First things first: there is always urine in a pool
It is not a pleasant thought, but it is a technical fact every pool professional works with. Even in a very well kept installation, organic matter arrives from the bathers' bodies: sweat, skin cells, sun cream, saliva and urine. Nobody has to urinate deliberately in the pool: what comes in on skin and swimwear is enough. The issue is not so much the quantity as what that organic matter does to the chlorine.
The chemistry: from urea to chloramines
Urine is mostly water, but it carries two key nitrogen compounds: urea and uric acid. Once they enter treated water, the free chlorine that should be available to disinfect reacts with that nitrogen instead of doing its job.
The result is chloramines, also called combined chlorine. They form in three stages, depending on the chlorine-to-nitrogen ratio and on pH:
- Monochloramine (NH₂Cl): the first to appear. It still has some disinfecting power, but far less than free chlorine.
- Dichloramine (NHCl₂): forms as chlorine keeps rising. It is the main source of that typical "indoor pool" smell.
- Nitrogen trichloride or trichloramine (NCl₃): the most volatile and irritating of the three. It causes stinging eyes and throat and builds up just above the water surface.
On top of that, the uric acid in urine reacts with chlorine to produce by-products such as cyanogen chloride and trichloramine itself, which is why urine is chemically a considerably worse input than sweat.
That chlorine smell is not chlorine
This is the most widespread misunderstanding in pool care. A pool with a strong "chlorine" smell does not have too much chlorine: it has too little. That smell is chloramines, chlorine already spent and combined with organic matter. A properly treated pool, with enough free chlorine and low combined chlorine, barely smells at all.
How to measure it: free, total and combined chlorine
A correct reading is not one figure but three:
- Free chlorine: what remains available to disinfect. Usual working range in a private pool: 1–2 ppm.
- Total chlorine: free plus combined.
- Combined chlorine (chloramines): total minus free. It should stay below 0.6 ppm; above that, smell and irritation appear.
A DPD reagent kit (DPD1 for free, DPD3 for total) or a photometer gives all three values in a few minutes. Cheap test strips, by contrast, rarely resolve combined chlorine properly.
The real effect on a sand filter
This is the part that is almost never explained. Chloramines and organic matter do not simply float about: they end up recirculating through the filtration equipment, and there they set off a chain of problems.
- Chlorine is consumed before it reaches the filter. With a high chlorine demand, the disinfectant is exhausted in the pool and the water reaches the filter still carrying unoxidised organic load.
- Biofilm forms between the sand grains. Without enough residual chlorine, sand stops being a hostile medium and becomes a substrate that bacteria and algae colonise.
- The sand compacts and cements. Body oils, sun cream, organic matter and limescale bind the grains into clumps.
- Channelling appears. Water finds preferential paths through the cemented bed and passes through the filter without really being filtered.
- Gauge pressure rises. And it no longer drops fully after backwashing, the classic sign of saturated sand.
- Backwashing stops working. Heavy clumps are not lifted by the wash flow and stay inside the vessel.
- The media's service life is cut short. A filter that should last four or five years can be useless in two.
What the owner sees is a pool that swallows chemicals, that turns cloudy easily after a busy weekend and that needs more and more filtration hours just to look the same.
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How to fix water with high chloramines
Once combined chlorine has risen, nudging the usual dose up achieves nothing: the molecule has to be broken. The correct procedure is:
- Shock chlorination to breakpoint. It requires roughly ten times the measured combined chlorine value. Only then are chloramines oxidised and destroyed; smaller doses simply create more of them.
- Correct pH before shocking. At high pH, chlorine loses much of its oxidising power. The working range is 7.2–7.6.
- Aerate the water. Waterfalls, water blades or simply aiming the return at the surface help to strip out trichloramine, which is volatile.
- Filter continuously during and after treatment, and backwash once it is finished.
- Non-chlorine oxidation as support. Potassium monopersulfate oxidises organic matter without adding more nitrogen, useful as a periodic boost.
- Partial water replacement. When uric acid and stabiliser levels are very high, replacing part of the volume is the fastest and cheapest route.
If after all that the filter pressure stays high and the water never quite clears, the problem is no longer in the water but in the media: it is time to change the filter sand.
Prevention: what actually reduces the problem
- Shower before swimming. The single most effective measure and the most ignored: it removes much of the sweat, residual urine and sun cream.
- Toilet breaks, especially with children and in community pools.
- Wait after applying sun cream and, where possible, choose water-resistant products.
- Keep free chlorine steady, never letting it fall to zero between treatments.
- Enough filtration hours, matched to water temperature and bather load.
- Control the stabiliser (cyanuric acid): above 75 ppm it blocks chlorine and makes everything above worse.
The myth of the dye that reveals urine
No product turns the water a different colour around someone who urinates in the pool. It is a useful urban legend for keeping children honest, but no manufacturer sells anything of the sort. The only thing that reveals urine is a combined chlorine test.
How we work at AquaGarden Alicante
On the maintenance contracts we run from El Campello to Elche we routinely measure both free and total chlorine, not just one of them, precisely so that a rise in combined chlorine is caught early. When it appears, we treat the water and check the condition of the filter media before the sand is ruined. You can see the detail in our water treatment and pool maintenance services.