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    “With a minimum of materials and without an arsenal of chemicals, you can build an idyllic water oasis right in your own back yard and thwart summertime’s sultry dog days.” – Mother Earth News

    What are natural swimming pools?

    They’re constructed pools / ponds designed for swimming, but with room for nature. They typically have a swimming area and a ‘regeneration zone’, where the water is cleansed biologically (by plants and micro-organisms) and with physical filters, but without chlorine, ozone, copper compounds, UV radiation or ultrasonic devices, which are undesirable because they reduce the biological activity that a natural pool is intended to encourage. Microbes on plant roots and stones remove nutrients and contaminants, and free-swimming micro-organisms eat algae.

    Regeneration area with water and bog plants and plants with floating leaves, with a partition wall to separate it from the swimming area, but to allow the water to flow between the two areas.


    The modern natural swimming pool ‘movement’ is considered to have started with Gottfried Kern in Austria in 1954. He partitioned free-standing water from a ‘regeneration zone’, where plants and animals could live undisturbed, but could also venture out into the swimming area. The 60s saw a wave of chemically-treated swimming pools spread across Europe from the US, but in tandem with the wildlife gardening movement in the 70s, more and more people in central Europe were looking at installing natural pools for bathing. Installation companies were formed in the late 80s, and a new sector developed in the 90s.

    Two very different methods of digging the hole for your pool.

    Legal and technical barriers had to be overcome to enable the step from private to public natural pools. The first public natural swimming pool, created by Biotop, was opened in Klosterneuburg, Austria, in 1991. Natural swimming pools arrived in the UK in 2001 – the first was at a private residence in Gloucestershire by Michael Littlewood. In 2015, the UK’s first public natural swimming pool was opened at King’s Cross in London.


    Pools can have different levels of wildlife-friendliness. Some pools, although unchlorinated, have vertical walls, with no escape route for amphibians, and intensively-filtered water that reduces plant growth.

    Bird’s-eye view of a natural pool.

    Wolfram Kircher lists four main types of natural swimming pool, by increasing complexity (and therefore cost):

    1. Swimming pond: just a body of water with shallower edges (the regeneration zone) for water plants (including reeds and wetland plants, submerged plants and plants with floating leaves) that do all the filtering, but without a pump. This is the most cost-effective model, but you’ll need quite a large area (at least 120m²) for it to work effectively.

    2. Swimming pond with surface flow: a skimmer or overflow gutter allows water to flow from the surface to a pump, which oxygenates and delivers the water back through the regeneration area to the swimming area. This provides a constant nutrient flow for the plants. At least 50% of the surface area should be planted, and the overall area should be at least 100m².

    3. Pool with ‘technical wetlands’ that operate like a reed bed: a pump circulates water to a vertical- or horizontal-flow gravel bed that traps nutrients and makes them available to plant root systems (usually reeds), which keep the gravel beds permeable. Water passes through the beds back to the swimming area. You could even pump water to the top of a series of gravel-filled terraces, creating a rock garden to filter the water.

    4. Pool with biofilm-accumulating substrate filter (BSF): this is a gravel bed whose coarser stones are kept aerated, and through which circulating water can percolate quickly, so that they become covered in a ‘biofilm’ of micro-organisms that remove nutrients and pathogens from the water filtering through it. No reeds or other plants are required (and in fact would find it difficult, because nutrients are taken by the biofilm). Water filtered through the biofilm filter bed is pumped back to the swimming area.

    Types 3 and 4 can be much smaller than types 1 and 2. You can also have pools that combine features from two or more of these models.

    DIY swimming pond that was dug by hand and is kept clean by a combination of water plants and an ingenious home-made biofiltration system; cost – around £2000 in total.

    What are the benefits of natural swimming pools?

    For people:

    To guarantee pathogen-free water, traditional swimming pools are usually treated with chlorine, chlorine dioxide, mineral salts, organic biocides or ozone. Chlorine increases the risk of asthma and allergies, especially for kids. It is also a potential carcinogen. Regular exposure to chlorine may cause reproductive disorders and birth defects as well as skin ageing. A natural swimming pool does not require added chemicals. Water purification is achieved via physical and biological processes.

    Nothing better than plunging into a freshwater pool on a hot summer day – for humans and non-humans.

