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Natural Pools in Bali: Chlorine-Free Swimming That Gives Water Back

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On 9 September 2025, parts of Bali received 385 mm of rain in twenty-four hours (ClimaMeter). Six of the island’s eight regencies flooded. Water sat two to three metres deep in Denpasar neighbourhoods, the Simpang Dewa Ruci underpass became a three-metre lake, and at least eighteen people died. More than 6,000 families were displaced (Bali Solve). It was the island’s worst flooding in more than a decade.

Six months later, in the dry season, wells in south Bali were pulling up salt.

That contradiction, too much water in September and not enough in October, is the single most important thing to understand about building anything in Bali right now. And it changes how you should think about the pool you put in the ground.

This article makes an argument: that natural pools in Bali are not simply a prettier, gentler alternative to chlorine. Designed properly, they are a small piece of water infrastructure, and Bali urgently needs thousands of small pieces of water infrastructure.

New to the concept? Start with Natural Pools 101 or our visual guide to how a natural pool works.

The Problem Nobody Puts in the Brochure

A conventional swimming pool in Bali does three things to the island’s water, and all three are bad.

It takes. Filling a 40,000-litre pool draws from the same shallow aquifer your neighbours drink from. Evaporation in the tropics then demands constant top-up, and most pools are drained and refilled every few years.

It poisons what it returns. Backwash water and drained pool water, loaded with chlorine, cyanuric acid, algaecides and metal sequestrants, typically goes into a soak pit, a drain, or a river. In an island where 260 of 400 rivers have run dry (NOW! Bali), the ones still flowing are carrying our chemistry.

It seals the ground. A pool, its deck, its plant room and its surrounding paving replace soil that used to absorb rain with surfaces that shed it. Multiply that by every villa built in Canggu, Pererenan and Ubud in the last decade and you have a meaningful contribution to the flood risk that killed eighteen people last September.

Chlorine: What the Research Actually Says

Let’s be precise, because vague “chemicals are bad” arguments are easy to dismiss.

Chlorine is a genuinely effective disinfectant. That is not in question. The problem is what it becomes when it meets the organic matter that swimmers bring with them: sweat, sunscreen, skin cells, urine.

Disinfection by-products (DBPs). When chlorine reacts with that organic load, it forms trihalomethanes, haloacetic acids and chloramines. A landmark 2010 study in Environmental Health Perspectives identified over 100 distinct DBPs in pool water, many never previously reported anywhere, and found the water mutagenic in laboratory assay (Richardson et al., 2010). A large Spanish case-control study of 1,219 cases and 1,271 controls, published in the American Journal of Epidemiology, found swimming in chlorinated pools associated with an odds ratio of 1.57 for bladder cancer (95% CI: 1.18–2.09), alongside elevated risk from long-term household exposure to trihalomethanes (Villanueva et al., 2007). This is observational epidemiology, not proof of causation, and the absolute risks are small. But it is not nothing, and it is repeatedly replicated.

That “clean pool smell” is not chlorine. It’s chloramines, the by-product of chlorine binding with nitrogen compounds. A strong smell means a dirty pool, not a clean one. Chloramines are respiratory irritants, and a substantial body of research led by Belgian toxicologist Alfred Bernard has linked heavy chlorinated-pool attendance in children and adolescents to increased lung epithelium permeability and higher asthma prevalence (European Respiratory Journal, Environment International).

And chlorine does not kill the thing most likely to make your guests sick. The US Centers for Disease Control analysed 208 treated-water outbreaks between 2015 and 2019. Cryptosporidium caused 49% of outbreaks with a confirmed cause and 84% of the cases. Why? Because at 1 ppm free chlorine, the CDC-recommended minimum, Cryptosporidium oocysts “can survive for >7 days in water” (CDC MMWR, 2021).

Read that again. The chemical you tolerate stinging eyes for is largely ineffective against the leading cause of pool-borne illness.

“But Ours Is a Salt Pool”

This is the most common objection we hear in Bali, and it rests on a misunderstanding that the pool industry has been in no hurry to correct.

A salt water pool is a chlorine pool. A salt chlorine generator passes an electrical current through salted water and splits it, producing chlorine gas which dissolves into hypochlorous acid: the exact same sanitiser you’d get from a bucket of granules. As Mahoning County Public Health puts it in its operator guidance, “salt pools are still sanitized using chlorine.” The generator simply manufactures it on site rather than having it delivered.

Which means salt pools still produce chloramines. They still produce disinfection by-products, and because saline water carries bromide, brominated DBPs, which are of particular toxicological concern, become part of the picture (Journal of Environmental Sciences).

They add three problems of their own:

Salt pool reality
Salinity 3,000–5,000 ppm sodium chloride in the water (Salt water chlorination)
Discharge Every backwash and drain-down puts brackish water into Balinese soil or groundwater
Corrosion Salt attacks natural stone, grout, metal fixtures and rendering, and Bali builds in all four
Upkeep Weekly testing, pH correction with hydrochloric acid, cell descaling, cell replacement roughly every five years

In a coastal island where saltwater intrusion into freshwater aquifers is already happening and is, in IDEP Foundation’s words, “forever non-reversible” (IDEP), quietly discharging saline water into the ground is a strange thing to market as the eco-friendly option.

