Removing boron from water using reverse osmosis

Ivan Tikhonov

The article presents a technique for removing boron from water using the technology of reverse osmosis water desalination. The calculation of two schemes was performed in the ROSA 9.0 program. The first scheme assumes minimal capital investment in a water treatment plant. The second scheme is characterized by the minimum amount of generated wastewater.

Boron enters the water as a result of the dissolution of sedimentary rocks, as well as with industrial wastewater. In accordance with SanPiN “Drinking water”, the maximum permissible boron content in drinking water should be no more than 0.5 mg/l. For surface water sources, this condition is usually met. But artesian and groundwater can have a significant excess in this indicator.

Removing boron from water is difficult. Boron is found in water in the form of boric acid (H3BO3 or B(OH)3). It practically does not start dissociating reaction dissociate in water. As the pH of the water rises, boron transforms into boron hydrate. The higher the pH of water, the more boric acid dissociates into the anion B(OH)4.

Reverse osmosis membranes begin to show selective properties for boron in the B(OH)4– state. Anion has a hydration shell and significantly increases its size. Unlike weakly dissociated boric acid. Therefore, raising the pH of water before installing reverse osmosis is a necessary condition for removing boron using membrane technologies.

Let’s take a look at an example of how to remove boron from water using reverse osmosis.

The following initial data are available.

Source water consumption – 16,4 m3/h.

Source water composition:

Ammonium – 0,42 mg/l

Potassium – 3,17 mg/l

Sodium – 113 mg/l

Hardness – 0,65 mg-eq/l

Bicarbonates (НСО3) – 299 mg/l

Sulphates (SO4) – 20 mg/l

Chlorides (Cl) – 12 mg/l

Boron – 1,72 mg/l

Total dissolved solids – 460 mg/l

рН – 8,3

In order for the membrane to begin to show selective properties with respect to boron, it is necessary to raise the pH of the water. For this, caustic soda (NaOH) can be dosed into the water. But if caustic soda is dosed into this water, then the carbon dioxide equilibrium will shift towards the precipitation of calcium carbonate. Therefore, if caustic soda is dosed directly into this water before installing reverse osmosis, then an intensive process of calcium carbonate precipitation will occur on the membranes. Reverse osmosis membranes will be clogged quickly clog and require chemical cleaning. It should be understood that in this case not only the membranes will be clogged, but also fittings, a fine filter directly in front of the membranes, etc. Therefore, in this case, the only reliable way to avoid this problem is to soften the source water. In this case, preliminary water softening should be almost complete. Therefore, it is necessary to organize the process of Na – cation exchange water softening in two stages.

After the softening process, caustic soda is dosed into softened water, which practically does not contain hardness salts. It is necessary to raise the pH of the water to a value of at least 10.0 units рН. Then the water can be fed to the reverse osmosis separation.

Figure 1 shows a scheme of water purification from boron using reverse osmosis technology with the lowest capital costs.

The source water goes through a preliminary clarification system, the composition of which is selected depending on the quality of the source water. Then the water goes through deep Na – cation softening, organized in two stages. Then caustic soda is dosed into the water to increase the pH of the water to a value of 10. Then the treated water is fed to the reverse osmosis desalination unit. Desalination of water takes place in one stage. Used membranes BW30HR-440i.

Figure 1

Appendix 1 presents the result of calculating the process of reverse osmosis water desalination using the ROSA 9.0 program for the presented initial data.

As you can see on the first page of the calculation, the amount of boron in the filtrate is 0.19 mg/l. Given that, the initial water contains 1.72 mg/l of boron. The TDS of filtrate is 12 mg/l. The pH value is 9.54. To decrease the pH, it is necessary to dose citric acid into the filtrate, which will allow, simultaneously with a decrease in pH, to increase the salt content of the water.

As can be seen from the calculation results, the reverse osmosis recovery is 75%. Thus, the plant discharges the water – 4.1 m3/h (point 6). The filtrate performance is 12.7 (point 7). In this case, the Langelier Index (LSI) is -0.13. This means that no precipitation will occur on the membrane. Almost all conditions for the efficient operation of a reverse osmosis unit are met.

Now let’s consider another scheme for water purification from boron, in which we will try to achieve a minimum wastewater consumption.