    Of course, very few natural pools will be large enough for lane swimming, but they’re fine for swimming and splashing about in the fresh air, and they’re great as plunge-pools after a sauna.

    For nature:

    A natural swimming pool will provide habitat for wildlife. Not everyone will be happy to share their swimming space with frogs or water beetles, but they’ll try to get out of your way, and if you’re a nature lover, you’ll take to it easily. The environment will benefit from the fact that chlorine isn’t used, and it will save water too, as, unlike conventional swimming pools, total water changes are not required.

    Unrolling the pool EPDM liner.

    What can I do?

    Type 1 and 2 pools can easily be built DIY – you don’t need much more knowledge than for a normal garden pond. You could also have a go at type 3, but you’ll probably need a professional for type 4. You’ll need to consult a good book for more detailed information on construction, stocking with plants and maintenance.

    This is a conversion from a conventional pool – chlorine out, plants in.


    Here are the (basic) construction stages:

    Planning: choose the type, location, size, depth and partitioning of the pool. Think about filtering equipment and adjacent decks, paths, planting areas etc.

    Excavation: you can hire a mechanical digger, or you could dig it by hand, using picks and shovels. It’s a big job – definitely a case of green gym. The swimming area needs to be at least 2m deep.

    Underlay and sealing: EPDM (synthetic rubber) is considered more environmentally-friendly than PVC liner, and definitely more so than concrete – plus it’s UV resistant. It will need an underlay – old carpets can come in handy.

    Build superstructures: add things above the liner, like stone steps, foundations for wooden decking or bridges, boulders or flag stones on the bottom of the pool, and around the edge of the liner; you can also start filling the main swimming area with water to tension the liner.

    Build regeneration zones: 50-150cm deep, and separated from the main swimming area with an underwater barrier wall that allows water to flow over the top of it into the swimming area; introduce plants (beginning in the deepest zone), then fill up with water in stages.

    Construct the capillary barrier: the edge of the liner must be above the level of the surrounding soil, and not in contact with it. This is to stop the drier soil sucking water from the pool, and also to prevent influx of surface water after heavy rainfall, which might wash in nutrients and pollutants.

    You won’t need planning permission for a natural swimming pool unless you’re in an area of outstanding natural beauty (AONB) or a conservation area, although even then you might not. Best to contact your local authority if you’re in one of those areas. Note that existing conventional pools can be converted to natural pools – plants replace chlorine when it comes to keeping the water pathogen-free.

    DIY pool with a solar-powered filtration system, costing a fraction of the price of a conventional swimming pool, and providing a home for newts and other wildlife. This booklet explains how it was built.

    Welcoming wildlife

    Stocking with animals is not recommended. Fish will raise the phosphorus levels and increase the likelihood of algae. Wildlife, however, will come on its own. A few days after filling the pool, you might get an algal bloom that could make the water murky; but soon after that, the first microscopic animals such as water fleas and daphnia will appear to eat the algae and the pool will clear again. Their predators – water-insects and arthropods – will inevitably follow: pond skaters walk over the water surface thanks to their hydrophobic feet; great diving beetles fly in, as do whirligig beetles with their fantastic dances on the surface of the water. Later, dragonflies will lay their eggs, and their larvae will go hunting with their extending mandibles. Later still, frogs, toads and newts will find their way overland to lay their eggs. All very exciting if you enjoy observing nature.

    After you’ve built your pond, you may start to get a build-up of algae, but soon daphnia (above) will arrive to eat it; and after that, other pond life will turn up to eat the daphnia, and an ecosystem will develop.

    Plants are important for filtering in pools of type 1 and 2. This includes a densely planted area with submerged plants and floating-leaf plants. Plants withdraw nutrients, raise oxygen levels and provide a habitat for beneficial micro-organisms that kill pathogens. Plants for technical wetland areas (type 3 pools) are mostly helophytes such as common reed (Phragmites australis), cattails (Typha spp.), irises (e.g. Iris pseudacorus) or sedges (e.g. Carex elata). In this case the roots and rhizomes keep the filter aggregate permeable for water percolation and promote the development of beneficial micro-organisms. In pools with fast percolation (biofilm accumulating substrate filters, type 4) plants are only for decoration. Consult specialist books for the range of native plants that are available.

