Reverse osmosis membranes are often described as the “heart” of a filtration system. If that’s true, a 40/40 RO membrane is one of the most common hearts beating away in residential, light commercial and small industrial installations. Yet most users know surprisingly little about how these membranes actually perform, how to optimise them, or what to expect in terms of PFAS removal.
This guide walks through the performance of 40/40 RO membranes in practical terms: what they are, how they work, what affects their efficiency, and how to run them in a way that delivers reliable contaminant removal – including PFAS – without wasting water or energy.
What exactly is a 40/40 RO membrane?
A “40/40” RO membrane is a standard spiral-wound element with:
- 40 inches (about 1.02 m) of length
- 4 inches (about 10.16 cm) of diameter
This format is widely used in:
- Point-of-entry systems for homes and small buildings
- Light commercial systems (cafés, small labs, car washes)
- Pilot systems for industrial or municipal treatment
Inside the pressure vessel, feed water flows across the membrane surface. Under pressure, water molecules pass through the semi-permeable polyamide layer, while most dissolved salts, organic molecules and many PFAS are rejected and stay in the concentrate (waste) stream.
Because of their size and flow capacity, 40/40 elements often serve as a bridge between small under-sink units and large 8-inch industrial membranes. They’re compact, relatively affordable, and can be arranged in arrays to scale up treatment capacity.
Key performance metrics you should watch
To manage a 40/40 membrane effectively, you need to track a few core performance indicators. These are the numbers that tell you whether your membrane is healthy, efficient, and actually protecting users from contaminants such as PFAS.
1. Permeate flow rate (flux)
Permeate flow is simply the volume of treated water produced, usually expressed as:
- Litres per hour (L/h) or gallons per day (GPD)
- Or as flux: L/m²·h (litres per square metre of membrane per hour)
Typical 40/40 membranes for brackish water or tap-water polishing are often rated around 2,000–2,500 GPD at specific test conditions (e.g., 25°C, 150 psi, 2,000 ppm NaCl). Real-world values are often lower because:
- Feed water is cooler than 25°C
- Operating pressure is lower
- Water chemistry is more complex than a lab NaCl solution
Monitoring permeate flow over time is crucial. A slow, steady drop usually indicates fouling or scaling. A sudden jump can mean a damaged membrane – which is bad news for PFAS and other contaminant removal.
2. Salt rejection (and why it matters for PFAS)
Salt rejection (%) indicates how effectively the membrane blocks dissolved ions:
Rejection = (1 – (Permeate TDS / Feed TDS)) × 100
For a healthy 40/40 RO element treating relatively low-salinity feed, you might expect 96–99% salt rejection under normal operating conditions.
While PFAS are not “salts” in the traditional sense, high salt rejection usually correlates with good overall membrane integrity and structure. If salt rejection drops significantly, you should assume other contaminants – including PFAS – may be slipping through as well.
3. Recovery rate
Recovery is the percentage of feed water that becomes permeate:
Recovery (%) = (Permeate flow / Feed flow) × 100
Higher recovery means less wastewater, which is attractive in terms of water efficiency. But push recovery too high and you concentrate salts, organics and PFAS on the membrane surface, increasing fouling, scaling and the risk of performance loss.
Typical recovery for single 40/40 elements often sits around 35–50%, depending on feed quality and pre-treatment. Multi-element arrays can be designed for higher system-wide recovery with careful staging.
4. Differential pressure (ΔP)
ΔP is the pressure drop from inlet to outlet on the feed/concentrate side. It’s a direct indicator of hydraulic resistance inside the element. A rising ΔP usually means:
- Particulate fouling (suspended solids, iron, colloids)
- Biofouling (biofilm growth on spacers and membrane)
- Scaling (mineral precipitates like calcium carbonate)
Each manufacturer will give a baseline ΔP and a threshold increase (e.g., 15% or 1 bar over baseline) at which you should consider cleaning. Ignoring ΔP trends is a fast way to shorten a membrane’s life and lose PFAS removal efficiency.
What affects 40/40 membrane performance in practice?
Even the best membrane element is only as good as the conditions you operate it under. Four variables dominate performance: feed water quality, pressure, temperature and pH.
