{"id":5588,"date":"2026-08-14T22:40:05","date_gmt":"2026-08-14T21:40:05","guid":{"rendered":"https:\/\/flusso.hr\/bio-bazeni\/"},"modified":"2026-08-14T22:48:52","modified_gmt":"2026-08-14T21:48:52","slug":"bio-bazeni","status":"publish","type":"post","link":"https:\/\/flusso.hr\/en\/bio-bazeni\/","title":{"rendered":"Bio pools: how they work, how much they cost and do they work in Dalmatia"},"content":{"rendered":"

A bio pool (natural pool) is a chlorine-free pool in which the water is purified by a planted regeneration zone. Croatian regulation for public biological pools<\/strong> (NN 59\/2020) requires that this zone be at least 50 % surface<\/strong>, depth at least 0.8 meters<\/strong> and water temperature up to 26 \u00b0C<\/strong>; does not apply to private pools (Art. 2), but is the only domestic reference framework. This means approximately twice the water surface and different maintenance \u2014 not less maintenance.<\/p>\n

Immediately, what you should know about the author of the text: Flusso builds only concrete pools and pool equipment. We do not build natural pools.<\/strong> and this is not an offer for their construction. The text exists because people regularly ask us "is it possible without chlorine", and Croatian content on this topic is mostly romantic and almost never numerical. Below are the facts \u2014 Croatian regulations, German guidelines and three published studies on water quality \u2014 on the basis of which you can decide for yourself. If you are just choosing the type of pool, start with a complete guide to building a swimming pool in Croatia<\/a>.<\/p>\n

What was a pool and how does it work?<\/h2>\n

The bio pool has two zones: swimming area<\/strong> and regeneration zone<\/strong> planted with aquatic and marsh plants. Water circulates between them, purified by microorganisms on the roots and substrate \u2014 the same process that works in a healthy lake, compressed into a yard-sized container. There is no chlorine because it would kill the very organisms that clean the water.<\/p>\n

Croatian legislation explicitly defines this type of pool. The Ordinance on Sanitary, Technical and Hygienic Conditions of Swimming Pools (Official Gazette 59\/2020<\/a>), Art. 3, states: "a biological pool is an outdoor pool in which the preparation of pool water is carried out by natural biological processes in an ecosystem of microorganisms, plants and indigenous small animals"<\/em>.<\/p>\n

The key word in that definition is ecosystem<\/em>. A classic pool is a tank where you control the chemistry and flow \u2014 if something goes wrong, you add chlorine and the problem is solved in a few hours. A bio pool is a living system with biological inertia: it reacts for days and weeks, and your regulators are nutrients, temperature and light. The technology in it (pump, skimmer, bottom filter through the substrate) does not disinfect the water but helps the biology work.<\/p>\n

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Key:<\/strong> According to Article 2 of the same Ordinance, the regulation is does not apply to swimming pools owned by citizens that are not intended for public use<\/strong>. So you don't have these obligations for a private bio-pool \u2014 but NN 59\/2020 is the only Croatian document that even sets figures for a biological pool, so it's the best reference framework you have.<\/p>\n<\/div>\n