    Constructing a gravel bed for a ‘biofilm’ filter (type 4 pool, see above).


    Maintenance is necessary for all types of pool. The accumulated mud has to be removed with a suction pond cleaner at least once per year. They’re quite cheap, and can transfer the pond mud to the garden, where it will be a good fertiliser.

    Plants must be pruned back each autumn to remove nutrients. Some weeding might have to be done too, as well as possibly removing accumulations of filamentous algae.

    Skimmers and/or pumps, if used, will include mechanical filters that capture leaves, dirt, pollen etc. These need to be cleaned out – keep checking them and you’ll work out how regularly they need to be cleaned (which will change depending on the season).

    Yellow flag irises can add beauty to natural swimming pools
    Yellow flag irises love wet conditions and will add beauty to your pool, as well as helping to keep the water clean.

    Occasionally, the biofilm on the gravel in type 4 pools will reach its final phase of ageing. Particles die off and are flushed into the open pool area, making the water murky. To prevent this, backwashing is required. The pump is switched off for a week to kill all biofilm microbes. Then, the direction of water flow in the distribution pipes is reversed – instead of pumping water into the filter, it is drawn off and allowed to trickle away or irrigate the garden. This discharged water will initially be brownish and murky or smell mouldy. As soon as it becomes clear and loses any unpleasant smell, the pump can be switched back to its original setting, and a new biofilm can begin to establish itself. Backwashing is recommended at least twice a year, in spring and autumn. An additional backwashing can be beneficial in mid-summer.

    Thanks to Wolfram Kircher, author of How to Build a Natural Swimming Pool for information.

    The specialist(s) below will respond to queries on this topic. Please comment in the box at the bottom of the page.

    Wolfram Kircher is an international expert on natural swimming pools and author of How to Build a Natural Swimming Pool. He teaches landscape architecture and planting design at Anhalt University, Bernburg, Germany. His research into low-maintenance naturalistic plantings on extreme substrates, which made an important contribution to “new German style” planting, puts habitat planting in reach of amateur gardeners. He lectures widely.

    The views expressed here are those of the author and not necessarily lowimpact.org's


    • 1Donna July 22nd, 2019

      hi. Great article. If you are converting an existing concrete/tiled pool, do you place the liner over the concrete/tiles, or have to remove either/both? thanks

    • 2Wolfram Kircher July 26th, 2019

      Hi Donna, best will indeed be to put the liner over the concrete/tiles. I suppose you want to use the concrete basin as swimming zone and add a regeneration zone marginally? in this case you should be able to build the edge of the total NSP at least 20 cm higher than the concrete basin’s edge or you should remove at least the upper 20 cm of the concrete framework (hard job). Welding the sealing in such a form without producing inappropriate folds is not easy. If you are not experienced with welding PVC (or EPDM even more difficult) you should engage a professional for this.

      an other idea is to pump the water from the pool to a separate regeneration basin apart from it.

      good luck!

    • 3Karen August 30th, 2019

      HI Wolfram,

      Fantastic video! I thought I had seen them all but yours is the best and simplest. If I follow your method and also increase the plant activity, I still have the problem of a 5k liner for the size of your pool. There are EDPM liners on Ebay for $500. What exactly are the requirements (both are UV and same thickness) that explains the difference in price? I would love to be able to email you directly as you are the FIRST person to do these cost effectively and beautifully (hoping the pool is still in good condition!!)


    • 4Dave Darby September 6th, 2019

      Karen – Wolfram is away until sep 18. I’ll ask him to respond when he’s back. Cheers.

    • 5Wolfram Kircher October 7th, 2019

      Hi Karen

      Thanks for your commendation, but none of the ponds in the two videos on Dave’s website is mine – sorry… They use external filter systems, which is ok of course, but I prefer filter zones which are a part of the natural swimming pool (NSP) itself (as explained in my book). I also do not know what you mean with “5k liner”. Is this a PVC material? EPDM is a synthetic caoutchouc rubber material. It is much more elastic and more environmental friendly than PVC if once the NSP must be dismantled. On the other hand it is not too difficult to weld PVC yourself whilst connecting EPDM sheets should better be done from experts.