Bali’s Water Emergency, in Numbers

The flooding and the drought are the same story told twice.

  • 60% of Bali’s water catchments are drying up. In some areas the water table has fallen more than 50 metres in roughly a decade (IDEP Foundation).
  • 260 of Bali’s 400 rivers have run dry, and Lake Buyan, the island’s second largest freshwater reserve, dropped from 72 metres in 2011 to 38 metres in 2021 (NOW! Bali).
  • Fresh water aquifers have been drawn down to less than 20% of capacity (IDEP).
  • A star-rated hotel room consumes around 800 litres of water a day, against roughly 200 litres per person for a Balinese household (The “Good Tourism” Blog).
  • Bali received more than 4.6 million foreign arrivals between January and August 2025, exceeding the island’s own population of 4.4 million (The Japan Times).

Meanwhile the surfaces that used to catch the rain are disappearing. WALHI Bali’s analysis of the Sarbagita region found that between 2018 and 2023, Tabanan lost 2,676 hectares of farmland, Gianyar 1,277 hectares, Badung 1,100 hectares and Denpasar 785 hectares.

WALHI’s executive director Made Krisna Dinata offered the figure that reframes everything: one hectare of paddy field holds up to 3,000 tonnes of water when flooded to just seven centimetres (WALHI, via Tempo).

Bali did not simply lose farmland. It lost a distributed, gravity-fed, thousand-year-old stormwater retention system, the subak, and replaced it with concrete. Add roughly 4,200 tonnes of daily waste, less than half of which reaches landfill, and over 200 tonnes of debris choking river mouths on the night of the flood, and the September disaster stops looking like bad luck.

The rainfall was extreme. ClimaMeter’s attribution analysis found conditions like these are now up to 7% wetter than in the past, and that “natural variability alone cannot explain the increase in precipitation.” But the damage was built.

The Sponge City Idea

There is a body of thinking that addresses exactly this, and Bali should be borrowing from it.

The sponge city concept, developed by the late Chinese landscape architect Kongjian Yu, inverts a century of drainage orthodoxy. Instead of moving water away as fast as possible through pipes, channels and concrete, a sponge city is designed to absorb rainfall where it falls, hold it, filter it through soil and vegetation, and release it slowly, recharging groundwater on the way down.

The toolkit is unglamorous and effective: permeable paving, bioswales, constructed wetlands, retention ponds, green roofs, restored river edges, infiltration wells.

The evidence is real but not magical, and it’s worth being honest about both halves. Yu’s Benjakitti Forest Park in Bangkok stayed dry through a one-in-ten-year rainfall event in 2022 while much of the surrounding city flooded. But Zhengzhou, a flagship Chinese sponge city, was overwhelmed by record rainfall in 2021 with 292 deaths (Global Voices). Sponge infrastructure reduces the frequency and severity of flooding. It does not repeal physics.

The lesson from that mixed record is the one that matters most for Bali: sponge capacity only works at scale, distributed across thousands of individual properties. A single park cannot absorb a regency. Ten thousand properties each holding their own rainfall can.

Indonesia already knows this. Badung Regency has required building owners to install infiltration measures since Regent Regulation No. 24 of 2010, on a formula of one unit per 20 m² of building footprint plus one for each additional 7 m². Badung’s own legal database files that regulation under lubang resapan biopori — 10cm bored biopore holes rather than metre-scale infiltration wells (Badung JDIH), though it was widely reported at the time as a sumur resapan requirement (Antara Bali). Either way, enforcement has been another matter. And the Bali Water Protection Program, run by IDEP Foundation with Bali State Polytechnic, has demonstrated that a single well-designed recharge well can return up to 41,000 litres of groundwater per hour during rainfall.

That is the number to hold onto when you look at your garden.

Where Natural Pools Come In

Here is the connection that most people miss. A natural swimming pool is, structurally, a sponge city component that you happen to swim in.

Consider what a properly designed natural pool actually is:

A permanent body of water that is never drained. Conventional pools are emptied and refilled to reset water chemistry. A natural pool’s ecosystem regenerates continuously, so the water stays in the system for years. The single largest water saving is the one you never see.

A planted wetland attached to your house. The regeneration zone, the gravel-and-reed bed that does the filtering, is functionally identical to the constructed wetlands that sponge city designers install for stormwater treatment. Aquatic plants and the biofilm on gravel strip nutrients from the water. Research on public natural swimming pools found external biological filters achieving 93–99% bacterial elimination, and, notably, that zooplankton-mediated removal of protozoan parasites ran around four times faster than in a chlorinated pool (IWA Water Supply, 2019) — precisely the Cryptosporidium problem chlorine cannot solve.

Storm capacity. A natural pool with a designed freeboard and a planted margin can accept a significant rainfall surge rather than overflowing into the street. It is retention volume that is already in the ground.

