Water filtration by Reverse Osmosis (RO)
Reverse osmosis is a widely used technology for the desalination of water. Reverse osmosis is absolutely useful in technical applications under controlled conditions. However, the use in the home raises some questions we answer below.
What is reverse osmosis?
Reverse osmosis was originally developed in California in collaboration with the State of Israel to turn saltwater into drinking water. In reverse osmosis, a membrane made of polyamide (PA, see Wikipedia entry here) is used. The plastic is polymerized on a porous support material in such a way to create the largest possible surface. The first rectangular support is then glued to make a bag and the bag is then attached to a perforated pipe. The PA layer is located outside on the bag.
Then the bag is rolled around the tube which is facing forward, poking out from behind the rolled-up bag. A curtain-like spacer is then wrapped to allow the water to flow between the layers of the rolled bag. A spacer is also built in on the clean water side to allow the water in the bag surrounding the perforated pipe to flow to the tap.
That’s an awful lot of plastic offering bacteria plenty of room to grow, even on the purified side of the pipe.
How are substances transported through the reverse osmosis membrane?
The transport of water and dissolved particles takes place through these three mechanisms:
- A differential pressure between the inlet and clean water sides forces the water through the reverse osmosis membrane
- Ions in the water (hardness, salt, etc..) and dissolved organic substances pass through the membrane due to the difference in concentration between the inlet and clean water sides
- Germs and particles pass through the membrane due to defects in the polyamide layer, which can happen when dust particles get trapped in the coating of the membrane.
While the reverse osmosis membrane is very good at catching ions, it is not so good at catching dissolved organic matter. Depending on the type of pesticide or other dissolved organic matter, the retention rate is only between 20% and 80%.
How does reverse osmosis affect the water’s equilibrium?
Change of chemical equilibrium
Cold water that comes out of our tap is usually in chemical equilibrium, which means that no dissolved substances are being released and no substances are being absorbed from its environment. In some regions (for example, the foothills of the Alps, the water has excess calcium in it making it hard, while in other regions the water tries to absorb minerals as it flows by (for example, in the Bavarian Forest).
If one removes the minerals dissolved in water, this changes the water’s pH level and it will try to absorb new minerals to restore its equilibrium. The same thing happens when you change the pressure or temperature of the water. In reverse osmosis, H2O is extracted from the water and that has consequences.
Consequences of extracting H2O in reverse osmosis
- The concentration of the dissolved components on the feed side increases, causing a faster transfer of material through the reverse osmosis membrane
- On the feed side, the previously dissolved ions precipitate as crystals and create a layer on the membrane. This not only reduces the flow of the membrane dramatically, but the concentration of salts directly can theoretically increase infinitely, allowing a lot of dissolved substances to pass through.
- The water on the permeate side of the reverse osmosis now has a lower pH and stays in this state at room temperature in the pipes and reservoirs of the home’s water system until it is consumed.
Technical details for reverse osmosis
How much water does reverse osmosis consume?
Conventional reverse osmosis equipment used in domestic settings consume up to 4 liters of water to produce one liter of drinking water. The waste water is discarded into the trap of the sink.
- 1-4 gallons of water consumption for 1 gallon of drinking water
If each person consumes on average 1 gallon of water a day, a 4-person household will go through 17 gallons of water to produce their daily drinking water requirement! This is almost enough to fill a bathtub.
What happens in reverse osmosis if I don’t use any water?
When in a state of standstill, the same pressure initially holds on both sides of the membrane. The concentrate on the feed side of the membrane is no longer removed, but instead results in a substantially increased concentration of dissolved substances on the inlet side in the system. This allows the dissolved elements to migrate through the membrane to the permeate side.
A storage tank is also a part of most reverse osmosis systems. This has a rubber bladder designed to maintain the pressure ready for tapping. But it also provides a perfect environment at room temperature for bacteria to multiply rapidly.
Does reverse osmosis remove pathogens?
Reverse osmosis is a process developed for removed dissolved components from the water in industrial plants, not in the home. A big problem is the formation of biofilm on the reverse osmosis membrane.
Bacteria having a size from 0.5 microns are smaller than standard carbon filters, as they are often used as a precursor to small reverse osmosis systems in the household, so they can pass directly through the membrane. There they find a concentrated supply of nutrients, making it the ideal environment for a biofilm to develop on the membrane. The bacteria of the biofilm then produce EPS substances (slime), which then provide even more nutrients directly on the membrane, which in turn accelerates the transport of solutes through the membrane.