How much space does a bio pool require?<\/h2>\n

Much more than people expect. The regeneration zone is not a decoration on the edge \u2014 it is filtration and takes up from approximately 30 % of surface area in the most technically equipped systems to 50\u201360 % for classic designs without pump<\/strong>The German FLL guidelines divide natural pools into five types based on this proportion.<\/p>\n\n\n\n\n\n\n\n\n\n
Type (FLL)<\/th>\nRegeneration zone<\/th>\nMin. size<\/th>\nTechnique<\/th>\n<\/tr>\n<\/thead>\n
Type 1<\/td>\n\u2265 60 %<\/td>\n~120 m\u00b2<\/td>\nwithout pump<\/td>\n<\/tr>\n
Type 2<\/td>\n\u2265 50 %<\/td>\n~100 m\u00b2<\/td>\npump + skimmer<\/td>\n<\/tr>\n
Type 3<\/td>\n\u2265 40 %<\/td>\n~80 m\u00b2<\/td>\n+ floor filter through substrate<\/td>\n<\/tr>\n
Type 4<\/td>\n\u2265 40 %<\/td>\n~60 m\u00b2<\/td>\nstrong pump, full volume daily<\/td>\n<\/tr>\n
Type 5<\/td>\n\u2265 30 %<\/td>\n~50 m\u00b2<\/td>\ntwo-chamber system, pressure filter<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Source: FLL classification of types of natural pools<\/a>. For Type 1, sources differ (50 % or 60 %), so take it as a range. Croatian regulation is stricter than most types: Art. 17. NN 59\/2020 requires that the biological treatment part be at least 50 %<\/strong> surfaces, "completely planted with selected vegetation"<\/em> and inaccessible to swimmers.<\/p>\n

\"Share<\/figure>\n

Let's translate that into a number that decides whether the pool will fit in the yard. Let's take as a benchmark the most requested dimension of a concrete pool in our country, 8 \u00d7 4 m \u2014 so 32 m\u00b2 of swimming area<\/strong>With a regeneration zone of 40 %, a total of about 53 m\u00b2 of water surface is needed, with 50 % about 64 m\u00b2<\/strong>, and with 60 % full 80 m\u00b2<\/strong>If you adhere to the Croatian criteria of at least 50 %, count on 64\u201380 m\u00b2<\/strong>.<\/p>\n

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This is the real obstacle:<\/strong> In a typical urban yard in Dalmatia, there is simply no water surface of 64\u201380 m\u00b2. Measure the plot before discussing the price. Reducing the regeneration zone \u201cbecause there is no room\u201d does not result in a smaller bio-pool, but a pool that fails to purify itself.<\/p>\n<\/div>\n

What should the water in a biological pool be like?<\/h2>\n

Different than in a classic pool and measured by other quantities: instead of chlorine and redox potential, the key phosphorus, transparency and temperature<\/strong>. NN 59\/2020, Annex I, Table 2 prescribes these limit values for public biological pools:<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Parameter<\/th>\nLimit value<\/th>\n<\/tr>\n<\/thead>\n
Water temperature<\/td>\nmax 26 \u00b0C<\/strong><\/td>\n<\/tr>\n
Total phosphorus<\/td>\nmax 20 \u00b5g\/L<\/strong><\/td>\n<\/tr>\n
Transparency<\/td>\n\u2265 2m<\/td>\n<\/tr>\n
Escherichia coli<\/td>\nmax 100 cfu\/100 ml<\/td>\n<\/tr>\n
Enterococci<\/td>\nmaximum 50<\/td>\n<\/tr>\n
Pseudomonas aeruginosa<\/td>\nmaximum 10<\/td>\n<\/tr>\n
Oxygen saturation<\/td>\n\u2265 80 %<\/td>\n<\/tr>\n
Minimum depth (Art. 21)<\/td>\n0.8 m; waterfowl and fish prohibited<\/td>\n<\/tr>\n
Sampling (Art. 26)<\/td>\nat least once a week<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
\"Comparison<\/figure>\n

The Austrian guideline IOB \/ \u00d6NORM L 1128 goes even lower: for Swimming pool<\/em> allows up to 35 \u00b5g\/l of phosphorus, and for Nature pool<\/em> \u2014 more technically equipped version \u2014 maximum 10 \u00b5g\/l<\/strong>The same guideline also states the intake: each bather injects into the water about 100 mg of phosphorus per day<\/strong>.<\/p>\n

A comparison of orders of magnitude explains the sensitivity of the system. In a classic pool, free chlorine is measured in milligrams<\/strong> per liter (0.2\u20131.0 mg\/l according to Table 1 of the same Ordinance); in biological phosphorus is measured in micrograms<\/strong>, a thousand times smaller unit. A calculation on a napkin: 100 mg of phosphorus in 100 m\u00b3 of water is exactly 1 \u00b5g\/L, so four bathers a day for a week ingest about 28 \u00b5g\/L \u2014 more than the Croatian limit of 20 \u00b5g\/L, if the system doesn't take anything out.<\/p>\n