A recharge point, if you build one in. This is the part that turns a nice pool into infrastructure. Overflow from heavy rain, instead of being piped to a drain, is directed to an aquifer recharge pit: a gravel-filled shaft that lets clean, chemical-free water infiltrate down toward the water table. Because the water carries no chlorine, no cyanuric acid and no salt, it is water you can responsibly put back into the ground. You cannot do this with a chlorine pool. You cannot do this with a salt pool. That is the whole argument in one sentence.

This is why we design for aquifer recharge and smart overflow as standard at Bali Natural Pools, never as an upsell — and why we fit recharge pits on every site where the ground conditions allow it. On some sites they won’t be practical, and we’d rather say so than sell you something that doesn’t work.

Habitat. Dragonflies (whose larvae eat mosquito larvae), birds, frogs and pollinators colonise the regeneration zone within a season. In a landscape that has lost most of its wetland, small water bodies matter more than their size suggests.

What This Means If You Own or Manage Property in Bali

The regulatory ground is moving. Following the September 2025 floods, Governor Koster announced a review of building along four major rivers, enforcement action on zoning violations, and land-conversion rules aligned to Bali’s hundred-year plan. Proposals to restrict new hotel and villa construction in the most saturated districts are back on the table. Whatever the final shape, the direction is clear: water-neutral or water-positive development is going to stop being a marketing angle and start being a condition of approval.

There is a commercial case too, and it is not subtle:

  • Differentiation that photographs well. In a market where every listing has a rectangle of blue tile, a spring-fed lagoon is the first image in the gallery. More on that in will a natural pool get me more Airbnb bookings?
  • The guest experience arms race. Eco-conscious travellers are actively filtering for it, and no one has ever written a review complaining that the water didn’t sting their eyes.
  • Lower running costs. No chemical purchases, no repeat draining and refilling, no salt cell replacement every five years. See our breakdown for villa owners.
  • Longevity. Neither chlorine nor salt is degrading your stone, grout and fixtures.
  • Cultural coherence. Bali’s relationship with water, the subak, the water temples, Tri Hita Karana, is about reciprocity. A pool that returns water to the aquifer is a more honest expression of that than a sign asking guests to reuse their towels.

The Honest Limits

We would rather you trust us than be impressed by us, so:

A natural pool is not a flood defence. It will not save a property built in a river’s floodplain, and nothing in this article should be read as suggesting otherwise. Its contribution is incremental, and it only becomes significant when many properties do it.

Natural pools need correct design and commissioning. The same research that shows excellent protozoan removal also found that biological systems clear E. coli more slowly than chlorine does. That is a design and circulation problem with well-understood solutions — regeneration zone sizing, flow rates, hydraulic residence time — but it means this is not a build for a general contractor improvising.

There is a maturation period. An ecosystem takes a season to establish. It is not instant.

And recharge is site-dependent. Soil permeability, depth to water table and contamination risk all have to be assessed. A recharge pit in the wrong place is useless at best.

Frequently Asked Questions

Are natural pools in Bali safe to swim in? Yes, when correctly designed. Biological filtration in natural pools has been shown to eliminate 93–99% of bacteria and to remove chlorine-resistant protozoan parasites substantially faster than chlorinated water. Europe has operated public natural bathing facilities under formal standards for decades.

Will a natural pool breed mosquitoes? No. Mosquitoes need still, stagnant water. A natural pool circulates continuously and supports dragonfly larvae, backswimmers and other predators that consume mosquito larvae. See our FAQ on mosquitoes.

Can I convert my existing chlorine or salt pool? Usually, yes. The existing shell often becomes the swimming zone, with a regeneration zone added alongside. Salt pools need a thorough flush and, sometimes, remediation of salt-damaged surfaces first.

Does a natural pool use less water than a chlorine pool? Considerably less over its life, because it is never drained and refilled. Evaporation still occurs, and shaded, planted margins reduce it relative to an exposed tiled pool.

How does a natural pool help with flooding? Through retention volume, permeable planted margins that absorb rather than shed rainfall, and, where site conditions allow, an aquifer recharge pit that infiltrates overflow into the ground instead of sending it to a storm drain. Chemical-free water is the precondition for all of it.

The Bottom Line

Bali is running two water crises simultaneously: an aquifer being drained faster than the rain can refill it, and a landscape that has been paved to the point where rain runs off instead of soaking in. September 2025 showed what the second one costs. The dry-season wells show what the first one costs.

Every conventional pool built on this island makes both problems slightly worse. It takes water from a depleted aquifer, returns it contaminated, and seals more ground.

A natural pool, designed with recharge, does the opposite. It holds water, cleans it biologically, supports life, and puts clean water back into the ground.

One pool will not save Bali. That is not the claim. The claim is that Bali is going to be rebuilt over the next twenty years regardless, and that we get to decide whether the thousands of pools built during that rebuild are extraction points or recharge points.

Ours give back. We also give 1% of revenue to a Bali-based charity of your choice, because water infrastructure isn’t the only thing this island needs.

If you’re building, renovating, or you’ve just looked at your salt cell invoice and wondered whether there’s a better way, book a free 30-minute online consultation. We’ll talk through your site and your soil, come and walk it if it looks promising, and tell you honestly what’s possible.

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