Used vs New Water Filtration

Water Filtration  · 

Reverse osmosis membranes and the rejection test

The membrane is the expensive part of a reverse osmosis unit and the one part you cannot see. A membrane exposed to chlorine, run dry, or left fouled with iron and scale can look intact and still pass almost everything. The practical test is a dissolved solids reading on both sides of it. Measure the feed at a nearby tap, then measure the permeate from the unit’s own faucet after it has run long enough to clear the storage tank. Rejection is the gap between the two readings: a membrane in good order holds back most of the dissolved solids in the feed, while a spent one lets them through and the permeate reads close to the feed. A handheld meter is enough for this; it is a trend instrument rather than a lab test, but the comparison is the right one.

Permeate flow is the second signal. A thin, slow stream can mean a fouled membrane, a failing booster pump, low feed pressure or a waterlogged tank, so check flow with the tank empty and again with it full. Ask what the unit was fed with. If the seller cannot say whether the supply was chlorinated, assume the membrane has seen chlorine and price the replacement into the offer before you make it.

Housings, sumps and cracks

Inspect every plastic pressure part. The usual failure is a hairline crack at the shoulder or in the threads of a filter sump, or a crack near the inlet port of an RO manifold. Clear housings cloud and embrittle with age and ultraviolet light, and a housing that has gone chalky is more likely to split under line pressure than one that is still glassy. Check the threads on both the sump and the head for cross-threading and stripped plastic, and check the o-ring groove for nicks, grit and gouges. A sump that a previous owner forced on cross-threaded will weep no matter how carefully the next owner seats it.

Then confirm the housing takes a cartridge that is still sold. Many units use a standard ten-inch or twenty-inch cartridge in a slimline or big-blue body, and those are easy to keep running for years. A proprietary housing tied to a discontinued cartridge is a dead end once the last spare is gone, and the seller’s remaining cartridges are not a supply. A used unit with a cracked sump and an obsolete cartridge is scrap with a pump attached to it.

O-rings, seals and the parts that wear

Every seal on a treatment unit is a wear item. Housings close on a flat gasket or an o-ring; reverse osmosis systems add brine seals around the membrane, lip seals in the pump head, a flow restrictor in the drain line and check valves on the permeate outlet. A drip at a sump is almost never the housing. It is a flattened o-ring that has taken a set, a grain of sand sitting in the groove, or a groove that was never cleaned.

Treat every seal on a used unit as a replacement item, along with the sediment and carbon pre-filters and the post-filter if one is fitted. Then check the two valves that decide whether the unit behaves. A leaking check valve lets the storage tank push permeate back down the drain, which shows up as a drain line that never stops running. A stuck autoshutoff valve keeps the pump running against a full tank, which is the fault that quietly destroys booster pumps and membranes.

Pumps, boosters and what the noise tells you

A booster pump should hum steadily while it runs and stop when the storage tank is full and the faucet is closed. Grinding or rattling points at a worn pump head or air in the line. Rapid on-off chatter points at a failing pressure switch or a tank that cannot hold pressure. A pump that runs continuously and never shuts off points at a failed autoshutoff valve, a closed tank valve or a tank with no air charge left.

Demand pumps that sit on the permeate line sound different from booster pumps on the feed line, so identify the type before calling a noise a fault. Check the transformer, the wiring and any float switch for corrosion, and check whether the unit is sold with its power supply. A pump is a replaceable part and often a reasonably cheap one; a pump that has been run dry for a season usually is not the only casualty, because the membrane behind it has been fed at low pressure the whole time.

Pressure tanks and precharge

Most reverse osmosis systems store water in a bladder tank with a charge of compressed air behind the bladder. The precharge does the work: as permeate enters, it compresses the air, and that pressure pushes water out when the faucet opens. The charge is checked with a tyre gauge at the Schrader valve with the system depressurised and the tank drained. Checking it with the tank full of water gives a reading of water pressure, not air charge, and tells you nothing useful.

A tank that has lost its charge, or whose bladder has ruptured, is heavy, holds very little usable water and makes the unit short-cycle: it fills, shuts off and restarts every time a glass is drawn. Shake it and weigh it. A tank that sloshes and feels full with the system off has lost either its air or its bladder. Some tanks have a replaceable bladder and can be brought back; sealed tanks are replaced. Check that the air valve is not corroded and that the tank connection will actually come off the fitting, because a seized connection can turn a cheap tank swap into a new manifold.

Obsolete cartridges, stagnant water and the real arithmetic

Before any used system is worth the trip, confirm the consumables are still made. Cartridge families retire, housings get revised, and a unit that takes a discontinued proprietary cartridge is only as good as the seller’s remaining stock. Look up the part number and confirm replacements come from more than a single listing, then check whether the membrane is a standard size or a proprietary one, because a proprietary membrane with no supply is the end of the unit.

Hygiene matters as much as the mechanics. A unit that has sat with water in the housings, the tubing and the tank grows biofilm on every wetted surface, and the inside of the storage tank is the worst of it because it is dark, wet and hard to reach. Assume a used system needs a full cleanout: new cartridges, a new membrane if the rejection test fails, housings and tubing flushed and disinfected, and the tank sanitised or replaced. Add that list up before deciding. The parts you have to replace — membrane, tank, pump, housings — are most of what the unit is, so a used purchase is only a bargain when the frame, faucet and fittings are sound and the consumables were replaced recently.

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FAQ

How do I test a reverse osmosis membrane before buying a used system?

Take a dissolved solids meter, read the feed water at a nearby tap, then read the permeate from the unit’s faucet after it has run a while. Permeate close to feed means the membrane is finished. Check as well that permeate flow is a steady stream rather than a dribble, with the storage tank empty and again when it is full.

Can a used water softener be reused?

Usually yes, and more easily than a used reverse osmosis unit, because there is no membrane to fail. The resin may be fouled with iron or damaged by chlorine, and the control valve, brine line and injector need checking. A softener that sat with brine in the tank should be sanitised and the resin inspected before it goes back into service.

Is a waterlogged pressure tank worth repairing?

If the tank has a replaceable bladder and the shell and fittings are sound, a new bladder and a recharged air valve can bring it back. Sealed tanks with a ruptured bladder are replaced, and a tank whose connection is corroded onto the fitting may not come off without damaging the surrounding parts.

What is the biggest hidden cost of buying used treatment equipment?

The consumables. A membrane, a pressure tank or a set of housings can cost as much as the used unit did, and if the cartridge family has been discontinued you may be buying a device that cannot be kept running at all. Price those parts before agreeing a figure.