Other reference numbers: FLL lists a capacity of 10 m\u00b3 of water per visitor<\/strong>, optimal pH 7.6\u20138.5 and carbonate hardness 5\u201314 \u00b0dH, and IOB warns that concrete can raise pH to 12<\/a>, while nitrification is inhibited above pH 8.8.<\/p>\n

Does a bio pool work in the Dalmatian climate?<\/h2>\n

This is the most important question for this region and one that Croatian texts do not answer. Temperature is the greatest limitation of a natural pool, and Dalmatia is the most demanding. The FLL guideline states that water temperature 30 cm below the surface should not exceed 23 \u00b0C<\/strong>, because higher temperatures favor pathogens; exceeding them is tolerated up to five consecutive days<\/strong>Croatian legislation is more lenient and allows 26 \u00b0C<\/strong>.<\/p>\n

Now check out the DHMZ climatological norms for the station Split-Marjan (1948\u20132024)<\/a>: average air temperature in June 23.4 \u00b0C<\/strong>, in July 26.2 \u00b0C<\/strong> with 353.3 hours of sunshine<\/strong>, in August 25.9 \u00b0C with 328.0 hours. The July average in Split reaches the Croatian upper limit for the biological pool, and the June average already exceeds the FLL threshold of 23 \u00b0C.<\/p>\n

\"Split-Marjan<\/figure>\n
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Fair about the limits of this data:<\/strong> air temperature is not water temperature and we do not equalize it. There are no water temperature measurements in natural pools in Dalmatia in the available sources, so it is above comparison of the climate context with prescribed thresholds<\/em>, not proof that the pool will not work.<\/p>\n<\/div>\n

What follows from this? Not that a natural pool in Dalmatia is impossible \u2014 we have no source for such a claim. It follows that three design levers are mandatory here, not optional: shading<\/strong> (the regeneration zone on the southern exposure without shade is heated all day), depth<\/strong> (larger volume changes temperature more slowly; the prescribed 0.8 m is an absolute minimum, not a goal) and flow<\/strong> (stronger circulation equalizes temperature and prevents stratification of shallow zones.) In addition, a larger regeneration zone provides a greater reserve \u2014 the closer the system is to types 1 and 2, the more stable it is in summer.<\/p>\n

Was the pool really maintenance-free?<\/h2>\n

It is not, and that is the most persistent myth on this topic. A bio pool does not have chemistry, but it does have seasonal works that a classic pool does not have at all<\/strong>: algae removal, sludge suction from the bottom, mowing of aquatic and marsh plants<\/strong> and phosphate removal.<\/p>\n

Mowing is the most important item and the one that most people skip. Plants don't destroy nutrients \u2014 they do bind to their biomass<\/em>If you don't take the cut grass outside, it will die, decompose and return phosphorus to the water. Mowing and removal of biomass are the main routes through which nutrients leave the system.<\/strong>without it, phosphorus accumulates year after year.<\/p>\n

Algae are therefore not bad luck with the weather \u2014 they are nutrient excess indicator<\/strong>Algal blooms mean that phosphorus intake (bathers, leaves, dust, birds) is greater than excretion, and the solution is not a preparation but a reduction in intake and an increase in excretion.<\/p>\n

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Realistic comparison:<\/strong> with a classic pool, maintenance is predictable and can be left to us \u2014 with us \u20ac300\u2013400\/month<\/strong> depending on the size, along with water testing, shell cleaning, vacuuming and engine room servicing. With bio pools, the work is seasonal and gardening-like, difficult to describe as a flat rate because it does not depend on the volume of water but on how much the plant zone has grown.<\/p>\n<\/div>\n

Is the water in the natural pool safe?<\/h2>\n

Published measurements give a mixed picture. Three studies worth knowing before making a decision:<\/p>\n