Feed water quality
Feed water is rarely just “water plus a few salts”. In real installations, it can contain:
- Hardness (Ca, Mg) that drives scaling
- Iron and manganese that oxidise and foul membranes
- Silica, which is notoriously difficult to remove and can form hard deposits
- Natural organic matter (NOM), which promotes biofouling and colour
- PFAS and other emerging contaminants
Pre-treatment is therefore critical. For a 40/40 RO, good practice usually includes:
- Particle filtration (e.g., 5 µm cartridge or multimedia filter) to remove suspended solids
- Activated carbon to remove chlorine (which damages polyamide) and reduce organic load
- Softening or anti-scalants where hardness and scaling risk are high
- Biological control (e.g., UV) in systems prone to biofouling
Where PFAS are a concern, upstream granular activated carbon (GAC) can also help reduce the PFAS load on the membrane and improve long-term stability.
Pressure
Applying pressure is how RO “pushes” water through the membrane against the osmotic gradient. For a 40/40 element treating relatively low-salinity groundwater or municipal water, operating pressures often fall between 8–16 bar (120–230 psi), depending on design.
Under-pressure reduces permeate flow and can compromise rejection if flux drops below the membrane’s optimal range. Over-pressure increases energy consumption and can risk compaction or mechanical damage.
The performance sweet spot is generally where:
- Permeate flow meets design expectations
- Salt rejection remains stable and high
- Energy use per litre is acceptable
Temperature
Warmer water is less viscous and passes through the membrane more easily. As a rule of thumb, every 3°C increase in temperature can raise permeate flow by roughly 7–10%, but salt rejection may drop slightly.
Winter versus summer feed temperatures can therefore significantly change your apparent “membrane performance” even if the membrane itself has not changed at all. Temperature correction factors from the manufacturer are essential when interpreting data.
pH
Most polyamide RO membranes are designed for a pH operating range of around 2–11, but long-term operation is typically kept between pH 3–10 for membrane longevity.
pH affects:
- Scaling tendency (e.g., higher pH favours calcium carbonate scaling)
- Speciation of some contaminants, including certain PFAS
- Effectiveness of cleaning chemicals
Careful pH control during both operation and cleaning can significantly extend the life of a 40/40 element.
PFAS removal with 40/40 RO membranes
Can a 40/40 membrane remove PFAS effectively? In many cases, yes – particularly the longer-chain compounds like PFOA and PFOS. But the details matter.
What the evidence shows
Peer-reviewed studies and field data generally indicate:
- Long-chain PFAS (e.g., PFOA, PFOS) – often > 95–99% rejection by RO
- Some short-chain PFAS (e.g., PFBA, PFBS) – lower and more variable rejection, sometimes 70–95% depending on the specific compound and conditions
RO membranes reject PFAS mainly through size exclusion and electrostatic interactions. Long-chain PFAS are larger and tend to be more strongly rejected, while some short-chain PFAS are small enough to partially slip through.
Why membrane health is critical for PFAS control
Partially damaged or fouled membranes can show:
- Reduced salt rejection
- Increased permeability to organics
- Unpredictable PFAS breakthrough
In PFAS-sensitive applications (e.g., drinking water, lab water), relying on a single barrier is inherently risky. Good practice often includes:
- RO followed by polishing with GAC or ion exchange
- Regular testing of PFAS in feed and permeate
- Redundancy in the system design for critical applications
If a 40/40 RO membrane is part of your PFAS strategy, you should treat its performance monitoring as a non-negotiable safety measure rather than a maintenance nicety.
Operating a 40/40 system for efficiency and stability
Good design is only half the story. Day-to-day operation determines whether your 40/40 RO runs quietly in the background or becomes a constant source of problems.
Set realistic recovery targets
Running a single 40/40 element at 75–80% recovery might look efficient on paper, but in practice it can lead to rapid scaling and fouling unless feed water is exceptionally clean. For most users:
- Start with a conservative recovery (e.g., 35–45%)
- Monitor scaling/fouling indicators
- Increase recovery in small steps only if data supports it
Protect the membrane from oxidants
Standard polyamide RO membranes are highly sensitive to free chlorine and other oxidants. Even low levels over time can cause irreversible damage, reducing rejection and increasing PFAS breakthrough risk.