      Hope this helped a bit?

      good luck – Wolfram

    • 6Fergus December 3rd, 2019

      Hi Wolfram I am building a type 3 swimming pond and have had conflicting advice on the grade of gravels in the top down regeneration zone. I’d want to plant straight into the gravels and would welcome any advice you can give on sizes of gravel or different grades that would be best. Can you help?


    • 7Wolfram Kircher December 5th, 2019

      Hi Donna, if the tiles are still well embedded in the wall you can leave them and place first a protection membrane and then the liner directly above them. If you want to avoid huge folds I recommend to engage a special company to weld the liner accurately into the basin – or you try to do this yourself. DIY of course means: no warranty claims possible…

      good luck!

    • 8Zach Grant May 31st, 2020

      Hi Wolfram,

      Thank you for this article! I’m just beginning research into building a swimming pond within a 120 square meter area. I was hoping you could help with 3 basic “beginner” questions.

      1) Is heating the pond (at some minimal level) viable at the beginning and end of the season?

      2) Should we be concerned about too much heat in the summer due to evaporation or the effect on the regeneration zone? We live in a river valley in otherwise desert New Mexico (1500m altitude, avg July temperature = 34 C).

      And 3) Is it crazy to consider, shooting off from the main 2m deep swimming area, a lap swimming channel (1.2m depth and width) that would surround one of the shallow regeneration zones in the shape of a ring? Would these channels make it harder to circulate and keep the water clean/balanced etc? Or would the main concerns be prohibitive cost of the construction needed?

      Any advice would be much appreciated, thanks much!



    • 9Wolfram Kircher June 2nd, 2020

      Hi Zach

      1. heating – do you want to know if this may affect the plants? heating in autumn could indeed prevent an adequate preparation to the winter dormancy. If you have a frosty winter it could reduce the hardiness of some species.

      2. an aride climate is indeed stressy for the plants and the complete ecosystem. You should be very strict in reducing any phosphorus influx to achieve P-limitation against algae growth: hard water (carbonate enriched), pH around 8,3 and optimal oxygen saturation. regularly vacuuming mud is also recommended.

      3. I never saw such a construction. I think it can work, but you should not additionally include a pump to have a counter current system in the channel. This could cause an algae emergence at the entraces of the channel because of inconsistent water movement there.

      hope I could help a bit with this?

      good luck


    • 10Penny June 12th, 2020

      Hi Wolfram, I bought your book, which is great thank you. I have a couple of questions relating to our specifics here.

      We run an organic farm in Somerset – on a marsh, a semi wetland which has been drained by farmers for hundreds of years to use for grazing. The earth is heavy clay. The water table in the winter is 40cm below the surface. We have designed a swimming pond which is 25m x 25m, considering Type 1 or 2. The local groundworkers think that we should be able to dig a pond and literally paddle the clay to create a swimming pond. We would then plant the Regeneration zones appropriately and use a lot of gravel and a pump to ensure clarity of water.

      However, I’m reading p 130/131 of your book which strictly advises against allowing surface water (or land water) into the pond because of nutrients and pollutants. So if we don’t have a liner, we will be allowing surface and land water into the pond. But we are an organic farm, so we have no pollutants, and there is no organic matter in the clay. So will the influx of surface and land water matter to us?

      Grateful for your advice. If you are ever passing Somerset, please visit us, and we will give you some of our multi award winning cheese!

      Thank you and kindest regards Penny

    • 11Wolfram Kircher June 18th, 2020

      Hi Penny, what you want to build is not a swimming pond according to our definition, because it is not completely sealed against the earth. The problem is not the influx of pollutants but of phosphorus compounds, which would support algae growth. espacially in the shallow areas you may get a very green water and lots of filamentous algae. Maybe it helps, to build the edges as steep as possible to avoid shallow zones. eventually you could sopport this with underwater wall constructions (set without mortar) around the edge. – good luck! Wolfram

      PS: I discussed this also with my co-author Andreas Thon, he meant that your clay soil may effect phosphorus fixing if it has a certain iron-content – hope he is right with his prognosis…

    • 12Edward hannon June 26th, 2020


      I want to build a swimming pond in my garden in Leeds – Northern England !!

      The soil is heavy clay with a high water table and i have 2 large trees next to the area i want the pond !

