Bacteria regrowth in an RO storage tank
The membrane is not the problem — the tank is. Reverse osmosis removes micro-organisms along with almost everything else, and the water leaving the membrane is close to free of them. What it is not is protected: it then sits in a storage tank at ambient temperature, with no disinfectant residual and a large wetted surface, which is exactly the environment micro-organisms need in order to re-establish. Regrowth after an RO tank is therefore normal behaviour rather than a fault — and the answer is a barrier after the tank, not a better membrane.
My tank smells or tastes off after a while
Water that is fine fresh from the tap can be different after it has stood in the tank — see what the tank changes.
I already shock-disinfected the system
It works on what is suspended in the water, and less well on what is attached to a surface — the order that actually fixes it.
UV is a disinfection component. It has no media and no membrane, so it is not a removal stage — and it is not a stand-alone purifier. Everything dissolved is decided upstream of it.
Why it comes back, even after good filtration
The first reason is what reverse osmosis does. It is a removal process: contaminants are pushed away from the permeate, micro-organisms included. That is why the water leaving the membrane is the cleanest water in the building — and also why nothing downstream of it is defended.
The second reason is what a storage tank is. It holds water still, at room temperature, under no disinfectant residual, across a large wetted surface. Those four conditions together are not a fault in the design; they are a description of a tank. Micro-organisms that enter after the membrane — from the tank surface, from the fittings, from the line to the faucet — find nothing to stop them settling and multiplying.
| Assumption | Reality |
|---|---|
| A good membrane should keep the water clean | It cleans the water, but does not protect it. Removal happens once, at the membrane. Nothing downstream maintains that condition. |
| The bacteria must be coming through the membrane | Usually not. A membrane in good condition removes micro-organisms. Regrowth downstream is the more common explanation — and it is testable by sampling before and after the tank. |
| A tank is just a container | A tank is a reactor. Stored water at ambient temperature with no residual is the standard environment for regrowth on wetted surfaces. |
| Shock disinfection should settle it | Partly. It acts on what is in the water. What is attached to a surface inside a tank or a long pipe run is harder to reach and re-establishes afterwards. |
| A bigger or better tank would help | Not by itself. More stored volume usually means longer residence time, which is the wrong direction. |
Sample at two points: directly after the membrane or at the tank inlet, and at the faucet. If the first is clean and the second is not, the growth is happening in the tank or in the line after it — and the treatment belongs there too. If both are positive, the problem started upstream and the chain has to be looked at from the source.
What the tank changes about the water
Four conditions decide how fast a wetted surface is colonised. A storage tank supplies all four at once.
- No residual. Ultraviolet treatment is physical and adds nothing, and reverse osmosis removes what was dissolved. The water arriving in the tank therefore has no disinfectant in it to suppress anything that gets in afterwards.
- Residence time. Water in a tank is not passing through — it is waiting. Long standing times let a small number of organisms become a population, and let cells attach to surfaces instead of staying suspended.
- Ambient temperature. A tank sits at room temperature rather than at chilled or hot temperatures, which is the range in which micro-organisms grow most readily.
- Wetted surface. A tank wall, a bladder, a float, fittings and the first metres of outlet pipe are all surfaces in contact with non-chlorinated water. Attached growth is the normal outcome, and once established it is a persistent source feeding the water passing over it.
Pressure tank
A pressurised tank with a bladder or diaphragm holds water against air pressure. The bladder is a large wetted polymer surface, and the water behind it is in no hurry to leave.
Atmospheric tank
An open or vented tank exchanges air with the room and holds a larger volume of slowly moving water. Longer residence time and more surface both work in the same direction.
The line after it
Every metre of tubing between the tank and the faucet is wetted surface with no residual on it. A long run does not merely delay the water — it adds surface for growth to establish on.
A disinfection stage is not a filter, and the difference matters at the point of sale: nothing is removed from the water, and no protection follows the water out of the chamber.
The order that actually fixes it
The sequence matters, because two of these steps make the last one possible. Cleaning a tank that is about to be re-colonised from the line, or fitting a barrier ahead of the point where growth happens, both leave the problem in place.
- Find where it is growing. Sample at the tank inlet and at the faucet. The comparison is what tells you whether this is a membrane problem, a tank problem or a line problem — and only one of those is fixed by replacing the membrane.
- Clean or replace the wetted parts that are colonised. Where a tank, a bladder or a length of tubing has an established biofilm, physical cleaning or replacement is what removes it. A chemical shock alone is a weaker measure against attached growth than against suspended organisms.
- Shorten the run after the tank. Fewer metres of tubing between the tank and the faucet means less surface for growth and less water standing in the line between draws. This costs nothing and is often the step that changes the result.
- Put a barrier last, after the tank. A continuous in-line ultraviolet stage installed after the storage tank and before the faucet acts on the water as it moves to the point of use — after the components that seed it, not before them.
- Keep the water moving, and keep the stage maintained. Standing water is what makes a tank a growth environment. Regular draws, periodic cleaning, and replacing the lamp at the end of its rated life are what keep the arrangement working.
