What Damages an RO Membrane: Chlorine, Scale, Sediment and Low Pressure

An RO membrane rarely just wears out on its own. In almost every case where output slows down or the TDS reading on the display starts climbing, one of four things has reached the membrane: free chlorine that was never stripped out upstream, hardness minerals that have scaled its surface, sediment or bacterial film that has fouled it, or feed conditions, pressure, temperature, or a dry run, outside the window the system is specified for. Reading the symptom correctly, and ruling the stage-1 pre-filter in or out first, matters more than guessing at a cause.

• Free chlorine oxidises the membrane's thin polyamide layer directly, and that kind of damage does not reverse once it has happened.
• Hardness scale and sediment build up more gradually, and tend to concentrate on the reject side of the membrane rather than across its whole surface at once.
• A flashing stage-1 PPC indicator rules the pre-filter in as a suspect before the membrane itself needs blaming.
• Keeping the stage-1 cartridge current and running the system on treated mains water are the two habits that protect the membrane from one week to the next.

Why a Rising TDS Number or Slower Flow Doesn't Automatically Mean a Dead Membrane

A TDS reading that creeps up by a few points over several weeks is telling you something different than one that jumps overnight, and the same goes for output: water that comes out gradually slower points somewhere different than water that all but stops. Before assuming the membrane itself has failed, check the stage-1 PPC indicator on the display. It flashes red once the cartridge has less than five percent of its life left and stays solid red once replacement is due, and that single signal rules the pre-filter in or out as the explanation before you look any further.

What a Worn Membrane Looks Like

A membrane that has actually lost its ability to reject dissolved solids produces water with a TDS reading that keeps climbing even right after a stage-1 change, because the problem sits downstream of that cartridge. Output may also fall as the membrane's effective surface area shrinks under fouling or scale. The pattern to watch for is a TDS number that does not come back down once the sediment and carbon stage is fresh.

What a Clogged Pre-Filter Looks Like

A spent PPC cartridge mainly shows up as slower flow at the faucet, because the sediment layer is restricting how much water reaches the membrane in the first place, and the TDS reading is usually closer to normal since the membrane itself has not been compromised. If a fresh stage-1 cartridge restores both flow and the TDS reading, the membrane was never the problem. If the symptoms persist, the stage-2 membrane itself is the part worth checking next.

Chlorine Is the Fastest Way to Damage the Membrane

Thin-film composite membranes, the type used in the RO600, are built around a polyamide layer that free chlorine attacks on contact. Documented testing on this membrane chemistry describes the damage pattern as an initial loss of flux followed by rising salt passage: the membrane first produces less water, then starts letting more dissolved solids through, rather than failing all at once (source: WaterWorld, on protecting RO membranes from chlorine damage). That same source describes the damage as irreversible once it occurs, which is the stated reason the RO600's first stage pairs a carbon block with the sediment layer: stripping chlorine before the feed water reaches the membrane is the design purpose of putting a carbon-containing stage first, a point covered in more depth in why an RO system still needs a carbon stage. A pre-filter cartridge that has gone past its interval stops doing that job quietly, long before the flashing indicator catches up.

Hardness Scale Builds Up Gradually, on a Different Timeline

Scale forms when calcium or magnesium, and less often barium or strontium, are present alongside bicarbonate or sulfate in the feed water (source: Water Research Center, on reverse osmosis fouling). Unlike chlorine attack, scale concentrates on the reject side of the membrane, where minerals left behind by the water that does pass through become steadily more concentrated as the remaining water is sent to drain. The RO600's 2:1 pure-to-drain ratio sets how much concentration happens before that reject stream carries the minerals away, and scale tends to show up as a gradual fall in output over months rather than the sharper rejection drop chlorine causes. It reflects the feed water's own chemistry rather than how the system was used.

Sediment and Biological Fouling Clog the Membrane Surface

Rust flakes from piping, silt, and other particles that slip past a spent or missing pre-filter settle on the membrane surface and physically restrict flow, and the same thing happens with bacterial film when water sits untreated long enough for it to establish itself (source: Water Research Center, on reverse osmosis fouling). Spiral-wound membranes, the construction used across home RO systems, cannot be backwashed or air-scoured the way some other filter media can, so material that fouls the surface tends to stay there (source: Wikipedia, on reverse osmosis pretreatment). That is the main reason the stage-1 cartridge's sediment layer and carbon block matter even when the water tastes fine: they catch particles and organisms before either one reaches a part of the system that cannot be cleaned the same way.

Feed Pressure and Water Conditions Outside the Specified Range

The RO600 is specified for 1.4 to 5.5 bar of feed pressure and 4 to 38 degrees C of cold, treated mains water. Running well outside that window is a feed-water-quality issue rather than something the membrane is built to absorb indefinitely, and low feed pressure mainly shows up as reduced output and a less efficient split between permeate and reject water rather than as the kind of surface damage chlorine or scale causes. A booster pump running with no water supply to feed it is a pump problem first, and the manual's own instructions on connecting the feed line and checking the water supply are what prevent that situation; this article does not go beyond pointing to them.

What Actually Protects the Membrane Day to Day

Out of everything covered here, the habit a reader fully controls is changing the stage-1 cartridge on schedule, roughly every six months or after about 1,700 to 1,900 litres, before its carbon block stops stripping chlorine and its sediment layer stops catching particles.

Running the system on treated mains water rather than untreated well or surface water matters just as much, since that is the feed chemistry the membrane and its pre-filter were specified around. For a closer look at whether cleaning or replacing the membrane makes more sense once these symptoms show up, see whether an RO membrane can be cleaned. If a schedule-based approach appeals more than reacting to symptoms as they appear, how often to change the membrane and the other filters lays that out stage by stage. And when the issue turns out to be unrelated to the membrane altogether, such as a leak, an odd taste, or a flow problem elsewhere in the system, the broader troubleshooting guide for reverse osmosis systems covers that ground. Readers who want the fundamentals of how reverse osmosis works before troubleshooting damage can start with the full reverse osmosis explainer, and anyone whose main question is what their TDS number itself means, rather than why it changed, is better served by the dedicated guide to TDS readings.