That’s why an upstream activated carbon filter or dechlorination step is essential when treating chlorinated municipal feed water.
Stabilise operation – avoid shocks
Rapid pressure changes, sudden stops and starts, and frequent shutdowns can all stress the membrane and its spacers. Where possible:
- Ramp pressure up and down gradually during startup and shutdown
- Use automatic flush cycles to rinse the membrane with permeate or low-TDS feed before prolonged shutdown
- Maintain a stable operating window instead of constant manual adjustments
Monitoring, diagnostics and troubleshooting
Well-run systems treat monitoring as part of normal operation, not a last resort when something fails.
Parameters to log regularly
- Feed, permeate and concentrate pressures
- Feed and permeate conductivity or TDS
- Feed, permeate and concentrate flow rates
- Temperature
- pH of feed and permeate
From these, you can calculate:
- Salt rejection
- Recovery
- Specific flux (flow normalised for pressure and temperature)
- Differential pressure across the element
What common symptoms usually mean
- Gradual loss of permeate flow, stable rejection – likely fouling or scaling; time for cleaning and reviewing pre-treatment.
- Sudden drop in rejection, stable or increased flow – possible membrane damage, chemical attack (e.g., chlorine), or severe compaction.
- Rising differential pressure – particulates, biofouling or scaling building up in feed spacer channels.
- Higher flow in warmer months, lower in winter – usually just temperature effects; check against temperature-corrected performance curves.
Cleaning and maintenance of 40/40 membranes
Even well-designed systems will eventually foul. Proper cleaning and maintenance extend the useful life of 40/40 elements and help maintain PFAS removal efficiency.
When to clean
Most manufacturers recommend cleaning when:
- Permeate flow drops by 10–15% from baseline (after normalising for temperature)
- Differential pressure rises by 15–20%
- Salt rejection decreases by 5–10%
Waiting until performance has halved is not cost-effective; advanced fouling is harder to reverse and may permanently damage the membrane.
Types of cleaning
- Alkaline cleaning – targets organic fouling and biofilms (e.g., NaOH-based solutions with surfactants).
- Acid cleaning – targets inorganic scale (e.g., citric or phosphoric acid, or proprietary blends).
- Specialised cleaners – for silica or iron fouling where standard recipes are insufficient.
Cleaning protocols must respect the membrane’s pH, temperature and exposure limits. Overly aggressive conditions can strip away performance along with the fouling.
Storage and preservation
If a 40/40 membrane will be idle for more than a few days, you should:
- Flush it thoroughly with low-TDS water
- Use an approved preservative solution to prevent microbial growth
- Store it within the recommended temperature range and away from direct sunlight
Poor storage is a common reason for “mysterious” performance loss in otherwise new or lightly used membranes.
End-of-life, waste streams and PFAS responsibility
A 40/40 membrane may seem small in the context of global PFAS contamination, but how you handle it – and its brine – still matters.
Dealing with PFAS-rich concentrate
RO does not destroy PFAS; it concentrates them in the brine stream. Discharging that brine without understanding the regulatory framework and environmental risk simply moves the problem downstream.
Depending on jurisdiction and application, options may include:
- Sending PFAS-rich concentrate to regulated hazardous waste treatment
- Combining RO with downstream destruction technologies (still emerging and site-specific)
- Using additional polishing steps (e.g., adsorption) on the concentrate in sensitive cases
Disposal of spent membranes
RO elements are complex composites of plastics, adhesives and mesh materials. Today, most still end up in landfill, where PFAS and other adsorbed contaminants may be a concern.
Some manufacturers and specialised recyclers are developing take-back and recycling programmes for spent RO elements. When available, these options are preferable to simple disposal, especially for membranes used in PFAS treatment.
Ultimately, 40/40 membranes are a powerful tool – but not a magic wand. They are most effective when integrated with thoughtful pre-treatment, downstream safeguards and responsible waste management.
Used well, they can form a robust barrier against a wide range of contaminants, including many PFAS, helping to protect both human health and aquatic ecosystems. Used carelessly, they risk giving a false sense of security while quietly shifting persistent chemicals from one part of the water cycle to another.
The difference comes down to design, data, and a willingness to look at the whole system – not just the membrane cartridge inside the pressure vessel.