February 7th, 2011 by Tim Leave a reply »

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

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.

Digging the hole

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

First course of blocks

Block work complete

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

Gravel ready for planting

Green fingers

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.



  1. Anton says:

    Also look here for more info. Pools can be heated, but you need more oxygen then.

  2. Liam says:

    Hey Tim

    I love what you’ve done here and hope your pool is still doing well 7 years later!

    I just wanted to ask whether you filled your pool with rainwater or tap water? I’m in Adelaide, South Australia where rain is pretty scarce but I’ve read that tap water has high levels of Phosphor which is difficult to remove from the system.

  3. Tim says:

    It is indeed – not much has changed, best pool ever. Initially I used tap water to fill yes, although our water I think is generally very good quality as its mostly from mountain stream sources with little agriculture above. For the last year I have been topping it up with my rainwater system as we are in a nasty drought at the moment.

  4. Liam says:

    That’s brilliant, we actually have an overflow stream running by our house that comes down from a dam in the National Park behind our house so I might tap into that come Winter time and borrow some water!

    I’m married to a South African and her folks live in Jeffrey’s so I’ve been to PE a few times. They’re currently doing everything they can to conserve water just like you!

    Are you still using the same pump (

  5. Tim says:

    I am – and its still running- no service required to date.

  6. Taryn says:

    Hi Tim,
    We are also in PE and are considering a natural pool. Would you please give me a rough idea of the initial costs to install/build please? We are limited for space so ours would not be very big.

  7. Tim says:

    Just a guess – depends on so much R50 – R100 K ?

  8. Liam says:

    Hey Tim..I have a few more questions – would you prefer I post here or email you?

    Or feel free to tell me to leave you alone =P

  9. Tim says:

    Welcome to mail me – happy to help. I leave for GodZone in NZ in few days so may be delay in answering 🙂

  10. Bridget says:

    Hi Tim, Like Liam I am in Australia (Melbourne) so would really like to hear your answers as well 🙂

    And the cost doesn’t seem that high (we would also go David Butler’s way considering it would all be on one level) approximately AUD$5,000-$11,000 if I am correct?
    Thanks heaps 🙂

  11. Jo says:

    Hi Tim

    This is a very useful post, thank you.
    I’m planning an eco pool in East London and just wanted to know if the same design and materials would work if it was dug into the ground rather than being above ground?
    My father has successfully converted a chlorine pool into an eco pool by himself, but we don’t have construction experience/knowledge about what won’t crack or leak etc.

    Is your how-to-guide available?



  12. Tim says:

    Hi – sorry never got round to the how to guide just got too busy. I would suggest you get some engineering input if you do decide to go below ground, saturated soils are at least twice as heavy as water so the forces on the walls could potentially be greater when the pool is empty, but will all depend on the depth, soil conditions etc.

  13. Tim says:

    Going to depend on who is building it – our labour is dirt cheap – assume its not the same in Aus though ?

  14. Jacques says:

    Ho Tim

    Hope all is good on in PE.

    I was inspired by your posts to finally build a eco pool after putting it off for a while.

    I’m based in CT and am currently in the middle of my eco pool build. I was hoping you can give me some info on where you sourced the slotted pvc pipes. I cant seem to track any down here in Cape Town that are 40mm. Only106mm and larger.


  15. Tim says:

    Hi Jacques

    I just used 40mm PVC pipe and an angle grinder – many cuts with a thin blade – just note they very soon were clogged with rooots so I did away with them and have no pipe drainage below stones now


  16. Paul Fish says:

    Hi Tim,
    Nice project. Where did you house the pump? I’m guessing you have a submersible pump in a chamber sunk in gravel of the lower pool..?
    I’ve built a similar pool and and now looking to refine the design for the next one!

  17. Tim says:

    Hi Paul

    Have a look at the pictures – you will see I put the submersable in an old washing machine and dishwasher housing – works like a bomb. But yes its in the gravel area


  18. Paul Fish says:

    Hi Tim, Thanks for the reply! 🙂

    Do you have a problem with algae on the pool walls? I built a similar pool a while back for a client and found algae grew quite quickly. That pool flowed in the opposite direction and had a weir so we were able to attach a regular pool vac fitting to scrub the pool walls and vacuum up the algae, pumping it to the planted pool.

    The next pool I plan to have the flow the same as your pool, but I’m wondering if you’ve come up with any clever solutions to algae? Or, do you just let it be…? My client above was a Cape Town, city-type :))

    Cheers, Paul

  19. Tim says:

    Hi Paul – I dont think I have a problem – but many may. Put it this way there is always a thin layer of something – probaly algae so its slippery on the feet but water totally clear. Very seldom depending on weather – so say 3 times a year there can be a string algae outbreak in swimming area. This is quicklly solved by either use (swimming) – or a quick scrub. But I have never had the water not be clear.

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