FAQ & Beginners guide to reverse osmosis

If I ended up on this site january 2011, everything would have been very new to me. If you feel like this, this page is for you.

Isn't the tapwater good enough?

The water in EU has generally been ok for many years, but since year 2000 increasing amounts of harmful substances are found in the groundwater and drinking water, for example pesticides from agriculture. I pay a little to be sure to get pure drinking water and to avoid more kidney stones, and I had severe awful pain in 2011 from kidney stone attack.

Who purifies tapwater and why?

I know 3 groups of people who want clean water: aquarists, window cleaners and people like me who want clean drinking water without toxic substances.

Reverse osmosis is best

There are many different methods for purifying water, but everyone agrees that reverse osmosis is the best and most economical method to purify water. The method has been known since 1950 and therefore it is used both industrially and in private homes. There are plants which produce thousands of liters of water per hour.

The pollutants are washed away and lead out into the drain. In order to push the water through the membrane, most reverse osmosis plants need a water pressure of 3 bar.

Reverse osmosis removes all pollutants such as nitrate, nitrite, fungicides, herbicides, medicine residues, hormones, asbestos, bacteria etc. from the water.

Sailors have been using reverse osmosis to make drinking water from seawater since 1950, when the technique was 'found' by a guy in California. The beauty of the technique is that it makes use of the laws of physics. Many breweries, and other industries that need clean water, use it.

Who invented and developed reverse osmosis?

The possibility of reverse osmosis was first discovered by scientist and researcher Sourirajan at University of California in the 1950s. First used for desalination of sea water, reverse osmosis was so promising that the US government launched an extensive research program, which many large companies were involved in. During 8 years of research, and an investment of over $33 million, the most modern and most advanced water treatment technology of our time was born.

Filter and membrane

A reverse osmosis system typically consists of 2-3 filters followed by a membrane. The membrane is expensive, so you try to filter as much as possible before the water comes to the diaphragm to make it last as long as possible.


Without membrane, no reverse osmosis ! Both filters and membranes are designed as cylinders and are placed into a plastic container, and the vessels are connected with thin plastic hoses. As shown a membrane consists of many thin layers of film with ultra-small pores, which, in principle, only water molecules can slip through.

A reverse osmosis membrane has wholes of µ (0,0001µ). The unit is micron, or just µ. 1 µ er en milliontedel meter eller en tusindedel millimeter (0,001 millimeter).

Element µ
Reverse osmosis membranes holes 0,0001µ
Seasalt molecule 0,0007µ
Calcium carbonat molecule 0,0009µ
Nanofiltration removes pesticides from ground water, heavy metals and more  
Ultrafiltration is needed to remove all vira 0,001-0,1µ
Smallest virus 0,02µ
Microfiltration removes all bacteria, but not all vira 0,1-10µ
smallest bacteria 0,2µ
smallest particle, that the human eye can see 50µ
human hair 100µ

As mentioned above, the membrane is the most expensive part, and therefore one attempts to purify the water as much as possible before the water reaches the membrane.

BEFORE the membrane first a sediment filter is added and then one or two carbon filters and then the membrane, and the pure water comes out of the membrane housing, which also deflect the waste water.


Sediment filter (right) takes care of the coarsest particles such as sand, rust and lime, while one or more carbon filters (active carbon blockfilter and active carbonfilter) cleans smaller molecules such as chlorine, organic chemical products, etc..

An active coal block filter (far left) can absorb a little more than an active carbon filter (also called GAC, Granulate Active Coal), but is also a bit more expensive.

The membrane is placed in a membrane housing, and this therefore has two outputs: the farthest discharging the pure osmosis water. Membranes are produced with different capacity, identified by Gallons Per Day

Then the filters and the membrane are placed into the containers, called filter housings and membrane housings (left photo):

The houses are connected with small, white plastic tubes.

Answers to all questions about reverse osmosis

Here is a lot of good answers to typical questions, if you have an aquarium.

What happens to all the toxic substances that are separated from the water by the membrane?

The pollutants are automatically washed off the membrane and lead down the drain. Here lies the secret to the high efficiency and durability of the systems. The membrane works not as an ordinary filter, which only absorbs impurities, in which bacteria can grow, but has a self-cleaning mechanism that automatically removes the secluded impurities.

A simplified representation of the operation: tap water passed through the membrane, becomes divided into two streams. A stream permeates the membrane, leaving impurities back. The second flow flushes the impurities from the surface of the membrane, and then directly down the drain.

This prevents contaminants from settling on the diaphragm. Thus, it remains clean all the time, which ensures membrane durability. Depending on the model, waste water is around 2-4 times osmosis-water.

How to connect the plant?

See the different ways to connect the unitplant

Osmosis plant with or without tank?

The tank has a rubber bladder on the inside, which has a certain air pressure on one side. The water then pushes on the other side in and compresses air. If you then open the tap, then compressed air will push water out.

The membrane is a non-return valve on the incoming water side built in. Once the tank is full, and the tap is closed, the water in a so-called 4-way-valve pushes a flask and thus also closes on the waste water side. If there is no pressure, the plant begins working again. Actually, tank and faucet get their supply through a hose that is shared through a T-piece: The water flows through the T-piece, where there is least or no pressure

When the 10-liter tank is empty, a 125GPD plant WITHOUT a pump takes nearly an hour to fill the tank. A similar plant WITH a pump takes only half an hour. Video om vandtank

Water storage

It is recommended to store water in glass containers. IKEA has many cheap glass containers with a rubber seal

Detailed description of reverse osmosis:  what-is-reverse-osmosis

Read more about which filters and when to change them

purified-healthy-water.comContact • last update: 01 Jul 2017