A barrier before the tank cannot protect what is after it. That is the whole placement argument: growth happens in and after the storage tank, so the stage that addresses it belongs in and after the storage tank too — on the line that actually reaches the faucet.
Where the stage sits on the line
| Position | What it achieves |
|---|---|
| Before the membrane | Acts on raw feed water only. It cannot prevent regrowth downstream, because the tank and the outlet line come later. |
| Between membrane and tank | Treats water entering the tank, and then leaves it standing in the very environment that produces regrowth. The barrier is upstream of the problem. |
| After the tank, before the faucet | This is the position the arrangement is designed for. The stage acts on water after the components that seed it, on the line that reaches the point of use. |
| At the faucet, as a separate appliance | A point-of-use device at the outlet may be an option in some installs, but it introduces its own wetted housing after the barrier. The in-line stage on the permeate line is the simpler arrangement. |
The unit is specified for feed water that is already filtered and clear, within stated limits for turbidity, iron, hardness, pH, chlorine and temperature. Those limits are conditions of use — they describe when the lamp can do its job. They are not a statement about what the water will contain afterwards.
Three ways the fix misses
- The barrier is installed before the tank. The commonest version. The unit is fitted where there is space on the feed line, treats water on the way in, and the water then stands in a tank that seeds it again. The equipment is working and the problem is unchanged.
- A tank is cleaned and the line is not. The tank is emptied and disinfected, the outlet tubing is left as it is, and growth resumes from the surfaces that were not addressed. Cleaning the reservoir while leaving the distribution line is half a repair.
- Growth is attributed to the membrane, and the membrane is replaced. A new membrane on a system whose tank and line are colonised changes nothing that the owner can detect, because the membrane was not where the growth was.
Sample at two points. Clean or replace the wetted parts that are actually colonised. Shorten the run after the tank. Place a continuous barrier last, immediately before the point of use. Then the arrangement matches the mechanism instead of contradicting it.
Bacteria regrowth in an RO storage tank
Why do bacteria come back after an RO system?
Because reverse osmosis removes micro-organisms once, at the membrane, and does nothing to protect the water afterwards. Between the membrane and the faucet the water sits still in a storage tank, at room temperature, with no disinfectant residual and a large wetted surface. That is the environment in which regrowth on surfaces is expected.
Does a UV sterilizer before the tank stop regrowth?
No. It treats the water entering the tank and then leaves that water standing in the tank and the outlet line, which is where growth establishes. The unit addresses micro-organisms as water passes the lamp, so it has to be placed after the components that seed the water — the storage tank included.
Is regrowth a sign that the membrane has failed?
Usually not. A membrane that is passing its own checks has done its job. The useful test is a comparison: sample after the membrane and at the faucet. A clean first sample with a positive second one locates the growth downstream, which is where the treatment should go.
Will a bigger tank help?
Not by itself. More volume generally means longer residence time and more wetted surface, both of which work in the same direction as the problem. What helps is keeping the stored water moving, cleaning the tank on a routine, and placing a barrier after it.
What maintenance does the arrangement need?
Two things, on a schedule rather than on a symptom: cleaning of the tank and the quartz sleeve according to the manufacturer's instructions, and replacement of the lamp at the end of its rated life. The lamp carries a manufacturer design rating of 15,000 hours at not less than 60 per cent of initial output — a design rating, not a validated service-life claim.
Describe the system, not the symptom alone
Tank type, where the growth appears, draw frequency and the two sample results are what make a useful reply possible — the placement question answers itself once the layout is known.
- Models: ZXSJ-UVC-1GPM ｜ ZXSJ-UVC-3GPM
- Credential: CE conformity certificate DPCW/01068/06/KA/26 · EPA Establishment No. 105777-CHN-1 (facility registration)
- Scope: in-line UVC post-treatment for water that has already been filtered
- Samples: available for evaluation before a volume commitment
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Sources and scope
5 sources
- Treatment sequence and placement: the manufacturer's product documentation and installation manual, revision 01.03. Cited for: where the unit is installed, what it treats, and the feed-water conditions it requires.
- Feed-water limits quoted on this page (turbidity, iron, hardness, pH, chlorine, temperature): SKWEIT® UV Water Sterilizer instruction manual and product parameter table.
- Regrowth conditions — absence of disinfectant residual, residence time, ambient temperature and wetted surface: attributed to the general practice of distribution-system water quality management. Cited for: the statement that stored, unchlorinated water in contact with surfaces is the environment in which attached growth occurs.
- Removal performance of reverse osmosis: attributed to the process itself. Cited for: the statement that a membrane removes micro-organisms at the membrane, and provides no residual protection downstream of it.
- Lamp life: manufacturer design rating of 15,000 hours at not less than 60 per cent of initial output, per the supplier reliability documentation. Cited here as a design rating — not a validated service-life claim. Actual life depends on drive current, thermal conditions and water quality.