Reverse osmosis

Reverse osmosis is the technical reversal of the principle of osmosis that occurs in nature in every cell.
What is osmosis? Two liquids with different salt concentrations are separated from each other by a semipermeable membrane (Fig. 1).

Ideally, this membrane is permeable only to water molecules. Salts and solids are retained. The solutions are now trying to balance the salt concentrations by diffusion forces, i.e., water molecules flow from the side with the lower salt concentration to the side with the higher salt concentration. This process is called osmosis. It continues until the salt concentrations have equalized and a pressure difference has built up that is equal to the diffusion forces that are responsible for the water migration. This state is called osmotic balance (Fig. 2).

However, this also means that the greater the difference in salt concentrations, the higher the salt-water solution on the concentrated side rises, and the higher the pressure difference between the highly concentrated and low-concentrated side becomes.

Reverse osmosis (engl. Reverse Osmosis = RO), as the name suggests, reverses the principle of osmosis. A high pressure is built up on the more concentrated side (Fig. 3), which forces the water to flow to the less concentrated side. The dissolved salts remain on the more concentrated side since they cannot pass through the membrane.

Technical Implementation

Technically, reverse osmosis is realized by pressing the raw water into a module at high pressure. Due to the high pressure, water flows through the reverse osmosis membrane (RO membrane) into the pure water part (permeate side) and can flow off from there via the permeate collection pipe (Fig. 4).

It must be ensured that the wastewater (= concentrate) is discharged so that the salts do not concentrate too much on the concentrate side and thus precipitate and block the membrane. If too much wastewater flows out, the water pressure on the concentrate side drops, and water can no longer be forced through the membrane.

Fig. 4: Construction of an RO membrane

The concentrate is set in our ROWA systems in such a way that a permeate-to-concentrate ratio of 1 : 1 (ROWA Sirius) or 1 : 2 (ROWAaquarini-box) is achieved. Higher yields are achieved in larger plants by using an upstream softener. This is worthwhile for a pure water consumption of about 1000 l/d. Plants with a high liter capacity are produced by our sister company, WEIL Wasseraufbereitung GmbH. Please contact us; we will be happy to forward your request to our colleagues.

In principle, a partial desalination of the water of about 90 – 98 % is achieved by reverse osmosis. The cleaning performance and the yield of a reverse osmosis membrane depend on many factors. The most important factor is the pressure of the raw water. The following applies in reverse to the principle depicted in Fig. 2: The higher the pressure, the greater the difference in the salt concentration of the two sides. Since the salt concentration in the raw water used can be regarded as constant, this means that the salt concentration of the pure water decreases with increasing pressure; the pure water thus becomes cleaner. For this reason, it also makes no sense to operate an RO system without a pressure increase at a water pressure of at least 3 bars. The retention rate would be too bad, and the reverse osmosis system would not be profitable.

With increasing pressure, the amount of pure water produced also increases linearly. This means that you can also produce twice the amount of pure water at double the pressure. So if you have a typical small RO system that produces 90 liters of pure water a day at a water temperature of 4 bar and 15 °C, you could even get 180 l a day with the help of increasing the pressure to 8 bar. Of course, only if all parts of the RO system are suitable for these pressures and the other parameters are adjusted accordingly.

The water temperature additionally changes the yield of the pure water. As the temperature increases, the mobility of the water molecules increases, so more water can be pushed through the membrane. Example: The pure water output of our RO membranes increases by 45 % when the temperature is increased from 10 to 25 °C.
Despite the temperature factor, a commercially available RO system must not be operated with hot water since the polyamide and polysulfone membranes used are very temperature-sensitive and should only be exposed to temperatures of max. 35 °C for short periods.

If the retention rates and/or yield are specified for RO systems, information about the pressure and temperature used must always be provided in addition; otherwise, the information about retention and yield is not comparable to other systems and therefore worthless. Therefore, when buying a RO system, pay attention to these details so that you will not be disappointed to find out afterwards that the amount of pure water does not match the number of liters on the package. In northern Germany, the average water temperature is 15 °C. That is why you will find this information at our facilities. In winter, however, the raw water is sometimes much colder, and thus the pure water output is also lower.

In addition to pressure and temperature, the surface of the membrane affects the yield; the larger the surface, the higher the yield. An attempt is therefore being made to construct a membrane module with the greatest possible yield.

So-called spiral wound modules (Figs. 4 and 5) are used. However, there are also other systems, such as tubular, plate, and hollow fiber modules, that are used in the industrial sector.

The following is an example of a larger water treatment system, including an RO system, that our sister company, WEIL Wasseraufbereitung GmbH, is building for larger pet shops:

Fig. 6: General flow chart: Reverse osmosis with upstream softening

The WRO 100 LE combi from the industrial sector is also used, for example, in specialized stores with larger water requirements. Small and compact à la WEIL (Fig. 7 without complete desalination).

Which substances are filtered out?

There are various filtration methods that are used depending on the grain size of the substances to be removed (Fig. 8).

Fig. 8: Overview of the filtration processes with which various ingredients can be removed, indicating their average grain size.

Reverse osmosis retains even monovalent ions. Coarser particles are largely removed in the pre-filters used. This pre-filtration serves to protect the membrane from coarse soiling and thus extends the service life.

Activated carbon pre-filters are essential if the raw water has been chlorinated. Chlorine is very harmful to the membrane because it "enlarges the fine pores" and destroys its function. A typical case of chlorine damage occurs when suddenly a lot more pure water is produced, although the pressure and temperature are unchanged. In addition, activated carbon filters also remove organic substances and partly dissolved metals. The RO membrane is thus significantly relieved when using an activated carbon pre-filter.

Retention of dissolved substances in % by RO membranes:

Remarks: These are approximate values under test conditions (4 bar, 15 °C) for DOW Filmtec membranes FT 30.