      The area is south facing and gets decent amount of sun


      1. Will the over head trees be an issue ?

      2. Is high water table a problem ? – can it lift the pool liner ? is there a way round it ?



    • 13mfsaggau June 30th, 2020

      Hello Wolfram – I just bought your book, How to Build a Natural Swimming Pool, after reading reviews that said it thoroughly covered the science. Do you think an NSP will work in Michigan (northern U.S., hot humid summers/cold winters). I ask because I know of only two NSP attempts in northern midwest, but both ended up relying on UV sterilizers/clarifiers (one is a public pool in the City of Minneapolis, the other is the Tryon Farms community in Indiana.)

      Thank you so much for your willingness to take the time to answer question from us. Much appreciated.

      Michael Saggau

    • 14Wolfram Kircher July 20th, 2020

      dear Edward

      yes, trees may cause problems with inserting phosphorus through their leaves which fall into the water in autumn. Conifers are less critical. the softer and larger the leaves are, the more nutrients they usually release into the water. You can span a net over the basin in autumn. Most leaves are blown into the water through the wind, so that even a fence around the pool prevents many leaves from reaching the water.

      If a high ground water level effects water seeping into the excavation it must be pumped out more or less permanently during the construction works. Best is to insert a massive waterproof foundation and walling for the pool and not only a pond liner. a pool sealed with a plastic liner may be pushed in from the bottom if the ground water level rises

      good luck!


    • 15Wolfram Kircher July 20th, 2020

      dear Michael

      I never was in Michigan, but quite sure a NSP will work there too. I don’t know how the two NSPs which you mention were treated and how they were built. And of course it is up to your expectations – if you acept a little murkyness you may be happy with a model 1 or 2 NSP (see case studies in my book), if it shall be very clear I recommend model 4 with a quicly percolated filter zone. To be on the safe side I recommend to get in touch with a professional company. “Balena” is very good (www.balena-gmbh.de), I am not sure if they would work also in the USA, but maybe they would calculate the filter zone for you and you can buid it DIY according to their specifications. In USA you could try to be supported by “Total Habitat”, but as far as I know they work with the Technical Wetland principle only (model 3).

      I wish you good success


    • 16Polly Stephenson August 17th, 2020

      Hi Wolfram,

      Thank you for producing such a simple and informative web site. I have been planning a natural swimming pond (not pool) and I think I have finally settled on the design. It is 80m2 but at the moment is only 1.5m deep.

      Is this too shallow? What will the consequences be?

      Also I’m intending on manufacturing a sand and gravel filtration system (as well as the plant life) similar to the one Jonas has created. it will have a head height of 2.5m with a waterfall return into the pond.

      Would you recommend a submerged or surface pump?

      Are there pond pumps capable of supplying enough water?

      I have read about the min/max flow rate for pools, saying there should be 2 turnovers per day.

      What is acceptable for a natural swimming pond?

      Many thanks,


    • 17Leela August 18th, 2020

      Thank you for this very useful article and all the comments. I have a question of whether to use gravel or pebbles as my plant bed. I can’t seem to find the best answer, aeration and porosity being of importance. Can you please advise. Many thanks

    • 18Wolfram Kircher September 7th, 2020

      Dear Leela, usually fine gravel of 2-8 mm is a very suitable subbstrate for most water plants, including water lilies. In a natural swimming pool I recommend to cover this subbstrate with a 5 cm layer of nutrient poor clay. If there is any need for nutrients (pale leaves, too weak growth) you can insert fertilizer pellets into the gravel by pressing them through the clay layer. Then close the clay skin again and thus you encapsulate the nutrients in the substrate: the roots can absorb them but they can not escape into the free water, where they could push algae emergence.

      best wishes


    • 19Wolfram Kircher September 7th, 2020

      dear Polly

      1,5 m is ok if the filtration works well. turbulences caused by swimming may whirl up mud and cause murky water temporarily. For this reason some experts recommend 2 meters depth of the swimming zone.

      a water fall into the NSP can effect algae emergence in the area where you have not a fast running water but also not a standstill water. To avoid such inconsistant water zones waterfalls should be build in an extra basin adajacent to the NSP as exemplarily shown in case study 18 in my book “How to Build a Natural Swimming Pool”.

