RO Membranes

The membrane is the working heart of a reverse osmosis system. Under pressure its semi-permeable surface lets water molecules through while leaving most of the dissolved salts on the feed side. Filters hold back what is suspended in the water; the membrane separates what is dissolved. Four figures decide the choice of membrane: salt rejection rate, operating pressure, recovery rate and size. For the protective stages ahead of the membrane see our Water Filters category, and for the valves that run the system our Control Valves category. Membrane and housing prices vary with membrane size, manufacturer and pressure class; the current list is in the price table below.

How a Water Treatment Membrane Works

In natural osmosis water moves from the side with the lower salt concentration to the side with the higher one. In reverse osmosis that flow is reversed by pressure applied from outside: water is driven from the high-salt side through the semi-permeable film and taken off on the clean side. The salts that cannot pass through the film are carried away on the concentrate (drain) line.

This operating principle brings two consequences with it. First, sufficient feed pressure is essential for the system to work; if pressure is too low the membrane produces no water. Second, for every litre of clean water produced a certain amount of wastewater is formed; this is not a fault but a requirement of the principle. The wastewater protects the membrane by preventing salt from building up on its surface.

Four Factors That Decide Membrane Choice

1 — Salt rejection rate

This states how much of the dissolved salt the membrane holds back. A higher rejection rate means a lower TDS at the outlet. It is the deciding figure in applications such as laboratories, steam boilers and precision manufacturing.

2 — Operating pressure

Membranes are made in low-pressure (LP/ULP) and standard-pressure classes. Low-pressure membranes run on less pump energy and are preferred on low-salinity sources such as mains water. Seawater membranes are designed for far higher pressure; they cannot be swapped with a mains-water membrane.

3 — Recovery rate

This shows how much of the water entering the feed is taken off as clean water. Raising recovery reduces wastewater but increases the risk of salt and limescale building up on the membrane surface. On industrial lines this risk is managed with antiscalant dosing.

4 — Size and capacity

Membranes are made in standard sizes: 1812 for domestic use, 4040 (4 inch) for commercial use and 8040 (8 inch) for industry. Capacity is scaled through the number of membranes and the pressure vessel arrangement.

Domestic Membranes (1812)

This is the standard for under-sink units: 1.8 inch diameter, 12 inch length. Daily production capacity is stated in gallons (GPD). The higher the capacity, the shorter the tank fill time of the unit.

The membranes used in under-sink systems and the pressure vessel the membrane sits in:

Commercial Membranes (4040 / 4 inch)

Used for small and medium-sized businesses, laboratories and commercial kitchens. They are fitted into a pressure vessel singly or in series.

Industrial Membranes (8040 / 8 inch)

This is the standard for industrial reverse osmosis lines. The required capacity is reached by arranging membranes in series and in parallel inside pressure vessels; capacity is increased through the number of membranes.

• FilmTec 8″ Membrane (BW30 PRO-400) — 88,233.12 TL
• Toray TM720D-400 — 78,429.44 TL
• LG 8 inch RO Membrane BW 400 ES — 73,527.60 TL
• STAR LP8040 RO Membrane — 44,116.56 TL
• TechFlow 8040 LP-400 — 44,116.56 TL
• Vontron ULP22-8040 — 44,116.56 TL

The pressure vessel the membranes go into is chosen separately: RO 8″ Membrane Vessel — 5-element 80,880.36 TL. A five-element vessel carries five membranes connected in series and delivers high capacity on a single line.

Seawater Membranes

Seawater is many times more saline than mains water. Seawater membranes are therefore designed both for a higher salt rejection rate and for far greater pressure resistance. A mains-water membrane cannot be used on seawater; it is damaged in a short time.

Membrane Housing Sizes and Pressure Vessel Arrangement

Membranes do not work on their own; they are fitted inside a vessel built to take pressure. In a domestic unit this vessel carries a single membrane. On industrial lines several membranes are arranged in series inside one vessel; the concentrate of the first membrane becomes the feed of the second, and so on.

There are two ways to increase capacity. Series arrangement raises the recovery rate because the water passes through more than one membrane along the same line; the last membranes, however, work with a higher salt concentration and are stressed more. Parallel arrangement increases total output and keeps the load per membrane balanced.

In practice industrial systems use the two together: several vessels are connected in parallel, and inside each vessel the membranes are arranged in series. RO 8″ Membrane Vessel — 5-element is a typical example of this logic; five membranes work in series in a single vessel.

• Series arrangement: Recovery rises, wastewater falls, the last membranes are stressed more.
• Parallel arrangement: Total capacity rises and the load per membrane stays balanced.
• Combined arrangement: This is the standard approach on industrial lines.

