Hot Tub Biofilm

Biofilm is a layer of microbial growth on the inside surfaces of the plumbing. It is not in the water, which is the single most important thing to understand about it: a water sample can test perfectly while the pipework is coated, and nothing you add to the water in normal doses reaches it.

Which of these is you?

• White or gray flakes appear when the jets start
• A musty smell that survives shocking
• Sanitizer disappears for no visible reason
• The shell and waterline feel slippery

Flakes when the jets start

Why this happens. Established biofilm sloughs off in pieces when flow increases. Turning the jets on after a quiet period dislodges fragments that then float in the water, typically white, gray or brownish and slightly slimy rather than crisp.

How to confirm it. They appear at jet startup rather than gradually, and they reappear after you net them out. Flakes that are crisp and white and do not recur are more likely scale or disintegrating filter media.

The fix. A full purge. Netting the flakes out treats the visible fraction of something that is mostly still inside the pipes.

A musty smell that survives shocking

Why this happens. A chemical, chlorine-like smell means chloramines and responds to shocking. A musty, earthy or faintly rotten smell is biological and does not, because the source is not in the water being shocked.

How to confirm it. Shock properly, wait the label time, retest, and smell it again. Chloramine smell goes. A biofilm smell comes back within a day, and is often strongest in the first few seconds after the jets start.

The fix. Purge, drain, wipe, refill. There is no water-side treatment for this.

Sanitizer disappearing for no reason

Why this happens. Biofilm consumes sanitizer. A tub that will not hold a residual despite correct dosing, with no unusual bather load to explain it, is usually feeding something in the pipework.

How to confirm it. Dose to the CDC’s stated level for hot tubs,^[source] retest after a full circulation cycle, and retest again the following day. A sharp unexplained drop with the tub unused points at demand rather than at dosing.

The fix. Purge. Continuing to add sanitizer to a tub with an established biofilm is feeding a problem rather than treating it, and it is the case where the CDC’s guidance on maintaining disinfectant in hot tubs matters most.^[source]

Slippery shell and waterline

Why this happens. The same growth, on surfaces you can reach. Slippery rather than merely wet is the signature, and it usually means what is on the visible surfaces is also in the pipes.

How to confirm it. Run a finger along the waterline and just below it. If it feels slick, you have your answer.

The fix. Purge and drain, then wipe every reachable surface while the tub is empty. Wiping alone gets the visible fraction and leaves the rest.

The purge procedure

1. Do not drain first. A purge product needs water to circulate through the pipework. Draining first defeats the whole exercise.
2. Remove the filter cartridge. It will trap what comes loose, and on a first purge it will be beyond saving anyway.
3. Add the purge product to the label dose for your volume.
4. Run every jet, on every setting, for the stated time. Cycle the air controls too. The point is to get flow through every branch of the plumbing, including the ones that normally sit still.
5. Expect the water to look unpleasant. On a tub that has never been purged, it is genuinely alarming, and that is the product working.
6. Drain completely.
7. Wipe the shell, waterline, jet faces and the cover underside while the tub is empty. This step is not optional: everything the purge released is sitting on those surfaces.
8. Refill, and fit a clean cartridge. On a heavy first purge, fit a new one.
9. Balance and sanitize from scratch, following first chemicals after a refill.

On a badly affected tub, a second purge on the fresh fill is worth doing. It should come out far cleaner, and if it does not, the first purge only reached the easy parts.

Is hot tub biofilm dangerous?

Biofilm protects the organisms inside it from disinfectant, which is exactly why it matters rather than being merely unpleasant. The CDC’s hot tub guidance is built around maintaining a disinfectant residual, and a tub whose plumbing is consuming that residual is a tub that is not maintaining it.^[source] Read the CDC’s own page rather than our summary of it; it is free and it is the authority. If anyone who has used the tub becomes unwell, that is a matter for a doctor.

Stopping it coming back

• Purge at every water change, not once a year. A routine purge on a maintained tub takes an hour and shows almost nothing, which is the point.
• Hold your sanitizer residual consistently. Biofilm establishes during the gaps.
• Run the jets regularly, even when the tub is not being used. Stagnant branches are where it starts.
• Keep the filter clean. A loaded cartridge means less water meeting the sanitizer per cycle.
• Change the water on schedule. How often to change hot tub water works out the interval.