      2 turnovers a day can be ok, but I would not use this attribute to calculate an NSP. It is up to the type or model which you choose: Filtering only by plants (Hydrobotanical system) in model 1 means that there is no pump – water movement is only caused by thermal convection.

      A sand filter can be used as a Technical Wetland planted with reed vegetation framing the swimming zone. A quickly percolated filter can allow a biofilm to grow on the grain surfaces. For this you need coarser substrate (> 2mm grain size) and > 500 liters influx per m² filter surface per hour. Such a filter should cover 20 % of the whole NSP surface if it is the only area of water treatment. A Technical Wetland should cover > 30 % plus a Hydrobotanical System. – much to explain about these things – you find it in detail in my book.

      Even an underwater pump must be placed in an extra chamber apart from the NSP for safety reasons. only if it is a 12 or 24 volt pump you can put it directly into the swimming basin.

      good luck


    • 20cornishwoodsman October 5th, 2020

      Hi Wolfram,

      I think my pool is a type 2- built from the ideas by the dvd by David Pagan-Butler.

      I have an algal problem which I think is a result of the hessian bags used to build the retaining wall degrading and over-nutrifiing the water.

      We are off-grid and struggle to keep the circulation system running all the time, which obviously doesn’t help.

      Would appreciate a chat about how we might resolve this issue.



    • 21Dave Darby October 17th, 2020

      cornishwoodsman – email me your details at dave at lowimpact dot org and I’ll pass them on to Wolfram.

    • 22brian Sowell November 4th, 2020

      I am building a type 4 swimming pool/pond. The pool will be a 30 foot diameter 4′ deep CMU block with a 45 mil EPDM liner and on one side the pool will return to the pond via an 8 foot wide x 4″ tall waterfall into the pond. The pond sides will be approx. 4″ deep and the middle approx. 24″ deep and is the same size as the pool, but totally different shape. The pond drain will have several 4″ black perforated sock pipes in various shapes to filter the entire pond bottom under 8″ of gravel. All will circulate through a pump and return to the pool via return jets and a water wheel attached to the pumphouse. I will aerate the pond with a fountain I installed in the middle of the pond and a waterwheel through a small creek bed at the pumphouse.

      There will be a bridge across the narrow section of the pond that will go to a covered outdoor kitchen. I was quoted $50,000.00 to construct the pool and pond only, but I decided to do it myself and it cost a total of $5,500.00 not including the patios and outdoor kitchen. Of course I own my own backhoe, so digging the holes cost nothing but fuel. All the work I did by myself and it took 4 weeks to complete the project not including the pumphouse, patios or outdoor kitchen. All this was done weekdays after work and on weekends. These are pretty simple to do if you have some knowledge of construction and how things mechanically work and flow.

      My question is what size gravel should I use for the pond bottom filter? I am looking at a mid size stone between pea gravel and 57 stone sizes. Is this size good for this pool/pond?

      I would post pictures but I don’t see a way to post them.

    • 23Wolfram Kircher November 8th, 2020

      dear Brian, congratulations for your success so far! for a biofilm accumulating filter bed (> 0,5 m³ per m² per hour influx) use a gravel 2-4 or 2-8 mm. More details you may find in our book “How to Build a Natural Swimming Pool”. You write that you want to build a waterfall into the pond. Please be aware, that in a certain distance to where the water flushes into the pond you will have inconsistant limnological conditions (neither floating water nor standstill water), which may promote filamentous algae to proliferate. to smother these algae you can plant water lilies or helophytes in that area, or you construct the waterfall as a “pondless water feature” in an extra water body – visually lokking like a part of the total system but with a barrier between waterfall feature and pond. Good luck, Wolfram

    • 24brian Sowell November 17th, 2020

      Thank you so much Wolfram. I am using a gravel that is Granite approx. 1/4 – 3/8″. You mentioned the waterfall going into the pond. The waterfall is actually going into a separate small pond area that will spill over into the pool then thirty foot away the pool will spill into the pond over a 4″ tall x 8′ wide waterfall. There will be approx. 8″ of gravel on the pond liner with 4″ of water on that around the outer sides of the pool with 18″ of water in the middle. Do I need any soil for planting the plants or do I just plant them into the small gravel size I am using? I am using the waterfall to help with oxygenation and there will also be a fountain in the middle of the pond to help.