Where the Membrane Sits in a Reverse Osmosis System

The membrane sits in the middle of the system and every stage ahead of it is there to protect it. When that order is broken, the most expensive part — the membrane — is damaged.

In this chain the filter stages, booster pump and pressure tank must be considered together with the membrane. On industrial lines a stage of pre-softening is also added to reduce the limescale load on the membrane.

Membrane Count on an Industrial Line

On an industrial reverse osmosis line the number of membranes is set by calculation, not by guesswork. Three inputs decide it: the target daily output, the feed water analysis and the target recovery rate.

As the salinity of the feed water rises, the pressure and the number of membranes needed for the same output rise with it. In the same way, as the recovery target rises the concentration on the concentrate side increases and the risk of precipitation grows; at that point antiscalant dosing and pre-softening come into play.

When more capacity is needed, the first reflex should not be to look for a bigger membrane. 8040 is already the industrial standard; growth is achieved by increasing the number of membranes and vessels of the same class. This approach makes spare part supply easier and also prevents the whole system from stopping when one membrane fails.

Membrane Cleaning and Maintenance

When an industrial membrane loses performance it is not replaced straight away; cleaning is tried first. In this operation, known as chemical cleaning, the deposit on the membrane surface is dissolved with suitable solutions and flushed out of the system.

The decision to clean is made by measurement. A drop in output under the same feed conditions, a rise in the pressure differential or a decline in outlet quality all point to the need for cleaning. The type of deposit also decides the cleaning chemical: acidic solutions are used for limescale and mineral precipitation, alkaline solutions for organic and biological fouling.

These chemicals are applied at the correct dose and in a controlled manner.

Cleaning is not applied to domestic 1812 membranes; membranes of this size are treated as consumables and replaced when their life is over. Cleaning makes sense in the 4040 and 8040 classes, where the membrane cost is high.

• Take measurements: Output, pressure differential and outlet quality must be on record.
• Choose the right chemical: If the type of fouling is identified wrongly, cleaning will not help.
• Follow the dose and the duration: Too much chemical can damage the membrane surface.
• Verify after cleaning: If the values do not return to their earlier level, the membrane has reached the end of its life.

The Water Analysis to Do Before Choosing a Membrane

The choice of membrane depends far more on the characteristics of the feed water than on the brand of the unit. The same membrane can run for years without trouble in one building, while the same model tires quickly a few kilometres away. What makes the difference is the water itself.

These are the basic values to look at before choosing:

• TDS (total dissolved solids): Determines how much salt the membrane will separate and the pressure required. A TDS Meter at 1,225.46 TL is enough for the measurement.
• Hardness: The amount of calcium and magnesium shows the risk of precipitation on the membrane surface. If it is high, pre-softening is needed. It can be measured with a Water Hardness Test Kit at 1,176.44 TL.
• pH: A very low or very high pH can fall outside the working range of the membrane material. pH Test Kit 588.22 TL.
• Turbidity and suspended solids: Determines how strong the pre-filtration needs to be.
• Iron and manganese: In well water in particular they form deposits on the membrane surface; pre-treatment is needed.
• Free chlorine: Directly harmful to the membrane; the capacity of the carbon stage is chosen accordingly.

A membrane choice made without knowing these values rests on guesswork. Industrial applications call for a laboratory analysis, while for domestic and small commercial use the trio of TDS, hardness and pH is usually enough to point in the right direction.

Common Symptoms and Their Causes

This category lists membranes and membrane vessels. When you send us the values of your feed water and your target output, we can decide the right class together.

The Difference Between Membrane, Softening and UV

All three methods improve the water but they solve different problems; they are not substitutes for one another.

The reverse osmosis membrane separates the dissolved salts and lowers the TDS value. It is used for drinking water quality, pure water production and process water preparation. Water softening, by contrast, removes only calcium and magnesium — that is, hardness — through ion exchange; it does not lower the TDS value. This is the answer to a limescale problem, and it protects the membrane at the same time. UV (ultraviolet) treatment is a stage aimed at reducing microbiological risk; it does not change the chemical content of the water and removes neither salt nor hardness.

In practice these three are most often designed to work together: softening ahead of the membrane reduces the limescale load, the membrane separates the dissolved salts, and the UV stage is placed at the final step when it is needed. What decides which combination is required is the water analysis and the intended use.

What Determines Membrane Life

Protected properly, a membrane runs for years; under the wrong conditions it is finished in a few months. Four factors set its life, and all four have to do with the rest of the system.

