Point-of-use water filters for legionella

Point-of-use water filters are 0.2 micron membrane cartridges fitted at taps and showers. They physically retain Legionella and Pseudomonas as water leaves the outlet, giving immediate protection. They are a supplementary control, never a replacement for temperature management or the written scheme of control.

What a point-of-use filter actually does

A point-of-use filter is a cartridge that fits directly onto a tap or shower outlet. Inside is a pleated or hollow-fibre membrane, typically rated at 0.2 micron absolute, which is the sterilising grade used to hold back bacteria. As water passes through, Legionella pneumophila and Pseudomonas aeruginosa are physically too large to cross the membrane and are retained on its surface. The water delivered at that outlet is therefore free of those organisms for the rated life of the cartridge.

People search for these devices as a legionella filter, a legionella tap filter or a point-of-use filter, and they all describe the same thing: a physical barrier at the very end of the line. It helps to separate point-of-use (POU) filters, fitted at the outlet, from point-of-entry (POE) filters fitted where the supply enters a building. Only a POU filter protects the water actually delivered to the person using the tap or shower.

Where filters are used: augmented care and high-risk outlets

The main home for point-of-use filters is augmented care, a term used in HTM 04-01 for clinical areas where patients are far more vulnerable to waterborne infection. This includes transplant units, intensive care, neonatal units, burns units and haematology or oncology wards. In these settings the tolerated bacterial count at the outlet is effectively zero, so a filter provides an immediate physical barrier while any remedial work on the system continues.

Filters are also used tactically: as a temporary measure during an outbreak, on individual outlets with intractable contamination that flushing and temperature cannot clear, and on showers or basins used by people who are especially at risk. In healthcare, water is still expected to be delivered at around 55°C at unfiltered outlets, so the filter supplements the temperature regime rather than standing in for it.

HSE ACOP L8 and HSG274 Part 2

Supplementary control, not a substitute for temperature

The most common and most dangerous misconception is that fitting filters lets you relax the temperature regime or the risk assessment. It does not. Under ACOP L8 and HSG274, temperature is the primary control for legionella in water systems: cold water below 20°C, hot water stored at 60°C and delivered at 50°C, or 55°C in healthcare, because the bacteria multiply between 20°C and 45°C.

A point-of-use filter treats the symptom at a single tap. It does nothing about the reservoir of bacteria and biofilm living in the pipework, the calorifier or any dead legs feeding the rest of the building. Remove the filter and the underlying risk is exactly as it was. This is why the HSE and filter manufacturers position these devices as a supplementary or temporary control that sits alongside temperature control and the written scheme of control, never as a replacement for them.

Filter lifespan, changeover and record-keeping

Point-of-use filters have a finite rated life, commonly between 14 and 92 days depending on the product and the water quality. Two things happen as that life runs out. The membrane gradually blocks and flow drops, and the retained bacteria on the upstream face, together with any biofilm forming on the wetted housing, become a contamination risk in their own right. A filter left on past its rated life can end up making an outlet worse.

For that reason every filter must be changed on the manufacturer's schedule, with the date of fitting and the date due recorded. This turns filters into a recurring management commitment rather than a fit-and-forget fix, and the changes belong on your monitoring schedule alongside the rest of the control tasks.

Filters, Pseudomonas and biofilm: what they catch and miss

The 0.2 micron membrane that stops Legionella also retains Pseudomonas aeruginosa, which is why the same filters appear on augmented care outlets where Pseudomonas is a named risk under the HTM 04-01 Part B guidance. For a severely immunosuppressed patient, a physical barrier at the tap can matter as much for Pseudomonas as it does for Legionella.

What filters do not do is treat the system. They do not remove dissolved chemicals or bacterial endotoxins, and they cannot clear the biofilm that lines pipework upstream. Biofilm can also form on the housing itself if the change schedule slips. A point-of-use filter is a barrier at the outlet, not a treatment for the water system behind it, so it works alongside cleaning, flushing and temperature control rather than replacing any of them.

How filters fit into the risk assessment and written scheme

In a legionella risk assessment, filters are recorded, not assumed. The assessment should log which outlets are filtered, the reason each filter is fitted, the change schedule and who is responsible, and it should still identify the underlying cause of the contamination and the plan to fix it. Filters buy time for that remedial work; they are not the end of it.

Sampling and action levels continue to apply across the system. Under HSG274 Part 2, results between 100 and 1,000 CFU per litre call for review and results above 1,000 CFU per litre call for immediate action, whether or not filters are fitted at individual outlets, as set out on our page on sampling and action levels. Recording all of this in one place is exactly what a written risk assessment is for. Our free legionella risk-assessment template gives you a structured place to capture where filters are fitted, why, and the temperature regime they are supporting.