      I would like to post some pictures of the work I am doing, but not sure how to send pictures to you.

    • 25Wolfram Kircher January 14th, 2021

      dear Brian – generally I recommend gravel 2-8 mm as a substrate. It can be covered with a 5 cm layer of clay (i.e. Nymulat pellets which expand in water and result in such a layer). If the plants show nutrient deficiency symptoms you can insert fertilizer pellets through the clay layer. The clay encapsulates the nutrients within the gravel layer, so that the water stays oligotrophyc whilst the roots can benefit from the fertilizer.

      Using grantite gravel is ok if you aim a carbon limited system (pH below 7, soft water). This is proper in areas with oceanic or alpine climate with high rainfall amounts. In most cases it is more reliable to aim at a Phosphorus limitation (high pH > 8, hard water), which is supported by limestone gravel. To comment the gravel sizes and distances – I feel a bit overchallenged – sorry, but I am German and completely confused with feet, inches, ‘ and ”… – can you transform this for me into mm or cm? please send your photos to [email protected], but have mercy with me if I do not answer immediately

    • 26Blake March 29th, 2021

      Wolfram, reading the article above it mentions having to suction out accumulated mud. Why would I have mud in a closed system with gravel beds? I am looking to build a swimming pond on a hillside with the “pool” at the top and filtration fields below.

    • 27Wolfram Kircher April 7th, 2021

      dear Blake, you will always have mud in a water body even without plants growing inside. Mud comes not only from falling in pollen, leaves, insects and diverse particles, but also from onmnipresent microorganisms (plankton). Periodically some of these microbs die off and sink to the ground. Some of the microbs are floating green algee. Together with filamentous green algee they photosynthesize CO2 and/or HCO3- ions. In effect the pH increases and this leads to a sedimantation of limestone mud (CaCO3), which can’t staydissolved under such conditions.

    • 28Sue January 19th, 2022

      Dear Wolfram,

      we have got a large, I assume natural) pond (around 100 m2) sitting on clay soil. It is only fed by rainwater and has an overflow into a smaller pond. Last summer was wet enough for it not to dry out, but with changing climate we can’t rely on it. And the neighbours are saying that two summers in a row it had recently dried out. I would love to turn it into a natural swimming pool, but the price has scared me off. Now I read your blog!

      Is it possible to turn this rainwater pond into a natural swimming pool?

    • 29Wolfram Kircher January 29th, 2022

      dear Sue, I understand that do do not fill any water into your pond except what naturally comes from above? It surely can be converted into a NSP. But for more advice I needed more details: how deep is the pond, what about its profile – in steps or continuously sloping? how thick is the clay layer? would you like to insert a plastic sheet as an additional sealing? where is the pond?

      to keep it free from noxious algea you presumably could try to achieve a limitation of carbon by furthermore using rainwater. If necessary you could build a cistern to collect rainwater from your roof.

    • 30Carmen Greenlees January 29th, 2022

      Dear Wolfram,

      Thank you so much for your book which had been brilliant. I am just about to embark on building a natural swimming pond in Wiltshire UK. We are on chalk, and it will be 15m by 9m with a swim zone of 10m by 4. I am looking to build a type 1 pond mainly, using natural filtration with the addition of circulation pump, to help draw water through gravel bed.

      I have also watched David pagan butlers video and am thinking of using his solar powered pump design. My questions are :

      1. David used gravel, covered by geo textile, and then subsoil as a plant substrate. We are on chalk and therefore unlikey to have much subsoil or clay. Can we plant directly into gravel without any soil or clay at all, or should we have a layer of subsoil on top of gravel?

      2. I was hoping we could use the simple air pumps, as per David butler, with pvc piping drawing water through the gravel bed as our main filter, probably 2. Do you think this is adequate?

      Many thanks for your help,

      Carmen Greenlees.

    • 31Wolfram Kircher January 30th, 2022

      Dear Carmen, I suppose gravel with 2-8 mm grain size is a proper substrate, presumably better than subsoil. You can use limestone material if it is not too soft, otherwise it could eventually release traces of phosphorus contamination.

      I am not familiar with David pagan butlers pump technology, sorry. For a proper working type 1 NSP a pump is not necessary, if 65 percent of the total area is densely planted with submerse and floating leave plants, but this seems to be the case with yours.

