What's really in your hot tub water — and how to keep it genuinely clean

What are total dissolved solids, and why do they build up?

Total dissolved solids — TDS — is the cumulative weight of every substance dissolved in water that isn't water itself. Minerals, metals, salts, organic compounds, and every chemical you've ever added all contribute to one relentlessly climbing number. Calcium, magnesium, bicarbonate, sodium, chlorides, sanitiser residues, body oils, sweat, cosmetics — all of it dissolves and stays in the water.¹

In a hot tub, TDS accumulation is structural and inescapable. Every time water evaporates at 37–40°C, it leaves its dissolved load behind. Every chemical correction adds dissolved mass. Every bather session contributes organic load that either gets removed by your filter or reacts chemically and adds to TDS. The rate of accumulation is directly linked to three things: how fast your water heats, how well your filter performs, and how intelligently you manage the organic load between sessions.²

Putting 1,500 ppm in context

To understand what the BISHTA ceiling actually represents, you need to see it against every other type of water humans interact with daily — from the purest natural sources to water that is definitively undrinkable.

The BISHTA ceiling of 1,500 ppm sits dramatically above the WHO's 300 ppm drinking water recommendation and the EPA's 500 ppm limit — comfortably in territory every major health body would classify as unsuitable for consumption. That doesn't automatically make it unsafe to soak in. But it raises a question the industry has never properly answered: when did "not as bad as seawater" become an acceptable wellness benchmark?

Cloudiness, broken chemistry, and the organic load problem

Does high TDS cause cloudy water?

Not directly — and that's precisely what makes it insidious. Cloudiness is caused by suspended particles: undissolved calcium, biological matter, and algae — things too large for the filter to catch efficiently.⁷ TDS is dissolved, not suspended, so a tub can carry extremely high TDS while looking perfectly clear. This false reassurance is one of the most dangerous aspects of TDS accumulation. What high TDS does directly cause is foam — BISHTA acknowledges this explicitly as a sign that TDS has reached a point where the water's surface tension has changed.⁸

The sanitiser efficiency collapse

As TDS climbs, chlorine and bromine become progressively less effective. Dissolved substances create molecular interference — forming a chemical barrier around bacteria and organic matter that prevents free chlorine reaching its targets.⁹ Crucially, chloramines — the irritating by-products of chlorine reacting with bather waste — accumulate faster in high-TDS water. They cause stinging eyes, skin irritation, harsh chemical odour, and rashes. The CDC confirms chloramines, not free chlorine itself, cause these effects.^10,11

The organic load cycle — the root cause of rapid TDS rise

Every bather introduces organic matter: body oils, sweat, sunscreen, cosmetics, dead skin cells. In a typical portable hot tub holding 800–1,200 litres, two bathers create an organic concentration equivalent to approximately 200 people in a standard swimming pool.¹² This organic load triggers two damaging outcomes simultaneously. First, it depletes free chlorine — the sanitiser is consumed reacting with organics rather than killing pathogens, so you dose again, adding more dissolved chlorine salts. TDS rises. Second, the organics that chlorine doesn't fully destroy bond with it to form chloramines, which then persist in the water as inert dissolved load — adding to TDS without providing any sanitation benefit.

This is the cycle that drives rapid TDS accumulation. Break it, and the water stays fresher for longer. Understanding how to break it — with better filtration, smarter treatment chemistry, and enzyme-based organic removal — is what separates genuinely clean water from water that just passes an operational compliance check.

Non-chlorine shock, enzymes, and the honest difference between them

Managing TDS effectively means addressing the organic load that drives it. There are two complementary tools that make a meaningful difference — and it's worth being precise about exactly what each one does and doesn't do, because the industry often conflates them.

Non-chlorine shock (MPS / potassium monopersulfate)

Non-chlorine shock, most commonly potassium monopersulfate (MPS), is an oxygen-based oxidiser. What it does exceptionally well is break apart chloramines — the combined chlorine compounds responsible for virtually all the discomfort associated with hot tub water.¹³ When MPS oxidises chloramines, it converts them to a form that can be eliminated or restored as useful free chlorine. Your existing sanitiser works harder without adding more of it — a direct brake on the TDS contribution from repeated chlorine dosing.¹⁴

However — and this is a nuance the industry rarely discusses — MPS itself contributes to TDS. As a dissolved potassium salt compound, each dose adds dissolved mass to the water. It is not a TDS reducer. It is a TDS rate-of-growth reducer: by keeping chloramines from building up and reducing the need for repeated chlorine top-ups, it slows the TDS accumulation trajectory without reversing it. It also adds no chlorine residual, causes no bleaching, and allows re-entry to the water within 30 minutes of dosing — all meaningful practical advantages.¹⁵

Enzyme-based treatment — the more direct TDS tool

Enzyme treatments work through a fundamentally different mechanism, and they are the more powerful tool specifically for TDS management. Natural enzymes — derived from plant extracts — act as biological catalysts, breaking down non-living organic matter: body oils, sunscreen, sweat residues, cosmetics, and bather waste into smaller molecules that can then be physically removed by filtration.¹⁶

The critical distinction: chlorine and bromine sanitise organics but don't remove them. They oxidise contaminants and leave the resulting dissolved compounds behind as TDS. Enzymes break organics down and make them filterable — meaning they exit the water through the filter rather than accumulating as dissolved load. This is what makes enzyme treatment the most direct intervention available for slowing TDS accumulation short of a drain and refill.¹⁷

