The reverse osmosis system

One of the most important aspects of the aquarist is to raise fish, plants and invertebrates in an environment that reproduces the original biotope as faithfully as possible.

The first aspect to consider is certainly that of water, especially if our goal is to breed particularly sensitive and not very adaptable species.

In Italy tap water, mainly in large urban centres, often contains high quantities of dissolved substances, which far exceed the Northern European standards.

It is not uncommon to find traces of heavy metals and chlorine; the carbonate hardness and the total hardness with values ​​enormously above those optimal for an aquarium, the presence of polluting salts such as nitrates, silicates, and phosphates; PH values ​​ranging between 7 and 8 (and which in any case in the aquarium, given the high concentration of buffer substances, would stabilize on definitely alkaline values).

A good rule of thumb, to understand if it is possible to use your own tap water, is to check its chemical values ​​(KH, GH and NO 3 are sufficient or, if you have a conductivity meter, MS) and compare them with a table indicating the parameters of the biotope to be reproduced, such as those reported in many books or in the following table.

If the values ​​found are obviously incorrect, it becomes necessary to use demineralised water which can be purchased at all specialized aquarium shops (attention to the demineralised water which is used for irons and batteries is not suitable), or it can be produced directly in your own home.

In some cases, such as in the freshwater breeding of African Cichlids, Poecilidae, Carassi or other hard water fish, it will be sufficient to let the tap water rest for 24 hours and add a water conditioner to eliminate chlorine and metals.

The same can be said for the marine aquarium if robust predatory fish such as moray eels, triggerfish, groupers, etc. are kept. (although in this case, it would be better to at least cut the tap water with half of demineralized water).

Conversely, if we wanted to reproduce a South American or Asian river, with many plants and demanding fish such as Discus or dwarf Cichlids, the use of demineralised water becomes extremely advisable, and in the case of reef aquariums, perhaps with Sclerattinia and Acropora, or new fascinating “extreme” freshwater aquariums (Japanese bonsai aquariums to be clear) absolutely mandatory.

It should also be remembered that tap water with high concentrations of dissolved substances, in addition to debilitating the most sensitive fish and leading to the deterioration and premature death of the most delicate invertebrates and plants, due to the presence of certain salts, increases the probability of proliferation of algae, especially diatoms, cyanobacteria, and brush black.

The most effective and currently most used method in aquariology to treat and demineralize tap water is certainly the reverse osmosis system, which has almost definitively supplanted the use of ion exchange resins, which are much more complex to manage.

The reverse osmosis system is nothing more than a device consisting of a semi-permeable membrane and two pre-filters (one mechanical and one activated carbon), which is connected to the home water supply is able to produce water almost completely free of dissolved substances.

Pressurized tap water is first passed through the activated carbon filter, which in practice has the function of dechlorinating, then through the cotton-wrapped thread cartridge, which retains the coarser impurities (up to 5 microns) which could end up in the semipermeable membrane and clog it (for example the particles of the activated carbon itself), and finally through the osmotic membrane which eliminates from 80 to 98% of all the other substances (salts, heavy metal ions, organic compounds, but also any pesticides, bacteria, etc.).

At the outlet there will be two types of water, the demineralized one (called permeate) which we can collect in a tank, and the waste one, concentrated in substances extracted from tap water, which instead must be eliminated.

The quality and quantity of water produced are determined by various factors, first of all by the size and type of membrane. There are two types of membranes, the membranes in CTA (cellulose acetate) which are cheaper but short-lived, because they are easily attacked by bacteria which settle in the event of periods of inactivity, and the membranes in TFC-Thin Film Composity (aromatic polyamide) a little more expensive but absolutely more reliable.

Other important factors are the operating pressure, which generally must range from a minimum of 3 to a maximum of 8 bar (the higher the pressure, the greater the quantity and the better the quality of the water produced), the temperature (the is the temperature the greater the quantity of water produced) and clearly the original quality of the water to be treated.

If the water pressure in your water mains is lower than 3 bar (for example if you live in apartments on high floors), and you are not satisfied with the quality of the water produced, a pump can be installed upstream of the system to increase pressure. Reverse osmosis systems used in aquariums (non-professional) produce on average from 30 to 200 litres of demineralised water per day.

The installation of a reverse osmosis system is very simple, any place equipped with a water connection is suitable, for example, the bathroom, the kitchen, the terrace or the garden (if kept outside, during the winter, the freezing of the water contained in the membrane which would cause irreparable damage).

If you have the possibility (perhaps during the design of the aquarium at the same time as that of a new home), it will be very convenient to bring the water supply and discharge and install the system, near the aquarium, especially if you intentionally create an open tank with an automatic refilling system for the evaporated water.

To connect the system, simply connect the inlet pipe to a tap (the pitch is generally 34 of an inch but adapters for other threads are often supplied), and insert the waste water pipe into a drain ( for example that of a sink).

Once the tap is opened, the membrane will start producing demineralised water after a few seconds. It is recommended to always throw away the first 4/5 litres of water because it is not perfectly purified, and in the case of a new appliance, it is essential to let it work for a couple of hours to extract the substances introduced during the production phase for the conservation of the membrane.