      An additional pump is possible of course, so why not try it with the David pagan butlers system?

      Good luck!

    • 32Ian Guyver July 14th, 2022

      Hello Wolfram, thankyou for your comprehensive book ‘How to build a natural swimming pool’ and for the kind generousity of your time. Two small questions: I would like to know which submerged tall grass helophyte species is shown in the picture on page 46 of the book (also page 8). It is the picture taken from Erlebnisgarten Schleitzer in Munich. It looks like it has strong upright growth like Phragmities which is desirable from an aesthetic design point of view. I am a garden designer and we are working on a project in Devon UK and I also wondered if you recommended any companies for NSP design and install. Kind regards, Ian

    • 33Wolfram Kircher July 22nd, 2022

      dear Ian, on p 46 and 8 you see a NSP from Schleitzer Gardencenter in Munich. the marginal zone is planted mainly with Typha latifolia. For small and medium sized ponds I recommend the smaller species Typha shuttleworthii, unfortunatelly it is often not correctly labelled and you receive instead of the desired species Typha l (Bristol)axmannii, which is also smaller than T. latifolia, but grows much denser. If you want to create a vertical structured reed zone with only 1 species, T. laxmannii is great, for combinations with ground cover plants etc. (Caltha, Myosotis, Lysimachia nummularia…) or even taller species (Lythrum, Iris…) T. shuttleworthii is better.

      Maybe Clearwater Revival can help with your NSP planning? http://www.clear-water-revival.com,

      or Will Woodhouse (Cambridge) https://naturalswimmingpools.com

      I wish you great success!


    • 34Hrvoje Bota January 10th, 2023

      Hi i am building a natural pool cca 80-100 m2.

      What type of gravel i can use and in what lawers?

      First lawer 20-30 cm gravel 2-4 cm, and then on top for planting 10 cm lawer 2-8 mm of sand or?

      Thank you

    • 35Wolfram Kircher January 18th, 2023

      Dear Hrvoje Bota

      If you can get 2-4 mm (not cm!) Gravel – perfect for a quickly percolated filter body! 2-8 is also ok

      You can use the same Material for the planting, However: if you have a small filter area only (< 20% of the pond surface) it is better not to plant anything to allow an undisturbed percolation.

      Good luck!

    • 36rosie June 4th, 2023

      Hello, last year we built a natural swimming pool, mostly following David Pagan Butler’s guides. It filled with rain water over the winter, however it has been so dry this year that it is getting lower and lower and we are beginning to lose plants. Is it possible to top it up with tap water if we get some kind of de-chlorination? Any advice would be greatly appreciated. Thanks

    • 37Justin July 18th, 2023

      Hi, thank you for sharing your knowledge. I’m in the middle of a type 4 top/down filtration pool construction and trying to decide the dimensions of my aggregate filter surface area. My pool is roughly inside diameter of 18x22 ft, with max dept of 6 ft. My filter currently is 18x1.6 ft and 3.6 ft deep( ~30 ft ^2). I’m worried I cannot get a high enough percolation rate with my 3in pvc perf pipe and 1.5 hp variable speed pump. By decreasing the surface area by half and increasing the depth of the filter to 6 ft, the needed flow rates of 500L/m^2/hr should be more easily achieved? Or is my current design capable of pulling 500 L/m^2/hr? Thank you

    • 38Wolfram August 17th, 2023

      Justin, decreasing the surface and compensating that by increasing the depth is a suitable option - however, it may be more difficult to backwash the system completely. maybe you could insert two suction hoses: one at the bottom of the bed and a second one 3 feet below surface (if you have 6 feet depth in total). But the better variant will be to use a greater overall surface of the filter and a sufficiently powerful pump.

      hope this helps? good luck!

    • 39Penny August 28th, 2023

      Any suggestions for keeping the snapping turtles and water snakes out of the pool?

    • 40Wolfram August 28th, 2023

      Penny, seems you live in tropical climate- for such year-round warm areas I recommend to build natural swimming pools as two chamber systems: swimming area constructed like a normal pool, rectangular and possibly covered if not in use. The regeneration area can be several meters away to keep undesirable animals there and not in the swimming area

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