The downstream effects compound: lower organic load means less chlorine consumption, which means fewer repeat doses, which means less chemical contribution to TDS. Lower organic load also means fewer chloramines forming, reducing skin irritation and sanitiser demand simultaneously. Some enzyme-based regimens, when combined with good filtration, can extend drain-and-refill intervals significantly beyond a standard chemical-only drain schedule — though changing water when TDS exceeds 500 ppm remains best practice for genuinely fresh water.¹⁸

Used together, non-chlorine shock and enzyme treatment form a complementary system. MPS handles the immediate aftermath of a soak — oxidising chloramines, restoring sanitiser efficiency, clearing the water fast. Enzymes handle the ongoing organic baseline — breaking down what builds up between sessions, reducing the raw material that becomes chloramines in the first place. The combination keeps TDS accumulation rate substantially lower than chlorine-only maintenance.

How the organic load flows — and where each treatment intervenes

Your filter is the frontline — and most filters are losing the battle by week two

Enzyme treatment can only be as effective as the filter that catches the particles enzymes break down. If enzymes are reducing organic contaminants to smaller, filterable particles — but the filter is a degraded paper cartridge that lost meaningful performance in its first fortnight — those particles simply pass back through the water column, react with sanitiser, and add to TDS anyway. The enzyme did its job. The filter failed.

Standard paper filter cartridges degrade under the combined thermal and mechanical stress of hot tub operation. Their effective filtration rating declines continuously from the point of installation. By week three or four, a paper cartridge is filtering at a fraction of its rated performance — not because it's dirty, but because the paper membrane is structurally compromised. The biological and chemical load it fails to catch remains in the water, feeds the organic cycle, accelerates chloramine formation, and drives TDS upward.

Permanent stainless-steel filtration changes this entirely. Consistent 30-micron mechanical filtration — not declining over time, not degrading after a few weeks — means the particles enzymes break down are genuinely captured and removed. The filter works identically on day one and day 300. That consistency is what makes the enzyme-filtration combination genuinely effective.

Why your heater's performance directly affects water freshness

Slow heaters create longer heating cycles. Longer heating cycles mean more evaporation per session. More evaporation means more dissolved load left behind, and a faster TDS accumulation rate. A heater taking three to four hours to reach temperature generates significantly more evaporative TDS contribution per session than one completing the job in under an hour. At 31kWh output and a full heat cycle of approximately one hour, the SUPA Heater minimises total evaporative loss per use — keeping water fresher between changes and making every other element of your water care system work more efficiently.

Why dissolved chemical load in hot water matters for your skin

The skin is a barrier — but not a perfect one, particularly at hot tub temperatures. Research confirms that a 10°C rise in skin surface temperature approximately doubles the transdermal flux of chemical compounds through the skin barrier.¹⁹ Vasodilation at hot tub temperatures (37–42°C) increases blood flow to surface capillaries, reducing lipid barrier resistance in the stratum corneum. The substances most readily absorbed are lipophilic compounds with molecular weights below 1,000 daltons — exactly the profile of chlorination by-products and dissolved organic residues that accumulate as TDS climbs.²⁰

Chlorine compounds also strip the skin's natural sebum layer. Hypochlorous acid — the active form of chlorine in water — reacts with surface skin lipids, removing the protective oil film that maintains barrier integrity. The result is dryness, itching, and in cases of repeated exposure, accelerated skin ageing.²¹ Non-chlorine shock and enzyme treatment both reduce the chloramine burden in the water directly — and in doing so, directly reduce the harshest element of the chemical environment your skin is soaking in.

What we believe the 1,500 ppm limit was designed for — and what it wasn't

BISHTA's guidance and the HSG282 standard were developed primarily to address Legionella risk.²² The TDS ceiling is a drain-and-refill trigger — the point at which water is considered too degraded for safe commercial operation. It is an operational compliance threshold, not a wellness benchmark. When you see 1,500 ppm described as "acceptable" in hot tub literature, what that means is: acceptable to avoid a Legionella enforcement action. It doesn't mean it's optimal for the person soaking in it.

What genuinely clean hot tub water looks like — and the system to achieve it

The industry's current standards were designed around operations. What they are not is a framework for the wellness of the person in the water. The gap between "operationally compliant" and "genuinely clean" is exactly where SUPA Products operates — and it's closable with the right system.

A consumer-centred water standard should target a TDS range closer to 200–500 ppm and treat anything above 500 ppm as a quality signal, not a routine milestone. Achieving that requires four things working together: fast heating to minimise evaporative TDS contribution per session (SUPA Heater, 31kWh, approximately one hour heat time); permanent, consistent filtration to physically remove what enzyme treatment breaks down (SUPA Filter, 30 micron stainless-steel); enzyme-based organic removal to break the chloramine formation cycle before it adds to TDS; and non-chlorine shock after each use to oxidise chloramines immediately, restore sanitiser efficiency, and keep water comfortable between enzyme doses.

None of these is a magic solution in isolation. Together, they represent a coherent, science-based system for keeping hot tub water genuinely fresh — not just technically compliant. That's the standard the industry hasn't set. It's the one consumers deserve.