pool but to prevent contamination from the surrounding soil. A black plastic pool liner with a minimum thickness of 45 mils is recommended; it's thick enough to resist tearing in rocky soil, and it absorbs sunlight to warm the pool. You can cover it with 3 to 4 inches of gravel in the regeneration zone, and the microorganisms that grow in the gravel will help purify the water. As an alternative, you can pack the soil inside the pool area with bentonite clay.
How to Build a Natural Swimming Pool
Step 1: Dig a Hole
You're obviously not going to do this part of the job with a shovel. Rent an excavator, preferably with an operator, which should cost in the neighborhood of $100 per hour, and dig out the hole. The section you dig out for the swimming area should be deeper than the plant zone; 6 feet is a good depth for the swimming area, while the plant zone shouldn't be any more than 18 inches deep at its deepest to ensure the aquatic plants get the sunlight they need. The edges of the pool should slope up to a depth of 2 to 3 inches to allow shoreline plants to grow.
If you intend to build submerged walls around the swimming area, you can dig out that section with an abrupt edge because the walls will prevent the soil from collapsing. Avoid abrupt edges if you don't plan to divide the swimming and regeneration zones with a partition. Keep in mind that you're going to have a lot of extra dirt, so make sure you have a plan for it. You can use it to build features around the pool or in other parts of the yard, but if you don't use it, you'll have to haul it away.
Step 2: Bury Underground Pipes and Wires
Depending on the water circulation scheme you intend to use, you may need to install underground water circulation pipes or conduit for electricity to power the pumps and any underwater lights you want. Now is the time to bury these with the stubouts sticking out into the pool area. This is also the time to install mesh filters between the swimming pool and the regeneration zone to catch debris and organic matter before it collects and interferes with the plants that form the biological filtration system.
Step 3: Construct the Swimming Area Partition
Erect a pool partition using a suitable material. One of the best DIY materials is RASTRA blocks, also known as Insulated Concrete Forms (ICF), which are relatively lightweight blocks made from a combination of concrete and polystyrene. They come in 10-foot stackable sections, and because they don't weigh much, they are easy for two people to handle. The layers of blocks are held together by rebar and concrete.
The purpose of the partition is to segregate the pool area and also to prevent the soil around the sides of the swimming pool from collapsing. The top of the wall should be several inches under the surface of the water when the pool is full to allow water to circulate between the swimming area and the regeneration zone. The partition is optional. You don't need one if you use the French natural swimming pond design because the swimming area and regeneration zone are naturally separated by virtue of being on opposite sides of the pool.
Step 4: Install the Pool Liner
Whether you use a black plastic liner or line the pool with clay, you'll want to tamp down the soil in the bottom of the pool. You can rent a plate compactor to do this, but it's often sufficient to use a lawn roller. If you pack the sides and bottom with clay, you'll also want to compact the clay.
To install a plastic liner, take it to the center of the pool, unfold it and walk the edges to the sides of the pool, wrapping it up and over the tops of the partition walls. Cut a slit for each plumbing or electrical stubout, slip the liner over the pipe and patch the slit with a patch kit or with waterproof tape. Trim the edges as needed around the perimeter of the pool and cover the plastic in the bottom of the plant zone with 4 to 5 inches of pea gravel or something similar.
You can also fill the bottom of the swimming area with gravel to promote decomposition of organic matter that falls in the pool and promote the growth of beneficial bacteria and other microorganisms, but if you want to take advantage of the warming effect of the black plastic, you can leave it uncovered.
Step 5: Install the Water Circulation Equipment
Hook up the circulation pumps to the pipe stubouts in the bottom of the pool and connect the electricity. Since this is a crucial step and is potentially hazardous if done incorrectly, it's best to hire an electrician or spa professional to make sure it's done properly.
Step 6: Prepare the Filtration Zone
Cover the gravel on the edges of the pool with 3 to 6 inches of soil to provide a growing substrate for the aquatic plants that will form the filtration system. It's a good idea to take a sample of the soil to your state health department and get it tested for contaminants, such as animal feces, before you use it. You want soil with a minimum of organic matter, keeping in mind that you want the plants to draw nutrients primarily from the water.
Step 7: Fill the Pool and Test the Circulation Equipment
Fill the pool with clean, chemical-free water, preferably from a natural source, such as a well or a spring. If all you have available is municipal water that contains chlorine and fluorine, you can use that because these sanitizing chemicals break down in sunlight and should mostly be gone by the time you introduce the plants.
Once the pool is full, turn on the circulation pumps and make sure they are working as expected. Leave them on for a week or so before introducing the plants. Use a skimmer to keep the surface of the water free of debris during this period as well as after the plants are in and have become established.
Step 8: Plant the Filtration System
Fill your garden pond with plants that are recommended for your climatic zone and that will enhance your landscaping but don't go too heavy on exotic ones. Submerged and floating plants should occupy the deepest part of the regeneration zone, while semiaquatic plants should occupy the shallower areas. Use stones and rocks freely around the edges of the pool to provide design interest and a substrate for moss, algae and creeping ground cover.
Choosing Plants for a Natural Swimming Pool
To help your pool get off to a quick start, a good percentage of the aquatic plants you add to your garden pond should be mature, and the bulk should be indigenous species, although there's nothing wrong with introducing a few exotic species that can grow in your area. The natural filtration the plants provide does not so much involve removing impurities from the water as it does consuming the nutrients that algae would otherwise use, so the greener and hardier the plants you use, the better. The plants also remove phosphorous, which is an important algae nutrient, and they oxygenate the water.
A natural swimming pool typically needs three types of plants: floating ones, such as water lilies and — if you live in a climate that can support them — lotus; emergent plants for the shoreline, such as cattails and other rushes and tall grasses, including pond hedge and swamp hibiscus; and underwater plants, which do the important work of oxygenating the water. You're creating more than your own backyard pond; you're creating an entire ecosystem, so the plants should be grouped in cohesive units that will support each other and attract frogs, dragonflies and other wildlife, which also play a part in maintaining the health of the system.
Even though the purpose of the filtration system is to compete with algae for nutrients, algae are nevertheless an important and unavoidable part of the ecosystem. Algae and phytoplankton are photosynthetic workhorses, and if left to their own devices, they will form a blanket on the water and starve all other organisms of sunlight. That's why you want to make the pool as large and as deep as possible and confine them to the regeneration zone where the other plants will keep them under control.