A significant problem is represented by possible flooding, as we have already seen a reverse osmosis system produces water very slowly, and it is not at all improbable to forget to close the flow before the tank used for storage fills up and overflows (once I had to reach the faucet in a dinghy).

A good precaution is to put the tank in the bathtub or in the shower tray, the more cautious can install a solenoid valve with a built-in timer directly on the tap (those for gardening are very reliable and cheap).

The osmotic membranes are delicate and over time tend to get clogged and if subjected to the action of chlorine degrade. Good maintenance of the two prefilters, which should be replaced every 4-6 months, is the best way to safeguard the life of the membrane for as long as possible.

There are “disposable” pre-filters, which are convenient but uneconomical, and inspectable pre-filters, definitely to be preferred, which allow only the filter materials to be replaced (activated carbon, which must always be rinsed beforehand under running water, and the wound wire cartridge).

A further precaution is not to leave the system inactive for too long, and at least every two weeks it is a good idea to let it work for a few hours, in this way, the membrane is rinsed and stagnant water and any bacterial settlements are eliminated.

Some systems are equipped with a discharge valve downstream to carry out the rinsing of the membrane more efficiently, this operation must be carried out as often as possible for about 10-15 minutes (some companies recommend a daily rinsing, and supply, connected to the drain valve, a solenoid valve managed by a timer which automatically performs the operation).

If you intend to decommission the system for a long period, a preservative substance (glycerin and sodium metabisulphite) must be introduced into the membrane to prevent it from being damaged due to inactivity.

Unfortunately, sooner or later the membrane of our system will have to be replaced (if the membrane is of good quality and if you carry out regular maintenance on the prefilters it can still last for years), to understand the level of degradation it is enough to periodically carry out the chemical analyzes of the water produced, the most immediate parameter to control is that of conductivity (mS) which indicates the total quantity of dissolved substances and which is carried out using a conductivity meter (quite expensive digital device), but also simple analyzes of KH, GH and Nitrates carried out with normal tests for aquarium use are indicative. Even in the event of clogging (the production of water decreases over time) it will need to be replaced or cleaned by the manufacturer’s assistance centres.

Pure demineralized water must not be used directly in the aquarium because it is too poor and aggressive, it will be necessary to partially reintegrate some of the essential substances that we had previously eliminated with the treatment.

It is possible to do this by cutting it with tap water (even if in this way we partially cancel the positive characteristics) or by using special supplements available on the market to bring the carbonate and total hardness to the correct values and to make trace elements, fertilizers, vitamins, colloids, etc.

Osmosis Osmosis
is a natural principle in which water is transferred through a semi-permeable membrane from a diluted saline solution to another with a higher concentration. A membrane is said to be semipermeable when it has holes large enough to let some molecules through, but too small to let others through.

For example, a membrane that allows the water molecules (solvent) to pass through, but not other molecules (solute) present in the solution, behaves like a semi-permeable membrane.

Like any diffusion process, osmosis originates from a chaotic movement of particles tending towards a state of equilibrium, in this case, it is an equilibrium between water molecules on the two sides of a membrane. The phenomenon was observed for the first time by the abbot JA Nollet who in 1748 observed that an animal bladder containing alcohol immersed in water swells until it bursts.

With the transfer of the water molecules into the more concentrated solution, a characteristic pressure arises in the compartment of the same, exerted by the solute molecules against the walls, which tends to oppose diffusion (osmotic pressure).

By applying pressure higher than the osmotic pressure in the compartment of the solution with the highest concentration, the process is reversed giving rise to the phenomenon of reverse osmosis, in this case, the water molecules pass through the membrane, while the salt molecules rejected. The organic molecules and almost all the microbial loads are also rejected.

Chemical values ​​table

ValueMilan tap water valuesValues ​​of Milan tap water treated with an osmosis systemAverage values ​​found in South American riversAverage values ​​found in rivers of Southeast AsiaOptimal values ​​in the aquarium
GH30th0,1°- 4°0,1°- 3°5th-10th
Nitrates30-50 mg/lLess than 10 mg/l0-tracks0-tracksLess than 20-30 mg/l
Silicates10-20 mg/l0-1 mg/l0-tracks0-tracksLess than 0.5 mg/l

Percentage of substance reduction using a reverse osmosis system.

ElementReduction percentageElementReduction percentage
Selenium93-98%Total hardness93-97%
Silica80-90%Carbonate hardness90-95%
Cyanide85-95%Parts in suspension90-96%

The data are indicative and may vary depending on the pressure, temperature and saline content of the feed water.

Technical table

Operating pressure3-8 bars
Temperature range1 – 30°C.
PH range2 – 11
Produced water/waste water ratio14
Percentage of substances eliminated80 – 98%

The data are indicative and may vary depending on the membrane used.

Elizabeth Canales is fond of marine life since childhood and got an aquarium as a gift from her father on her 6th birthday. Since then she started to take care of Coralia (her goldfish). Her love for fish made receive her B.S. in Marine Biology from the University of Washington and DVM from Delaware State University.

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