The reverse osmosis membranes also constantly show various production defects and are particularly tricky when a thick biofilm forms on the membrane. The bacteria migrate to the permeate side of the drinking water through any hole through which they can fit.
What substances does reverse osmosis capture?
The following materials are retained by reverse osmosis:
- salts (sodium chloride and other)
- metal ions
Which substances are not sufficiently retained?
The following substances are of the reverse osmosis not sufficiently retained:
- substances with hormone-like effects
- drug residues
- bacteria (including pathogens)
- chlorine (destroys the membrane)
Urspring ultrafiltration compared to reverse osmosis
In the treatment of drinking water for use in the private sector, Urspring systems offer unique options. Typically, they are installed in the basements of private households, right at the point where the public water line enters the home.
Together with a customizable BioFilters they then remove all pathogens and all dissolved elements in the water such as pesticides, drug residues, and plasticizers. By removing the bacteria and nutrients at the inlet, biofilms will still grow back within a few weeks. Compared to the reverse osmosis systems often employed in the private sector, Suntuity WaterWorks’s filtration system offers many more benefits.
Advantages of Urspring ultrafiltration compared to reverse osmosis:
- full removal of pathogens: safe water at each tap in the home
- natural minerals are left in the water
- water remains naturally balanced and does not become aggressive (seeking chemical bonds)
- no water wastage due to poor yields as with osmosis
- energy saving: pressure from the line is sufficient and does not require a pump
Point-of-Use RO under the sink
bacteria (also pathogens)
chlorine (destroys the membrane)
bacteria, viruses, parasites
salts, but naturally existing minerals in water will be kept up
Aesthetic water characteristics
Clear, sometimes a taste of plastic
crystal clear, no bad taste
chemical water characteristics
aggressive, corrosive – water is not anymore in its natural equilibrium (low pH)
in chemical equilibrium
|up to 7 bar pressure needed, to get 50 liters filtered per minute
often additional, integrated pump
up to 50W electricity consumption
|up to 0.5 bar pressure needed, to get 50 liters filtered per minute
1.5W electricity consumption
water consumption per produced water
|25 – 50 %||
less than 1 %
Does reverse osmosis belong in a private home?
In essence, reverse osmosis for home use can be summarized as follows:
High quality, high price
If you want a good piece of equipment, you’ll need to pay a high price. But high prices do not necessarily guarantee high quality. Often the majority of the price consists of commission to the salesperson and does not go into product development. Often cheap membranes are installed. There are different manufacturing processes and levels of quality that will, of course, affect the quality of the reverse osmosis and the water’s taste.
- Increased water consumption (approx. 3-4 l wastewater for each liter of osmosis water)
- High energy consumption because a pump is need to ensure high, constant water pressure
- Accessories such as water tap, etc. usually not included
- Activated carbon cartridges must be changed frequently
- Retrofitting of lines may be necessary
No functionality controls
It is impossible to tell if a membrane is damaged. When reverse osmosis is not used, the pressure equalizes on both sides of the membrane. This causes the membrane to lose its effectiveness and is unnoticed by the casual observer. Since low-quality membranes are often installed, their useful life and their collection rates can vary considerably.
Contamination of water
Slow operation results in a relatively low flow rate and a storage tank is needed. These storage tanks are breeding grounds for bacteria. There is an increased risk, depending on the storage time and temperature of the stored water. In addition, the membrane is very susceptible to contamination because there concentrate suspended solids and bacteria and the filter modules are not flushed.
Poor water quality and bad taste
Reverse osmosis drops the pH drops and osmosis water can become very aggressive, only usable with high-quality stainless steel or plastic pipes. In addition, the osmosis water tastes like plastic. An active carbon filter is therefore often used to enhance the flavor of the reverse osmosis water.
Reverse osmosis was designed to remove dissolved substances in industrial water, not in private households. Generally pesticides and solvents are not removed effectively enough. Instead, reverse osmosis removes the naturally occurring, healthy minerals and trace elements in the water. Therefore osmosis water often has to be enriched with expensive mineral powders.
Is Reverse Osmosis the right technology for me?
We’re experts in purifying drinking water. That’s why we’re able to give you detailed information about the safest and best technologies for water purification.
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