• Chlorine contact: It wears the membrane surface and the damage cannot be reversed. For that reason the carbon stage must sit ahead of the membrane and must be replaced on time.
• Sediment: If the sediment filter is not doing its job, suspended solids settle on the membrane surface and reduce its permeability.
• Limescale and salt precipitation: As the recovery rate rises, the concentration on the concentrate side increases and the risk of precipitation grows. On industrial lines antiscalant at 9,803.68 TL is dosed to delay this risk.
• Drying out: A membrane left without water for a long period permanently loses capacity. On systems that are to be taken out of service, keeping the membrane wet matters.

Knowing When a Membrane Needs Replacing

Membrane replacement is a decision to be made by measurement, not by the calendar. Typical life on mains water is 2–3 years, but feed quality changes that period noticeably.

The most reliable indicator is the TDS value of the outlet water. Note the value you measure while the unit is new; that is your reference point. If the outlet TDS rises noticeably over time, the membrane is worn out. With a TDS Meter at 1,225.46 TL you can keep track of this yourself.

A drop in flow does not on its own point to the membrane. The pre-filters, the pressure tank charge and the pump must be checked first. When a membrane is worn out it is usually not the flow but the water quality that deteriorates.

Wastewater Ratio and Recovery

Reverse osmosis systems produce wastewater as a requirement of how they work. This water protects the membrane by sweeping away the salts that collect on its surface. Reducing the wastewater ratio to zero is not possible; the aim is to keep the ratio at a reasonable level without harming the health of the system.

Three things decide recovery: the salinity of the feed water, the operating pressure and the class of the membrane. As salinity rises, the reasonable recovery rate falls. On industrial lines, pre-softening and antiscalant dosing are used together to raise recovery; part of the wastewater can be put back to use in suitable processes.

On domestic units, rather than trying to change the wastewater ratio directly, it is better to focus on keeping the membrane and the pre-stages healthy. A blocked pre-filter has a negative effect on the wastewater ratio as well.

Choosing Brand and Model

At Water Point we work with FilmTec, Toray, LG, STAR, TechFlow and Vontron products on the membrane side. What decides the brand is not prestige but the fit with the class the system was designed for.

With standard-sized consumables such as filter cartridges, the use of equivalents is common and usually causes no trouble. With membranes the situation is different: salt rejection rate, pressure resistance and permeability values change from model to model. Going outside the class the system was calculated for may look cheap in the short term, but it comes back as lower output, higher energy consumption and early failure.

• Mains water, low salinity: The low-pressure (LP/ULP) class is enough and saves pump energy.
• Processes that need high salt rejection: Standard-pressure, high-rejection membranes are preferred.
• Seawater: Only a seawater-class membrane can be used.
• Increasing capacity: Instead of a bigger membrane, increasing the number of membranes of the same class is usually the better answer.

What to Watch During a Membrane Replacement

• Release the pressure. Before opening the pressure vessel, shut off the line and wait for the pressure to drop.
• Fit it the right way round. The feed and concentrate direction of the membrane is fixed; a membrane fitted the wrong way round produces nothing.
• Check the O-rings. The end O-rings of the membrane stop the clean water from mixing with the concentrate. A worn O-ring raises the outlet TDS, and the membrane gets the blame.
• Replace the pre-stages at the same time. Fitting a new membrane behind old, blocked filters means wearing the new membrane out fast.
• Do not use the first water. After a new membrane is commissioned, the water produced first should be run to drain for a while.
• Take a reference measurement. Note the outlet TDS after the replacement; the next decision will be made against that figure.

Membrane and Housing Prices 2026

The prices below are the current values on the product pages and include VAT.

What Buying a Membrane from Water Point Gives You

• This group is consumable material; because their life depends on water quality and on use, they fall outside warranty cover.
• CE certified products — we work with products that fall within the scope of certification.
• ETBİS-registered e-commerce — we are registered in the Ministry of Trade system.
• Invoiced sales — a legal invoice is issued with every order.
• The right class match — we decide the right membrane class together, according to the feed water and the pressure of your system.
• Free site survey available — we establish by measurement whether the membrane is genuinely finished. see the details
• Türkiye-wide service network for industrial systems — for domestic units we serve İstanbul and the surrounding provinces (such as Kocaeli and Tekirdağ).

Address: Mehterçeşme Mah. 1919 Sk. No: 54/B Esenyurt / İstanbul.

Cannot find the spare part you are looking for?

Send us the brand and model of your unit and we will decide the right filter, membrane, valve or resin together. If the unit is purchased from us the site survey is free; if only a survey and measurement is requested, a site survey fee of 2,000 TL applies.