We have had the idea of constructing a natural swimming pool for some time now, but finishing some of the other house necessaries have taken preference. However with the deck finally completed and some of the other renovations complete, we decided to tackle the pool next.

Finished pool

This is the third pool we have “self built” and we used much of what we had learnt with the first two in this one. One major difference this time is that we decided that we wanted to build a natural swimming pool. What is a natural swimming pool you might ask? Well my definition is a swimming pool that doesn’t require the addition of any chemicals, relying on natural processes to keep the water clean and clear. If correctly designed, the idea is that pool becomes an extension of the garden and becomes part of your garden’s ecology.

The basic principle of how a natural swimming pool works is, in order to keep the water clear, you need to deprive the algae which is always present in the water of nutrients. This is achieved by the large number of plants and large areas of gravel that the water flows through. The plants and the bacteria that colonise their roots and stones, compete with the algae keeping the nitrate and phosphate levels low. Low levels of these nutrients means the algae simply cannot bloom and the water stays clear.

The idea of natural pools is not particularly new and they do seem to be gaining popularity, however getting reliable and useful information on how to tackle a natural swimming pool turned out to be quite a challenge. Whilst there are quite a number of sites which showcase the projects completed by pool contractors, as well as companies selling bio-filters and related equipment. We found they were very skimpy on details. Some forums do exist which did offer some advice, but generally these were more orientated towards Koi ponds.

This lack of available info is something which I am sure will change as natural swimming pools become more commonplace, especially here in South Africa, although I suspect most experts will probably keep the info close to their chests. A book that did help a bit was “Natural Swimming Pools: A Guide to Building by Michael Littlewood”, however I found it did lack much of the nitty-gritty on how to actually build a pool. So after much reading both along and between the lines here are the basic principles for what I believe will make a successful natural swimming pool:

• The planted area needs to be about the same surface area as the swimming area (ie. 50 / 50)
• Swimming area needs to be as deep as possible, at least 1.5m (increases volume of water without increasing area exposed to sunlight)
• The water needs to circulate slowly but constantly through the system.
• Total size should be as big as possible (Littlewood refers to a minimum area of 50 sq meters – we broke this rule and opted for an area of 25 sq meters due to space constraints)
• Water needs to be well oxygenated (plants and/or water action)
• You need to provide maximum surface area for beneficial bacteria to colonise ie. roots and gravel beds.

Because our garden was not big enough to accommodate a large organically shaped pool and also there is a significant level difference between the floor/deck and garden level, we decided to opt for a more contemporary design.  Here is the final Sketchup drawing showing the planned shape prior to me drawing up the final details. The physical sizes are as follows:

• Main swimming area : 2.4 m x 4.4 m – depth 2.4m with a continous step along the deck side.
• Intermediate pond: 1 m x 2.2 m – depth 1.2 m
• Lower pond : 0.8 m wide x +-10 m long – depth 0.6 m but varies from 0.1 m to about 0.4 m with the gravel inside.
• Total volume 25 500 litres

Pool Concept

Because we wanted the swimming area to be close to deck level we decided to terrace the pool with the planted sections stepping down to the garden level. This way there was some continuity between the deck / pool level and the grass lower down. One thing we did pick up from other designs was that the water is almost always flows from under the planted area up and towards the deeper swimming area. In our design this wasn’t really possible, but since we couldn’t find any logical reason way it would make a difference reversing the flow, we decided to try it out in reverse.

The Digging Starts

In terms of physical construction we decided on using standard 190mm wide masonry blocks, reinforced with reinforcing steel and concrete in-fill. The reason for this is we wanted to have narrow as possible walls, and this was the only way I could think of achieving this without going the shuttered reinforced concrete route. To waterproof the structure a glass-fibre reinforced resin system again proved to be a cost-effective solution. Prior to fibre glassing the block-work was simply plastered to the desired shape and detail and allowed to harden.

First course of blocks

Block work complete

A large part of any natural swimming pool is obviously the water plants and we were  very fortunate that my lovely Mum was easily persuaded to start collecting and cultivating plants. I think she even started collecting before we had started digging the pool. She has very green fingers and very soon had quite a collection of plants happily growing in some temporary containers. Apologies to the neighbours who had to put up with all the mosquitoes who obviously loved the still, stagnant water. It’s interesting to note that there are no mosquitoes in the swimming pool, which I am sure is mainly due to the running water and our collection of eight hungry mosquito fish.

Gravel ready for planting

Green fingers

In the end she managed to collect quite a variety of indigenous water plants which I believe have contributed largely to the success of the project. We had to create shallow and deeper sections to accommodate the different plant types, but it worked out well in the end as it makes it all the more interesting. Special thanks to Robs & Vaughn who also let us raid their reservoir of additional water plants for the deeper intermediate pond.

In order to get the water to flow through the gravel of the planted area we placed three slotted 40mm pipes running the entire length of the lower pond. These join and then feed into a stainless steel filter box (actually an old dishwasher with a washing machine drum inside). This ensures a steady slow flow of water through the gravel and the plant roots keeping the beneficial bacteria happy and fed, and the water clear. To circulate the water we used a Hailea H9000 submersible pond pump which delivers about 5000 l/h. This means the water is turned over every 5 hours or so but the energy consumption is only 105W.

In terms of costs, because of the additional planted area and the plants, the initial capital outlay is obviously much higher. However in terms of running costs there is obviously no comparison to a conventional pool and there definitely is no comparing the finished product!

Some of the fun things we included was a pool window (at the request of my 8 year old – see ‘Mermaid’ pic below), rainwater top up system with level control and a pool light. The pool has been running for almost 2 months now and apart from one week in the first month when it was pea green, the water has been perfectly clear. Slight variations in colour do occur after rain but are hardly noticeable. Total water consumption (apart from rainwater overflow) has been an average of 150 litres per month, not bad for the height of summer .

For more details and a step-by-step building, watch out for my how-to guide coming soon.