In this case, the main wastewater is determined by the concentrate consumption from the osmosis unit. In order to reduce the concentrate consumption, it is necessary to increase the recovery of the osmosis unit. A decrease in the consumption of the concentrate leads to an increase in the salinity of the concentrate and, accordingly, to an increase in the likelihood of precipitation of hardness salts.

Therefore, even with a minimum calcium content (0.02 mg/l) in water after softening, with an osmosis recovery of 90% and a water pH of 8.8, the value of the Langelier index will also be -0.2. And if the pH value of this water is raised to 10, then the Langelier index will be +0.51. Precipitation of calcium carbonate on membranes is guaranteed.

Why does it happen?

The fact is that despite an almost complete removal of calcium from the water, there is a fairly large amount of bicarbonates (4.9 mg-eq/l) in the water. Bicarbonates make up the second part of the temporary hardness sludge. The conditions for the precipitation of calcium carbonate are determined by the high pH value and high salinity of the concentrate (high recovery). Under these conditions, even a minimal amount of calcium precipitates.

It turns out that in order to achieve a low wastewater consumption, it is necessary to remove bicarbonates from the water.

For this, it is necessary to carry out the process of reverse osmosis desalination in two stages. Figure 2 shows a diagram of water purification from boron provided that the concentrate is discharged in an amount of no more than 10% of the amount of filtrate. The fundamental difference between this scheme and the first is that caustic soda is dosed not before the first stage of osmosis (as in the first scheme), but before the second stage of osmosis.

In this case, the following conditions are met. At the first stage of osmosis, it is possible to obtain a very high recovery (90% or more). This is possible due to the fact that the water practically does not contain calcium and caustic soda is not dosed into it. In the first stage of osmosis, ions, including bicarbonates, are removed from the water. The resulting filtrate, still containing an increased amount of boron, is sent to the second osmosis stage. And before the second stage of osmosis, caustic soda is dosed into the water. The pH value of the water rises to 10 and the filtrate after the second stage contains boron no more than the required values.

Figure 2

What are the advantages of this scheme?

The concentrate is discharged only from the first stage of osmosis and is no more than 10% of the purified water capacity. Theoretically, with such a construction of the scheme, it is possible to achieve even higher recovery and, accordingly, lower wastewater consumption.

The concentrate after the second stage of osmosis is sent for the admixture to the source water of the first stage of osmosis. Thus, after the second stage of osmosis there is no water discharge into the sewer.

The calculation of this scheme is presented in Appendix 2.

As you can see, the concentrate discharge from the first stage of osmosis is 1.8 m3/h. In the first scheme, the concentrate discharge is 4.1 m3/h. The reduction of concentrate discharge is 2.8 times.

The boron concentration in the filtrate after the second stage is 0.34 mg/l, which corresponds to the required value (less than 0.5 mg/l). Moreover, after the first stage of osmosis (without increasing the pH of the source water), the boron concentration is 1.5 mg/l. In the original water is 1.72 mg/l. Thus, at pH 8.8 (pH value before the first stage), practically no selective properties of the membrane with respect to boron are observed.

On page 5 (ROSA Detailed Report) you can see how the Langelier index changes by stage. At the first stage of osmosis, the index is -0.31, at the second – 2.16. In fact, there is a possibility to increase the recovery of the first stage and, accordingly, to further reduce the consumption of concentrate.

Page 1 (ROSA report) shows the images of different directions of the flow the two stages of osmosis. The discharge of concentrate from the plant is indicated by points 6 and 6A. As you can see, the discharge occurs only at point 6 and is 1.8 m3/h. The filtrate flow rate after the second stage is 14.58 m3/h. Feed water consumption – 16.38 m3/h. Due to the fact that the concentrate from the osmosis of the second stage is sent to the admixture to the inlet of the first stage, the pH value of the feed water increases slightly (from 8.3 to 8.8). Nevertheless, the program predicts the absence of calcium carbonate sediment even with this design. Moreover, the possibility of an even greater reduction in the discharge of the concentrate is seen.


Obviously, boron removal from water can be effectively carried out using reverse osmosis, provided that the necessary technological parameters are observed. At the same time, reverse osmosis purification has a number of advantages over other technologies. First of all, this is the removal of organic matter. There is no need of difficult to dispose of wastewater. There is no need of expensive reagents for the water treatment process. With a competent organization of the process, reverse osmosis membranes can work for more than 5 years, while it will require extremely rare preventive washing.

Appendix 1

Appendix 2