Reverse Osmosis

Mains water needs some treatment before it can be used in our marine aquariums, because it contains chlorine, grit, rust and dissolved salts (TDS): sodium, potassium, calcium, magnesium, chlorides, sulphates, bicarbonates and sometimes nitrates, phosphates and silicates. The "dissolved salts" must be removed to get water with a TDS value of zero.

How the unit is built

As an illustration, I attach photo no. 1 (at the bottom of the page) showing a five-stage reverse osmosis unit:

• three housings hold the cartridges called sediment and carbon;
• a vessel holds the osmosis membrane;
• one cartridge (in this case the disposable version) contains the mixed-bed resins.

Reverse osmosis units generally come with a fitting for the washing-machine tap, which lets household tap water flow into the unit.

1. 1Tap mains water with chlorine, grit, rust and dissolved salts
2. 2Sediment cartridge stops rust, grit and particles: 1, 5 or 10 micron
3. 3Carbon block absorbs chlorine
4. 4Third housing the author only names sediment and carbon
5. 5Membrane in the vessel holds back about 94-98% of dissolved salts
6. 6Resins mixed-bed or colour-changing: remove what is left
7. 7Waste what the membrane does not let through
8. 8RO water the goal is a TDS of zero

Each stage removes something: first the solids, then the chlorine, then the salts, finally what is left.

Sources: [1]

The pre-filters

The water comes out of the household tap and first passes through the first pre-filter, which holds a cartridge that stops sediment (usually a 5-micron one is used, but 1- and 10-micron versions are also available), after which it passes through the next pre-filter containing the carbon block. Note that these pre-filters (disposable cartridges or housings holding the cartridges) only hold back solid impurities. In particular, there are the following types of cartridge, which are best replaced every six months:

1. Sediment cartridge, available in 1, 5 and 10 micron versions; it removes rust, grit and all suspended particles. (See fig. 2)
2. Cartridge containing activated carbon block to absorb chlorine. (See fig. 3)

The reverse osmosis process

The pre-filters above cannot absorb or hold back chemical and bacterial impurities, and this is where the "reverse osmosis" process comes in, using osmosis membranes able to remove most of the mineral salts in the water, making it similar to distilled water.

In practice, the "reverse osmosis" process exploits the semi-permeability of the membranes to separate from the water the substances dissolved in it, known as TDS. (See fig. 4)

It should also be said that, through "semi-permeability", the membranes can remove inorganic substances and some organic ones; in fact they are an effective barrier against micropollutants, pesticides, pyrogens, viruses and bacteria, should these be present in municipal mains water.

In particular, "semi-permeability" needs a water pressure of no less than 3.5 bar; otherwise you need a booster pump (matched to the type of membrane in use), which will raise the pressure in the vessel holding the membrane (see fig. 5).

To recap: water from the mains is treated by the pre-filters mentioned above, then reaches the vessel holding the osmosis membrane, which holds back and rejects about 94-98% of the dissolved salts.

1. 1Pressure at least 3.5 bar, otherwise a booster pump is needed
2. 2Semi-permeable membrane sits in the vessel and lets water through
3. 3Dissolved salts (TDS) about 94-98% stays behind
4. 4Permeate still contains traces of salts
5. 5Waste carries away what was held back

The membrane holds back the salts, not the water: that is why it needs pressure behind it.

Sources: [1]

The membranes: 50, 75 and 100 GPD

Note that reverse osmosis units for producing aquarium water come with three types of membrane: 50, 75 and 100 GPD:

1. 50 GPD produces about 190 litres/day, with a rejection rate of about 98% of the TDS in our mains water;
2. 75 GPD produces about 220 litres/day, with a rejection rate of about 96% of the TDS in our mains water;
3. 100 GPD produces about 240 litres/day, with a rejection rate of about 94% of the TDS in our mains water.

I would like to point out that the production figures above, in litres of RO water, vary with the TDS, temperature and water pressure in our mains supply.

Membrane	Litres per day (author)	Salts rejected (author)	Gallon conversion only
50 GPD	about 190	about 98%	about 189 litres
75 GPD	about 220	about 96%	about 284 litres
100 GPD	about 240	about 94%	about 379 litres

The first three columns are the author's figures. The last is a simple calculation: 1 US gallon = 3.785 litres.

Sources: [4]

The post-filters

Note that the water produced by these reverse osmosis membranes contains traces of dissolved salts that must be removed with one or more post-filters and/or cartridges, which in all cases contain mixed-bed or colour-changing resins able to remove silicates, nitrates and phosphates; see fig. 6, mixed-bed resins, and fig. 7, colour-changing resins.

If your tap water is very hard

If we find that our household tap water has a TDS above about 600, I recommend buying and fitting a water softener, containing ion-exchange resins. These resins can hold back large amounts of calcium and magnesium and, through a chemical reaction, release sodium ⁵. In this case, the softener should go at the start of our reverse osmosis unit, or at the inlet of the household water supply, in which case we will also extend the life of our plumbing, such as the boiler coil and tap cartridges.

1. 1Water softener contains ion-exchange resins
2. 2Calcium and magnesium the resins hold back large amounts
3. 3Sodium the resins release it in exchange
4. 4RO unit receives water already softened
5. 5Household inlet the softener serves the whole supply
6. 6Plumbing boiler coil and taps last longer

Two possible places: before the RO unit, or at the household inlet to protect the plumbing too.

Sources: [5]

Photo no. 1

author: Ciro Murino

Sources

1. U.S. Environmental Protection Agency (2026). Overview of Drinking Water Treatment Technologies. EPA.
2. Centers for Disease Control and Prevention (2022). Avoid Contaminated Water During Travel. CDC.
3. Gohil J.M., Suresh A.K. (2017). Chlorine attack on reverse osmosis membranes: Mechanisms and mitigation strategies. Journal of Membrane Science 541: 108-126.
4. National Institute of Standards and Technology (2008). NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity (volume). NIST.
5. U.S. Environmental Protection Agency, WaterSense (2026). Cation Exchange Water Softeners. EPA.